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Development of hydro-mechanical deep drawing
Journal of Materials Processing Technology 83 (1998) 14 – 25
Development of hydro-mechanical deep drawing S.H. Zhang *, J. Danckert Department of Production, Aalborg Uni6ersity Fibigerstraede 16, DK-9220 Aalborg, Denmark Received 16 June 1997
Abstract The hydro-mechanical deep-drawing process is reviewed in this article. The process principles and features are introduced and the developments of the hydro-mechanical deep-drawing process in process performances, in theory and in numerical simulation are described. The applications are summarized. Some other related hydraulic forming processes are also dealt with as a comparison. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Deep drawing; Hydro-mechanical; Numerical simulations
1. Introduction Hydro-mechanical deep drawing is a new sheet metal forming technology originating from hydroforming technology. It combines the features of both traditional deep-drawing technology and hydroforming technology. In hydro-mechanical deep drawing the limit drawing ratio (LDR) values can reach 2.8 compared with the traditional deep-drawing process where the LDR values are only about 2.2. Since its inception, the hydro-mechanical deep-drawing process has found increasing application in industry, such as application in automobile and aircraft manufacture. After continuous improvement and innovation, much specialized equipment and many devices have also been devised and put into use. The hydro-mechanical deep-drawing process can replace some other metal forming processes, because this process can improve the quality of the product without losing productivity. Hydro-mechanical deep-drawing technology was first developed in 1890 [1]. However, the real development began after the second world war. Early research work began mainly in Germany and Japan. Japanese researchers began investigating the process in 1955. Kasuga et al. first proposed pressure-lubricated deep drawing and was in charge of the research work in this field from 1958 to 1964. After the mid 1970s, * Corresponding author. Tel.: + 45 96 358958; fax: + 45 98 153030; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00039-9
Nakamura and Nakagawa [2] began their research work on the hydraulic counter-pressure fluid-forming process (the FF method). The radial-pressure deepdrawing method was proposed during this period. German researchers began their work in the 1950s [3]. A seal-ring on the die face was used in order to prevent the leakage of fluid between the die and the blank [2–4]. Dutch researchers presented the daalderop method in 1961 and American researchers began research on the aqua-draw method in 1973. AP & T of Sweden began researching the process and devices in the 1960s, supplying specialized equipment and devices [1]. Larsen [5], a Danish researcher, published a paper on work in Denmark in 1977, the research at present being continued by Danckert andersen et al. [6,7] at Aalborg University and Bay [8,9] at the technical University of Denmark. Chinese researchers began their work on this technology at an earlier date (D.C. Kang, private communication, Jan. 1997). Kang is in charge of the research work at Harbin Institute of Technology. He has been co-operating with Nakamura of Japan since 1980. Kang investigated the hydro-mechanical forming process of box workpieces, parabolic shells and also the hydro-mechanical ironing process. Guo, another researcher at Harbin Institute of Technology, who stayed in Japan with Nakamura for a period, has researched the forming accuracy of hydro-mechanically drawn cups, boxes and tapered cups. A new press of 2000 kN capacity has
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been installed recently at Harbin Institute of Technology, especially for hydro-mechanical drawing. Also, specialized facilities will be installed on a press of 5000 kN capacity in a Hong Kong firm. The research work of this technology is also being carried out at Jinan Research Institute of Casting and Forging Machinery [1] and at some other universities [10]. In recent years, hydro-mechanical deep-drawing technology has been developing quickly in some other countries also, for example, France, USA, Russia, Israel, Switzerland, Korea and even Kuwait. Tirosh et al. [11 – 17] have published a series of research papers on theoretical analysis and experiment in Israel, whilst Gelin et al. have published some papers [18 – 20] on their research work in France. According to the analysis by Amino and Bakagawa [21], the hydro-mechanical deep-drawing process has the following features: (a) it has a friction-holding effect (friction forces are produced between the blank and the punch and serve as part of the forming force); (b) it has a resistance-reduction effect (the friction resistance between the flange and the die is reduced because of the flowing out of the fluid); (c) it has an initial extension effect (extension in the vicinity of the die shoulder portion is caused by pre-bulging, the thickness thus becoming more uniform: by using pre-bulging the material over the die cavity is initially stretched, causing a more uniform thickness distribution). Because of the above three fracture-prevention effects, this process can improve the fracture limit. The process also has a wrinkle-prevention effect; the unsupported portions are subjected to bulging pressure, causing circumferencial tensile stresses, which prevents wrinkles from occurring. The hydro-mechanical deep drawing process has the following advantages [1,3 – 5,21,22]: (a) Due to the fracture-prevention effects and the wrinkle-prevention effect, the product may have a higher drawing ratio (the LDR is increased), the forming steps are reduced, a sizing tool and a sizing operation are unnecessary, the limitations of the quality and size of the product are overcome and costs are reduced. (b) Due to the forced contact between the blank sheet and the punch and the lubrication effect of the liquid between the flange and the die, less damage is suffered by the flange, the deep drawing products have a better surface quality, wrinkles can be suppressed, the dimensional accuracy is high and the wear of the dies is clearly reduced [14]. (c) Because of the friction-holding effect, local thinning is lighter and the thickness distribution is more uniform. (d) As the female die is replaced by fluid pressure, only a punch is used and the drawing operations are reduced. Therefore, the process is cost effective and can be used in small batch production and even in sheetmetal property tests.
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(e) The process can be used for the manufacture of complex-shaped workpieces and for the deep drawing of some materials that are not suitable for intermediate heat treatment. The process is flexible. When drawing automotive body panels higher deformation may easily be obtained to increase the stiffness of the product [23]. However, the process has its drawbacks, the main drawback being that a higher drawing force and a higher blank-holding force are needed compared with those of conventional drawing technology [3,18].
