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A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components
Accepted Manuscript A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components Kailun Zheng, Denis J. Politis, Liliang Wang, Jianguo Lin PII:
S2588-8404(18)30012-X
DOI:
10.1016/j.ijlmm.2018.03.006
Reference:
IJLMM 8
To appear in:
International Journal of Lightweight Materials and Manufacture
Received Date: 6 February 2018 Revised Date:
26 March 2018
Accepted Date: 27 March 2018
Please cite this article as: K. Zheng, D.J. Politis, L. Wang, J. Lin, A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components, International Journal of Lightweight Materials and Manufacture (2018), doi: 10.1016/j.ijlmm.2018.03.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components Kailun Zhenga, Denis J. Politisa,*, Liliang Wanga, Jianguo Lina
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a) Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
Abstract
Aluminium alloys are being increasingly utilised in the automotive and aerospace industries to reduce the weight of vehicles. Extensive research has been conducted to overcome the poor ductility of aluminium alloys at
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room temperature and improve formability of the materials, to enable complex-shaped panel components to be manufactured. To this end, this paper contains a comprehensive review of widely used forming processes for aluminium alloys, under cold, warm and hot forming conditions, and the material characteristics and equipment
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used for each process. Based on a detailed analysis from the view of industrial requirements, recent progress in experimentation techniques are reviewed addressing the limitations and improvements of specific forming processes. Furthermore, material modelling methods at both cold and elevated temperature forming conditions have been presented. In addition, finite element (FE) simulations with the implementation of material models are discussed. This review article intends to provide a systematic guide for process designers to choose the most
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appropriate sheet forming technique for specific industrial applications.
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Keywords: Aluminium alloys, forming, complex-shaped, panel components, modelling
Corresponding author: Denis J. Politis, [email protected]
ACCEPTED MANUSCRIPT A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components
Abstract
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Aluminium alloys are being increasingly utilised in the automotive and aerospace industries to reduce the weight of vehicles. Extensive research has been conducted to overcome the poor ductility of aluminium alloys at room temperature and improve formability of the materials, to enable complex-shaped panel components to be manufactured. To this end, this paper contains a comprehensive review of widely used forming processes for aluminium alloys, under cold, warm and hot forming conditions, and the material characteristics and equipment
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used for each process. Based on a detailed analysis from the view of industrial requirements, recent progress in experimentation techniques are reviewed addressing the limitations and improvements of specific forming processes. Furthermore, material modelling methods at both cold and elevated temperature forming conditions
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have been presented. In addition, finite element (FE) simulations with the implementation of material models are discussed. This review article intends to provide a systematic guide for process designers to choose the most appropriate sheet forming technique for specific industrial applications.
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Keywords: Aluminium alloys, forming, complex-shaped, panel components, modelling
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Contents Abstract................................................................................................................................................................... 1 1. Introduction ........................................................................................................................................................ 4 2. Lightweight aluminium alloys and forming techniques ..................................................................................... 5
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2.1 Lightweight aluminium alloys ...................................................................................................................... 5 2.1.1 Applications in automotive and aerospace industries ................................................................................ 5 2.1.2 Characteristics and comparisons ................................................................................................................ 6
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2.2 Review of sheet metal forming techniques ................................................................................................... 7 2.2.1 Cold forming.......................................................................................................................................... 8
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2.2.1.1 Cold stamping ................................................................................................................................. 8 2.2.1.2 Cryogenic forming .......................................................................................................................... 9 2.2.1.3 Sheet hydroforming ...................................................................................................................... 10 2.2.1.4 Incremental sheet forming ............................................................................................................ 12 2.2.2 Elevated forming techniques ............................................................................................................... 13 2.2.2.1 Warm stamping ................................................................................................................... 14
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3. Recent progress of experimentation ................................................................................................................. 20 3.1 Warm stamping........................................................................................................................................... 20
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3.1.1 Raw material candidates ...................................................................................................................... 20
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3.1.2 Process variables .................................................................................................................................. 22 3.1.2.1 Heating.......................................................................................................................................... 22 3.1.2.2 Forming ........................................................................................................................................ 23
3.2 Warm sheet hydroforming .......................................................................................................................... 24 3.3 Hot gas forming .......................................................................................................................................... 24 3.3.1 Quick plastic forming .......................................................................................................................... 24 3.3.2 Superplastic forming ............................................................................................................................ 25 3.4 Hot stamping............................................................................................................................................... 26 4. Material modelling and numerical simulations................................................................................................. 34 2
ACCEPTED MANUSCRIPT 4.1 Cold forming condition............................................................................................................................... 34 4.1.1 Fundamentals of material model.......................................................................................................... 34 4.1.2 Improvements on material model under cold forming condition ......................................................... 35 4.1.3 Modelling of anisotropic and kinematic hardening ............................................................................. 36
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4.1.3.1 Anisotropy for earing prediction ................................................................................................... 36 4.1.3.2 Kinematic hardening ..................................................................................................................... 36 4.2 Elevated temperature forming conditions ................................................................................................... 37
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4.2.1 Phenomenological models ................................................................................................................... 37 4.2.2 Physical based material model ............................................................................................................. 37
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4.3 Unified material model ............................................................................................................................... 38 4.3.1 Hot stamping (HFQ®) .......................................................................................................................... 38 4.3.2 Superplastic forming ............................................................................................................................ 39 4.4 Model application ....................................................................................................................................... 40 4.4.1 Hot stamping........................................................................................................................................ 40
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4.4.2 Superplastic forming ............................................................................................................................ 41 5. Conclusions and future research trends ............................................................................................................ 42
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References ............................................................................................................................................................ 44
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1. Introduction The concerns of environmental protection and stringent requirements of greenhouse gas emissions have driven transportation industries to reduce the weights of their products [1]. Applying lightweight materials, such as aluminium and magnesium alloys or composites [2], to replace traditional steel raw material candidates [3], has become an efficient and popular approach to achieve weight targets. In addition, the development of battery
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electrical vehicles (BEVs) has also required the usage of lightweight materials to compensate for the insufficient energy density of current batteries and extend the range of BEVs [4]. Among the family of lightweight materials, aluminium alloys have been extensively used in the automotive and aircraft body structures [5], due to the advantages of low density, high strength to weight ratio, good corrosion resistance and relatively low cost compared with composites [6].
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For automobile applications, the manufacture of panel components has been the subject of significant research as they occupy a large proportion of the mass of body structures, as shown in Fig. 1(a) [7]. Previous usage of high strength aluminium alloys has been restricted by the poor ductility at room temperature. Recently, the rapid
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development of forming techniques has also contributed to the manufacture of complex-shaped high strength aluminium alloy panel components, resulting in their applications being significantly extended [8]. Body in white structures have been manufactured from aluminium alloys in some luxury vehicles, such as Jaguar Land Rover and Audi [9]. An overall summary and evaluation of recent progress of forming techniques driven by industrial demands is currently lacking.
This paper provides a systematic and comprehensive review of the forming techniques used to form complex-
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shaped aluminium alloy panel components. Material characteristics, processing variables, specific equipment used, and advantages and disadvantages of each forming technique are also presented. Moreover, recent research progress to address the identified disadvantages, such as new experimentation and theoretical modelling techniques are also reviewed. Finally, conclusions on the latest state of the art forming processes are drawn summarising the recommended applications of each forming technique. Finally, through a review of the
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literature, the paper aims to provide process designers and researchers a guide to selecting appropriate forming
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techniques and processing parameters and also identify the trends of future scientific research.
Figure 1. Applications of aluminium alloy panel structures in an automobile and aircraft: (a) Audi TT coupé [7] and (b) Airbus 380 [10]
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2. Lightweight aluminium alloys and forming techniques 2.1 Lightweight aluminium alloys Wrought aluminium alloys are the most widely used raw material candidates for panel structures. Nowadays, non-heat treatable AA5xxx, and heat treatable AA6xxx, AA7xxx and AA2xxx, are popular candidates for automotive and aircraft industries. This section briefly reviews the current applications of these alloys and their
techniques. 2.1.1 Applications in automotive and aerospace industries
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characteristics and properties, enhancing the analysis and evaluations of forming these alloys using different
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Typical applications of different alloys in an automobile are summarised in Table 1. Non-heat treatable aluminium alloys, such as AA5754 (Al-Mg), are widely used for internal body structures to compensate for the insufficient strength. The strengthening mechanism of this alloy is strain hardening, normally by cold working
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during fabrication, in association with solution hardening. Therefore, AA5xxx sheet components are normally cold stamped, in the annealed(O), as fabricated (F) or strain hardened (H) conditions with greater ductility. In comparison, medium or high strength aluminium alloys, AA6xxx and AA7xxx, are potential material candidates for outer panel structures. These alloys are precipitation hardened, and their strength arises from the typical T6 heat treatment for the automotive industry. However, the current use of high strength alloys, like AA7xxx, is still limited for automotive vehicles.
In terms of the aircraft industry, AA7xxx and AA2xxx are the main material candidates due to their high
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strength and good corrosion resistance. The corrosion resistance property of AA7xxx is poor at T6 condition with peak strength, which results in such alloys being applied in an over-aged condition, such as T73. Al-Li alloys were first developed in the 1920s. To date, the third generation Al-Li is believed to be at a mature stage and capable of being used extensively. Compared with other aircraft alloys, Al-Li alloys exhibit the advantages
aircraft [10].
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of low density, high stiffness and good fatigue resistance, which are widely used in the wing structures of an
The applications of a specific alloy are predominantly determined by their mechanical properties. However, it
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should be noted that, the work-ability of an alloy can also determine its application. For example, the extensive usage of AA5xxx aluminium alloys results from such alloys being soft and easily formable. For high strength aluminium alloys, which have similar strength levels to conventional mild steel, the ductility is poor at room temperature which results in these alloys being unable to sustain higher deformations and be manufactured to complex-shaped components. The corresponding applications are restricted, and advanced forming techniques are required.
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AA5052
AA5754
AA6016-T4
Al-Mg-Si
AA6111-T4 Al-Zn-Mg
AA7055T7751 AA7075-T73
Al-Zn-Mg
AA2024-T3 AA2199-T8
Strength/Density (Pa/(kg.m-3))
Applications
2680
72000
2670
86000
Interior panels and components, truck bumpers and body panels [11] Inner body panels, splash guards, heat shields, air cleaner trays and covers, structural and weldable parts, load floor [11] Outer panels and structural sheets (Europe) [12] Outer and inner panels (North America) [12] Potential applications for A pillar and B pillar [15][16] Case of fuselage [17]
2700
81500
2710
103000 [13]
2780
127000 [14]
2860
222000
2810
180000
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Aircraft
AA7020-T6
Density (kg/m3)
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Automotive
Main chemical composition Al-Mg
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Alloy grade
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Al-Cu-Mg
2780
1740000
Al-Cu-Li
2640
152000
Upper wing skins, stringers and horizontal/vertical stabilizers Upper wing structure [17] Fuselage and lower wing structure [18]
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2.1.2 Characteristics and comparisons
Essentially, the differences of mechanical properties of different alloys arises from the addition of alloying
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elements. As each element has a different solute ability within the aluminium matrix, the type, quantity and combination of alloying elements results in variations in microstructure and mechanical properties. For instance, AA2xxx alloys have lower fracture toughness than those of AA7xxx with similar yield strength level, as larger sizes of inter-metallic compounds exist in the AA2xxx alloys. To improve the fracture toughness, the levels of iron, silicon or copper can be reduced to avoid the formation of large and brittle compounds. Table 2 summarises the main compositions of typical wrought aluminium alloys in literature considering the slight difference in element percentage of different commercial aluminium alloy supplier. Although various alloys have been commercialised, this paper only focuses on commonly and widely used materials for automotive and aircraft industries. Progress made to develop novel aluminium alloy grades with new additional elements, and corresponding metallurgy are not taken into account.
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ACCEPTED MANUSCRIPT Table 2. Main chemical compositions of some aluminium alloys (wt%) Mg 3.0 Cu 4.5 Si 1.05 Zn 5.4
Mn 0.24 Mg 1.5 Mg 0.8 Mg 2.2
Fe 0.26 Mn 0.5 Mn 0.68 Cu 1.4
Si 0.03 Si 0.41 Fe 0.26 Fe 0.22
Cu 0.02 Fe 0.40 Cu 0.04 Cr 0.19
Ni <0.01 Zn 0.20 Zn 0.02 Si 0.07
Ti <0.01 Ti 0.12 Cr 0.01 Mn 0.04
Zn <0.01 Cr 0.07 Ti 0.01 Ti 0.02
Al Bal Al Bal Al Bal Al Bal
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AA5754 [19] AA2024 [20] AA6082 [21] AA7075 [22]
Fig. 2 summarises the mechanical properties of a series of commonly used aluminium alloys. Compared with low strength AA5754, AA6082 exhibits significantly improved strength, which makes this alloy the preferred grade in the automotive industry. In comparison, AA2024 and AA7075 exhibit even greater strength, which
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enables them to be used in aircraft applications. In addition, for heat treatable aluminium alloys, heat treatment temper is also a significant factor determining the application of an alloy. Although the strength of AA7075 in T73 condition (overaged) is less than that in T6 condition (peak age), the corrosion resistance property is
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improved due to the growth of precipitates when the alloy is overaged. The large precipitates contribute to corrosion resistance in the severe service environment of an aircraft. For automobiles, the service environment of components is not as severe, and hence aluminium alloys are normally heat treated to the peak strength to
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meet vehicle safety requirements.
Figure 2. Comparisons of mechanical properties of commonly used alloy and different heat treatment temper [23][19]. 2.2 Review of sheet metal forming techniques To evaluate and enhance the analysis of a specific forming technique, raw material selection, which depends on alloy grade and heat treatment temper needs to be considered, as the microstructure and mechanical properties of a material play an intrinsic role on the capability of individual forming techniques. This section briefly 7
ACCEPTED MANUSCRIPT reviews the forming techniques and corresponding material characteristics for commonly used commercial aluminium alloys. 2.2.1 Cold forming 2.2.1.1 Cold stamping Cold stamping using rigid dies is believed to be the most commonly used forming technique for the manufacture
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of aluminium alloy components, especially for non-heat treatable AA5xxx. For heat treatable alloys, such as AA6111 shown in Fig. 3, the ductility in T6 condition is very poor which is not beneficial for producing complex-shaped structures. To address this disadvantage, W-temper forming [24] or stamping at T4 temper condition can be used. W-temper refers to the super saturated solid solution state after solution heat treatment [25]. Alloys in such a state exhibit lower strength but improved ductility, as shown by the dash line in Fig. 3.
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However, the strength of W-temper or T4 formed components needs to be increased by additional heat treatment, to restore the microstructure and mechanical properties of T6 temper. However, springback [26] and quenchinginduced distortions [27] may be experienced during the additional heat treatment, especially for high strength
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alloys [28]. The loss of dimensional accuracy from cold stamped aluminium alloys presents difficulties in component assembly in a mass production setting.
Besides the proper selection of initial blank heat treatment temper to improve draw-ability, multi-stage deep drawing can also be used to manufacture high aspect ratio components [29]. However, multi-stage deep drawing is deemed not suitable for automotive components, due to the lower production efficiency and irregular shapes of automobile parts. Lubrication is another feasible approach to reduce interface friction and enhance the ability to produce a good quality part [30]. In addition, lubricant enables tool life to be extended by reducing wear [31].
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Rao and Wei [32] investigated the lubrication performance of an environmentally-friendly and low cost boric acid film by comparing conventional commercial solid and liquid lubricants. Higher sheet metal draw-ability and lower forming forces were found to be the direct benefits that can be achieved. Recently, to remove the post-cleaning of lubricant operation, surface coating techniques, especially low frictional carbon-based coatings
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have been extensively investigated on a laboratory scale [33] [34]. However, for large cold stamped aluminium alloy parts, lubricants are not indispensable considering the much lower strength of aluminium alloys compared
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Figure 3. Characteristics of cold stamping using rigid dies: (a) Stress-strain curves of different alloys and different tempers [35][36], (b) Springback defect [26] and (c) Shape distortion due to additional heat treatment [27].
2.2.1.2 Cryogenic forming
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Cryogenic forming [37] is motivated by reducing the discontinuous deformation of stamping AA5xxx at room temperature. The discontinuous deformation is marked by the formation of deformation bands that not only leave undesirable traces on the surface of panels, but also reduce the ductility of the alloy [38]. The phenomenon is known as the Portevin-Le Chatelier (PLC) effect [39]. To reduce the negative impact of the PLC effect, aluminium alloys can be formed at cryogenic temperatures (below room temperature) selectively. Schneider et
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al. [40] performed tensile tests at temperatures ranging from 25 °C to -196 °C and strain rates from 1.7 × 10-3 to 6.6 × 10-2 for aircraft alloys. Fig. 4 shows the variations of yield strength, ultimate tensile strength and fraction elongations at different temperatures. Strength increases with decreasing temperature for all alloy candidates,
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while ductility of EN AW-7021 exhibited a drop at -196 °C. Glazer et al. [41] also observed the improved strength toughness, strain hardening rate and tensile elongation with decreasing temperatures for AA2090-T81. Welpmann et al. [42] tested the low temperature deformation of Al-Li alloy, AA8090, from room temperature to -196 °C. Yield strength, uniform strain and elongation to failure were gradually improved as the test temperatures decreased. Such enhanced properties conclude the potential of forming complex-shaped components at cryogenic temperatures of aluminium alloys, although the process is more technologically challenging than elevated temperature forming for automotive industry [11]. Feasibility of cryogenic forming aerospace alloys still exists considering the smaller volumes and price insensitivity of aerospace components. Formed sheet metal blanks normally experience at least a tensile deformation in one direction. The mechanism of cryogenic forming is believed to be associated with the PLC effect caused by Cottrell-atmospheres and dislocation movements. When the alloy in a W temper is exposed to a cryogenic temperature, the diffusivity of
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ACCEPTED MANUSCRIPT pre-dissolved additional atoms is generally decelerated, which results in the formation of Cottrell-atmospheres of the same intensity during the same period of time being unlikely. Then, the PLC effect is reduced correspondingly and the elongation is increased. In terms of the strength, at cryogenic temperatures, the activation energy is decreased. Consequently, the force required to move a dislocation through the crystal lattice can be increased. Based on the above discussion, aged conditions with precipitates capable of promoting the
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formation of Cottrell-atomospheres, such as T4 or T6, are not ideal states for cryogenic forming processes [40].
Figure 4. Characteristics of cryogenic forming, (a) Improved mechanical properties of different alloys and (b) Substitutional solute atoms at stationary forest dislocations generate Cottrell-atmospheres [40].
2.2.1.3 Sheet hydroforming
Sheet hydroforming is a relatively new sheet metal forming technology originating from the well-established hydroforming technology, that was first developed in the 1890s [43]. A typical forming method among sheet hydroforming techniques is hydrodynamic deep drawing, as schematically shown in Fig. 5. Instead of the conventional deep drawing with a die cavity, oil or other pressurising liquid medium are utilised to press the sheet metal tightly onto the punch when it is drawn into the die by the rigid punch. In this stage, the friction between the sheet metal and die is reduced as a result of the liquid medium in the die cavity, which flows out
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ACCEPTED MANUSCRIPT between the upper surface of the die and the sheet metal. This fluid flow results in a lubrication effect that reduces frictional forces. By this process, the sheet metal can be drawn to a greater depth and drawing ratio (DR) value of sheet metal can be increased, as shown in Fig. 5(b), and the quality of the part can be improved [62]. Several novel process modifications have been made based on the hydrodynamic deep drawing to increase the ability of manufacturing complex-shaped components. Nakamura and Nakagawa proposed radial pressure assisted hydraulic counter pressure deep drawing [44] and reverse redrawing under hydraulic counter pressure
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with its radial pushing [45] in 1985, which enables the significant increase of draw-ratio. Kolleck and Cherek [46] proposed an active hydrodynamic deep drawing to form large extensive deep drawn parts. In this process, a preforming stage is involved prior to hydrodynamic deep drawing. Springback can be reduced and resistance of denting can be strengthened using such a process.