2. Process principles and classification Hydro-mechanical deep drawing has various names, e.g. Hydromec, Aqua-draw, fluid former, drawing with counter pressure or hydraulic counter-pressure deep drawing, deep drawing, process with fluid-pressure assisted method, hydraulic forming and even just Hydroform or hydroforming, each representing its different features. In fact, the hydro-mechanical deep-drawing process is a kind of soft-tool forming technology or flexible-forming method originating from hydroforming technology. Soft-tool forming technologies have been used widely in industries because of their simple equipment and convenient devices, their low energy consumption, their fine product quality and their cost effectiveness. Soft-tool forming technologies include rubber-die forming and fluid-hydraulic forming. Hydraulic forming is used widely in sheet metal forming and tube forming and it includes soft punch forming (in which pressurized water or other fluid media such as oil is taken as the punch, whilst the female die is a rigid body) and soft-die forming (the pressurized water or other fluid media is taken as the female die, the punch being a rigid body). The former is usually referred to as hydro-bulging by most researchers and the latter is usually referred to as the hydro-mechanical deep-drawing process. However, some researchers call it the hydroforming process (especially when using a thin rubber diaphragm between the blank and the fluid). The latter process, i.e. the hydro-mechanical deep-drawing process, will be discussed in detail in this article. Hydroforming (hydrobulging) has been reported most recently in automobile industry, especially for tube forming and the forming of panel parts [24–27]. The hydro-mechanical deep-drawing process can be divided into the hydrostatic type and the hydrodynamic type on the basis of the feature of the fluid pressure. The hydro-bulging process can also have the two different situations according to the loading. In most cases, hydrobulging is in the hydrostatic state, but there exist some cases, such as explosive forming with fluid as the medium, where the fluid is in the dynamic state. The hydro-mechanical deep-drawing process has different performances during operation. Fig. 1 (a) shows
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Fig. 1. Schemes of the hydromechanical deep-drawing process.
the conventional deep-drawing process, where LDR is somewhere between 2.2 and 2.3 [5], although the theoretical prediction should be about 2.715 [11]. The main reason for the reduction in LDR is due to the friction
between the blank flange and the die. The fluid pressure is used the in hydro-mechanical deep drawing process mainly so that friction can be significantly reduced and so that the clamping (or friction-holding effect) between
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Fig. 1. (Continued)
the punch and the blank can prevent fracture of the blank at the punch corner, thus enabling the LDR value to be increased. However, the fluid can be used in different ways, therefore the LDR values can be increased to different levels. Usually the LDR values can be increased to about 2.7 within one stroke by means of the hydro-mechanical drawing process, whilst the LDR can be increased to 3.2 for the radial hydro-mechanical deep-drawing process in one step. It is reported that the LDR values may be increased to about 6 for the radial-pressure deep-drawing method (two steps). The hydro-mechanical deep-drawing process may be sub-divided into the following different types.
2.1. Deep-drawing process with fluid-assisted blank holding This process is shown in Fig. 1(b) and (c). The
traditional blank holder is replaced by a fluid pressure with a thin rubber diaphragm transferring the pressure to the blank flange, a fluid blank-holding device being required. The friction can be reduced considerably and the LDR can reach 2.3; the productivity is reduced a little in this case. This process is referred to as the hydro-mechanical deep-drawing process, deep drawing with a fluid-pressure-assisted blankholder or hydrostatic deep drawing in [11,12,15,16].
2.2. Hydro-forming process As in Fig. 1(d), when the female die is totally replaced by a fluid pressure, the pressure is transferred by a thin rubber diaphragm between the blank and the fluid. The punch determines the final shape of the workpiece. The fluid pressure in the chamber can be controlled by a device (a valve or a pump). The fluid
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pressure is very important because wrinkles will appear if the pressure is not sufficiently high. In the case of too-high a pressure, the workpiece may be damaged by rupture. This process is referred to as the hydro-form or hydroforming (HF) process by researchers [4,11,28,29]. It is also called flexforming, or internal high-pressure metal forming [28 – 30].
2.3. The typical hydro-mechanical deep drawing process As in the former section, in the hydro-mechanical deep-drawing process when the female die is replaced by fluid pressure and the final shape of the workpiece is determined by the punch, an O-ring is used for preventing the flowout out of the fluid on the flange. The function of the blank holder is almost the same as in the conventional deep drawing [4]. The process is named hydro-mechanical deep drawing, or Hydromec which was the name of the process when German researchers obtained a patent [3,4] for it (see Fig. 1 (e)).
2.4. Hydrodynamic deep drawing process If the fluid below the blank can flow out at a high speed, then the fluid pressure will change quickly with increase of the punch stroke. In such a situation the process is a hydrodynamic deep-drawing one, as can be seen in Fig. 1 (f) and (g). This process shown in Fig. 1 (g) is complex, but it can be used for forming different workpieces, for example a cup with variable thickness can be produced by using a ball die that can rotate freely. A hexagon cup can also be formed [11]. It is unnecessary to control the fluid pressure and a thin rubber diaphragm is not necessary. This process was proposed by the Japanese researchers Kazuga et al. as pressure-lubricated deep-drawing in 1958 and also referred to as aquadraw deep drawing or the daalderop method, in which the thin rubber diaphragm is replaced by a hydrodynamic fluid [20]. This has proven to be cost effective. The process is also referred to as hydromechanical forming [5] or hydro-forming [31,32].
2.5. The radial-pressure deep-drawing method or the hydro-rim process This process was advanced by Nakamura and Nakagawa (Fig. 1 (i) and (j)) [2,5]. Some other researchers also carried out investigations into this process [33]. The process is a modification of the hydrodynamic deep-drawing process, including the direct method (Fig. 1 (i)) and an indirect method (Fig. 1 (j)). The LDR values can reach 3.2– 3.3 [6] in one step. In [5] a 0.8 mm thick and 30 mm diameter cup was drawn with a DR of 3.3, the pressure being 25.0 MPa.
3. Experimental research into the process
3.1. Progress in process performance and principle in6estigation Buerk [3] reported work on hydro-mechanical drawing in 1967; he used a seal ring between the blank flange and the die. It was concluded that the deformation is a combination of stretching, compressing and bending. An upward bead is usually formed at the free portion of the blank due to the fluid pressure, which increases very rapidly whilst the punch moves down. The upward bead causes radial and tangential stresses in the flange area. The actual drawing takes place over the bead, but not over the drawing radius on the die and the LDR can reach a value of 3.0 or greater. The seal ring is made of a wear-resisting plastic and it is preferred that a copper ring be placed on top of the plastic cord for very high pressure. Buerk also proposed different combinations of the hydro-mechanical drawing, the redrawing (second draw) and the reverse draw (counter draw) processes and also a combination of blanking and drawing or other combinations. Larsen [5] made some investigations into the hydrodynamic process (aquadraw) in 1977. Oil was used as the pressure fluid medium. When the opening pressure (threshold pressure) is reached, the oil is pressed out under the blank flange; the oil flow acts as a lubricant and the friction between the blank flange and the die is a minimum. At the same time, the critical area moves from the bottom of the workpiece to the area around the die radius. The fluid chamber can be pressurized by a pressure pump (called a preliminary pressure pump) before the punch moves down to the blank. If a preliminary pressure pump is provided then no contact between the blank and the die radius exists, the deformation is better and time is saved due to the deformation occurring before reaching the opening pressure. The pressure can be regulated by a valve. The strain distribution, the optimal preliminary pressure and the effect of the die radius on the process were also determined experimentally. Becker and Bensmann [4] reported a modification to the hydromec process in which the seal ring is dispensed with, an undulating die-ring or undulating blank-holder being used instead. This process is in fact a hydrodynamic process (see Fig. 1 (h)). A dual-action die was designed for the predraw and the redrawing process. Nakamura and Nakagawa [2] carried out some investigations into the fracture mechanism of the workpieces during forming and how to suppress the fractures. Their work has contributed a great deal to the applications of the process in Japan. Their work proved that pre-bulging before hydro-mechanical forming can increase the LDR considerably. They proposed the re-
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drawing process, which can perform conventional drawing and hydro-mechanical deep drawing within one stroke, the LDR values reaching 4.9 [5]. Nakamura and Nakagawa [6] recently published a paper reporting hydro-mechanical drawing in detail and systematically. The principles, the process features, the equipment and the applications are introduced and analyzed. Yang et al. [33] made some modifications to the radial-pressure deep-drawing method when forming very long cups with conventional predraw and hydromechanical redrawing process. A separated radial-pressure system was used for the slight difference between the optimal radial pressure and the optimal chamber pressure to obtain a very high drawing ratio and ironing was used after redrawing the cups to secure a good surface quality and fine thickness distribution. The drawing ratio reached 4.46 in their experiments and the quality of the product proved to be very good. Yossifon and Tirosh [13] proposed a new set-up which in principle is a deep-drawing process with fluid-assisted blank holding (see Fig. 1(c)). The set-up looks like a hydroforming set-up, but pressure is exerted only upon the flange, a rigid die and a rigid punch still being in use. Yossifon and Tirosh [17] investigated deep drawing with fluid pressure assisted being blankholder. The process seemed feasible and had some features different from those of conventional deep-drawing and hydro-forming/hydro-mechanical forming, a sound working-pressure path being suggested. Anderson [6,7] performed some experimental investigations into the hydro-mechanical forming behavior of high strength steel workpieces, the relationship between the shape accuracy of the workpieces, the material properties and process parameters being determined by experiment. It is indicated that the accuracy is affected by the residual stresses and the anisotropy of the sheet blanks. The shape accuracy is determined and compared by measuring the radii of workpieces drawn conventionally and hydro-mechanically and the strain distribution is determined by the grid-method and a computer-vision based analysis system. Hydro-mechanical drawing was first put into use at SMG Engineering Germany [6]. SMG Engineering [23] developed hydro-mechanical drawing and referred to it as active hydro-mechanical forming. It includes: (a) pre-bulging; (b) closing of the dies; and (c) increase of the pressure. Large automotive outer panels can be produced by this technology. Large outer panel parts need high stiffness to avoid buckling or vibration during running and thus many measures have been taken in order to enhance the stiffness. The new technology
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can be used for forming complex parts; the deformation can be increased so that the strain hardening is greater, the stiffness can be increased whilst the quality is improved, weight can be reduced and the costs can be lowered. The active hydro-mechanical forming technology can be applied to all large-surface outer panels: hoods, deck lids, roofs, doors and fenders. In the active hydro-mechanical process, an oil-in-water emulsion is used as the active fluid medium, prebulging is used before deep drawing and then the fluid is pressurized to a particular pressure during pressing. Nakamura, et al. [34] presented techniques used in the hydro-mechanical drawing of automotive body panels. They discussed how to integrate drawing and bending, how to insist on using only one die half (avoiding using the lower die), how to improve the surface quality (eliminating surface torsion) and how to reduce wrinkles. ABB [28,29] reported that fluid deep-drawing (hydroforming) can produce drawn parts such as boxes, cups and parabolic shapes in one step and that hydroforming allows 50% greater DR and 50% less material thinning than for conventional deep-drawing.
3.2. Other progress Tirosh et al. [30] proposed the process hydrostatic ironing, in which the ironing process is assisted by hydrostatic pressure in order to increase the friction between the punch (or mandrel) and the workpiece. The punch force can thus be reduced and even omitted (with a threshold pressure) and the limit ironing ratio (LIR) can be increased considerably. Asakura et al. [36] carried out some investigations on deep drawing utilizing lateral fluid pressure (Fig. 2 (a)), high pressure being exerted upon the perimeter of the blank whilst only a lower punch force is needed. The LDR reached 4 and 6 in two experiments on hard aluminium workpieces. The punch force decreases with the increase of the lateral fluid pressure. When the lateral fluid pressure is sufficiently high, the punch force is close to zero, therefore experiments without a punch were also performed. When the die diameter is within a strict range, the drawing experiments failed due to buckling of the workpieces. It is worth noting that a new fluid-pressure-assisted deep-drawing process was reported recently [35], which is referred to as an HBU process (high-pressure sheet metal forming). In this process, the fluid pressure plays the role of the punch, in contrast to hydro-mechanical forming. Two workpieces are formed in each work-cycle and the female dies determine the final shapes of the workpieces (Fig. 2 (b)). This process is suitable for the forming of complex sheet parts and small lot production. The process time may be a little longer and accurate positioning at the dies may be difficult.
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Fig. 2. Some new developments in hydromechanical forming processes.
4. Theoretical investigation and numerical simulation of the process
4.1. Theoretical in6estigation Larsen [5] proposed an equation for calculating the opening pressure in the chamber to lift the blank free from the die radius in aquadraw: 2 ln b P= 2kt Rd where P is the pressure, k is the yield stress in pure shear, t is the blank thickness, Rd is the die radius and b is the drawing ratio. Ressener and Hora [38] analyzed the hydromec process by the energy-stability criterion and the stresses and the strains occuring throughout the deep drawing being analyzed by iteration. The LDR can be predicted considering the fluid pressure and other forming conditions. Yossifon and Tirosh [12] proposed a concept MDR (maximum drawing ratio) for the hydroforming process, because excessive pressure can cause rupture and insufficient pressure may cause wrinkling (buckling). The pressure-path has a working zone without failure and two critical pressure values can be calculated under different conditions. A good process can be successfully carried out through the pressure passage without touching the two critical lines. The two critical pressure values may be increased and the passage may be widened by reducing the friction or changing other process conditions. The MDR reaches a minimum value between n=0.1 and 0.2, this having been verified by experiments. Rupture occurs on the portion near the die radius (not the punch radius as in conventional drawing) and wrinkles occur at the unsupported portion between the flange and the punch.