Blankholding force and liquid pressure are critical process parameters in sheet hydroforming, which determine
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the flange wrinkling and tearing [47]. Yossifon and Tirosh [48] developed a fluid assisted blankholder to be used in conventional deep drawing, which is effective for preventing flange wrinkling. Damborg and Jensen [49] achieved adjustable blankholding force in the hydrodynamic deep drawing using a closed-loop control system.
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However, it was found that the variable blankholding force has little effect on the forming. Double-sided hydroforming [50] enhances the formability of sheet due to the fluid pressure increasing the hydrostatic pressure imposed on the sheet. The hydrostatic stress state is able to suppress the void expansion and growth, and thereby delays the fracture initiation [51]. Chen et al. [52] proved that appropriate liquid pressure can be used for manufacturing wrinkling-free parts from AA2219 both experimentally and analytically. Besides the above investigations of process variables for sheet hydroforming, novel contributions have also been made in areas
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such as tooling [53] and viscous pressure medium [54], which were not discussed in detail in this section. In addition, sheet hydroforming can also be integrated with other forming processes, such as stamping [55] and stretch forming [56].
The advantages of sheet hydroforming can be summarised as follows: Friction at the flange can be reduced due to fluid lubrication [57] and Friction retention effect on the
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punch is affected [58].
Draw-ability can be significantly increased [43].
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Reduced surface defects of the formed part due to the liquid pressure medium [59].
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Fewer dies are used thus reducing the tool cost [59].
Despite all the advantages, there are several drawbacks with sheet hydroforming. Compared with conventional stamping processes, the manufacturing efficiency is low especially for the automotive industry [60] and the manufacturing cost per part is high. Although sheet hydroforming is mainly used for closed-form revolutionary components, with shape distortion and springback being eliminated, there is an additional heat treatment process required to restore the component strength resulting in additional cost. Extensive research has been performed on sheet hydroforming, resulting in a very mature process that is widely used in industry [61].
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Figure 5. Sheet hydroforming: (a) Comparisons between conventional deep drawing and sheet hydroforming
2.2.1.4 Incremental sheet forming
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[43], and (b) Improved draw ratio using sheet hydroforming [62].
Incremental forming was first introduced by Matsubara in Japan in 1993 [63], with an extensive review of incremental sheet forming developments being reported by Emmens et al. [64]. The process can be sub-divided into different types according to the tool configurations [65], as shown in Fig. 6. Among them, single point incremental forming (SPIF) is the most commonly used. During the SPIF process, the blankholder is utilised for clamping and holding the sheet blank in position. The backing plate supports the sheet and its opening defines
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the working area of the single point forming tool. The rotating single point forming tool is utilised to progressively shape the sheet into a component and its path is generated by a CNC machining centre. During the forming process, there is no backup die used to support the bottom surface of the deformed sheet [66]. Echrif and Hrairi [67] reviewed the recent research and progress of incremental sheet forming, mainly on methods and
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potential applications. The advantages were summarised as below: The SPIF can be executed in a conventional CNC machine, hydraulic press is not required.
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Design changes can be performed efficiently and easily by the programming of CNC machine.
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Parts are produced directly from a CAD file, and no male and female dies are required. The forming load is small and formability can be increased.
Given these advantages, the main concerns with this technique are: (1) the production efficiency, which severely restricts its application for the automotive industry; (2) thinning of the formed sheet and (3) limited geometry accuracy such as accumulated step change errors and springback. In addition, the surface quality can be another issue as a result of the tooling component sliding and scratching the sheet surface, although this depends on the surface quality of tool and forming speed. To address these drawbacks, Araghi et al. [68] proposed a hybrid process, by combination of the SPIF and stretch forming, which enables a reduced processing time and improved uniformity of thickness after forming. Lubrication is also an effective approach to address the issue of surface quality. Jawale et al. studied the effect of lubrication on SPIF from the microstructure point of view [69]. The SEM images from different lubrication states were compared and it was found that there was some 12
ACCEPTED MANUSCRIPT influence of lubrication on the surface quality. It was observed that the marks left by the forming tool and the development of valley like structures along the grain boundaries were the cause for increased surface roughness. Azevedo et al. [70] tested a series of lubricants of SPIF aluminium 1050 and DP780, and proved that using
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lubricant guarantees a better surface finish of SPIF aluminium rather than steel.
Figure 6. Schematics of incremental sheet forming with various tooling configuration [65] 2.2.2 Elevated forming techniques
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To address the poor ductility of aluminium alloys and increase the ability of manufacturing complex-shaped components, forming at elevated temperatures becomes a feasible solution. Alloy ductility is increased and material strength is reduced with increasing forming temperatures. Different elevated temperature forming techniques, such as warm forming, quick plastic forming, superplastic forming and hot stamping, exist and have
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been extensively investigated and used for industrial applications. Fig. 7 shows the classification map of different elevated forming techniques according to the approximate temperature and strain rate for reference [75]. For aluminium alloys, elevated temperature forming techniques
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can be further divided into warm or hot forming conditions with traditional forming processes classified into these categories, for example warm hydroforming [57] and incremental forming [71]. Normally, warm forming refers to forming material at temperatures below the recrystallization temperature [72] and above 0.3 times the
melting temperature (0.3 ) [73]. For hot forming, temperatures are selected to be above the recrystallization temperature and normally below the solvus temperature. Semi-solid forming [74] is not considered in this
review. This section reviews commonly used elevated forming techniques for aluminium alloys based on the warm or hot classification criterion.
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Recrystallization
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2.2.2.1 Warm stamping
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Figure 7. Classifications of elevated forming techniques [75]
Stamping sheet alloys at warm forming condition using conventional rigid dies is the most commonly used warm forming technique. The process can be either isothermal or non-isothermal depending on the die temperatures. Fig. 8(a) shows a typical non-isothermal warm forming process with a non-uniform temperature distribution. In this process, the sheet blank can be heated to elevated temperatures using an external furnace or
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hot dies [76]. The punch is cooled using pre-machined water cooling channels. The advantages of this nonisothermal process are summarised as follows: 1.
Material ductility and forming limit can be increased at warm forming temperatures, as shown in Figs. 8(c) and 8(d) [77] [78] .
Springback defect can be reduced compared with cold stamping, resulting in a good dimensional
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accuracy of formed part [79].
Non-uniform temperature distribution contributes to increase draw-ability [80].
4.
Manufacturing efficiency is high compared with warm hydroforming and warm incremental forming
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processes.
However, there also exist some limitations of the warm forming technique, which are briefly listed as follows: 1.
Warm forming is extensively being researched in a laboratory setting, with no well-known press shops implementing the process [11].
2.
Warm forming is believed to be unsuitable for high-strength heat treatable aluminium alloys, since heating might affect the alloy microstructure and deteriorate post-form strength [81].
3.
The interfacial friction is increased resulting in high temperature lubricants being required [82].
It should be noted that, lubrication exhibits two opposite effects [83], the positive effect of lubrication is the reduced tool wear and improved product surface quality, while the negative effect is that the process is
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lubricant affect the productivity and makes the process uneconomical.
Figure 8. Characteristics of warm stamping using rigid dies: (a) Schematic of non-isothermal warm stamping [84], (b) Comparisons of cold and warm formed components [85], (c) Improved uniaxial ductility of warm
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forming [77] and (d) Improved forming limits of warm forming condition [78] Warm sheet hydroforming
The relatively complex die set-up of hydroforming results in a sheet blank that is unable to be heated using an
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external furnace and subsequently transferred to cold dies. In order to hydro-form aluminium alloys at elevated temperatures, the sheet blank is simultaneously heated with forming dies using an external furnace [86] or heating bands [87]. The punch can be kept cool if blankholders are self-heated [88]. Compared to cold hydroforming, warm hydroforming utilises the improved formability of aluminium alloys at elevated temperatures, which is effective for high strength aluminium alloys [89], such as AA6xxx and AA7xxx with poor ductility at room temperature. There are several key challenges to be addressed for warm hydroforming: 1.
Forming temperature is dependent on the pressurizing medium sustainable temperature. The conventional oil medium temperature is below 300 °C.
2.
An optimal non-isothermal temperature distribution needs to be determined within the tooling [90], similar to warm forming using rigid dies.
3.
Warm hydroforming is currently being developed to achieve reduced number of manufacturing steps and part consolidation [88]. 15
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2.2.2.3 Warm/hot sheet incremental forming
Warm/hot incremental sheet forming is an improvement from conventional incremental sheet forming by utilisation of a heating device to improve the formability of aluminium alloys. Compared with stamping using rigid dies, a relative simple set-up of incremental forming enables various kinds of flexible heating methods to be used. Ji and Park [71] selected a heating method of hot air blowers to heat the AZ31 sheet blank. Duflou et al.
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[91] used the laser-assisted local heating method for single point incremental forming a TiAl6V4 sheet. The method is believed to be highly efficient and costly. Fan et al. [92] proposed an electrical heating method to heat the blank using material resistance and external electrical power. Considering the sheet blank experiences thickness variations due to plastic deformation during forming, temperature control by adjusting current density can be challenging. Surface finish and geometrical accuracy are two main problems due to the extreme high
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temperature at local areas [93] when using a localised electrical heating method. Ambrogio et al. [94] used a conduction heating approach for forming the AZ31 sheets. A heater band was mounted at the external surface of the tools and the blank was indirectly heated using the hot tools. Compared to the local heating approach, this
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method lacks energy efficiency. It should be noted that the above heating methods can also be combined to increase the heating efficiency. •
Hot gas forming
Hot gas forming refers to blanks utilising a pressurised gas medium. From this point of view, hot gas forming can be further divided into superplastic forming (SPF) which has already been extensively investigated, and quick plastic forming (QPF) which was developed by General Motors [95], as shown in Fig. 9(a). The major
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difference between SPF and QPF is the strain rate and raw material candidate. SPF deforms aluminium alloys at a strain rate exhibiting maximum ductility, while QPF process aims to deform aluminium alloys at strain rates significantly greater than those in SPF to reduce the processing time. In addition, SPF requires raw material that is intrinsically superplastic with fine grains. Generally, SPF is designed to form low volume highly complex-
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shaped components, while QPF is designed for producing parts of less complexity but of high productivity. Table 3 summarises the main differences between QPF and SPF.
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Table 3 Comparisons of SPF and QPF processes [96]
Process variables
Market features
Manufacturing system
Super plastic forming Strain rate/Temperature chosen for exploiting maximum material ductility •
Aerospace Low volume (100 panels per year) • High price points product • Hand rework common for dimensional and surface quality
Low level automation, manual handling Low capital, high piece cost
Typical panels 16
Quick plastic forming Strain rate/Temperature chosen to maximize productivity consistent with final quality Automotive • High volume (tens of 1000s panels per year) • Low price point product (Consumer goods) • Emphasize first time dimensional and surface quality High level automation, automatic handling Higher investment, lower piece cost Moderate shapes (More complex than
ACCEPTED MANUSCRIPT Extreme shapes High forming strains-back pressure to limit cavitation Maximum mechanical properties
automotive metal stamping) Moderate forming strains-back pressure not required Moderate post-form strength
In terms of SPF, as shown in Fig. 9(b), the blank is heated to a pliable state, 0.5 − 0.6 , then pressurised by
gas to deform the sheet blank into the die cavity. The high temperatures allow the alloy to be elongated or stretched, to a much greater degree without rupture than would be observed in cold or warm forming processes.
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Furthermore, finer details of the part can be successfully formed, which requires less overall forming force compared to conventional forming methods. Typical features of SPF can be summarised as follows: 1.
SPF is an isothermal forming process. Therefore, the whole tooling set-up is normally located within a furnace to guarantee the blank and die have the same temperature, resulting in difficulties in automation.
Extremely large plastic deformation could be observed, which is ideal for forming very complex panel
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components, as shown in Fig. 10(a).
Sheet blank should have fine grains and grain growth rate must be slow during forming, as shown in Fig. 10(b).
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The productivity of SPF is low.
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3.
A number of aluminium alloys have been proved to exhibit superplasticity. Table 4 summarises the process variables and superplastic features of commercial alloys with superplasticity.
Table 4 Compositions of superplastic aluminium alloys and corresponding superplastic forming conditions [97] Composition, wt%
2004 5083 7475 8090 2090
Al-6Cu-0.4Zr Al-4.5Mg-0.7Mn-0.1Zr Al-5.7Zn-2.3Mg-1.5Cu-0.2Cr Al-2.4Li-1.2Cu-0.7Mg-0.1Zr Al-2.5Cu-2.3Li-0.12Zr
SPF temperature (°C) 460 500-520 515 530 530
Strain rate (/s) ~10-3 ~10-3 2×10-4 5×10-4 ~10-3
Elongation (%) 800-1200 ~300 800 1000 500
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Alloy
Figure 9. Schematics of hot gas forming, (a) Quick plastic forming [95] and (b) Superplastic forming [98].
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Figure 10. Typical features of superplastic forming: (a) Schematic superplastic forming mechanism [99], and (b) EBSD maps of superplastic forming an Al-Zn alloy [100]. Compared with superplastic forming aluminium alloys, the requirements of alloys used in QPF are not strict.
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QPF incorporates much more automation compared to conventional SPF processes for producing dimensionally accurate components directly from a forming cell. The QPF process is highly optimized around each specific
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component to minimize the cycle time and maximize productivity. Although the shape complexity is less than SPFed components, forming at elevated temperatures still enables poor formability of aluminium sheet materials to be overcome compared to cold stamping conditions. •
Hot stamping with rigid dies
Recently, to extend the applications of aluminium alloys in the transportation industry, especially the automotive industry, hot stamping of high-strength heat treatable aluminium alloys at even higher forming temperatures than conventional warm forming has become feasible and efficient. One of these processes, named Hot Form and Quench (HFQ®), was proposed by Lin et al. [101], and is believed to be a leading-edge technique in this area [75]. Fig. 11 illustrates the temperature profile of a blank in the HFQ® process [102]. Initially a blank is heated to its Solution Heat Treatment (SHT) temperature and soaked for a specific time period to
dissolve the original coarse precipitates and soluble inclusions within the -Al matrix and obtain an optimum microstructure. Then the blank is quickly transferred to the press, stamped and held for a brief period between 18
ACCEPTED MANUSCRIPT the cold dies which quench the blank to lower temperatures (normally lower than the artificial ageing temperature for heat-treatable aluminium alloys). The blank is more ductile at elevated temperatures, and using cold die quenching can achieve a cooling rate rapid enough to prevent the formation of coarse secondary phase at grain boundaries and obtain a super saturated solid solution state in the formed part. For heat treatable aluminium alloys, the hot stamped part can be artificially aged to achieve higher strength to meet the design requirements of vehicle manufacturers. Beneficial characteristics of HFQ® can be summarised as follows: HFQ® is a hot stamping process integrated with heat treatment, which is independent of alloy grade and
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1.
initial heat treatment temper. Optimal microstructures can be controlled during hot forming and postformed strength can be guaranteed. 2.
Ductility can be maximized, which enables the successful forming of complex-shaped components and extends the application of aluminium parts in car body structures.
The combination of solution heat treatment, hot forming and quenching can reduce manufacturing
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3.
operations and improve productivity. 4.
Holding the formed part between cold dies while it cools eliminates springback and ensures part shape
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accuracy.
Figure 11. Hot Form and Quench (HFQ®) process [102][103]. Another typical hot stamping process for the manufacture of high strength heat-treatable aluminium alloy components was proposed by Maeno et al. [104]. Two major characteristics of this process are: initial T4 heat treatment temper and quick heating. The quick heating can be achieved by resistance heating of initial blanks 19
ACCEPTED MANUSCRIPT with the same cross-section area, or by furnace heating with a pre-designed thermal gradient. Such a process enables the removal of solution treatment found in HFQ® and the sizing processes to determine part accuracy in cold stamping. Aluminium alloy aircraft parts with a high strength and dimensional accuracy were successfully produced by this process. However, arising from the requirements of this process, there also exists several challenges that need to be addressed: •
The selection of raw material is not flexible. An initial T4 temper of alloys is required, which restricts
•
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its application using commercial aluminium alloys such as those in T6 or O tempers.
An extremely high heating rate is required for high quench-sensitive alloys to avoid the coarsening of precipitates during heating according to the Temperature-Time-Property curve, such as AA7075 and AA7050, which means a powerful heating facility is required.
•
The uniformity of temperature for a large initial blank using quick heating approach might be a
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potential concern.
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Figure 12. Hot stamping process of high-strength aluminium alloy aircraft parts using quick heating [104].
3. Recent progress of experimentation
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Sheet metal forming processes have been investigated for many years. Each forming technique has its own advantages and applications. In addition, the disadvantages of each results in there being no single forming
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technique suitable for all industry scenarios. Comprehensive reviews on specific manufacturing techniques have been performed in the literature [105][43][106][107]. Therefore, the following section focuses on recent experimental progress that has been made with elevated forming temperature techniques from the view of the analysis of advantages and limitations in previous sections. 3.1 Warm stamping
3.1.1 Raw material candidates Warm forming has been proved to be suitable for non-heat treatable aluminium alloys, while its application for heat-treatable aluminium alloys is controversial. Recently, research has been performed on warm forming of high strength heat-treatable aluminium alloys. Kumar and Ross [108] investigated formability and post-form strength of high strength AW7921-T4 using uniaxial tensile tests at different warm forming conditions. Fig. 13(a) shows the temperature effect on the post-formed strength and strain to failure. By increasing the warm 20
ACCEPTED MANUSCRIPT forming temperature, severe loss of strength was observed, while the ductility was significantly improved. The reasons for such trends was identified and explained by Huo et al. [109] through TEM observations, as shown in Fig. 13(b). For warm formed AA7075, the dominant phases at 200 °C were fine η′ and GP zones, while welldeveloped η′ and coarser η phases were found at 250 °C. These phases determined the post-formed and postpaint-bake mechanical properties. Specifically, 200 °C warm forming caused limited coarsening of matrix precipitates and generated a certain number of dislocations. The combined effect of the decreased precipitation
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strengthening and increased dislocation strengthening enables to preserve the inherent high strength of 7075-T6. However, 250 °C forming led to severely decreased hardness due to the rapidly coarsened matrix precipitates, resulting in the loss of strength. Therefore, it can be found that temperature plays a dominant role in determining precipitation and corresponding mechanical properties. In general, the increase of warm forming temperature is detrimental for guaranteeing post-formed strength, and the temperature effect varies with different alloy grade
improved within a temperature range of 140 °C and 220 °C.
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and initial temper. Hui et al. [110] found that the draw-ability and stretch-ability of AA7075 can be significantly
AA7xxx belongs to the highly quench-sensitivity alloy category, which exhibit sensitive response of precipitates
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during heating and cooling. In comparison, AA6xxx is not as sensitive, which might experience different responses on formability and post-formed strength under warm forming conditions. Mahabunphachai and Koç [111] utilised hot bulging tests to evaluate the formability of AA6061. The formability was increased at 300 °C and a low strain rate. However, the post-formed strength was not evaluated. Fan et al. [112] tested the drawability and post-formed strength of an Al-Mg-Si alloy within a temperature range of 25 °C and 500 °C. The limit drawing ratio increased as temperature rose to 200 °C and decreased afterwards. In addition, hardness peaked at 200 °C and 500 °C. Formation of coarse ´ precipitates were believed to cause the decrease in hardness at
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temperatures ranging from 250 to 450 °C. Rapid heating may avoid the coarsening of precipitates when warm forming aluminium alloys with an initial T4 temper [104]. To this end, systematic studies with an overall analysis on warm forming heat-treatable aluminium alloys considering effects of temperature, strain rate,
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heating rate, alloy grade and initial heat treatment temper are minimal.
Testing temperature (°C)
Testing temperature (°C)
(a)
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Figure 13. Warm forming of high strength heat treatable aluminium alloys: (a) Effects of warm forming temperatures on mechanical properties of AA7921 [108], and (b) Microstructure evolutions at different warm forming temperatures of AA7075 [109].