Yossifon and Tirosh [13] applied the energy method to the analysis of the buckling (wrinkles) phenomena and proposed a formulation. The buckling mode (with a number of wrinkles) can be predicted under particular pressure and strain and the general solution can be obtained under particular conditions. The number of wrinkles can vary under different pressures and usually increases with the increase of the fluid pressure. The lowest fluid pressure suppressing buckling was predicted and compared with the results from experiments, which established the validity of the formulation. In fact, this formulation demonstrates the relationship between the current blank geometry (b/t), the flange width (1− a/b) and the minimum fluid pressure. Tirosh and Konvalina [14] reported the LDR values for the hydrodynamic process using limit analysis in plasticity theory coupled with the flow analysis of viscous, non-inertial fluid. The LDR values are theoretically determined by the upper-bound and lower-bound methods. The stress in the flange floating are presented and the effect of die curvature on the LDR values, as well as the maximum effective cup height, are analyzed. Shirizly et al. [15] reported their work on limit analysis of the deep-drawing process with fluid-pressure blank-holding. The upper-and lower-bound results were obtained, with special emphasis on the geometry of the die profile. The blank is divided into five zones. Yossifon and Tirosh [16] analyzed the hydroforming process, with particular attention being paid to tensile rupture. The bank is divided into three portions, the stress and deformation are predicted and the working path between the rupture and the wrinkles is investigated. Hsu and Hsieh [32] analyzed the rupture and wrinkle loci in hemisphere punch hydroforming processes (in fact hydro-mechanical drawing). The blank was divided
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into three zones and the stress and the internal pressure were obtained so that the diagram of pressure – punch stroke could be formed on the basis of calculations. Bay and Malberg et al. [8,9] proposed the mastercurve concept (the hydromec process). A diagram showing the relative drawing-in and the actual drawing ratio can be determined by experiment or calculation under a constant fluid pressure (hydromec) and blankholder force and the feasibility of the hydro-mechanical drawing process with the same conditions can also be determined by this diagram. When the drawing-in curve intersects with the master curve, the workpiece will be subjected to rupture; if not, the process will be successfully carried out. Using this method, the hydro-mechanical forming of workpieces with complex shapes (cylindrical, conical or combined shapes) can be predicted. It is also predicted that the LDR value increases with the increase in the fluid pressure, due to the friction increase between the punch and the deformed workpiece with increased pressure. A paper published by Zhang and Li [10] presented both an analytical method and a computer program that can predict the process feasibilities according to the analytical calculated data and the experimental loading conditions. Gelin and Delassus [18] presented a theoretical calculation of the pressure under the blank holder for the aquadraw process. The stress in the flange and the thickness of the lubricant film are calculated. The velocity of the flange in the radial direction and the influence of the cavity volume on the process are demonstrated. They also introduced some other proposals for measuring the cavity pressure in Ref. [19], where they pointed out some disagreements with the experimental results in Ref. [18]. Some of their experimental results gave the pressure cavity evolution, which is dependent on the material behavior, the blank thickness, the die entrance radius and the drawing ratio. The cavity pressure and the blank-holding pressure are estimated theoretically and the winding angle around the die entrance radius is given. A new mechanical model was recently proposed by them [20] to determine the fluid pressure in the die cavity and the blank-holding pressure. It was discovered that the blank-holding pressure has little effect on the cavity pressure and therefore, a new model is set up by the equilibrium of the blank-holder. The blankholder pressure is low near to the blank edge. The model was proven to be better than previous ones when using aquadraw to draw stainless- and mild-steel workpieces. A finite-difference method analysis was made by El-Domiaty and Shabara [39] for the radial-pressure deep-drawing process. According to their analysis, the LDR can be increased by utilizing the radial pressure. A higher value of P/k (P, radial pressure; k, shear yield stress) can improve the LDR values, wich can rise to 2.86 for steel sheets and 3.5 for aluminium.
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Bayoumi [40] carried out a theoretical analysis and some experiments using brass, steel and annealed aluminium under different process conditions (different friction and punch roughness) to investigate the LDR values. The effects of material properties (n and R values) on the process are discussed. Hyun and Cho [41] proposed a method using an artificial neural network to predict the forming-pressure curve for hydroforming processes based on the upperbound analysis by Tirosh et al. [16]. The effects proved to be very satisfactory.
4.2. Numerical simulation Computer simulation has been used in the research and design of hydro-mechanical deep drawing process. Computer simulation has developed quickly in recent years and compared to experimental methods computer simulation has the advantages of low cost, short time, more information and well-illustrating graphics. Therefore, it is now widely used in metal forming. A finite-element analysis of the hydro-mechanical process (hydro-forming —in fact an aquadraw process) was reported recently [31]. The forming of an oil pan upper of an automotive engine was simulated using an explicit non-linear finite-element code on a Pentium-90 PC. Shell elements were used for simulating the blank, the punch, the die and the blank holder. The fluid-pressure path and the blank holding force path were determined by the simulation in order to obtain a successful process. The blank size and the draw-bead location could also be determined by the simulation, but the draw-bead shape and size could not be predicted using this code. Yang et al. [31] performed FEM simulations of the separate radial-pressure drawing of the long cups. A rigid–plastic FEM code employing Hill’s anisotropic theory was used. The numerical simulations predicted the optimal chamber pressure, the optimal radial pressure and the thickness distribution; the simulations can help in judging whether the process can be carried out successfully. FEM analysis of the axisymetrical deep drawing process was also performed by these authors [33]. Gelin and Delassus [18,19] performed FEM simulations of the aquadraw process of stainless steel and low-carbon steel workpieces. In the numerical analysis, a 3D explicit elastic–plastic code was used and the forming process with fluid pressure was simulated to predict the distribution of the thickness. In future, the fluid pressures in the chamber and beneath the flange are expected to be coupled with the deformation. FEM analyses of the hydroforming (hydrobulging) of tubular and complex automobile parts have been reported recently. Wu et al. [25] used the LS-DYNA3D code to analyse the hydroforming of complex tubular
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parts, the results proving good and the code being suitable for the analysis and design of the process. Berg et al. [26] developed a new shell element by the explicit FEM code to simulate the hydroforming of automobile parts. An FE analysis [42] was made of a flanging operation using a fluid cell press. This is the flanging process of sheet material by rubber die forming, which is somewhat similar to hydroforming. The fluid pressure is transferred through some layers of rubber membrane to the workpiece; the rubber plays the role of the punch, the blank is forced to contact the die and thus, flanging is performed. This analysis was carried out using the LS-DYNA3D code and the wrinkles on the flange were predicted in the simulation. An FE analysis was also arried out of the HBU process [37]; the 3D FEM code INDEED (developed in Germany) was used in the simulation of this process and the tool components and the O-ring seal behavior were simulated by a FEM code named MARC. The contact area and the wrinkles were predicted by the FEM simulation. Asakura et al. [43] performed a numerical simulation of the deformation process of punchless drawing with lateral fluid pressure using the matrix method. The deformation behavior of the blank and the contact behavior on the die shoulder under various tool conditions were analysed and the optimum die radius for the punchless drawing was determined theoretically. The matrix method is the early form of the finite-element method, the present authors using this method in their research around 1983.