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3.1.2 Process variables 3.1.2.1 Heating
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Heating history prior to warm forming may determine the subsequent deformation and post-formed strength of heat-treatable alloys. To obtain a carefully controllable heating stage, novel and robust heating devices are required. For sheet metal blanks, these include (a) commonly used furnace heating [113] and (b) resistance heating [114] as shown in Fig. 14. The applications of (c) contact heating [115] and (d) induction heating [75] methods are still limited to laboratory scale shapes. Furnace heating is the most mature heating method, and extensively used in hot stamping press shops. The significant advantages of furnace heating are: (1) uniform temperature distribution, (2) the system layout is easy to be arranged and the furnace easily integrated into a
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production line, (3) several blanks could be heated simultaneously and (4) suitable for mass production applications. However, several disadvantages exist including: (1) a relatively slow heating rate due to the low emissivity of aluminium alloys, and (2) sophisticated handling systems are required for integration with the forming process.
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To increase the heating rate, resistance and induction heating have been developed. Potential problems of resistance heating are: (1) non-uniform temperature distribution, (2) resistance heating is not suitable for complex-shaped initial blanks and (3) the electrical resistance of aluminium alloys is relatively low, according
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to Joule’s law, a greater power input has to be used. With regard to induction heating, induction coil design is critical in determining the heating rate. The heating efficiency of heating aluminium alloys depends on the frequency of power supply, induction coil design and layout. Induction heating is less efficient at heating nonmagnetic materials such as aluminium alloys, compared with steels. Moreover, there are still improvements needed to develop laboratory scale heating methods to a mass production setting. Rasera et al. [115] developed a contact heating method for hot stamping process which enabled a reasonably high heating rate, uniform temperature distribution and high efficiency to be obtained. In terms of heating aluminium alloys, target heating temperature is relatively lower, between 400 °C and 500 °C. Commonly used material for heating platens is steel material that is low cost. However, the temperature of the heating platens needs to be constantly monitored for mass production. For hot stamping aluminium alloys, a
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lower productivity.
Figure 14. Heating methods used for warm forming processes: (a) Furnace heating [113], (b) Resistance heating [114], (c) Contact heating [115] and (d) Induction heating [75]. 3.1.2.2 Forming
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Aluminium alloys heated and formed above 0.3 , result in the material deformation mechanism changing
from elastic-plastic to elastic-visocoplastic. The mechanisms involved in this region become complicated, involving several time-dependent parameters, such as diffusion, recovery/annihilation, recrystallisation and grain growth. The dominant deformation mechanisms in warm forming conditions are related to temperature
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[111], deformation rate [115] and grain size [116].
Temperature and strain rate effects on warm forming AA5xxx has been critically reviewed by Toros et al. [11].
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In general, formability, reflected by forming limit curves, is increased with increasing temperature and decreasing strain rate. Kumar et al. [117] systematically investigated the formability of AW-7020-T6 using uniaxial tensile, swift-cupping and cross-die deep drawing tests at a temperature range between 150 °C and 250 °C. AW-7020-T6 has been proved to be temperature and strain rate dependent through tensile tests, as the yield and ultimate tensile strength decreased with increasing temperature. True fracture strain (uniaxial tensile), limit draw ratio (swift-cupping) and limit drawing depth (cross-die deep drawing) increase with increasing
temperature above 150 °C due to dynamic recovery and dissolution of . It should be noted that, the draw-
ability at elevated temperatures is a very complex definition, which is related to not only alloy intrinsic
properties, such as alloy grade, grain size, heat treatment temper and responses to temperature and strain rate, but also external factors, including blankholding force and tooling condition (friction and heat transfer for nonisothermal scenario). Details of above factors are discussed along with hot stamping condition in Section 3.5.
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ACCEPTED MANUSCRIPT 3.2 Warm sheet hydroforming The temperature limit of pressurizing medium used in hydroforming determines the success of warm sheet hydroforming [118]. Oil and gas are two commonly used pressurising medium for warm sheet hydroforming. Oil, in comparison with gas, offers the advantages of lower compressibility, higher thermal capacity and pressurization, whereas its application is restricted due to its flammability. Hence, non-flammable gases are used for higher temperature applications. The temperature ranges of oil and gas, along with the required pressure of
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hydroforming an aluminium alloy, is given by Hartl [118] and presented in Fig. 15. Palumbo et al. [87] performed the warm hydroforming of age hardenable AA6xxx and found that the optimum working temperature was 200 °C and the exposure time of the material to the warm-forming temperature had to be minimized. In addition, an increased strain rate was beneficial due to strain rate hardening, in terms of die cavity filling and sheet thinning, to obtain sound products. Lang et al. [86] found that the pressure rate is a factor that can
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influence the deformation of material in warm hydroforming. To determine the optimal loading path of warm hydroforming, Choi et al. [119] proposed a methodology to determine profiles of the hydraulic pressure and the blank holder force under warm hydroforming conditions at different punch speeds. An adaptive FE analysis
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with fuzzy control algorithm was developed. Thinning, wrinkling, punch wall contact, die corner floating and conduction were used as criteria in the fuzzy control algorithm.
1400 1200
Internal pressure P (Bar)
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1000 800 600
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400 200 0
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0
100
200
300
400
500
Temperature T (°C)
Figure 15. Example of required internal pressure depending on temperature and strain rate for the forming of an aluminium alloy [118].
3.3 Hot gas forming 3.3.1 Quick plastic forming The QPF developed by General Motors has been successfully applied to the automotive industry and enables the manufacture of complex-shaped aluminium components at a rate of 100,000 per year. The technology has been successfully used on four production closures [95] and a review of this technology has been conducted by 24
ACCEPTED MANUSCRIPT Krajewski and Schroth [95]. Currently, the material candidates mainly concentrate on AA5083, AA6xxx alloys and magnesium alloys [120] depending on the specific applications in automobiles. Recently, the QPF technology has been used in the aerospace industry, mainly for forming titanium alloys [95] and Al-Li alloys [121]. Fan et al. [121] investigated the strengthening behaviour and microstructure evolutions of AA2195 using a hot gas forming process integrated with heat treatment, as shown in Fig. 16(a). The objective of this process was to avoid subsequent heat treatment and resultant thermal distortion. The sheet is formed into the required
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shape by high gas-pressure within several seconds after being solution heat treated, then cooled quickly within water-cooled dies or by some other cooling methods. Obvious recrystallization, as shown in Fig. 16(b), was observed during hot gas forming, which was believed to be initiated by the initial dislocation density and high forming temperature.
However, the current progress has concentrated on simulating the microstructure
evolutions in the process, and practical operation methods have not been developed. Wang et al. [122]
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performed hot gas bulging tests of friction stir welded (FSW) AA2024-T4 at a temperature of 400 °C. The deformation of FSW sheet was also concentrated in the weld zone during free bulging tests and the global formability of FSW sheet was poorer than that of the base metal. To date, quick plastic forming for aerospace
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alloys is still limited.
Figure 16 Quick plastic forming of Al-Cu-Li with integrated heat treatment: (a) Experimentation details and (b) EBSD maps of different locations on a hot bulged part [121].
3.3.2 Superplastic forming The major concern of SPF is the over-thinning that occurs in a complex-shaped component, as shown in Fig. 17b. The reason is that during SPF, the sheet blank is firmly clamped without being drawn-in and the material experiences pure bulging deformation using the superplasticity of the alloy. To address this drawback, some modifications have been made to SPF. Luo et al. [123] developed an advanced hybrid forming process by
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ACCEPTED MANUSCRIPT combing SPF with a hot draw mechanical pre-forming process. The advantage of this process is, using a hot draw pre-forming, a certain amount of material can be drawn into the die, and a more uniform thickness distribution is obtained in the subsequent SPF. As can be seen in Fig. 17(b), using such a process, the tight radius of the rectangular box can be successfully formed without rupture. Luckey Jr. et al. [124] proposed a twostage SPF process, which was demonstrated by practical forming trials of AA5083 to prove the ability of overcoming excessive thinning and necking. The operation of this process is to use gas pressure to form the
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blank into a preform die cavity of a single die, prior to the pressure being reversed to form the sheet into the final component cavity. The preforming of the blank contributes to form longitudinal geometry without severe
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thinning, while guaranteeing metal thickness in certain regions to improve the thickness profile of the final part.
Figure 17. Hot draw mechanical pre-forming superplastic forming (HDMP-SPF) [123]: (a) Schematic of
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HDMP-SPF and (b) Rectangular boxes formed by HDMP-SPF and SPF for AA5083. 3.4 Hot stamping
HFQ®, a hot stamping process for aluminium alloys, was patented in 2008. The complex temperature profile of this process results in a material that exhibits different microstructural evolutions and corresponding mechanical properties. Therefore, the conventional characterisation methods of aluminium alloys, like isothermal warm forming, are not suitable for the HFQ® process. 3.4.1 Material characterisation Ductility and formability of an alloy is normally characterised by uniaxial tensile and forming limit tests. Material in HFQ® experiences solution heat treatment first to obtain an optimal microstructure. Then, to characterise the ductility under a uniaxial stress state or forming limits under a biaxial stress state at different 26
ACCEPTED MANUSCRIPT temperatures, a fast cooling is required to quickly quench specimen to target temperatures. This results in conventional isothermal dome tests being unsuitable. Novel test facilities with capabilities of precisely controlled heating, cooling and deformation, as well as new test schemes, are required. Fig. 18 summarises two commonly used heating methods for uniaxial tensile tests, which enables to quickly heat a specimen at a fixed heating rate. Resistance heating in a Gleeble machine is believed to be the most commonly used method. However, the clamping grips are water cooled resulting in the temperature distribution
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on the specimen being non-uniform. The determination of gauge zone becomes an important issue. Induction heating is another popular heating method, which enables the heating device to be independent of the mechanical testing facility. However, the non-magnetic characteristic of aluminium alloys restricts the efficiency of induction heating. In addition, to achieve the complex heating path of HFQ®, air or water cooling devices are normally integrated into the test equipment.
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Both resistance and induction heating belong to the localised heating method category. There exists a temperature gradient within the longitudinal direction of the specimen due to the grip cooling effect [125], compared with environmental heating such as conventional furnace heating. To generate a uniform temperature
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distribution within the specimen using such local heating methods, and to avoid determining gauge zone, an advanced grip design was proposed by Ganapathy et al. [126], as shown in Fig. 19(a). The innovative design of the new grips developed enables uniaxial tensile tests under hot stamping conditions to be performed in a simpler way resulting in more accurate results. As can be seen in Fig. 19(b), the concept of self-resistance heating grips, which ensures better uniformity of temperature fields in test-pieces, can be potentially applied for tests of different alloys under complex plane-stress conditions, such as biaxial tensile, with modifications on the
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grip material and circulation designs.
Figure 18. Novel uniaxial testing facilities for hot stamping: (a) Resistance heating (Gleeble) [127] and (b) Induction heating [128].
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new grip.
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Figure 19. Advanced Gleeble grip design [126]: (a) Schematic of grip design and (b) Stress-strain curve using
In terms of hot stamping complex-shaped components, a sheet metal blank experiences variations of
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temperature, strain rate and strain path. Using uniaxial ductility only is insufficient to evaluate formability. The forming limit diagram (FLD) is conventionally used to evaluate the formability of sheet metals [129], which comprises a set of forming limit curves enabling the boundary between uniform deformation and the start of plastic instability leading to material failure to be identified. Similar with hot tensile tests under HFQ®, robust test devices are required to achieve heating and cooling functions.
The FLD is commonly determined using either MK tests [130] or cruciform tests [131]. Bariani et al. [132]
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presented an innovative experimental set-up based on conventional Nakajima tests, enabling the forming limits of hot stamping high strength steel to be evaluated. An induction heating method, as shown in Fig. 20(a), was utilised. With the combination of air cooling, the actual phase transformation kinetics for materials under deformation conditions can be simulated with controls of temperature and strain path variations to be achieved. Such a device also has potential to be used in determining the FLD of aluminium alloys at hot stamping
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conditions. Shao et al. [133] established a novel biaxial test system and scheme for optimising cruciform specimens specifically used for the resistance heating. Using such a novel system, for the first time, the forming
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limit data of an alloy can be generated at various temperatures, strain rates and strain paths and forming limits predicted under hot stamping conditions.
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Figure 20. Novel experimental set-ups of determining forming limit diagrams at hot stamping conditions: (a) Elevated Nakajima tests using induction heating [132] and (b) Cruciform tests using resistance heating [133].
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3.4.2 Process variables
From the view of transferring laboratory research to industrial production, temperature and strain rate (forming speed) are two key variables in hot stamping aluminium alloys. Fig. 21(a) summarises the temperature effect on alloy ductility of various commercial wrought aluminium alloys. For aerospace alloys, such as AA2024, AA7075 and Al-Li alloy AA2060, strain to failure increases with increasing temperature initially, and then decreases afterwards. The maximum ductility emerges at temperatures lower than the solution heat treatment temperature. The sharp decrease of ductility is believed to be caused by the low melting phases within the matrix. Wang et al. [134] performed SEM observations of AA2024 and found that when the temperature exceeds 450 °C, softening of grain boundary occurs due to the solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles. For alloys preferred for automotive applications, such as AA5754 and AA6082, ductility continuously increases with increasing
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ACCEPTED MANUSCRIPT temperature until the solution heat treatment temperature, which demonstrates that hot stamping such alloys directly after solution heat treatment (SHT) results in a maximum ductility. Forming speed is another critical influencing factor for two main reasons. Firstly, strain rate hardening is the dominant mechanism of deforming aluminium alloys at elevated temperatures. Different forming speeds correspond to different flow stress levels that determine the uniformity of deformation and success of forming. Secondly, the heat transfer between hot blank and cold dies results in the forming process being non-isothermal.
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The heat exchange time is determined by the forming speed and punch stroke, which plays an interesting role on the failure location and deformation uniformity. Mohamed et al. [102] established a process window of hot stamping AA6082 as a function of forming rate, as shown in Fig. 21(b). El Fakir et al. [135] and Zheng et al.
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[136] also concluded that a higher forming speed is beneficial for obtaining a uniform deformation.
Figure 21. Process variable effects on the hot deformation of aluminium alloys, (a) Temperature effect on the ductility of different alloys [102][137][134], and (b) Forming speed effect on the failure mode of AA6082 [102]
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3.4.3 Novel tooling techniques
The high temperature of material using hot stamping results in lower material stress level and less hardening
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feature due to recovery, which affects material flow in the flange and induces localized thinning. To address these potential problems, improvements on the tribological and thermal properties of die surfaces determining flange material flow and temperature field within a blank during hot stamping, are feasible approaches. Compared with extensive research on the tooling of hot stamping high strength steel, considering the lower strength, greater heat transfer phenomenon and greater aluminium alloy adhesion tendency, novel tooling techniques are required for hot stamping aluminium alloys. Zheng et al. [136] proposed a macro-textured tool surface concept and these textured tool surfaces were investigated in bending [136] and drawing [138] types of forming process at both cold and hot stamping conditions [139], as shown in Fig. 22. The advantages of using texture design on the tool surfaces are: (1) less contact on the flange between hot blank and cold dies is beneficial for preserving the flange temperature. The more uniform temperature contributes to obtain a more uniform material deformation within the blank. (2) The material that is absent of contact with die surfaces experiences no friction, and this material is easier to be drawn into the die to assist the remaining material to 30
ACCEPTED MANUSCRIPT flow. (3) The required blankholding force of suppressing flange wrinkling is low. For the type of bending deformation, as shown in Fig. 22(a), material undergoes one-dimension however the textured design may induce buckling on the material that does not experience blankholding force. To control buckling and provide guidelines to tool designers, a series of buckling models, based on beam [138] [139], plate [138] and shell [140]
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assumptions, under cold [138], warm [141] and hot stamping [139] conditions were established.
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Figure 22. Textured tool surfaces for hot stamping dies [136] [138]. To improve the tribological performance of tooling and achieve the objective of minimising or eliminating lubricant, Dong et al. [142] proposed a methodology of duplex-treated tooling surfaces for hot stamping dies. In this treatment, low-cost tool material candidates, such as cast iron, can be utilised as substrate tool material, as the forming load of hot stamping aluminium alloys is not as high as that of high strength steel. Then, plasma
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treatments, such as plasma-nitriding or plasma-carburising, are performed on the tool surfaces to further increase the tool surface strength. Finally, to improve the tribological properties, different coatings, such as WC: C coating, are applied on the top layer. Fig. 23(a) illustrates the microstructure of a duplex-treated tool. A series
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of tribological tests were performed for different tool surface conditions using conventional pin on disc tests. The friction coefficient can be reduced significantly compared with untreated tool surfaces, as shown in fig. 23(b). It is interesting to note that, PNC treated tool surface has no significant effect on reducing friction coefficient. The function of PNC treatment is mainly for increasing tool surface strength, which contributes to using cheap tool material, like cast iron, to replace conventional expensive hot working steels. Zheng et al. [143] further tested the lubrication performance of duplex treated tool surfaces in a practical deep drawing process. The results have shown that the advanced WC: C Diamond-like Carbon (DLC) coating could significantly reduce the use of lubricant for hot stamping.
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Figure 23. Duplex surface treated hot stamping die surface, (a) SEM photo of WC: C coated & PNC treated
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G3500 die [144], and (b) Friction coefficient evolution of treated tool surface at hot stamping condition [142]. 3.4.4 Characterisation methods of interface properties under hot stamping condition
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Interfacial heat transfer coefficient (IHTC) and friction coefficient (CoF), representing the thermal and tribology properties of die surfaces are crucial for the accuracy of finite element simulations [145]. Therefore, characterisation methods of these two properties become important. In terms of the interfacial heat transfer coefficient, two commonly used methodologies, named as one-dimensional (1D) closed-form calculation [146] and inverse FE method [147] are shown in Fig. 24. Xiao et al. [148] utilised the 1D closed-form calculation to calculate the interfacial heat transfer coefficients of hot stamping AA7075 at different contact pressures and lubrication conditions. The results have shown that IHTC values increased with increasing contact pressure and
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use of lubricant. Ying et al. [146] performed a systematic investigation on the IHTC of HFQ® forming AA7075T6. Effects of contact pressure, surface roughness and lubrication conditions were evaluated. In addition, the obtained IHTC was used in the simulation process to be validated against a typical U-type experimental model. The finite element assist method is used to compare IHTC values implemented in FE with experimentally
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recorded values to compare temperature histories. To precisely measure the temperature evolutions of a specimen and tools during hot stamping, Liu et al. [149] developed a novel experimental facility based on the Gleeble testing machine, as shown in Fig. 24b. The IHTC values between AA7075 and three different tool
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materials were characterized. In addition, utilising this methodology, Liu et al. [149] further used this method to investigate the effects of lubricant on the IHTC during hot stamping AA6082 with different thicknesses. The IHTC was increased using a graphite lubricant to fill the vacancies of asperities at the contact interface. The increase in IHTC when applying lubricant compared to air is approximately 26% and 20% for the 2 and 3 mm thick specimens respectively. In addition, it was also found that the IHTC decreases dramatically from a stable value when the lubricant layer thickness is lower than 20 µm at different contact pressures.
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Figure 24. Methods of determining interfacial heat transfer coefficients, (a) One-dimensional closed-form
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method [146] and (b) Inverse FE method [147].
A precise determination of friction coefficient (CoF) is very challenging as the hot stamping process introduces
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non-isothermal contact. In addition, CoFs are also related to material pair, temperature, sliding speed, sliding distance, lubrication and surface roughness. A robust test method and new test scheme are required to include these influencing factors. Fig. 25 summarises typical methods used to determine interfacial friction coefficients, which can be potentially used for hot stamping processes. The Pin on disc method [142] is believed to be the most commonly used method to determine the CoF and wear during sliding. However, the contact condition at the interface is different with actual conditions experienced in hot stamping. In addition, the range of test variables, such as sliding speed, is dependent on the capability of test equipment. Shi et al. [150] developed
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simple test equipment used to determine friction coefficient under warm forming condition, as shown in Fig. 25b. The whole test rig was mounted within a furnace to heat both tools and blank to elevated temperatures. A sufficiently high sliding speed and distance can be obtained with the combination of hydraulic press. However, the contact condition still a point to surface contact. Hot strip drawing [151] is believed to be the most similar test method to simulate the actual contact during hot stamping. However, previous research mainly concentrated
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sheets.
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on investigating the tribology of steel with limited research being performed on hot stamping aluminium alloy
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Figure 25. Methods of determining interfacial friction coefficient of hot stamping aluminium alloys, (a) Pin on disk [142], (b) Strip drawing on pin tool [150], and (c) Hot strip drawing [151]
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4. Material modelling and numerical simulations For sheet stamping processes, the development of material modelling has mainly concentrated on the modelling of stress-strain response, hardening and failure of materials under different forming conditions. The developed material models can be implemented into different finite element software to give comprehensive and precise
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simulations to predict sheet metal deformation.
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4.1 Cold forming condition
4.1.1 Fundamentals of material model Materials experience elastic-plastic deformation, with the uniaxial stress-strain relationship at room temperature deformation conditions given in Eq. (1), where E represents the Young’s modulus, ε e , ε p and ε T represent the elastic, plastic and total strain respectively. σ represents the effective stress. A plane stress assumption is normally used for sheet forming process, enabling the uniaxial case to be written as Eq. (2).
σ = Eε e = E(ε T − ε p )
(1)
σ ij = Dijkl ε ije = Dijkl (ε ijT − ε ijp )
(2)
In FE analysis, the total strain is obtained from nodal displacements, which are achieved by solving global system equations. Therefore, to calculate stress, the calculation of plastic strains becomes vitally important. Plastic strain can be calculated from constitutive relationships, as given in Eq. (3). 34
ACCEPTED MANUSCRIPT dε ijp = H
∂f (σ ij ) ∂σ ij
df
(3)
f represents plastic potential, H is a scalar coefficient related to hardening. For associated flow theory, plastic potential is assumed to be yield locus, while for non-associated flow theory, the assumption is invalid. For cold forming, hardening is due to an increase in the dislocation density due to plastic straining, resulting in
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the flow stress σ s increasing with plastic strain. Lin [152] proposed a complete material model of an initially isotropic elastic-plastic solid subject to monotonic uniaxial loading:
(4)
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n σ −σY ε p = sgn(σ ), with σ = σ s K σ = E(ε T − ε p )
The above expression is derived based on dislocation density, following the form of Ramberg-Osgood equation [153]. Other commonly used phenomenological hardening laws can be also used, which are summarized in
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Table 5.
Table 5 Commonly used empirical constitutive models of cold forming Model Ludwik [154]
Expression
Swift [129]
σ = C(m + ε n )
Voce [155]
σ = C(1 − me −nε ) σ
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Ramberg-Osgood [153]
σ = Cε
Descriptions C is a constant stress, n is a strain hardening exponent C , m and n are empirical constants
n
σ ε = 1 + α E σ0
m −1
e is the exponential constant, others are constants σ 0 is a nominal yield stress, α is a dimensionless constant
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4.1.2 Improvements on material model under cold forming condition In terms of cold stamping complex-shaped aluminium alloy components, two particular issues need to be
•
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carefully considered and modelled to improve the accuracy of model predictions. Aluminium alloy sheet blanks are normally produced by rolling with strong anisotropy feature due to the texture of orientation of materials. The anisotropy feature has an important effect on the material
flow and earing of sheet stamping components [156].
•
For stamping a complex-shaped panel component, the material is subjected to bending and unbending conditions. In additional, drawing and redrawing processes might be utilised to obtain a greater drawing depth. In such scenarios, kinematic hardening become important for the prediction of residual stress and springback after stamping.
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ACCEPTED MANUSCRIPT 4.1.3 Modelling of anisotropic and kinematic hardening 4.1.3.1 Anisotropy for earing prediction Extensive research has been conducted to model the anisotropy of sheet stamping. Hill proposed a quadratic yield function as a generalization of the von Mises yield function for anisotropic materials in 1948 [157]. However, the well-known yield function is concluded to be unsuitable for highly anisotropic aluminium alloys [158]. Various types of anisotropic models have been developed and proposed to consider different aspects of
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anisotropy by Hill [159][160][161], Hosford [162] and Barlat et al. [163][164][165]. A comprehensive review of these models was given by Habraken [166]. The above classical anisotropic yield functions are widely used in engineering applications. Recently, Stoughton and Yoon [167] proposed a new anisotropic model based on the non-associated flow theory, which is a significant improvement on the isotropic hardening non-associated flow theory proposed in 2002 [168]. Based on Stoughton’s non-associated hardening model, Lee et al. [169]
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proposed a non-associated flow rule which simply couples quadratic and non-quadratic yield functions to describe the evolution of yield surface. The contribution of the non-quadratic part is to control the curvature of
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the whole model, while the quadratic part is to describe anisotropic hardening throughout a deformation history. The model comparison shows that the present model can follow the anisotropic hardening of AA5182-O and AA6022-T43.
To validate the anisotropic models, Yoon et al. [156] implemented the Yld2004 yield function into finite element simulations to predict earing in a deep drawn cup. The predicted and experimental cup height profiles (earing profiles) with six ears are shown to be in excellent agreement. Earing for strongly textured aluminium alloys, AA6111-T4, were also predicted by Yoon et al. [170] using the Yld 2000 anisotropic yield function. An
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improvement in prediction was observed using Yld 2000 compared with conventional Hill 48 yield criterion. It should be noted that, this review focused only on macroscopic yield functions, microscopic yield functions based on crystal plasticity are not taken into account. 4.1.3.2 Kinematic hardening
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In many sheet forming processes, especially those that include draw-bead and bending/unbending, the sheet metal undergoes cyclic deformation. Cyclic effects caused by this type of deformation cannot be predicted with
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the common isotropic hardening law, resulting in a suitable kinematic hardening law being required. A series of constitutive models have been proposed considering kinematic hardening together with anisotropic properties and cyclic deformations. Teng et al. [171] combined Mroz’s anisotropic hardening rule to calculate the springback of sheet stamping. Wu [172] extended the Hill 48 anisotropic criterion to include mixed hardening for sheet metals. Chung et al. [173] and Lee et al. [174][175] formulated the Yld 2000 plane stress yield function with modified Chaboche-type combined isotropic–kinematic hardening law to consider the Bauschinger effect and transient behaviour. In these studies, the prediction capability of springback was improved. Taherizadeh et al. [176] proposed an anisotropic material model based on non-associated flow rule and mixed isotropic–kinematic hardening (Armstrong–Frederick formulation), and implemented it into a userdefined material (UMAT) subroutine for ABAQUS with Hill’ 48 yield function being selected. This model was further compared with other advanced models using different yield functions [177] .
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ACCEPTED MANUSCRIPT 4.2 Elevated temperature forming conditions Aluminium alloys exhibit viscoplastic behaviours when they are deformed at a temperature T / Tm > 1 / 3 , where
Tm is the melting temperature of alloy. Therefore, viscoplastic theories should be used for elevated temperature forming conditions of aluminium alloys. When the material is deformed at high temperatures, material properties, such as flow stress, are dependent on forming temperature, strain rate, strain and strain rate history
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and hence constitutive models of forming aluminium alloys at elevated temperatures become vitally important. In this section, phenomenological (empirical) and physical-based material models are reviewed. 4.2.1 Phenomenological models
Phenomenological plastic constitutive models are widely used in modelling the forming processes of alloys at
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high-strain-rates and temperatures. Table 6 summarises a series of commonly used phenomenological models. Table 6 Commonly used phenomenological models
Khan–Huang– Liang (KHL) [181]
Descriptions K is the strength coefficient, n is the strain hardening coefficient, and m is the strain rate hardening coefficient. k is the threshold stress A is the yield stress at the reference temperature Tr and strain rate !
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+ 1 + ∗ 1 − ∗ ∗ = /! ∗ = − " / − " #$ = %& ' %$ '
equivalent plastic strain and strain rates. f1
&
&/$
1 = &/ = $/ = >? @ + A? @ + 1B
B is strain rate hardening coefficient, n is the strain rate hardening exponent C and m are material constants.
J2 is the flow stress, ε p and ' are
( + )1 − , ! - ./0 1 *!+ − ∗ = 1sinh67 89:−;/< = = 89:;
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Zener-Hollomon (Arrehnius) [182]
= +
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Khan and Huang (KH) [180]
Expression =
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Model Fields and Bachofen (FB) [178] Modified FB Johnson-Cook (JC) [179]
C
and f are functions of equivalent plastic strain and plastic strain rate respectively. A , B , n0 , n1 , C and m are material constants, D0P equals to 106 /s.
Q is the activation energy, R is the ideal gas constant. A , α and n are material constants.
4.2.2 Physical based material model Phenomenological models can be used to model strain hardening at elevated temperature forming conditions, which do not take into account microstructure evolutions. In terms of some particular forming processes, such as SPF, where recrystallisation takes place, dislocation-free grains are generated, which results in the reduction of the dislocation density and material softening. Therefore, in order to precisely model various microstructure evolutions for a specific forming process, physical mechanisms-based material models are required.
37
ACCEPTED MANUSCRIPT Physically-based models containing mechanisms of deformation such as dislocation dynamics and thermal activation have been developed in the literature. These include: Zerilli-Armstrong (ZA) [183], Hybrid model based on neuro-fuzzy and physically based component [184][185], Dynamic recrystallization (DRX) [186], Preston-Tonks-Wallace (PTW) [187], Voyiadjis-Almasri (VA) [188], Bonder-Partom (BP) [189], and Cellular Automaton (CA) [190]. It should be noted that, most of above physically-based models were focused on extreme loading conditions or materials other than aluminium. The applications of modelling aluminium alloy
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deformed at different elevated temperature forming techniques were not performed. 4.3 Unified material model
In terms of forming aluminium alloys at elevated temperatures, various microstructure evolutions, such as dislocation accumulation, recovery, recrystallisation, grain growth, grain refinement, precipitate dissolution,
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formation and growth, interact with each other and affect the macroscopic mechanical properties. In addition, microscopic damage during large deformation processes is also important for predicting rupture in sheet metal forming, especially for hot stamping and superplastic forming. Internal state variables have been used for a long
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time to represent damage or hardening of alloys. Lin and Liu [191] developed a set of unified mechanism-based constitutive equations for modelling different microstructure evolutions during warm/hot forming with the concept of internal state variables. In this section, unified constitutive equations of hot stamping and superplastic forming processes were reviewed. These two sets of equations can be further implemented into finite element software, which is discussed in the next section. 4.3.1 Hot stamping (HFQ®)
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The set of unified viscoplastic constitutive equations of HFQ® is based on the fundamental framework proposed by Lin and Dean [192] and Lin et al. [193]. The equations are given below [194]: = ) '
E D HIHJ EFG K
,
(6) (7) (8) (9)
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< = 0.5L̅ !.N L̅ L̅ = 1 − L̅ |' | − L̅ P & |' |RP Q = 1 − QRS σ = E(1 − ω )(ε T − ε p )
(5)
Where Eq. (5) is the traditional power-law viscoplastic flow formulation, which considers the effects of damage
ω on viscoplastic flow. Eq. (6) represents the evolution of material hardening, R , which is a function of the normalised dislocation density ρ defined by ρ = ( ρ − ρi ) / ρ m , where ρi is the dislocation density of the material at the initial state and ρ m is the maximum (saturated) dislocation density of the material. Hence, the range of normalised dislocation density, ρ , is from 0 to 1. Eq. (7) represents the rate of accumulation of dislocations. Eq. (8) represents damage evolution for the uniaxial formulation, which is a function of plastic
strain rate ' . Eq. (9) is Hooke’s law for a simple uniaxial state. In this equation set, n2 and η3 are temperature-
independent parameters, while the material parameters K , k , n1 , B , A , C , E , η 1 , and η2 are temperature
dependent, given in Eq. (10)
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ACCEPTED MANUSCRIPT K = K 0 exp(QK RT ) k = k0 exp(Qk RT ) n1 = n10 exp(Qn RT ) A = A0 exp( − QA RT ) B = B0 exp(QB RT )
(10)
C = C 0 exp( − QC RT ) E = E0 exp(QE RT )
η1 = η01 exp(Qη RT )
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1
η 2 = η 01 exp( −Qη RT ) 2
4.3.2 Superplastic forming
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A set of SPF damage constitutive equations for AA7475 based on the power-law have been introduced by Lin et al. [195]. Damage variable, ductile void growth damage and effective damage are included in this equation set as shown below. '
\
$/VWX.Y 0 [[ Z
?
\
− 1@^
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' = TT U1 +
0 ]]
/1 − %Z − < − Hd 〉b c − < − Hd ' 〉b c TT = 〈 < = e; − <|' | c = c HfX + |' |c Hg j/$ %h 2 % %Z %h = c + )%Z − h , __ c c
__ = 〈
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'
%Z = *& %ZZ& ' HZP + *$ ' ZS cosh*j ' & ' for %h = 0 = .. $ & ' for %h > 0 = n%Z .. − %h o
(11) (12) (13) (14) (15) (16)
(17)
$VW
= pq − '
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(18)
Eqs. (13)-(15) represent the steady-state flow of a viscoplastic material. In the early stage of deformation, the
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overall material hardening is due to grain growth rate and increase of dislocation density. Eq. (14) represents the isotropic hardening variable
R , which is related to the accumulation of dislocations and expansion of the yield
surface during plastic deformation. b and Q are material constants. Eq. (15) represents the evolution of average grain size, d . In this equation, γ 0 and φ represent the effects of isothermal and plastic strain-induced grain growth respectively. α and β are material constants. At the late stage of deformation, softening due to micro-damage dominates resulting in the decrease of flow stress. Grain-boundary sliding and grain rotation are two important mechanisms in SPF. The relative movements result in void nucleation and growth at grain boundaries. A cavitated cylindrical material assumption is utilised, which is similar to the intergranular void growth under high stress creep. Details of the formulation of this set of equations is given in the work by Lin et al. [195]. Eq. (13) represents the steady-state creep rate of power-law zone. k is the initial yield stress and
µ
is the material constant. Eqs. (16) and (17) represent the evolutions of 39
ACCEPTED MANUSCRIPT void volume fraction and effective damage respectively, where f d is the effective void volume fraction, ranging from 0 to 1, and f h is the value of void volume fraction. f d = 0 means no effective damage is incurred while f d = 1 signifies failure. Eq. (12) represents the plastic strain rate with voids. The plastic strain rate is given in Eq. (11), and represents the cavity spacing .
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4.4 Model application 4.4.1 Hot stamping
The set of constitutive equations for hot stamping aluminium from Eq. (5) to Eq. (10) in section 4.3, can be calibrated using a series of uniaxial tensile tests at different temperatures and strain rates to determine material constants. Fig. 26(a) shows the comparison between model fitting (solid lines) and experimental data (symbols).
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The calibrated material constants are given in Table 7. The developed material model can then be implemented into FE software via subroutine, such as Abaqus, Pamstamp and Mark. El Fakir et al. [135] performed numerical simulations of HFQ forming AA5754 with the implementation of material model. Fig. 26c shows the plastic
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strain contours of a computed component with a comparison to the successfully part. As can be seen in Fig. 26b, good correlation with a deviation of less than 5% was achieved between the thickness distribution of the
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simulated and experimentally formed parts, verifying the capability of the model.
Table 7 Material constants determined from calibration [135] C 0 (s-1)
217.8 QC (J/mol)
6669.49 η 03
60999.8 Qη 3 (J/mol)
4.381
9469.6
B 01 (MPa)
k 0 (MPa)
K 0 (MPa)
η 01
η 02
6.75E+18 Q k (J/mol)
3.3932 Q K (J/mol)
0.0846 Qη 1 (J/mol)
0.03203 Qη 2 (J/mol)
1.7211 Qn1 (J/mol)
11181.5 A0
34630.3 Q A (J/mol)
26837.6 n2
17594.1 n01
12993 B02
1.9996
2898.3
0.3
3.56E-01
6.005
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E 0 (MPa)
13211 Q E (J/mol)
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Figure 26 Finite element simulations of hot stamping AA5754 [135]. 4.4.2 Superplastic forming
To determine the material constants from Eq. (11) to Eq. (18) under SPF conditions, experimental stress-strain and grain size data, shown in Figs. 27(a) are necessary. To obtain the best fitting between the equations and experimental data, optimisation methods are recommended. Table 8 presents an example set of material constants for SPF Al-Zn-Mg alloy at a temperature of 515 °C. From these constants, it was found that good
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correlations between experimental and computational results are obtained for both the flow stress and grain size evolutions [195]. The developed constitutive equations can be subsequently implemented into FE simulations to simulate practical SPF forming. Fig. 27b shows the field plots of state variables for Al-Zn-Mg, at strain rates 2 × 10-4 and 2 × 10-3 s-1 respectively [196].
K
0.2912
85.1712 D1
φ
5.5E-5
n
µ
b
Q
α
1 D2
1.6236 D3
0.1537 d1
7.6744 d2
69.5748
1
30.0013
3.3
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k
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Table 8 Material constants for Al-Zn-Mg at 515 °C [195]
8.145E-9
41
β
6.9E-2 d3
γ0 2.4 E
2.6 l0
1.7658
1.0E3
3.6347
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Figure 27 Finite element simulations of superplastic forming Al-Zn-Mg, (a) comparisons between experimental
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results and model fitting [195], and (b) Field plots of f h and fe for Al-Zn-Mg with target strain rate,8 rsr = 2 × 10Hu and 2 × 10−3 s -1 , respectively [196].
5. Conclusions and future research trends
A detailed review of forming techniques for manufacturing complex-shaped high strength aluminium panel components was conducted in this paper. The paper has identified and discussed research studies for improving and addressing the current limitations, both experimentally and theoretically, for a range of forming techniques. The current state of the art forming technologies have shown that:
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ACCEPTED MANUSCRIPT (1) Non-heat treatable aluminium alloys, AA5xxx, are widely used for automotive body structures, while the use of high strength heat treatable alloys, such as AA7xxx, is increasing. However, the current applications of high strength alloys are restricted due to the poor ductility at room temperature, requiring novel forming techniques to be utilised. (2) Cold stamping in alloys in T conditions is believed to be unsuitable for high strength aluminium alloys. Wtemper forming is an alternative way of manufacturing some components with lower complexity of geometries,
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which is preferred by industries considering the mature cold stamping production line. The springback is relatively low due to the less yield strength of material in W condition. Sheet hydroforming and incremental forming techniques enable the manufacture of more complex structures, while the production efficiency is not as high as conventional cold stamping and their applications are restricted.
(3) Forming at elevated temperatures increases alloy ductility. Warm forming techniques, including warm
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stamping, warm hydroforming and warm sheet incremental forming, have been extensively investigated. However, warm forming temperatures may deteriorate favourable microstructures, and additional heat
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treatments are required to restore component strength for heat-treatable aluminium alloys. (4) For hot forming techniques, from the view of productivity, SPF is mainly used for manufacturing complexshaped components in the aerospace industry, while QPF is concentrated on the automotive industry and has already been widely applied. HFQ® is a novel process integrating hot forming with heat treatment, which is believed to be a leading technique in this area.
(5) Extensive research has been carried out on formability and post-formed strength of alloys at different
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elevated temperatures which are affected by alloy grade, heat treatment temper, process variables, and tooling. (6) The novelty and robust capabilities of HFQ® promotes new characterisation methods and test schemes to be developed to evaluate formability of aluminium alloys under hot stamping conditions. In addition, tooling of HFQ®, focusing on heat transfer, coating and surface texturing techniques, was well developed to assist the
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manufacture of complex-shaped panel components.
(7) Material models under different forming techniques need to consider specific process characteristics, such as non-isothermal, variations of strain, strain rate and strain path and microstructure evolutions during forming.
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The constitutive models can enhance the prediction accuracy of FE simulations. Based on the above drawn conclusions, the future research of manufacturing complex-shaped high strength aluminium components should focus on four objectives: (1) maximize the process ability to achieve the manufacture of complex-shaped requirements with the industrial developments of component designs and new material candidates; (2) guarantee the post-formed strength of aluminium components to meet increasing and stringent requirements with the increasing occupation of aluminium alloy usage among body structures of vehicles; (3) increase the dimensional accuracy of component geometries considering the assembling requirements of volume production of aluminium components; (4) reduce the overall cost of manufacturing aluminium components to enhance the competitiveness of aluminium alloys with traditional steels.