5. Equipment and devices Ordinary presses can be used, either single or double acting. Hydraulic presses are the most suitable because of the constant drawing speed and independently separate control of the motion [3]. Mechanical eccentric presses (crank-driven straightsided press, e.g. with 2-point slide suspension) were tested for the hydro-mechanical drawing process and it is established that the mechanical press can also be used. The punch force undergoes a slight increase if the punch speed is increased compared with the process on a hydraulic press. The testing machine can work at a speed of from 24 to 41 strokes per minute [4]. Specialized equipment and devices for the hydro-mechanical deep drawing process have been devised at AP&T Sweden and at some other companies. The AP&T hydraulic presses [1,46,47] use welded bodies and frame structures and have three or four cylinders, i.e. the principal cylinder, the triple cylinder, the cusion cylinder and a fourth cylinder have a comparatively high pressure, which can perform some additional
working operations, such as blank-holding and blankrejection. The AP&T hydraulic presses can be used in hydro-mechanical deep drawing, as well as in other metal forming processes. The AP&T hydro-mechanical forming devices are independent of the hydraulic presses and can be installed on different AP&T hydraulic presses. These devices have their independent hydraulic systems, which can be used for supplying the fluid, for pressurizing, for pressure-keeping, for pressure unloading and for oil filtering. The hydraulic pressure can be adjusted or programme-controlled with the stroke and can change from 20 to 120 MPa. Three types of AP&T hydro-mechanical forming devices are supplied: (a) the numerical control type, which has independent power system and oil-filtering system, in which the pressure can be adjusted continuously or controlled with the movement of the cylinder, these types of devices being suitable for the forming of complex sheet workpieces; (b) the pressure-limit type, which has a pressure-restraining circuit, where the peak pressure is present by a pressure-restraining valve and can be adjusted manually; and (c) the lifting pressure type, in which the device has a lifting-pressure cylinder at the device bottom, the upper or down movement of the cylinder changing the chamber volume so that the pressure can be changed, this type being suitable for the forming of small complex workpieces. Specialized equipment has also been devised in Amitio Press Technical Center, Japan [2]. A conventional hydraulic press with double-action oil cylinders was first used in Japan. The press was equipped with an ejector, a liquid supplying unit to the die chamber, an automatic hydraulic counter-pressure control unit and an automatic blank-holding force-control device. Many of these items of equipment can control the pressure freely. A single-action press with a cusion cylinder that is simple and convenient for hydro-mechanical forming has also been on the Japanese market since 1983. About 50–60 hydro-mechanical forming machines had been installed in Japan up to 1987 of which about a half were in production whilst others were in research institutes. No seal ring is provided for these machines. A special press for active hydro-mechanical forming is also being developed at SMG Engineering, this being a double-action press equipped with hydromec facilities [23], consisting of a hydromec unit and a quick die change system. SMG intended to supply presses that can perform active hydro-mechanical forming, hydroforming and die try-out for conventional deep-drawing. Lagan Press Sweden (a member of ABB Group) [28] supplies equipment for hydro-mechanical drawing. ABB Metallurgy supplies Qintus presses for hydroforming up to a pressure of 200 MPa [28]. Qintus fluid-cell presses are available in different sizes with maximum forming pressures of 100, 140 and 200 MPa,
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all parts exposed to high stresses being wound with prestress wire. When the pressures exceed 100 MPa, the axial forces in the press tray are absorbed by a prestressed frame.
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The hydro-mechanical deep-drawing process has found many applications in industry because of its obvious features in process, product quality and economical aspects. The applications are reported mostly in Japan. Some applications are summarized in the following.
produced by this process, e.g. a fender trim, of 0.8 mm thickness, 650 mm length and 420 mm width was produced by this process. Nakamura et al. [34] reported the automotive body parts produced at Toyota Motor Corporation by hydro-mechanical drawing (referred to as liquid-pressure press-forming in their paper). Eleven major parts were shown, including fenders, quarter tails and door inners. As demanded by the diversifying market, personalized vehicles can be produced inexpensively and of good quality, by the commercialisation of the hydro-mechanical deep-drawing process. (d) Kitchen utensils and electrical household applications. Applications in this field began earlier, but less detailed descriptions have been published.