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DOI:
10.1016/j.ijlmm.2018.03.006
Reference:
IJLMM 8
To appear in:
International Journal of Lightweight Materials and Manufacture
Received Date: 6 February 2018 Revised Date:
26 March 2018
Accepted Date: 27 March 2018
Please cite this article as: K. Zheng, D.J. Politis, L. Wang, J. Lin, A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components, International Journal of Lightweight Materials and Manufacture (2018), doi: 10.1016/j.ijlmm.2018.03.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components Kailun Zhenga, Denis J. Politisa,*, Liliang Wanga, Jianguo Lina
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a) Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
Abstract
Aluminium alloys are being increasingly utilised in the automotive and aerospace industries to reduce the weight of vehicles. Extensive research has been conducted to overcome the poor ductility of aluminium alloys at
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room temperature and improve formability of the materials, to enable complex-shaped panel components to be manufactured. To this end, this paper contains a comprehensive review of widely used forming processes for aluminium alloys, under cold, warm and hot forming conditions, and the material characteristics and equipment
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used for each process. Based on a detailed analysis from the view of industrial requirements, recent progress in experimentation techniques are reviewed addressing the limitations and improvements of specific forming processes. Furthermore, material modelling methods at both cold and elevated temperature forming conditions have been presented. In addition, finite element (FE) simulations with the implementation of material models are discussed. This review article intends to provide a systematic guide for process designers to choose the most
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appropriate sheet forming technique for specific industrial applications.
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Keywords: Aluminium alloys, forming, complex-shaped, panel components, modelling
Corresponding author: Denis J. Politis, [email protected]
ACCEPTED MANUSCRIPT A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components
Abstract
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Aluminium alloys are being increasingly utilised in the automotive and aerospace industries to reduce the weight of vehicles. Extensive research has been conducted to overcome the poor ductility of aluminium alloys at room temperature and improve formability of the materials, to enable complex-shaped panel components to be manufactured. To this end, this paper contains a comprehensive review of widely used forming processes for aluminium alloys, under cold, warm and hot forming conditions, and the material characteristics and equipment
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used for each process. Based on a detailed analysis from the view of industrial requirements, recent progress in experimentation techniques are reviewed addressing the limitations and improvements of specific forming processes. Furthermore, material modelling methods at both cold and elevated temperature forming conditions
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have been presented. In addition, finite element (FE) simulations with the implementation of material models are discussed. This review article intends to provide a systematic guide for process designers to choose the most appropriate sheet forming technique for specific industrial applications.
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Keywords: Aluminium alloys, forming, complex-shaped, panel components, modelling
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Contents Abstract................................................................................................................................................................... 1 1. Introduction ........................................................................................................................................................ 4 2. Lightweight aluminium alloys and forming techniques ..................................................................................... 5
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2.1 Lightweight aluminium alloys ...................................................................................................................... 5 2.1.1 Applications in automotive and aerospace industries ................................................................................ 5 2.1.2 Characteristics and comparisons ................................................................................................................ 6
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2.2 Review of sheet metal forming techniques ................................................................................................... 7 2.2.1 Cold forming.......................................................................................................................................... 8
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2.2.1.1 Cold stamping ................................................................................................................................. 8 2.2.1.2 Cryogenic forming .......................................................................................................................... 9 2.2.1.3 Sheet hydroforming ...................................................................................................................... 10 2.2.1.4 Incremental sheet forming ............................................................................................................ 12 2.2.2 Elevated forming techniques ............................................................................................................... 13 2.2.2.1 Warm stamping ................................................................................................................... 14
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3. Recent progress of experimentation ................................................................................................................. 20 3.1 Warm stamping........................................................................................................................................... 20
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3.1.1 Raw material candidates ...................................................................................................................... 20
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3.1.2 Process variables .................................................................................................................................. 22 3.1.2.1 Heating.......................................................................................................................................... 22 3.1.2.2 Forming ........................................................................................................................................ 23
3.2 Warm sheet hydroforming .......................................................................................................................... 24 3.3 Hot gas forming .......................................................................................................................................... 24 3.3.1 Quick plastic forming .......................................................................................................................... 24 3.3.2 Superplastic forming ............................................................................................................................ 25 3.4 Hot stamping............................................................................................................................................... 26 4. Material modelling and numerical simulations................................................................................................. 34 2
ACCEPTED MANUSCRIPT 4.1 Cold forming condition............................................................................................................................... 34 4.1.1 Fundamentals of material model.......................................................................................................... 34 4.1.2 Improvements on material model under cold forming condition ......................................................... 35 4.1.3 Modelling of anisotropic and kinematic hardening ............................................................................. 36
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4.1.3.1 Anisotropy for earing prediction ................................................................................................... 36 4.1.3.2 Kinematic hardening ..................................................................................................................... 36 4.2 Elevated temperature forming conditions ................................................................................................... 37
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4.2.1 Phenomenological models ................................................................................................................... 37 4.2.2 Physical based material model ............................................................................................................. 37
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4.3 Unified material model ............................................................................................................................... 38 4.3.1 Hot stamping (HFQ®) .......................................................................................................................... 38 4.3.2 Superplastic forming ............................................................................................................................ 39 4.4 Model application ....................................................................................................................................... 40 4.4.1 Hot stamping........................................................................................................................................ 40
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4.4.2 Superplastic forming ............................................................................................................................ 41 5. Conclusions and future research trends ............................................................................................................ 42
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References ............................................................................................................................................................ 44
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1. Introduction The concerns of environmental protection and stringent requirements of greenhouse gas emissions have driven transportation industries to reduce the weights of their products [1]. Applying lightweight materials, such as aluminium and magnesium alloys or composites [2], to replace traditional steel raw material candidates [3], has become an efficient and popular approach to achieve weight targets. In addition, the development of battery
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electrical vehicles (BEVs) has also required the usage of lightweight materials to compensate for the insufficient energy density of current batteries and extend the range of BEVs [4]. Among the family of lightweight materials, aluminium alloys have been extensively used in the automotive and aircraft body structures [5], due to the advantages of low density, high strength to weight ratio, good corrosion resistance and relatively low cost compared with composites [6].
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For automobile applications, the manufacture of panel components has been the subject of significant research as they occupy a large proportion of the mass of body structures, as shown in Fig. 1(a) [7]. Previous usage of high strength aluminium alloys has been restricted by the poor ductility at room temperature. Recently, the rapid
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development of forming techniques has also contributed to the manufacture of complex-shaped high strength aluminium alloy panel components, resulting in their applications being significantly extended [8]. Body in white structures have been manufactured from aluminium alloys in some luxury vehicles, such as Jaguar Land Rover and Audi [9]. An overall summary and evaluation of recent progress of forming techniques driven by industrial demands is currently lacking.
This paper provides a systematic and comprehensive review of the forming techniques used to form complex-
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shaped aluminium alloy panel components. Material characteristics, processing variables, specific equipment used, and advantages and disadvantages of each forming technique are also presented. Moreover, recent research progress to address the identified disadvantages, such as new experimentation and theoretical modelling techniques are also reviewed. Finally, conclusions on the latest state of the art forming processes are drawn summarising the recommended applications of each forming technique. Finally, through a review of the
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literature, the paper aims to provide process designers and researchers a guide to selecting appropriate forming
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techniques and processing parameters and also identify the trends of future scientific research.
Figure 1. Applications of aluminium alloy panel structures in an automobile and aircraft: (a) Audi TT coupé [7] and (b) Airbus 380 [10]
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2. Lightweight aluminium alloys and forming techniques 2.1 Lightweight aluminium alloys Wrought aluminium alloys are the most widely used raw material candidates for panel structures. Nowadays, non-heat treatable AA5xxx, and heat treatable AA6xxx, AA7xxx and AA2xxx, are popular candidates for automotive and aircraft industries. This section briefly reviews the current applications of these alloys and their
techniques. 2.1.1 Applications in automotive and aerospace industries
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characteristics and properties, enhancing the analysis and evaluations of forming these alloys using different
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Typical applications of different alloys in an automobile are summarised in Table 1. Non-heat treatable aluminium alloys, such as AA5754 (Al-Mg), are widely used for internal body structures to compensate for the insufficient strength. The strengthening mechanism of this alloy is strain hardening, normally by cold working
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during fabrication, in association with solution hardening. Therefore, AA5xxx sheet components are normally cold stamped, in the annealed(O), as fabricated (F) or strain hardened (H) conditions with greater ductility. In comparison, medium or high strength aluminium alloys, AA6xxx and AA7xxx, are potential material candidates for outer panel structures. These alloys are precipitation hardened, and their strength arises from the typical T6 heat treatment for the automotive industry. However, the current use of high strength alloys, like AA7xxx, is still limited for automotive vehicles.
In terms of the aircraft industry, AA7xxx and AA2xxx are the main material candidates due to their high
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strength and good corrosion resistance. The corrosion resistance property of AA7xxx is poor at T6 condition with peak strength, which results in such alloys being applied in an over-aged condition, such as T73. Al-Li alloys were first developed in the 1920s. To date, the third generation Al-Li is believed to be at a mature stage and capable of being used extensively. Compared with other aircraft alloys, Al-Li alloys exhibit the advantages
aircraft [10].
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of low density, high stiffness and good fatigue resistance, which are widely used in the wing structures of an
The applications of a specific alloy are predominantly determined by their mechanical properties. However, it
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should be noted that, the work-ability of an alloy can also determine its application. For example, the extensive usage of AA5xxx aluminium alloys results from such alloys being soft and easily formable. For high strength aluminium alloys, which have similar strength levels to conventional mild steel, the ductility is poor at room temperature which results in these alloys being unable to sustain higher deformations and be manufactured to complex-shaped components. The corresponding applications are restricted, and advanced forming techniques are required.
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ACCEPTED MANUSCRIPT Table 1. Lightweight properties and applications of typical aluminium alloys in automotive and aircraft industries
AA5052
AA5754
AA6016-T4
Al-Mg-Si
AA6111-T4 Al-Zn-Mg
AA7055T7751 AA7075-T73
Al-Zn-Mg
AA2024-T3 AA2199-T8
Strength/Density (Pa/(kg.m-3))
Applications
2680
72000
2670
86000
Interior panels and components, truck bumpers and body panels [11] Inner body panels, splash guards, heat shields, air cleaner trays and covers, structural and weldable parts, load floor [11] Outer panels and structural sheets (Europe) [12] Outer and inner panels (North America) [12] Potential applications for A pillar and B pillar [15][16] Case of fuselage [17]
2700
81500
2710
103000 [13]
2780
127000 [14]
2860
222000
2810
180000
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Aircraft
AA7020-T6
Density (kg/m3)
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Automotive
Main chemical composition Al-Mg
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Alloy grade
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Industry
Al-Cu-Mg
2780
1740000
Al-Cu-Li
2640
152000
Upper wing skins, stringers and horizontal/vertical stabilizers Upper wing structure [17] Fuselage and lower wing structure [18]
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2.1.2 Characteristics and comparisons
Essentially, the differences of mechanical properties of different alloys arises from the addition of alloying
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elements. As each element has a different solute ability within the aluminium matrix, the type, quantity and combination of alloying elements results in variations in microstructure and mechanical properties. For instance, AA2xxx alloys have lower fracture toughness than those of AA7xxx with similar yield strength level, as larger sizes of inter-metallic compounds exist in the AA2xxx alloys. To improve the fracture toughness, the levels of iron, silicon or copper can be reduced to avoid the formation of large and brittle compounds. Table 2 summarises the main compositions of typical wrought aluminium alloys in literature considering the slight difference in element percentage of different commercial aluminium alloy supplier. Although various alloys have been commercialised, this paper only focuses on commonly and widely used materials for automotive and aircraft industries. Progress made to develop novel aluminium alloy grades with new additional elements, and corresponding metallurgy are not taken into account.
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ACCEPTED MANUSCRIPT Table 2. Main chemical compositions of some aluminium alloys (wt%) Mg 3.0 Cu 4.5 Si 1.05 Zn 5.4
Mn 0.24 Mg 1.5 Mg 0.8 Mg 2.2
Fe 0.26 Mn 0.5 Mn 0.68 Cu 1.4
Si 0.03 Si 0.41 Fe 0.26 Fe 0.22
Cu 0.02 Fe 0.40 Cu 0.04 Cr 0.19
Ni <0.01 Zn 0.20 Zn 0.02 Si 0.07
Ti <0.01 Ti 0.12 Cr 0.01 Mn 0.04
Zn <0.01 Cr 0.07 Ti 0.01 Ti 0.02
Al Bal Al Bal Al Bal Al Bal
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AA5754 [19] AA2024 [20] AA6082 [21] AA7075 [22]
Fig. 2 summarises the mechanical properties of a series of commonly used aluminium alloys. Compared with low strength AA5754, AA6082 exhibits significantly improved strength, which makes this alloy the preferred grade in the automotive industry. In comparison, AA2024 and AA7075 exhibit even greater strength, which
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enables them to be used in aircraft applications. In addition, for heat treatable aluminium alloys, heat treatment temper is also a significant factor determining the application of an alloy. Although the strength of AA7075 in T73 condition (overaged) is less than that in T6 condition (peak age), the corrosion resistance property is
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improved due to the growth of precipitates when the alloy is overaged. The large precipitates contribute to corrosion resistance in the severe service environment of an aircraft. For automobiles, the service environment of components is not as severe, and hence aluminium alloys are normally heat treated to the peak strength to
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meet vehicle safety requirements.
Figure 2. Comparisons of mechanical properties of commonly used alloy and different heat treatment temper [23][19]. 2.2 Review of sheet metal forming techniques To evaluate and enhance the analysis of a specific forming technique, raw material selection, which depends on alloy grade and heat treatment temper needs to be considered, as the microstructure and mechanical properties of a material play an intrinsic role on the capability of individual forming techniques. This section briefly 7
ACCEPTED MANUSCRIPT reviews the forming techniques and corresponding material characteristics for commonly used commercial aluminium alloys. 2.2.1 Cold forming 2.2.1.1 Cold stamping Cold stamping using rigid dies is believed to be the most commonly used forming technique for the manufacture
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of aluminium alloy components, especially for non-heat treatable AA5xxx. For heat treatable alloys, such as AA6111 shown in Fig. 3, the ductility in T6 condition is very poor which is not beneficial for producing complex-shaped structures. To address this disadvantage, W-temper forming [24] or stamping at T4 temper condition can be used. W-temper refers to the super saturated solid solution state after solution heat treatment [25]. Alloys in such a state exhibit lower strength but improved ductility, as shown by the dash line in Fig. 3.
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However, the strength of W-temper or T4 formed components needs to be increased by additional heat treatment, to restore the microstructure and mechanical properties of T6 temper. However, springback [26] and quenchinginduced distortions [27] may be experienced during the additional heat treatment, especially for high strength
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alloys [28]. The loss of dimensional accuracy from cold stamped aluminium alloys presents difficulties in component assembly in a mass production setting.
Besides the proper selection of initial blank heat treatment temper to improve draw-ability, multi-stage deep drawing can also be used to manufacture high aspect ratio components [29]. However, multi-stage deep drawing is deemed not suitable for automotive components, due to the lower production efficiency and irregular shapes of automobile parts. Lubrication is another feasible approach to reduce interface friction and enhance the ability to produce a good quality part [30]. In addition, lubricant enables tool life to be extended by reducing wear [31].
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Rao and Wei [32] investigated the lubrication performance of an environmentally-friendly and low cost boric acid film by comparing conventional commercial solid and liquid lubricants. Higher sheet metal draw-ability and lower forming forces were found to be the direct benefits that can be achieved. Recently, to remove the post-cleaning of lubricant operation, surface coating techniques, especially low frictional carbon-based coatings
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have been extensively investigated on a laboratory scale [33] [34]. However, for large cold stamped aluminium alloy parts, lubricants are not indispensable considering the much lower strength of aluminium alloys compared
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to steel.
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Figure 3. Characteristics of cold stamping using rigid dies: (a) Stress-strain curves of different alloys and different tempers [35][36], (b) Springback defect [26] and (c) Shape distortion due to additional heat treatment [27].
2.2.1.2 Cryogenic forming
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Cryogenic forming [37] is motivated by reducing the discontinuous deformation of stamping AA5xxx at room temperature. The discontinuous deformation is marked by the formation of deformation bands that not only leave undesirable traces on the surface of panels, but also reduce the ductility of the alloy [38]. The phenomenon is known as the Portevin-Le Chatelier (PLC) effect [39]. To reduce the negative impact of the PLC effect, aluminium alloys can be formed at cryogenic temperatures (below room temperature) selectively. Schneider et
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al. [40] performed tensile tests at temperatures ranging from 25 °C to -196 °C and strain rates from 1.7 × 10-3 to 6.6 × 10-2 for aircraft alloys. Fig. 4 shows the variations of yield strength, ultimate tensile strength and fraction elongations at different temperatures. Strength increases with decreasing temperature for all alloy candidates,
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while ductility of EN AW-7021 exhibited a drop at -196 °C. Glazer et al. [41] also observed the improved strength toughness, strain hardening rate and tensile elongation with decreasing temperatures for AA2090-T81. Welpmann et al. [42] tested the low temperature deformation of Al-Li alloy, AA8090, from room temperature to -196 °C. Yield strength, uniform strain and elongation to failure were gradually improved as the test temperatures decreased. Such enhanced properties conclude the potential of forming complex-shaped components at cryogenic temperatures of aluminium alloys, although the process is more technologically challenging than elevated temperature forming for automotive industry [11]. Feasibility of cryogenic forming aerospace alloys still exists considering the smaller volumes and price insensitivity of aerospace components. Formed sheet metal blanks normally experience at least a tensile deformation in one direction. The mechanism of cryogenic forming is believed to be associated with the PLC effect caused by Cottrell-atmospheres and dislocation movements. When the alloy in a W temper is exposed to a cryogenic temperature, the diffusivity of
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ACCEPTED MANUSCRIPT pre-dissolved additional atoms is generally decelerated, which results in the formation of Cottrell-atmospheres of the same intensity during the same period of time being unlikely. Then, the PLC effect is reduced correspondingly and the elongation is increased. In terms of the strength, at cryogenic temperatures, the activation energy is decreased. Consequently, the force required to move a dislocation through the crystal lattice can be increased. Based on the above discussion, aged conditions with precipitates capable of promoting the
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formation of Cottrell-atomospheres, such as T4 or T6, are not ideal states for cryogenic forming processes [40].
Figure 4. Characteristics of cryogenic forming, (a) Improved mechanical properties of different alloys and (b) Substitutional solute atoms at stationary forest dislocations generate Cottrell-atmospheres [40].
2.2.1.3 Sheet hydroforming
Sheet hydroforming is a relatively new sheet metal forming technology originating from the well-established hydroforming technology, that was first developed in the 1890s [43]. A typical forming method among sheet hydroforming techniques is hydrodynamic deep drawing, as schematically shown in Fig. 5. Instead of the conventional deep drawing with a die cavity, oil or other pressurising liquid medium are utilised to press the sheet metal tightly onto the punch when it is drawn into the die by the rigid punch. In this stage, the friction between the sheet metal and die is reduced as a result of the liquid medium in the die cavity, which flows out
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ACCEPTED MANUSCRIPT between the upper surface of the die and the sheet metal. This fluid flow results in a lubrication effect that reduces frictional forces. By this process, the sheet metal can be drawn to a greater depth and drawing ratio (DR) value of sheet metal can be increased, as shown in Fig. 5(b), and the quality of the part can be improved [62]. Several novel process modifications have been made based on the hydrodynamic deep drawing to increase the ability of manufacturing complex-shaped components. Nakamura and Nakagawa proposed radial pressure assisted hydraulic counter pressure deep drawing [44] and reverse redrawing under hydraulic counter pressure
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with its radial pushing [45] in 1985, which enables the significant increase of draw-ratio. Kolleck and Cherek [46] proposed an active hydrodynamic deep drawing to form large extensive deep drawn parts. In this process, a preforming stage is involved prior to hydrodynamic deep drawing. Springback can be reduced and resistance of denting can be strengthened using such a process.
Blankholding force and liquid pressure are critical process parameters in sheet hydroforming, which determine
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the flange wrinkling and tearing [47]. Yossifon and Tirosh [48] developed a fluid assisted blankholder to be used in conventional deep drawing, which is effective for preventing flange wrinkling. Damborg and Jensen [49] achieved adjustable blankholding force in the hydrodynamic deep drawing using a closed-loop control system.
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However, it was found that the variable blankholding force has little effect on the forming. Double-sided hydroforming [50] enhances the formability of sheet due to the fluid pressure increasing the hydrostatic pressure imposed on the sheet. The hydrostatic stress state is able to suppress the void expansion and growth, and thereby delays the fracture initiation [51]. Chen et al. [52] proved that appropriate liquid pressure can be used for manufacturing wrinkling-free parts from AA2219 both experimentally and analytically. Besides the above investigations of process variables for sheet hydroforming, novel contributions have also been made in areas
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such as tooling [53] and viscous pressure medium [54], which were not discussed in detail in this section. In addition, sheet hydroforming can also be integrated with other forming processes, such as stamping [55] and stretch forming [56].
The advantages of sheet hydroforming can be summarised as follows: Friction at the flange can be reduced due to fluid lubrication [57] and Friction retention effect on the
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•
punch is affected [58].
Draw-ability can be significantly increased [43].
•
Reduced surface defects of the formed part due to the liquid pressure medium [59].
•
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Fewer dies are used thus reducing the tool cost [59].
Despite all the advantages, there are several drawbacks with sheet hydroforming. Compared with conventional stamping processes, the manufacturing efficiency is low especially for the automotive industry [60] and the manufacturing cost per part is high. Although sheet hydroforming is mainly used for closed-form revolutionary components, with shape distortion and springback being eliminated, there is an additional heat treatment process required to restore the component strength resulting in additional cost. Extensive research has been performed on sheet hydroforming, resulting in a very mature process that is widely used in industry [61].
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Figure 5. Sheet hydroforming: (a) Comparisons between conventional deep drawing and sheet hydroforming
2.2.1.4 Incremental sheet forming
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[43], and (b) Improved draw ratio using sheet hydroforming [62].
Incremental forming was first introduced by Matsubara in Japan in 1993 [63], with an extensive review of incremental sheet forming developments being reported by Emmens et al. [64]. The process can be sub-divided into different types according to the tool configurations [65], as shown in Fig. 6. Among them, single point incremental forming (SPIF) is the most commonly used. During the SPIF process, the blankholder is utilised for clamping and holding the sheet blank in position. The backing plate supports the sheet and its opening defines
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the working area of the single point forming tool. The rotating single point forming tool is utilised to progressively shape the sheet into a component and its path is generated by a CNC machining centre. During the forming process, there is no backup die used to support the bottom surface of the deformed sheet [66]. Echrif and Hrairi [67] reviewed the recent research and progress of incremental sheet forming, mainly on methods and
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potential applications. The advantages were summarised as below: The SPIF can be executed in a conventional CNC machine, hydraulic press is not required.
•
Design changes can be performed efficiently and easily by the programming of CNC machine.
• •
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Parts are produced directly from a CAD file, and no male and female dies are required. The forming load is small and formability can be increased.
Given these advantages, the main concerns with this technique are: (1) the production efficiency, which severely restricts its application for the automotive industry; (2) thinning of the formed sheet and (3) limited geometry accuracy such as accumulated step change errors and springback. In addition, the surface quality can be another issue as a result of the tooling component sliding and scratching the sheet surface, although this depends on the surface quality of tool and forming speed. To address these drawbacks, Araghi et al. [68] proposed a hybrid process, by combination of the SPIF and stretch forming, which enables a reduced processing time and improved uniformity of thickness after forming. Lubrication is also an effective approach to address the issue of surface quality. Jawale et al. studied the effect of lubrication on SPIF from the microstructure point of view [69]. The SEM images from different lubrication states were compared and it was found that there was some 12
ACCEPTED MANUSCRIPT influence of lubrication on the surface quality. It was observed that the marks left by the forming tool and the development of valley like structures along the grain boundaries were the cause for increased surface roughness. Azevedo et al. [70] tested a series of lubricants of SPIF aluminium 1050 and DP780, and proved that using
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lubricant guarantees a better surface finish of SPIF aluminium rather than steel.
Figure 6. Schematics of incremental sheet forming with various tooling configuration [65] 2.2.2 Elevated forming techniques
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To address the poor ductility of aluminium alloys and increase the ability of manufacturing complex-shaped components, forming at elevated temperatures becomes a feasible solution. Alloy ductility is increased and material strength is reduced with increasing forming temperatures. Different elevated temperature forming techniques, such as warm forming, quick plastic forming, superplastic forming and hot stamping, exist and have
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been extensively investigated and used for industrial applications. Fig. 7 shows the classification map of different elevated forming techniques according to the approximate temperature and strain rate for reference [75]. For aluminium alloys, elevated temperature forming techniques
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can be further divided into warm or hot forming conditions with traditional forming processes classified into these categories, for example warm hydroforming [57] and incremental forming [71]. Normally, warm forming refers to forming material at temperatures below the recrystallization temperature [72] and above 0.3 times the
melting temperature (0.3 ) [73]. For hot forming, temperatures are selected to be above the recrystallization temperature and normally below the solvus temperature. Semi-solid forming [74] is not considered in this
review. This section reviews commonly used elevated forming techniques for aluminium alloys based on the warm or hot classification criterion.
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Recrystallization
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temperature
•
2.2.2.1 Warm stamping
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Figure 7. Classifications of elevated forming techniques [75]
Stamping sheet alloys at warm forming condition using conventional rigid dies is the most commonly used warm forming technique. The process can be either isothermal or non-isothermal depending on the die temperatures. Fig. 8(a) shows a typical non-isothermal warm forming process with a non-uniform temperature distribution. In this process, the sheet blank can be heated to elevated temperatures using an external furnace or
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hot dies [76]. The punch is cooled using pre-machined water cooling channels. The advantages of this nonisothermal process are summarised as follows: 1.
Material ductility and forming limit can be increased at warm forming temperatures, as shown in Figs. 8(c) and 8(d) [77] [78] .
Springback defect can be reduced compared with cold stamping, resulting in a good dimensional
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2.
accuracy of formed part [79].
Non-uniform temperature distribution contributes to increase draw-ability [80].
4.
Manufacturing efficiency is high compared with warm hydroforming and warm incremental forming
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3.
processes.
However, there also exist some limitations of the warm forming technique, which are briefly listed as follows: 1.
Warm forming is extensively being researched in a laboratory setting, with no well-known press shops implementing the process [11].
2.
Warm forming is believed to be unsuitable for high-strength heat treatable aluminium alloys, since heating might affect the alloy microstructure and deteriorate post-form strength [81].
3.
The interfacial friction is increased resulting in high temperature lubricants being required [82].
It should be noted that, lubrication exhibits two opposite effects [83], the positive effect of lubrication is the reduced tool wear and improved product surface quality, while the negative effect is that the process is
14
ACCEPTED MANUSCRIPT unsuitable for mass production. The pre-brushing on the tools and post-cleaning of formed components of
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lubricant affect the productivity and makes the process uneconomical.
Figure 8. Characteristics of warm stamping using rigid dies: (a) Schematic of non-isothermal warm stamping [84], (b) Comparisons of cold and warm formed components [85], (c) Improved uniaxial ductility of warm
•
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forming [77] and (d) Improved forming limits of warm forming condition [78] Warm sheet hydroforming
The relatively complex die set-up of hydroforming results in a sheet blank that is unable to be heated using an
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external furnace and subsequently transferred to cold dies. In order to hydro-form aluminium alloys at elevated temperatures, the sheet blank is simultaneously heated with forming dies using an external furnace [86] or heating bands [87]. The punch can be kept cool if blankholders are self-heated [88]. Compared to cold hydroforming, warm hydroforming utilises the improved formability of aluminium alloys at elevated temperatures, which is effective for high strength aluminium alloys [89], such as AA6xxx and AA7xxx with poor ductility at room temperature. There are several key challenges to be addressed for warm hydroforming: 1.
Forming temperature is dependent on the pressurizing medium sustainable temperature. The conventional oil medium temperature is below 300 °C.
2.
An optimal non-isothermal temperature distribution needs to be determined within the tooling [90], similar to warm forming using rigid dies.
3.
Warm hydroforming is currently being developed to achieve reduced number of manufacturing steps and part consolidation [88]. 15
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2.2.2.3 Warm/hot sheet incremental forming
Warm/hot incremental sheet forming is an improvement from conventional incremental sheet forming by utilisation of a heating device to improve the formability of aluminium alloys. Compared with stamping using rigid dies, a relative simple set-up of incremental forming enables various kinds of flexible heating methods to be used. Ji and Park [71] selected a heating method of hot air blowers to heat the AZ31 sheet blank. Duflou et al.
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[91] used the laser-assisted local heating method for single point incremental forming a TiAl6V4 sheet. The method is believed to be highly efficient and costly. Fan et al. [92] proposed an electrical heating method to heat the blank using material resistance and external electrical power. Considering the sheet blank experiences thickness variations due to plastic deformation during forming, temperature control by adjusting current density can be challenging. Surface finish and geometrical accuracy are two main problems due to the extreme high
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temperature at local areas [93] when using a localised electrical heating method. Ambrogio et al. [94] used a conduction heating approach for forming the AZ31 sheets. A heater band was mounted at the external surface of the tools and the blank was indirectly heated using the hot tools. Compared to the local heating approach, this
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method lacks energy efficiency. It should be noted that the above heating methods can also be combined to increase the heating efficiency. •
Hot gas forming
Hot gas forming refers to blanks utilising a pressurised gas medium. From this point of view, hot gas forming can be further divided into superplastic forming (SPF) which has already been extensively investigated, and quick plastic forming (QPF) which was developed by General Motors [95], as shown in Fig. 9(a). The major
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difference between SPF and QPF is the strain rate and raw material candidate. SPF deforms aluminium alloys at a strain rate exhibiting maximum ductility, while QPF process aims to deform aluminium alloys at strain rates significantly greater than those in SPF to reduce the processing time. In addition, SPF requires raw material that is intrinsically superplastic with fine grains. Generally, SPF is designed to form low volume highly complex-
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shaped components, while QPF is designed for producing parts of less complexity but of high productivity. Table 3 summarises the main differences between QPF and SPF.
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Table 3 Comparisons of SPF and QPF processes [96]
Process variables
Market features
Manufacturing system
Super plastic forming Strain rate/Temperature chosen for exploiting maximum material ductility •
Aerospace Low volume (100 panels per year) • High price points product • Hand rework common for dimensional and surface quality
Low level automation, manual handling Low capital, high piece cost
Typical panels 16
Quick plastic forming Strain rate/Temperature chosen to maximize productivity consistent with final quality Automotive • High volume (tens of 1000s panels per year) • Low price point product (Consumer goods) • Emphasize first time dimensional and surface quality High level automation, automatic handling Higher investment, lower piece cost Moderate shapes (More complex than
ACCEPTED MANUSCRIPT Extreme shapes High forming strains-back pressure to limit cavitation Maximum mechanical properties
automotive metal stamping) Moderate forming strains-back pressure not required Moderate post-form strength
In terms of SPF, as shown in Fig. 9(b), the blank is heated to a pliable state, 0.5 − 0.6 , then pressurised by
gas to deform the sheet blank into the die cavity. The high temperatures allow the alloy to be elongated or stretched, to a much greater degree without rupture than would be observed in cold or warm forming processes.
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Furthermore, finer details of the part can be successfully formed, which requires less overall forming force compared to conventional forming methods. Typical features of SPF can be summarised as follows: 1.
SPF is an isothermal forming process. Therefore, the whole tooling set-up is normally located within a furnace to guarantee the blank and die have the same temperature, resulting in difficulties in automation.
Extremely large plastic deformation could be observed, which is ideal for forming very complex panel
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components, as shown in Fig. 10(a).
Sheet blank should have fine grains and grain growth rate must be slow during forming, as shown in Fig. 10(b).
4.
The productivity of SPF is low.
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3.
A number of aluminium alloys have been proved to exhibit superplasticity. Table 4 summarises the process variables and superplastic features of commercial alloys with superplasticity.
Table 4 Compositions of superplastic aluminium alloys and corresponding superplastic forming conditions [97] Composition, wt%
2004 5083 7475 8090 2090
Al-6Cu-0.4Zr Al-4.5Mg-0.7Mn-0.1Zr Al-5.7Zn-2.3Mg-1.5Cu-0.2Cr Al-2.4Li-1.2Cu-0.7Mg-0.1Zr Al-2.5Cu-2.3Li-0.12Zr
SPF temperature (°C) 460 500-520 515 530 530
Strain rate (/s) ~10-3 ~10-3 2×10-4 5×10-4 ~10-3
Elongation (%) 800-1200 ~300 800 1000 500
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Alloy
Figure 9. Schematics of hot gas forming, (a) Quick plastic forming [95] and (b) Superplastic forming [98].
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Figure 10. Typical features of superplastic forming: (a) Schematic superplastic forming mechanism [99], and (b) EBSD maps of superplastic forming an Al-Zn alloy [100]. Compared with superplastic forming aluminium alloys, the requirements of alloys used in QPF are not strict.
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QPF incorporates much more automation compared to conventional SPF processes for producing dimensionally accurate components directly from a forming cell. The QPF process is highly optimized around each specific
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component to minimize the cycle time and maximize productivity. Although the shape complexity is less than SPFed components, forming at elevated temperatures still enables poor formability of aluminium sheet materials to be overcome compared to cold stamping conditions. •
Hot stamping with rigid dies
Recently, to extend the applications of aluminium alloys in the transportation industry, especially the automotive industry, hot stamping of high-strength heat treatable aluminium alloys at even higher forming temperatures than conventional warm forming has become feasible and efficient. One of these processes, named Hot Form and Quench (HFQ®), was proposed by Lin et al. [101], and is believed to be a leading-edge technique in this area [75]. Fig. 11 illustrates the temperature profile of a blank in the HFQ® process [102]. Initially a blank is heated to its Solution Heat Treatment (SHT) temperature and soaked for a specific time period to
dissolve the original coarse precipitates and soluble inclusions within the -Al matrix and obtain an optimum microstructure. Then the blank is quickly transferred to the press, stamped and held for a brief period between 18
ACCEPTED MANUSCRIPT the cold dies which quench the blank to lower temperatures (normally lower than the artificial ageing temperature for heat-treatable aluminium alloys). The blank is more ductile at elevated temperatures, and using cold die quenching can achieve a cooling rate rapid enough to prevent the formation of coarse secondary phase at grain boundaries and obtain a super saturated solid solution state in the formed part. For heat treatable aluminium alloys, the hot stamped part can be artificially aged to achieve higher strength to meet the design requirements of vehicle manufacturers. Beneficial characteristics of HFQ® can be summarised as follows: HFQ® is a hot stamping process integrated with heat treatment, which is independent of alloy grade and
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initial heat treatment temper. Optimal microstructures can be controlled during hot forming and postformed strength can be guaranteed. 2.
Ductility can be maximized, which enables the successful forming of complex-shaped components and extends the application of aluminium parts in car body structures.
The combination of solution heat treatment, hot forming and quenching can reduce manufacturing
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operations and improve productivity. 4.
Holding the formed part between cold dies while it cools eliminates springback and ensures part shape
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accuracy.
Figure 11. Hot Form and Quench (HFQ®) process [102][103]. Another typical hot stamping process for the manufacture of high strength heat-treatable aluminium alloy components was proposed by Maeno et al. [104]. Two major characteristics of this process are: initial T4 heat treatment temper and quick heating. The quick heating can be achieved by resistance heating of initial blanks 19
ACCEPTED MANUSCRIPT with the same cross-section area, or by furnace heating with a pre-designed thermal gradient. Such a process enables the removal of solution treatment found in HFQ® and the sizing processes to determine part accuracy in cold stamping. Aluminium alloy aircraft parts with a high strength and dimensional accuracy were successfully produced by this process. However, arising from the requirements of this process, there also exists several challenges that need to be addressed: •
The selection of raw material is not flexible. An initial T4 temper of alloys is required, which restricts
•
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its application using commercial aluminium alloys such as those in T6 or O tempers.
An extremely high heating rate is required for high quench-sensitive alloys to avoid the coarsening of precipitates during heating according to the Temperature-Time-Property curve, such as AA7075 and AA7050, which means a powerful heating facility is required.
•
The uniformity of temperature for a large initial blank using quick heating approach might be a
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Figure 12. Hot stamping process of high-strength aluminium alloy aircraft parts using quick heating [104].
3. Recent progress of experimentation
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Sheet metal forming processes have been investigated for many years. Each forming technique has its own advantages and applications. In addition, the disadvantages of each results in there being no single forming
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technique suitable for all industry scenarios. Comprehensive reviews on specific manufacturing techniques have been performed in the literature [105][43][106][107]. Therefore, the following section focuses on recent experimental progress that has been made with elevated forming temperature techniques from the view of the analysis of advantages and limitations in previous sections. 3.1 Warm stamping
3.1.1 Raw material candidates Warm forming has been proved to be suitable for non-heat treatable aluminium alloys, while its application for heat-treatable aluminium alloys is controversial. Recently, research has been performed on warm forming of high strength heat-treatable aluminium alloys. Kumar and Ross [108] investigated formability and post-form strength of high strength AW7921-T4 using uniaxial tensile tests at different warm forming conditions. Fig. 13(a) shows the temperature effect on the post-formed strength and strain to failure. By increasing the warm 20
ACCEPTED MANUSCRIPT forming temperature, severe loss of strength was observed, while the ductility was significantly improved. The reasons for such trends was identified and explained by Huo et al. [109] through TEM observations, as shown in Fig. 13(b). For warm formed AA7075, the dominant phases at 200 °C were fine η′ and GP zones, while welldeveloped η′ and coarser η phases were found at 250 °C. These phases determined the post-formed and postpaint-bake mechanical properties. Specifically, 200 °C warm forming caused limited coarsening of matrix precipitates and generated a certain number of dislocations. The combined effect of the decreased precipitation
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strengthening and increased dislocation strengthening enables to preserve the inherent high strength of 7075-T6. However, 250 °C forming led to severely decreased hardness due to the rapidly coarsened matrix precipitates, resulting in the loss of strength. Therefore, it can be found that temperature plays a dominant role in determining precipitation and corresponding mechanical properties. In general, the increase of warm forming temperature is detrimental for guaranteeing post-formed strength, and the temperature effect varies with different alloy grade
improved within a temperature range of 140 °C and 220 °C.
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and initial temper. Hui et al. [110] found that the draw-ability and stretch-ability of AA7075 can be significantly
AA7xxx belongs to the highly quench-sensitivity alloy category, which exhibit sensitive response of precipitates
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during heating and cooling. In comparison, AA6xxx is not as sensitive, which might experience different responses on formability and post-formed strength under warm forming conditions. Mahabunphachai and Koç [111] utilised hot bulging tests to evaluate the formability of AA6061. The formability was increased at 300 °C and a low strain rate. However, the post-formed strength was not evaluated. Fan et al. [112] tested the drawability and post-formed strength of an Al-Mg-Si alloy within a temperature range of 25 °C and 500 °C. The limit drawing ratio increased as temperature rose to 200 °C and decreased afterwards. In addition, hardness peaked at 200 °C and 500 °C. Formation of coarse ´ precipitates were believed to cause the decrease in hardness at
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temperatures ranging from 250 to 450 °C. Rapid heating may avoid the coarsening of precipitates when warm forming aluminium alloys with an initial T4 temper [104]. To this end, systematic studies with an overall analysis on warm forming heat-treatable aluminium alloys considering effects of temperature, strain rate,
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heating rate, alloy grade and initial heat treatment temper are minimal.