6.1. Applications in Japan
6.2. Applications in other countries
More than 300 kinds of workpieces were formed by the hydro-mechanical forming process in Japan up to 1987 [2,5]. The materials cover mild steel sheets, aluminium sheets, titanium alloys and stainless steel sheets, with a thickness of 0.2 – 3.2 mm. High-strength steel sheets and mild-steel sheets coated with Teflon (a kind of sandwich sheet) have also been used. The sizes range from 30×30×30 mm to 1200 ×1000 × 250 mm. The application fields are mostly related to reflectors of lighting equipment, aircraft parts and automobile parts. Nakamura and Nakagawa [6] reported many examples of automobile body panels formed by hydro-mechanical deep drawing presses in a recent paper. (a) Reflectors of lighting equipment. These parts are usually made of aluminium sheet and have rather complicated shapes and large sizes. They usually need 6 forming steps by the conventional spinning process, but can be formed in one stroke by the hydro-mechanical forming process. In addition, the inner surface quality and the thickness distribution is much better. The die cost is low, which is suitable for small-lot production. The production rate of a automobile light reflector is 8 pieces per minute and one machine can produce 200000 pieces ever month. (b) Aircraft parts. These are subjected to high safety standards, thus local thinning should be avoided. The hydro-mechanical deep-drawing process can reduce the friction between the die and the blank-holder, so that the thickness distribution becomes much better. Some aluminium aircraft parts have been formed by this process instead of by the spinning process. A lip skin, with a thickness of 1.6 mm and a diameter of 1150 mm, was produced with a thinning of 12% in Japan. (c) Automobile parts. Because of the fewer forming steps and the omission of the female dies, some automobile parts have been formed by the hydro-mechanical deep drawing process. In addition to the manufacture of light reflectors for automobiles, some panel parts (autobody) of automobiles have also been
Conical hollow bodies such as reflectors were reported to be formed by hydro-mechanical process at the beginning of 1967 [3]. One draw is sufficient to form a conical hollow workpiece in the hydro-mechanical process, whereas five draws and a sizing are needed for conventional deep drawing (see Fig. 3). Recently, an oil pan upper was reported to be produced in the United States [31], being part of an automotive engine. A motor housing was formed by hydro-mechanical deep drawing process in 1977 [5] (DR= 2.76). In China, tractor oil-pans were manufactured by hydro-mechanical drawing in the 1970s [10]. In the early 1980s, stainless-steel cups with a depth of 200 mm were formed at a factory in Guangzhou. Some specialized presses have been devised in China. In addition to the materials used in Japan, some other materials such as pure copper and brass have also been tried out in other countries [4]. Drewes et al. [44] dealt with the potential of reducing car weight by using the hydro-mechanical deep-drawing or hydroforming process. Hydroforming is reported mainly to be used in automotive industry and tube forming industry. It is reported [45] that a Dutch steel company and a German automotive firm have been implemented using the hydroforming process to produce automotive parts such as exhaust manifolds and engine cradles, at an annual rate of 1.5 million pieces. The hydroformed parts can be very complex. They are lighter, cheaper and of high quality, particularly when high-strength steel is used. Tubular automobile parts are also reported to be hydroformed in some countries [25,26]. Hydroforming is reported to have been used by General Motors and Magna International Plant in Aalen, Germany, to produce automotive parts such as pipes [30]. A sixteen-hundred metric ton Siempelkamp hydroforming press has been introduced, which is said to be the first HF press built especially for automotive components. Huber and Bauer GmbH is reported to be
6. Applications in industry
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Fig. 3. Drawing of conical or parabolic hollow bodies.
a German-based manufacturer of high-pressure hydroforming equipment. Johannisson [28,29] introduced the applications of hydroforming in automotive industry. Hydroforming can be used for producing automotive deep-drawn parts, which makes small-scale production profitable; the main reason being that it requires only one rigid tool half. Flexforming is an unbeatable process for producing prototype parts. The rigid tool half can be made from inexpensive materials. Parts made from blanks of different thickness and materials can be produced using the same tools. The fluid-cell process allows several parts to be formed in the same press cycle. Draw clearance between tools is unnecessary. A car engine cradle arm and a car front fender made by this technology have been presented. Hydroforming can reduce tool costs (only one tool half), production cycle tool costs can be reduced by 75% and lead time reduced to 60% and the process is especially suitable for forming sheet metal parts of prototypes and small scale series production [29]. BMW, VOLVO, Mercedes-Benz and Lebranchu S.A. Paris are all equipped with quintus presses to carry out hydroforming production. The production of a truck body part, a rear fender and an excavator door has been introduced. Other users of flexforming in the automotive industry are Saab, GM, Audi, Ford, Peugeot, Citroen and Renault. The largest press in the world in terms of press force — 150000 t — began operation in Benz in early 1990.
7. Conclusions The deep drawing processes assisted with fluid pressure has been summarized in this article. The process principles and the process features and the development and research progress have been introduced in detail. It is believed that hydro-mechanical deep-drawing processes have a prosperous future in the sheet metal forming industry, but further work must be carried out in their process control and their applications.
Acknowledgements The authors are very grateful to M.R. Jensen for supplying materials.
References [1] Q. Wang, Hydromechanical deep-drawing, New Technol. New Process. 5 (1994) 23 – 24 (in Chinese). [2] K. Nakamura, T. Nakagawa, Sheet metal forming with hydraulic counter pressure in Japan, Annal. CIRP 36 (1987) 191– 194. [3] E. Buerk, Hydromechanical drawing, Sheet Met. Ind. (March 1967) 182 – 188. [4] H.J. Becker and G. Bensmann, Further development in hydromechanical deep-drawing, Developments in the drawing of Metals, Metal Society of London, 1983, pp 272 – 279. [5] B. Larsen, Hydromechanic forming of sheet metal, Sheet Met. Ind. (Feb 1977) 162 – 166.
S.H. Zhang, J. Danckert / Journal of Materials Processing Technology 83 (1998) 14–25 [6] J.F. Andersen, Shape accuracy and parameters influencing the shape accuracy using conventional and hydro-mechanical deep drawing, IDDRG ’94, Lisbon, Portugal, 16–17 May 1994, pp. 545 – 553. [7] J.F. Andersen, Hydromechanical forming of high strength steels, PhD dissertation, Aalborg University, Denmark, 1994. [8] N. Bay, S. Skytte Jensen, M.P. Malberg, S. Grauslund, Forming limits in hydromechanical deep drawing, Annal. CIRP 43 (1994) 253 – 256. [9] M.P. Malberg, S. Grauslund, N. Bay and S. SkytteJensen, Hydromechanical deep drawing-prediction of forming limits. [10] H. Zhang, T.Y. Li, Numerical simulation analysis on hydromechanical deep-drawing of cups, Forg. Stamp. Technol. 1 (1995) 33 – 37 (in Chinese). [11] J. Tirosh, A. Shirizly, S. Yossifon, On recent progress in deep drawing processes by fluid-pressure assisted methods, Adv. Technol. Plast. 1996, Proc. 5th Int. Conf. on Technology of Plasticity, Columbus, OH, 7 – 10 October 1996, pp 707–710. [12] S. Yossifon, J. Tirosh, The maximum drawing ratio in hydroforming processes, J. Eng. Ind. Trans. ASME 112 (1990) 47 – 56. [13] S. Yossifon, J. Tirosh, Buckling prevention by lateral fluid pressure in deep-drawing, Int. J. Mech. Sci. 27 (1985) 177 – 185. [14] J. Tirosh, P. Konvalina, On the hydrodynamic deep-drawing process, Int. J. Mech. Sci. 27 (1985) 595–607. [15] A. Shirizly, S. Yossifon, J. Tirosh, The role of die curvature in the performance of deep-drawing (hydro-mechnical) process, Int. J. Mech. Sci. 36 (1994) 121–135. [16] S. Yossifon, J. Tirosh, Rupture instability in hydroforming deep-drawing process, Int. J. Mech. Sci. 27 (1985) 559–570. [17] S. Yossifon, J. Tirosh, Deep drawing with a fluid pressure assisted blankholder, Proc. Inst. Mech. Eng. J. Eng. Manuf. B 206 (1992) 247 – 252. [18] J.C. Gelin, P. Delassus, Modelling and simulation of the aquadraw deep drawing process, Annal. CIRP 42 (1993) 305 – 308. [19] J.C. Gelin, P. Delassus, J.E. Fontaine, Experimental and numerical modelling of the effects of process parameters in aquadraw deep drawing, J. Mater. Process. Technol. 45 (1994) 329 – 334. [20] J.C. Gelin, P. Delassus, Modelling and optimal control of process parameters in the aquadraw deep drawing process, Adv. Technol. Plast. 1996, Proc. 5th Int. Conf. on Technology of Plasticity, Columbus, OH, 7–10 October 1996, pp. 831– 834. [21] H. Amino, T. Nakagawa, Application of hydraulic counter pressure fluid forming into car body sheet metal forming, SAE Technical paper Series 880365, Int. Congr. and Expo., Detroit, MI, 29 February – 4 March 1988, pp. 35–48. [22] T. Nakagawa, K. Nakamura, H. Amino, Various applications of hydraulic counter pressure deep drawing, 1996 Int. Conf. on Sheet Metal Forming Technology, Columbus, OH, 9–11 October 1996. [23] Martin Steinmetz, Active hydro-mechanical sheet forming, 1996 Int. Conf. on Sheet Metal Forming Technology, Columbus, OH, 9– 11 October 1996. [24] F. Dohmann, Ch. Hartl, Hydroforming—a method to manufacture light-weight parts, J. Mater. Process. Technol. 60 (1996) 669 – 676. [25] L.W. Wu, Y. Yu, Computer simulation of forming automotive structural parts by the hydroforming process, Proc. 3rd Int. Conf. Numisheet ’96, Dearborn, MI, 29 September–3 October 1996, pp. 324 – 329.