Testing temperature (°C)
Testing temperature (°C)
(a)
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Figure 13. Warm forming of high strength heat treatable aluminium alloys: (a) Effects of warm forming temperatures on mechanical properties of AA7921 [108], and (b) Microstructure evolutions at different warm forming temperatures of AA7075 [109].
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3.1.2 Process variables 3.1.2.1 Heating
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Heating history prior to warm forming may determine the subsequent deformation and post-formed strength of heat-treatable alloys. To obtain a carefully controllable heating stage, novel and robust heating devices are required. For sheet metal blanks, these include (a) commonly used furnace heating [113] and (b) resistance heating [114] as shown in Fig. 14. The applications of (c) contact heating [115] and (d) induction heating [75] methods are still limited to laboratory scale shapes. Furnace heating is the most mature heating method, and extensively used in hot stamping press shops. The significant advantages of furnace heating are: (1) uniform temperature distribution, (2) the system layout is easy to be arranged and the furnace easily integrated into a
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production line, (3) several blanks could be heated simultaneously and (4) suitable for mass production applications. However, several disadvantages exist including: (1) a relatively slow heating rate due to the low emissivity of aluminium alloys, and (2) sophisticated handling systems are required for integration with the forming process.
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To increase the heating rate, resistance and induction heating have been developed. Potential problems of resistance heating are: (1) non-uniform temperature distribution, (2) resistance heating is not suitable for complex-shaped initial blanks and (3) the electrical resistance of aluminium alloys is relatively low, according
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to Joule’s law, a greater power input has to be used. With regard to induction heating, induction coil design is critical in determining the heating rate. The heating efficiency of heating aluminium alloys depends on the frequency of power supply, induction coil design and layout. Induction heating is less efficient at heating nonmagnetic materials such as aluminium alloys, compared with steels. Moreover, there are still improvements needed to develop laboratory scale heating methods to a mass production setting. Rasera et al. [115] developed a contact heating method for hot stamping process which enabled a reasonably high heating rate, uniform temperature distribution and high efficiency to be obtained. In terms of heating aluminium alloys, target heating temperature is relatively lower, between 400 °C and 500 °C. Commonly used material for heating platens is steel material that is low cost. However, the temperature of the heating platens needs to be constantly monitored for mass production. For hot stamping aluminium alloys, a
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lower productivity.
Figure 14. Heating methods used for warm forming processes: (a) Furnace heating [113], (b) Resistance heating [114], (c) Contact heating [115] and (d) Induction heating [75]. 3.1.2.2 Forming
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Aluminium alloys heated and formed above 0.3 , result in the material deformation mechanism changing
from elastic-plastic to elastic-visocoplastic. The mechanisms involved in this region become complicated, involving several time-dependent parameters, such as diffusion, recovery/annihilation, recrystallisation and grain growth. The dominant deformation mechanisms in warm forming conditions are related to temperature
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[111], deformation rate [115] and grain size [116].
Temperature and strain rate effects on warm forming AA5xxx has been critically reviewed by Toros et al. [11].
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In general, formability, reflected by forming limit curves, is increased with increasing temperature and decreasing strain rate. Kumar et al. [117] systematically investigated the formability of AW-7020-T6 using uniaxial tensile, swift-cupping and cross-die deep drawing tests at a temperature range between 150 °C and 250 °C. AW-7020-T6 has been proved to be temperature and strain rate dependent through tensile tests, as the yield and ultimate tensile strength decreased with increasing temperature. True fracture strain (uniaxial tensile), limit draw ratio (swift-cupping) and limit drawing depth (cross-die deep drawing) increase with increasing
temperature above 150 °C due to dynamic recovery and dissolution of . It should be noted that, the draw-
ability at elevated temperatures is a very complex definition, which is related to not only alloy intrinsic
properties, such as alloy grade, grain size, heat treatment temper and responses to temperature and strain rate, but also external factors, including blankholding force and tooling condition (friction and heat transfer for nonisothermal scenario). Details of above factors are discussed along with hot stamping condition in Section 3.5.
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ACCEPTED MANUSCRIPT 3.2 Warm sheet hydroforming The temperature limit of pressurizing medium used in hydroforming determines the success of warm sheet hydroforming [118]. Oil and gas are two commonly used pressurising medium for warm sheet hydroforming. Oil, in comparison with gas, offers the advantages of lower compressibility, higher thermal capacity and pressurization, whereas its application is restricted due to its flammability. Hence, non-flammable gases are used for higher temperature applications. The temperature ranges of oil and gas, along with the required pressure of
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hydroforming an aluminium alloy, is given by Hartl [118] and presented in Fig. 15. Palumbo et al. [87] performed the warm hydroforming of age hardenable AA6xxx and found that the optimum working temperature was 200 °C and the exposure time of the material to the warm-forming temperature had to be minimized. In addition, an increased strain rate was beneficial due to strain rate hardening, in terms of die cavity filling and sheet thinning, to obtain sound products. Lang et al. [86] found that the pressure rate is a factor that can
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influence the deformation of material in warm hydroforming. To determine the optimal loading path of warm hydroforming, Choi et al. [119] proposed a methodology to determine profiles of the hydraulic pressure and the blank holder force under warm hydroforming conditions at different punch speeds. An adaptive FE analysis
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with fuzzy control algorithm was developed. Thinning, wrinkling, punch wall contact, die corner floating and conduction were used as criteria in the fuzzy control algorithm.
1400 1200
Internal pressure P (Bar)
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400 200 0
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0
100
200
300
400
500
Temperature T (°C)
Figure 15. Example of required internal pressure depending on temperature and strain rate for the forming of an aluminium alloy [118].
3.3 Hot gas forming 3.3.1 Quick plastic forming The QPF developed by General Motors has been successfully applied to the automotive industry and enables the manufacture of complex-shaped aluminium components at a rate of 100,000 per year. The technology has been successfully used on four production closures [95] and a review of this technology has been conducted by 24
ACCEPTED MANUSCRIPT Krajewski and Schroth [95]. Currently, the material candidates mainly concentrate on AA5083, AA6xxx alloys and magnesium alloys [120] depending on the specific applications in automobiles. Recently, the QPF technology has been used in the aerospace industry, mainly for forming titanium alloys [95] and Al-Li alloys [121]. Fan et al. [121] investigated the strengthening behaviour and microstructure evolutions of AA2195 using a hot gas forming process integrated with heat treatment, as shown in Fig. 16(a). The objective of this process was to avoid subsequent heat treatment and resultant thermal distortion. The sheet is formed into the required
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shape by high gas-pressure within several seconds after being solution heat treated, then cooled quickly within water-cooled dies or by some other cooling methods. Obvious recrystallization, as shown in Fig. 16(b), was observed during hot gas forming, which was believed to be initiated by the initial dislocation density and high forming temperature.
However, the current progress has concentrated on simulating the microstructure
evolutions in the process, and practical operation methods have not been developed. Wang et al. [122]
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performed hot gas bulging tests of friction stir welded (FSW) AA2024-T4 at a temperature of 400 °C. The deformation of FSW sheet was also concentrated in the weld zone during free bulging tests and the global formability of FSW sheet was poorer than that of the base metal. To date, quick plastic forming for aerospace
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alloys is still limited.
Figure 16 Quick plastic forming of Al-Cu-Li with integrated heat treatment: (a) Experimentation details and (b) EBSD maps of different locations on a hot bulged part [121].
3.3.2 Superplastic forming The major concern of SPF is the over-thinning that occurs in a complex-shaped component, as shown in Fig. 17b. The reason is that during SPF, the sheet blank is firmly clamped without being drawn-in and the material experiences pure bulging deformation using the superplasticity of the alloy. To address this drawback, some modifications have been made to SPF. Luo et al. [123] developed an advanced hybrid forming process by
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ACCEPTED MANUSCRIPT combing SPF with a hot draw mechanical pre-forming process. The advantage of this process is, using a hot draw pre-forming, a certain amount of material can be drawn into the die, and a more uniform thickness distribution is obtained in the subsequent SPF. As can be seen in Fig. 17(b), using such a process, the tight radius of the rectangular box can be successfully formed without rupture. Luckey Jr. et al. [124] proposed a twostage SPF process, which was demonstrated by practical forming trials of AA5083 to prove the ability of overcoming excessive thinning and necking. The operation of this process is to use gas pressure to form the
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blank into a preform die cavity of a single die, prior to the pressure being reversed to form the sheet into the final component cavity. The preforming of the blank contributes to form longitudinal geometry without severe
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thinning, while guaranteeing metal thickness in certain regions to improve the thickness profile of the final part.
Figure 17. Hot draw mechanical pre-forming superplastic forming (HDMP-SPF) [123]: (a) Schematic of
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HDMP-SPF and (b) Rectangular boxes formed by HDMP-SPF and SPF for AA5083. 3.4 Hot stamping
HFQ®, a hot stamping process for aluminium alloys, was patented in 2008. The complex temperature profile of this process results in a material that exhibits different microstructural evolutions and corresponding mechanical properties. Therefore, the conventional characterisation methods of aluminium alloys, like isothermal warm forming, are not suitable for the HFQ® process. 3.4.1 Material characterisation Ductility and formability of an alloy is normally characterised by uniaxial tensile and forming limit tests. Material in HFQ® experiences solution heat treatment first to obtain an optimal microstructure. Then, to characterise the ductility under a uniaxial stress state or forming limits under a biaxial stress state at different 26
ACCEPTED MANUSCRIPT temperatures, a fast cooling is required to quickly quench specimen to target temperatures. This results in conventional isothermal dome tests being unsuitable. Novel test facilities with capabilities of precisely controlled heating, cooling and deformation, as well as new test schemes, are required. Fig. 18 summarises two commonly used heating methods for uniaxial tensile tests, which enables to quickly heat a specimen at a fixed heating rate. Resistance heating in a Gleeble machine is believed to be the most commonly used method. However, the clamping grips are water cooled resulting in the temperature distribution
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on the specimen being non-uniform. The determination of gauge zone becomes an important issue. Induction heating is another popular heating method, which enables the heating device to be independent of the mechanical testing facility. However, the non-magnetic characteristic of aluminium alloys restricts the efficiency of induction heating. In addition, to achieve the complex heating path of HFQ®, air or water cooling devices are normally integrated into the test equipment.
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Both resistance and induction heating belong to the localised heating method category. There exists a temperature gradient within the longitudinal direction of the specimen due to the grip cooling effect [125], compared with environmental heating such as conventional furnace heating. To generate a uniform temperature
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distribution within the specimen using such local heating methods, and to avoid determining gauge zone, an advanced grip design was proposed by Ganapathy et al. [126], as shown in Fig. 19(a). The innovative design of the new grips developed enables uniaxial tensile tests under hot stamping conditions to be performed in a simpler way resulting in more accurate results. As can be seen in Fig. 19(b), the concept of self-resistance heating grips, which ensures better uniformity of temperature fields in test-pieces, can be potentially applied for tests of different alloys under complex plane-stress conditions, such as biaxial tensile, with modifications on the
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grip material and circulation designs.
Figure 18. Novel uniaxial testing facilities for hot stamping: (a) Resistance heating (Gleeble) [127] and (b) Induction heating [128].
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new grip.
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Figure 19. Advanced Gleeble grip design [126]: (a) Schematic of grip design and (b) Stress-strain curve using
In terms of hot stamping complex-shaped components, a sheet metal blank experiences variations of
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temperature, strain rate and strain path. Using uniaxial ductility only is insufficient to evaluate formability. The forming limit diagram (FLD) is conventionally used to evaluate the formability of sheet metals [129], which comprises a set of forming limit curves enabling the boundary between uniform deformation and the start of plastic instability leading to material failure to be identified. Similar with hot tensile tests under HFQ®, robust test devices are required to achieve heating and cooling functions.
The FLD is commonly determined using either MK tests [130] or cruciform tests [131]. Bariani et al. [132]
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presented an innovative experimental set-up based on conventional Nakajima tests, enabling the forming limits of hot stamping high strength steel to be evaluated. An induction heating method, as shown in Fig. 20(a), was utilised. With the combination of air cooling, the actual phase transformation kinetics for materials under deformation conditions can be simulated with controls of temperature and strain path variations to be achieved. Such a device also has potential to be used in determining the FLD of aluminium alloys at hot stamping
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conditions. Shao et al. [133] established a novel biaxial test system and scheme for optimising cruciform specimens specifically used for the resistance heating. Using such a novel system, for the first time, the forming
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limit data of an alloy can be generated at various temperatures, strain rates and strain paths and forming limits predicted under hot stamping conditions.
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Figure 20. Novel experimental set-ups of determining forming limit diagrams at hot stamping conditions: (a) Elevated Nakajima tests using induction heating [132] and (b) Cruciform tests using resistance heating [133].
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3.4.2 Process variables
From the view of transferring laboratory research to industrial production, temperature and strain rate (forming speed) are two key variables in hot stamping aluminium alloys. Fig. 21(a) summarises the temperature effect on alloy ductility of various commercial wrought aluminium alloys. For aerospace alloys, such as AA2024, AA7075 and Al-Li alloy AA2060, strain to failure increases with increasing temperature initially, and then decreases afterwards. The maximum ductility emerges at temperatures lower than the solution heat treatment temperature. The sharp decrease of ductility is believed to be caused by the low melting phases within the matrix. Wang et al. [134] performed SEM observations of AA2024 and found that when the temperature exceeds 450 °C, softening of grain boundary occurs due to the solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles. For alloys preferred for automotive applications, such as AA5754 and AA6082, ductility continuously increases with increasing
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ACCEPTED MANUSCRIPT temperature until the solution heat treatment temperature, which demonstrates that hot stamping such alloys directly after solution heat treatment (SHT) results in a maximum ductility. Forming speed is another critical influencing factor for two main reasons. Firstly, strain rate hardening is the dominant mechanism of deforming aluminium alloys at elevated temperatures. Different forming speeds correspond to different flow stress levels that determine the uniformity of deformation and success of forming. Secondly, the heat transfer between hot blank and cold dies results in the forming process being non-isothermal.
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The heat exchange time is determined by the forming speed and punch stroke, which plays an interesting role on the failure location and deformation uniformity. Mohamed et al. [102] established a process window of hot stamping AA6082 as a function of forming rate, as shown in Fig. 21(b). El Fakir et al. [135] and Zheng et al.
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[136] also concluded that a higher forming speed is beneficial for obtaining a uniform deformation.
Figure 21. Process variable effects on the hot deformation of aluminium alloys, (a) Temperature effect on the ductility of different alloys [102][137][134], and (b) Forming speed effect on the failure mode of AA6082 [102]
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3.4.3 Novel tooling techniques
The high temperature of material using hot stamping results in lower material stress level and less hardening
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feature due to recovery, which affects material flow in the flange and induces localized thinning. To address these potential problems, improvements on the tribological and thermal properties of die surfaces determining flange material flow and temperature field within a blank during hot stamping, are feasible approaches. Compared with extensive research on the tooling of hot stamping high strength steel, considering the lower strength, greater heat transfer phenomenon and greater aluminium alloy adhesion tendency, novel tooling techniques are required for hot stamping aluminium alloys. Zheng et al. [136] proposed a macro-textured tool surface concept and these textured tool surfaces were investigated in bending [136] and drawing [138] types of forming process at both cold and hot stamping conditions [139], as shown in Fig. 22. The advantages of using texture design on the tool surfaces are: (1) less contact on the flange between hot blank and cold dies is beneficial for preserving the flange temperature. The more uniform temperature contributes to obtain a more uniform material deformation within the blank. (2) The material that is absent of contact with die surfaces experiences no friction, and this material is easier to be drawn into the die to assist the remaining material to 30
ACCEPTED MANUSCRIPT flow. (3) The required blankholding force of suppressing flange wrinkling is low. For the type of bending deformation, as shown in Fig. 22(a), material undergoes one-dimension however the textured design may induce buckling on the material that does not experience blankholding force. To control buckling and provide guidelines to tool designers, a series of buckling models, based on beam [138] [139], plate [138] and shell [140]
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assumptions, under cold [138], warm [141] and hot stamping [139] conditions were established.
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Figure 22. Textured tool surfaces for hot stamping dies [136] [138]. To improve the tribological performance of tooling and achieve the objective of minimising or eliminating lubricant, Dong et al. [142] proposed a methodology of duplex-treated tooling surfaces for hot stamping dies. In this treatment, low-cost tool material candidates, such as cast iron, can be utilised as substrate tool material, as the forming load of hot stamping aluminium alloys is not as high as that of high strength steel. Then, plasma
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treatments, such as plasma-nitriding or plasma-carburising, are performed on the tool surfaces to further increase the tool surface strength. Finally, to improve the tribological properties, different coatings, such as WC: C coating, are applied on the top layer. Fig. 23(a) illustrates the microstructure of a duplex-treated tool. A series
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of tribological tests were performed for different tool surface conditions using conventional pin on disc tests. The friction coefficient can be reduced significantly compared with untreated tool surfaces, as shown in fig. 23(b). It is interesting to note that, PNC treated tool surface has no significant effect on reducing friction coefficient. The function of PNC treatment is mainly for increasing tool surface strength, which contributes to using cheap tool material, like cast iron, to replace conventional expensive hot working steels. Zheng et al. [143] further tested the lubrication performance of duplex treated tool surfaces in a practical deep drawing process. The results have shown that the advanced WC: C Diamond-like Carbon (DLC) coating could significantly reduce the use of lubricant for hot stamping.
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Figure 23. Duplex surface treated hot stamping die surface, (a) SEM photo of WC: C coated & PNC treated
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G3500 die [144], and (b) Friction coefficient evolution of treated tool surface at hot stamping condition [142]. 3.4.4 Characterisation methods of interface properties under hot stamping condition
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Interfacial heat transfer coefficient (IHTC) and friction coefficient (CoF), representing the thermal and tribology properties of die surfaces are crucial for the accuracy of finite element simulations [145]. Therefore, characterisation methods of these two properties become important. In terms of the interfacial heat transfer coefficient, two commonly used methodologies, named as one-dimensional (1D) closed-form calculation [146] and inverse FE method [147] are shown in Fig. 24. Xiao et al. [148] utilised the 1D closed-form calculation to calculate the interfacial heat transfer coefficients of hot stamping AA7075 at different contact pressures and lubrication conditions. The results have shown that IHTC values increased with increasing contact pressure and
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use of lubricant. Ying et al. [146] performed a systematic investigation on the IHTC of HFQ® forming AA7075T6. Effects of contact pressure, surface roughness and lubrication conditions were evaluated. In addition, the obtained IHTC was used in the simulation process to be validated against a typical U-type experimental model. The finite element assist method is used to compare IHTC values implemented in FE with experimentally
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recorded values to compare temperature histories. To precisely measure the temperature evolutions of a specimen and tools during hot stamping, Liu et al. [149] developed a novel experimental facility based on the Gleeble testing machine, as shown in Fig. 24b. The IHTC values between AA7075 and three different tool
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materials were characterized. In addition, utilising this methodology, Liu et al. [149] further used this method to investigate the effects of lubricant on the IHTC during hot stamping AA6082 with different thicknesses. The IHTC was increased using a graphite lubricant to fill the vacancies of asperities at the contact interface. The increase in IHTC when applying lubricant compared to air is approximately 26% and 20% for the 2 and 3 mm thick specimens respectively. In addition, it was also found that the IHTC decreases dramatically from a stable value when the lubricant layer thickness is lower than 20 µm at different contact pressures.
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Figure 24. Methods of determining interfacial heat transfer coefficients, (a) One-dimensional closed-form
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method [146] and (b) Inverse FE method [147].
A precise determination of friction coefficient (CoF) is very challenging as the hot stamping process introduces
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non-isothermal contact. In addition, CoFs are also related to material pair, temperature, sliding speed, sliding distance, lubrication and surface roughness. A robust test method and new test scheme are required to include these influencing factors. Fig. 25 summarises typical methods used to determine interfacial friction coefficients, which can be potentially used for hot stamping processes. The Pin on disc method [142] is believed to be the most commonly used method to determine the CoF and wear during sliding. However, the contact condition at the interface is different with actual conditions experienced in hot stamping. In addition, the range of test variables, such as sliding speed, is dependent on the capability of test equipment. Shi et al. [150] developed
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simple test equipment used to determine friction coefficient under warm forming condition, as shown in Fig. 25b. The whole test rig was mounted within a furnace to heat both tools and blank to elevated temperatures. A sufficiently high sliding speed and distance can be obtained with the combination of hydraulic press. However, the contact condition still a point to surface contact. Hot strip drawing [151] is believed to be the most similar test method to simulate the actual contact during hot stamping. However, previous research mainly concentrated
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sheets.
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on investigating the tribology of steel with limited research being performed on hot stamping aluminium alloy
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Figure 25. Methods of determining interfacial friction coefficient of hot stamping aluminium alloys, (a) Pin on disk [142], (b) Strip drawing on pin tool [150], and (c) Hot strip drawing [151]
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4. Material modelling and numerical simulations For sheet stamping processes, the development of material modelling has mainly concentrated on the modelling of stress-strain response, hardening and failure of materials under different forming conditions. The developed material models can be implemented into different finite element software to give comprehensive and precise
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simulations to predict sheet metal deformation.
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4.1 Cold forming condition
4.1.1 Fundamentals of material model Materials experience elastic-plastic deformation, with the uniaxial stress-strain relationship at room temperature deformation conditions given in Eq. (1), where E represents the Young’s modulus, ε e , ε p and ε T represent the elastic, plastic and total strain respectively. σ represents the effective stress. A plane stress assumption is normally used for sheet forming process, enabling the uniaxial case to be written as Eq. (2).
σ = Eε e = E(ε T − ε p )
(1)
σ ij = Dijkl ε ije = Dijkl (ε ijT − ε ijp )
(2)
In FE analysis, the total strain is obtained from nodal displacements, which are achieved by solving global system equations. Therefore, to calculate stress, the calculation of plastic strains becomes vitally important. Plastic strain can be calculated from constitutive relationships, as given in Eq. (3). 34
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∂f (σ ij ) ∂σ ij
df
(3)
f represents plastic potential, H is a scalar coefficient related to hardening. For associated flow theory, plastic potential is assumed to be yield locus, while for non-associated flow theory, the assumption is invalid. For cold forming, hardening is due to an increase in the dislocation density due to plastic straining, resulting in
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the flow stress σ s increasing with plastic strain. Lin [152] proposed a complete material model of an initially isotropic elastic-plastic solid subject to monotonic uniaxial loading:
(4)
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n σ −σY ε p = sgn(σ ), with σ = σ s K σ = E(ε T − ε p )
The above expression is derived based on dislocation density, following the form of Ramberg-Osgood equation [153]. Other commonly used phenomenological hardening laws can be also used, which are summarized in
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Table 5.
Table 5 Commonly used empirical constitutive models of cold forming Model Ludwik [154]
Expression
Swift [129]
σ = C(m + ε n )
Voce [155]
σ = C(1 − me −nε ) σ
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Ramberg-Osgood [153]
σ = Cε
Descriptions C is a constant stress, n is a strain hardening exponent C , m and n are empirical constants
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σ ε = 1 + α E σ0
m −1
e is the exponential constant, others are constants σ 0 is a nominal yield stress, α is a dimensionless constant
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4.1.2 Improvements on material model under cold forming condition In terms of cold stamping complex-shaped aluminium alloy components, two particular issues need to be
•
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carefully considered and modelled to improve the accuracy of model predictions. Aluminium alloy sheet blanks are normally produced by rolling with strong anisotropy feature due to the texture of orientation of materials. The anisotropy feature has an important effect on the material
flow and earing of sheet stamping components [156].
•
For stamping a complex-shaped panel component, the material is subjected to bending and unbending conditions. In additional, drawing and redrawing processes might be utilised to obtain a greater drawing depth. In such scenarios, kinematic hardening become important for the prediction of residual stress and springback after stamping.
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anisotropy by Hill [159][160][161], Hosford [162] and Barlat et al. [163][164][165]. A comprehensive review of these models was given by Habraken [166]. The above classical anisotropic yield functions are widely used in engineering applications. Recently, Stoughton and Yoon [167] proposed a new anisotropic model based on the non-associated flow theory, which is a significant improvement on the isotropic hardening non-associated flow theory proposed in 2002 [168]. Based on Stoughton’s non-associated hardening model, Lee et al. [169]
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proposed a non-associated flow rule which simply couples quadratic and non-quadratic yield functions to describe the evolution of yield surface. The contribution of the non-quadratic part is to control the curvature of
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the whole model, while the quadratic part is to describe anisotropic hardening throughout a deformation history. The model comparison shows that the present model can follow the anisotropic hardening of AA5182-O and AA6022-T43.
To validate the anisotropic models, Yoon et al. [156] implemented the Yld2004 yield function into finite element simulations to predict earing in a deep drawn cup. The predicted and experimental cup height profiles (earing profiles) with six ears are shown to be in excellent agreement. Earing for strongly textured aluminium alloys, AA6111-T4, were also predicted by Yoon et al. [170] using the Yld 2000 anisotropic yield function. An
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improvement in prediction was observed using Yld 2000 compared with conventional Hill 48 yield criterion. It should be noted that, this review focused only on macroscopic yield functions, microscopic yield functions based on crystal plasticity are not taken into account. 4.1.3.2 Kinematic hardening
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In many sheet forming processes, especially those that include draw-bead and bending/unbending, the sheet metal undergoes cyclic deformation. Cyclic effects caused by this type of deformation cannot be predicted with
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the common isotropic hardening law, resulting in a suitable kinematic hardening law being required. A series of constitutive models have been proposed considering kinematic hardening together with anisotropic properties and cyclic deformations. Teng et al. [171] combined Mroz’s anisotropic hardening rule to calculate the springback of sheet stamping. Wu [172] extended the Hill 48 anisotropic criterion to include mixed hardening for sheet metals. Chung et al. [173] and Lee et al. [174][175] formulated the Yld 2000 plane stress yield function with modified Chaboche-type combined isotropic–kinematic hardening law to consider the Bauschinger effect and transient behaviour. In these studies, the prediction capability of springback was improved. Taherizadeh et al. [176] proposed an anisotropic material model based on non-associated flow rule and mixed isotropic–kinematic hardening (Armstrong–Frederick formulation), and implemented it into a userdefined material (UMAT) subroutine for ABAQUS with Hill’ 48 yield function being selected. This model was further compared with other advanced models using different yield functions [177] .
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ACCEPTED MANUSCRIPT 4.2 Elevated temperature forming conditions Aluminium alloys exhibit viscoplastic behaviours when they are deformed at a temperature T / Tm > 1 / 3 , where
Tm is the melting temperature of alloy. Therefore, viscoplastic theories should be used for elevated temperature forming conditions of aluminium alloys. When the material is deformed at high temperatures, material properties, such as flow stress, are dependent on forming temperature, strain rate, strain and strain rate history
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and hence constitutive models of forming aluminium alloys at elevated temperatures become vitally important. In this section, phenomenological (empirical) and physical-based material models are reviewed. 4.2.1 Phenomenological models
Phenomenological plastic constitutive models are widely used in modelling the forming processes of alloys at
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high-strain-rates and temperatures. Table 6 summarises a series of commonly used phenomenological models. Table 6 Commonly used phenomenological models
Khan–Huang– Liang (KHL) [181]
Descriptions K is the strength coefficient, n is the strain hardening coefficient, and m is the strain rate hardening coefficient. k is the threshold stress A is the yield stress at the reference temperature Tr and strain rate !
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+ 1 + ∗ 1 − ∗ ∗ = /! ∗ = − " / − " #$ = %& ' %$ '
equivalent plastic strain and strain rates. f1
&
&/$
1 = &/ = $/ = >? @ + A? @ + 1B
B is strain rate hardening coefficient, n is the strain rate hardening exponent C and m are material constants.
J2 is the flow stress, ε p and ' are
( + )1 − , ! - ./0 1 *!+ − ∗ = 1sinh67 89:−;/< = = 89:;
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Zener-Hollomon (Arrehnius) [182]
= +
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Khan and Huang (KH) [180]
Expression =
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Model Fields and Bachofen (FB) [178] Modified FB Johnson-Cook (JC) [179]
C
and f are functions of equivalent plastic strain and plastic strain rate respectively. A , B , n0 , n1 , C and m are material constants, D0P equals to 106 /s.
Q is the activation energy, R is the ideal gas constant. A , α and n are material constants.
4.2.2 Physical based material model Phenomenological models can be used to model strain hardening at elevated temperature forming conditions, which do not take into account microstructure evolutions. In terms of some particular forming processes, such as SPF, where recrystallisation takes place, dislocation-free grains are generated, which results in the reduction of the dislocation density and material softening. Therefore, in order to precisely model various microstructure evolutions for a specific forming process, physical mechanisms-based material models are required.
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ACCEPTED MANUSCRIPT Physically-based models containing mechanisms of deformation such as dislocation dynamics and thermal activation have been developed in the literature. These include: Zerilli-Armstrong (ZA) [183], Hybrid model based on neuro-fuzzy and physically based component [184][185], Dynamic recrystallization (DRX) [186], Preston-Tonks-Wallace (PTW) [187], Voyiadjis-Almasri (VA) [188], Bonder-Partom (BP) [189], and Cellular Automaton (CA) [190]. It should be noted that, most of above physically-based models were focused on extreme loading conditions or materials other than aluminium. The applications of modelling aluminium alloy
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deformed at different elevated temperature forming techniques were not performed. 4.3 Unified material model
In terms of forming aluminium alloys at elevated temperatures, various microstructure evolutions, such as dislocation accumulation, recovery, recrystallisation, grain growth, grain refinement, precipitate dissolution,
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formation and growth, interact with each other and affect the macroscopic mechanical properties. In addition, microscopic damage during large deformation processes is also important for predicting rupture in sheet metal forming, especially for hot stamping and superplastic forming. Internal state variables have been used for a long
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time to represent damage or hardening of alloys. Lin and Liu [191] developed a set of unified mechanism-based constitutive equations for modelling different microstructure evolutions during warm/hot forming with the concept of internal state variables. In this section, unified constitutive equations of hot stamping and superplastic forming processes were reviewed. These two sets of equations can be further implemented into finite element software, which is discussed in the next section. 4.3.1 Hot stamping (HFQ®)
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The set of unified viscoplastic constitutive equations of HFQ® is based on the fundamental framework proposed by Lin and Dean [192] and Lin et al. [193]. The equations are given below [194]: = ) '
E D HIHJ EFG K
,
(6) (7) (8) (9)
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< = 0.5L̅ !.N L̅ L̅ = 1 − L̅ |' | − L̅ P & |' |RP Q = 1 − QRS σ = E(1 − ω )(ε T − ε p )
(5)
Where Eq. (5) is the traditional power-law viscoplastic flow formulation, which considers the effects of damage
ω on viscoplastic flow. Eq. (6) represents the evolution of material hardening, R , which is a function of the normalised dislocation density ρ defined by ρ = ( ρ − ρi ) / ρ m , where ρi is the dislocation density of the material at the initial state and ρ m is the maximum (saturated) dislocation density of the material. Hence, the range of normalised dislocation density, ρ , is from 0 to 1. Eq. (7) represents the rate of accumulation of dislocations. Eq. (8) represents damage evolution for the uniaxial formulation, which is a function of plastic
strain rate ' . Eq. (9) is Hooke’s law for a simple uniaxial state. In this equation set, n2 and η3 are temperature-
independent parameters, while the material parameters K , k , n1 , B , A , C , E , η 1 , and η2 are temperature
dependent, given in Eq. (10)
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ACCEPTED MANUSCRIPT K = K 0 exp(QK RT ) k = k0 exp(Qk RT ) n1 = n10 exp(Qn RT ) A = A0 exp( − QA RT ) B = B0 exp(QB RT )
(10)
C = C 0 exp( − QC RT ) E = E0 exp(QE RT )
η1 = η01 exp(Qη RT )
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1
η 2 = η 01 exp( −Qη RT ) 2
4.3.2 Superplastic forming
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A set of SPF damage constitutive equations for AA7475 based on the power-law have been introduced by Lin et al. [195]. Damage variable, ductile void growth damage and effective damage are included in this equation set as shown below. '
\
$/VWX.Y 0 [[ Z
?
\
− 1@^
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' = TT U1 +
0 ]]
/1 − %Z − < − Hd 〉b c − < − Hd ' 〉b c TT = 〈 < = e; − <|' | c = c HfX + |' |c Hg j/$ %h 2 % %Z %h = c + )%Z − h , __ c c
__ = 〈
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'
%Z = *& %ZZ& ' HZP + *$ ' ZS cosh*j ' & ' for %h = 0 = .. $ & ' for %h > 0 = n%Z .. − %h o
(11) (12) (13) (14) (15) (16)
(17)
$VW
= pq − '
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(18)
Eqs. (13)-(15) represent the steady-state flow of a viscoplastic material. In the early stage of deformation, the
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overall material hardening is due to grain growth rate and increase of dislocation density. Eq. (14) represents the isotropic hardening variable
R , which is related to the accumulation of dislocations and expansion of the yield
surface during plastic deformation. b and Q are material constants. Eq. (15) represents the evolution of average grain size, d . In this equation, γ 0 and φ represent the effects of isothermal and plastic strain-induced grain growth respectively. α and β are material constants. At the late stage of deformation, softening due to micro-damage dominates resulting in the decrease of flow stress. Grain-boundary sliding and grain rotation are two important mechanisms in SPF. The relative movements result in void nucleation and growth at grain boundaries. A cavitated cylindrical material assumption is utilised, which is similar to the intergranular void growth under high stress creep. Details of the formulation of this set of equations is given in the work by Lin et al. [195]. Eq. (13) represents the steady-state creep rate of power-law zone. k is the initial yield stress and
µ
is the material constant. Eqs. (16) and (17) represent the evolutions of 39
ACCEPTED MANUSCRIPT void volume fraction and effective damage respectively, where f d is the effective void volume fraction, ranging from 0 to 1, and f h is the value of void volume fraction. f d = 0 means no effective damage is incurred while f d = 1 signifies failure. Eq. (12) represents the plastic strain rate with voids. The plastic strain rate is given in Eq. (11), and represents the cavity spacing .
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4.4 Model application 4.4.1 Hot stamping
The set of constitutive equations for hot stamping aluminium from Eq. (5) to Eq. (10) in section 4.3, can be calibrated using a series of uniaxial tensile tests at different temperatures and strain rates to determine material constants. Fig. 26(a) shows the comparison between model fitting (solid lines) and experimental data (symbols).
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The calibrated material constants are given in Table 7. The developed material model can then be implemented into FE software via subroutine, such as Abaqus, Pamstamp and Mark. El Fakir et al. [135] performed numerical simulations of HFQ forming AA5754 with the implementation of material model. Fig. 26c shows the plastic
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strain contours of a computed component with a comparison to the successfully part. As can be seen in Fig. 26b, good correlation with a deviation of less than 5% was achieved between the thickness distribution of the
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simulated and experimentally formed parts, verifying the capability of the model.
Table 7 Material constants determined from calibration [135] C 0 (s-1)
217.8 QC (J/mol)
6669.49 η 03
60999.8 Qη 3 (J/mol)
4.381
9469.6
B 01 (MPa)
k 0 (MPa)
K 0 (MPa)
η 01
η 02
6.75E+18 Q k (J/mol)
3.3932 Q K (J/mol)
0.0846 Qη 1 (J/mol)
0.03203 Qη 2 (J/mol)
1.7211 Qn1 (J/mol)
11181.5 A0
34630.3 Q A (J/mol)
26837.6 n2
17594.1 n01
12993 B02
1.9996
2898.3
0.3
3.56E-01
6.005
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EP
E 0 (MPa)
13211 Q E (J/mol)
40
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Figure 26 Finite element simulations of hot stamping AA5754 [135]. 4.4.2 Superplastic forming
To determine the material constants from Eq. (11) to Eq. (18) under SPF conditions, experimental stress-strain and grain size data, shown in Figs. 27(a) are necessary. To obtain the best fitting between the equations and experimental data, optimisation methods are recommended. Table 8 presents an example set of material constants for SPF Al-Zn-Mg alloy at a temperature of 515 °C. From these constants, it was found that good
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correlations between experimental and computational results are obtained for both the flow stress and grain size evolutions [195]. The developed constitutive equations can be subsequently implemented into FE simulations to simulate practical SPF forming. Fig. 27b shows the field plots of state variables for Al-Zn-Mg, at strain rates 2 × 10-4 and 2 × 10-3 s-1 respectively [196].
K
0.2912
85.1712 D1
φ
5.5E-5
n
µ
b
Q
α
1 D2
1.6236 D3
0.1537 d1
7.6744 d2
69.5748
1
30.0013
3.3
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k
EP
Table 8 Material constants for Al-Zn-Mg at 515 °C [195]
8.145E-9
41
β
6.9E-2 d3
γ0 2.4 E
2.6 l0
1.7658
1.0E3
3.6347
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ACCEPTED MANUSCRIPT
Figure 27 Finite element simulations of superplastic forming Al-Zn-Mg, (a) comparisons between experimental
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results and model fitting [195], and (b) Field plots of f h and fe for Al-Zn-Mg with target strain rate,8 rsr = 2 × 10Hu and 2 × 10−3 s -1 , respectively [196].
5. Conclusions and future research trends
A detailed review of forming techniques for manufacturing complex-shaped high strength aluminium panel components was conducted in this paper. The paper has identified and discussed research studies for improving and addressing the current limitations, both experimentally and theoretically, for a range of forming techniques. The current state of the art forming technologies have shown that:
42
ACCEPTED MANUSCRIPT (1) Non-heat treatable aluminium alloys, AA5xxx, are widely used for automotive body structures, while the use of high strength heat treatable alloys, such as AA7xxx, is increasing. However, the current applications of high strength alloys are restricted due to the poor ductility at room temperature, requiring novel forming techniques to be utilised. (2) Cold stamping in alloys in T conditions is believed to be unsuitable for high strength aluminium alloys. Wtemper forming is an alternative way of manufacturing some components with lower complexity of geometries,
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which is preferred by industries considering the mature cold stamping production line. The springback is relatively low due to the less yield strength of material in W condition. Sheet hydroforming and incremental forming techniques enable the manufacture of more complex structures, while the production efficiency is not as high as conventional cold stamping and their applications are restricted.
(3) Forming at elevated temperatures increases alloy ductility. Warm forming techniques, including warm
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stamping, warm hydroforming and warm sheet incremental forming, have been extensively investigated. However, warm forming temperatures may deteriorate favourable microstructures, and additional heat
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treatments are required to restore component strength for heat-treatable aluminium alloys. (4) For hot forming techniques, from the view of productivity, SPF is mainly used for manufacturing complexshaped components in the aerospace industry, while QPF is concentrated on the automotive industry and has already been widely applied. HFQ® is a novel process integrating hot forming with heat treatment, which is believed to be a leading technique in this area.
(5) Extensive research has been carried out on formability and post-formed strength of alloys at different
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elevated temperatures which are affected by alloy grade, heat treatment temper, process variables, and tooling. (6) The novelty and robust capabilities of HFQ® promotes new characterisation methods and test schemes to be developed to evaluate formability of aluminium alloys under hot stamping conditions. In addition, tooling of HFQ®, focusing on heat transfer, coating and surface texturing techniques, was well developed to assist the
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manufacture of complex-shaped panel components.
(7) Material models under different forming techniques need to consider specific process characteristics, such as non-isothermal, variations of strain, strain rate and strain path and microstructure evolutions during forming.
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The constitutive models can enhance the prediction accuracy of FE simulations. Based on the above drawn conclusions, the future research of manufacturing complex-shaped high strength aluminium components should focus on four objectives: (1) maximize the process ability to achieve the manufacture of complex-shaped requirements with the industrial developments of component designs and new material candidates; (2) guarantee the post-formed strength of aluminium components to meet increasing and stringent requirements with the increasing occupation of aluminium alloy usage among body structures of vehicles; (3) increase the dimensional accuracy of component geometries considering the assembling requirements of volume production of aluminium components; (4) reduce the overall cost of manufacturing aluminium components to enhance the competitiveness of aluminium alloys with traditional steels.
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