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[26] H.J. Berg, P. Hora, J. Reissner, Simulation of Complex hydroforming process using an explicit code with a new shell formulation, Proc. 3rd Int. Conf. Numisheet ’96, Dearbon, MI, 29 September – 3 October 1996, pp. 330 – 335. [27] Conference Report: Hydroforming technology, Adv. Mater. Process. 151 (5) (1997) 50 – 53. [28] T.G. Johannisson, Flexforming — high-pressure sheet metal forming, ABB Rev. 6 (1989) 3 – 10. [29] T.G. Johannison, Flexforming — high-pressure sheet metal forming, ABB Rev. 5 (1990) 25 – 30. [30] J.N. Pennington, The promise of hydroforming, Mod. Met. 52 (7) (1996) 32 – 33. [31] J. An, A.H. Soni, Three dimensional computer simulation of a sheet metal hydroforming process, Adv. Technol. Plast. 1996, Proc. 5th Int. Conf. on Technology of Plasticity, Columbus, OH, 7 – 10 October 1996, pp. 827 – 830. [32] T.C. Hsu, S.J. Hsieh, Theoretical and experimental analysis of failure for the hemisphere punch hydroforming processes, Trans. ASME J. Manuf. Sci. Eng. 118 (1996) 434 – 438. [33] D.Y. Yang, J.B. Kim, D.W. Lee, Investigation into manufacturing of very long cups by hydromechanical deep drawing and ironing with controlled radial pressure, Annal. CIRP 44 (1995). [34] S. Nakamura, H. Sugiura, H. Onoe, K. Ikemoto, Hydromechanical drawing of automotive parts, J. Mater. Process. Technol. 46 (1994) 491 – 503. [35] J. Tirosh, D. Iddan, Silviano, Hydrostatic ironing-analysis and experiment, Trans. ASME J. Eng. Ind. 114 (1992) 237–243. [36] K. Asakura, N. Kobayashi, T. Hanamoto, M. Kohzu, An experimental study of the punchless drawing utilizing lateral fluid pressure, Bull. Univ. Osaka Prefecture 31A (1982) 171–181. [37] M. Kleiner, A. Gartzke, R. Kolleck, J. Rauer and T. Weidner, Finite element simulation for high-pressure sheet metal forming (HBU process) and tool construction, Adv. Technol. Plast. 1996, Proc. 5th Int. Conf. on Technology of Plasticity, Columbus, OH, 7 – 10 October 1996, pp. 975 – 983. [38] J. Ressner, P. Hora, Hydro-mechanical deep-drawing, Annal. CIRP 30 (1981) 207 – 209. [39] A. El-Domiaty, M.A. Shabara, Improvement of deep-drawability by radial pressurized fluid, Int. J. Mach. Tools Manuf. 35 (1984) 353 – 363. [40] M.R. Bayoumi, Deep-drawing of anisotropic work-hardening sheet metals into a high-pressure medium, J. Test. Eval. 12 (1984) 353 – 363. [41] B.S. Hyun, H.S. Cho, Prediction of forming pressure curve for hydroforming process using artificial neural network, Proc. Inst. Mech. Eng. J. Syst. Control Eng. 208 (2) (1994) 109 – 121. [42] K.B. Nielsen, N. Brannberg, L. Nilsson, Sheet metal forming simulation using explicit finite element methods, Proc. 2nd Euro. Conf. on Structural Dynamics: EURODYN ’93, Trondheim, Norway, 21 – 23 June 1993, pp. 625 – 633. [43] K. Asakura, N. Kovayashi, M. Kohzu, N. Suzuki, An analysis of deep drawing process in the punchless drawing utilizing lateral fluid pressure, Bull. Univ. Osaka Prefecture 32A (1983) 37–45. [44] E. Drewes, B. Engl, U. Techaven, Potential for lightweight car-body construction using steel, Techan. Mit. Kruup. 1 (1994) 25 – 32. [45] News, Hydroforming aims to lighten cars, Mag. Prof. Eng. 19 (1996) 8. [46] Q. Wang, Hydromechanical deep-drawing in AP&T, Forg. Stamp. Technol. 6 (1994) 39 – 42 (inChinese). [47] Q. Wang, Deep-drawing technology in AP&T, Forg. Stamp. Mach. 4 (1994) 39 – 42 (in Chinese).
Development of hydro-mechanical deep drawing S.H. Zhang *, J. Danckert Department of Production, Aalborg Uni6ersity Fibigerstraede 16, DK-9220 Aalborg, Denmark Received 16 June 1997
Abstract The hydro-mechanical deep-drawing process is reviewed in this article. The process principles and features are introduced and the developments of the hydro-mechanical deep-drawing process in process performances, in theory and in numerical simulation are described. The applications are summarized. Some other related hydraulic forming processes are also dealt with as a comparison. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Deep drawing; Hydro-mechanical; Numerical simulations
1. Introduction Hydro-mechanical deep drawing is a new sheet metal forming technology originating from hydroforming technology. It combines the features of both traditional deep-drawing technology and hydroforming technology. In hydro-mechanical deep drawing the limit drawing ratio (LDR) values can reach 2.8 compared with the traditional deep-drawing process where the LDR values are only about 2.2. Since its inception, the hydro-mechanical deep-drawing process has found increasing application in industry, such as application in automobile and aircraft manufacture. After continuous improvement and innovation, much specialized equipment and many devices have also been devised and put into use. The hydro-mechanical deep-drawing process can replace some other metal forming processes, because this process can improve the quality of the product without losing productivity. Hydro-mechanical deep-drawing technology was first developed in 1890 [1]. However, the real development began after the second world war. Early research work began mainly in Germany and Japan. Japanese researchers began investigating the process in 1955. Kasuga et al. first proposed pressure-lubricated deep drawing and was in charge of the research work in this field from 1958 to 1964. After the mid 1970s, * Corresponding author. Tel.: + 45 96 358958; fax: + 45 98 153030; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00039-9
Nakamura and Nakagawa [2] began their research work on the hydraulic counter-pressure fluid-forming process (the FF method). The radial-pressure deepdrawing method was proposed during this period. German researchers began their work in the 1950s [3]. A seal-ring on the die face was used in order to prevent the leakage of fluid between the die and the blank [2–4]. Dutch researchers presented the daalderop method in 1961 and American researchers began research on the aqua-draw method in 1973. AP & T of Sweden began researching the process and devices in the 1960s, supplying specialized equipment and devices [1]. Larsen [5], a Danish researcher, published a paper on work in Denmark in 1977, the research at present being continued by Danckert andersen et al. [6,7] at Aalborg University and Bay [8,9] at the technical University of Denmark. Chinese researchers began their work on this technology at an earlier date (D.C. Kang, private communication, Jan. 1997). Kang is in charge of the research work at Harbin Institute of Technology. He has been co-operating with Nakamura of Japan since 1980. Kang investigated the hydro-mechanical forming process of box workpieces, parabolic shells and also the hydro-mechanical ironing process. Guo, another researcher at Harbin Institute of Technology, who stayed in Japan with Nakamura for a period, has researched the forming accuracy of hydro-mechanically drawn cups, boxes and tapered cups. A new press of 2000 kN capacity has
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been installed recently at Harbin Institute of Technology, especially for hydro-mechanical drawing. Also, specialized facilities will be installed on a press of 5000 kN capacity in a Hong Kong firm. The research work of this technology is also being carried out at Jinan Research Institute of Casting and Forging Machinery [1] and at some other universities [10]. In recent years, hydro-mechanical deep-drawing technology has been developing quickly in some other countries also, for example, France, USA, Russia, Israel, Switzerland, Korea and even Kuwait. Tirosh et al. [11 – 17] have published a series of research papers on theoretical analysis and experiment in Israel, whilst Gelin et al. have published some papers [18 – 20] on their research work in France. According to the analysis by Amino and Bakagawa [21], the hydro-mechanical deep-drawing process has the following features: (a) it has a friction-holding effect (friction forces are produced between the blank and the punch and serve as part of the forming force); (b) it has a resistance-reduction effect (the friction resistance between the flange and the die is reduced because of the flowing out of the fluid); (c) it has an initial extension effect (extension in the vicinity of the die shoulder portion is caused by pre-bulging, the thickness thus becoming more uniform: by using pre-bulging the material over the die cavity is initially stretched, causing a more uniform thickness distribution). Because of the above three fracture-prevention effects, this process can improve the fracture limit. The process also has a wrinkle-prevention effect; the unsupported portions are subjected to bulging pressure, causing circumferencial tensile stresses, which prevents wrinkles from occurring. The hydro-mechanical deep drawing process has the following advantages [1,3 – 5,21,22]: (a) Due to the fracture-prevention effects and the wrinkle-prevention effect, the product may have a higher drawing ratio (the LDR is increased), the forming steps are reduced, a sizing tool and a sizing operation are unnecessary, the limitations of the quality and size of the product are overcome and costs are reduced. (b) Due to the forced contact between the blank sheet and the punch and the lubrication effect of the liquid between the flange and the die, less damage is suffered by the flange, the deep drawing products have a better surface quality, wrinkles can be suppressed, the dimensional accuracy is high and the wear of the dies is clearly reduced [14]. (c) Because of the friction-holding effect, local thinning is lighter and the thickness distribution is more uniform. (d) As the female die is replaced by fluid pressure, only a punch is used and the drawing operations are reduced. Therefore, the process is cost effective and can be used in small batch production and even in sheetmetal property tests.
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(e) The process can be used for the manufacture of complex-shaped workpieces and for the deep drawing of some materials that are not suitable for intermediate heat treatment. The process is flexible. When drawing automotive body panels higher deformation may easily be obtained to increase the stiffness of the product [23]. However, the process has its drawbacks, the main drawback being that a higher drawing force and a higher blank-holding force are needed compared with those of conventional drawing technology [3,18].
2. Process principles and classification Hydro-mechanical deep drawing has various names, e.g. Hydromec, Aqua-draw, fluid former, drawing with counter pressure or hydraulic counter-pressure deep drawing, deep drawing, process with fluid-pressure assisted method, hydraulic forming and even just Hydroform or hydroforming, each representing its different features. In fact, the hydro-mechanical deep-drawing process is a kind of soft-tool forming technology or flexible-forming method originating from hydroforming technology. Soft-tool forming technologies have been used widely in industries because of their simple equipment and convenient devices, their low energy consumption, their fine product quality and their cost effectiveness. Soft-tool forming technologies include rubber-die forming and fluid-hydraulic forming. Hydraulic forming is used widely in sheet metal forming and tube forming and it includes soft punch forming (in which pressurized water or other fluid media such as oil is taken as the punch, whilst the female die is a rigid body) and soft-die forming (the pressurized water or other fluid media is taken as the female die, the punch being a rigid body). The former is usually referred to as hydro-bulging by most researchers and the latter is usually referred to as the hydro-mechanical deep-drawing process. However, some researchers call it the hydroforming process (especially when using a thin rubber diaphragm between the blank and the fluid). The latter process, i.e. the hydro-mechanical deep-drawing process, will be discussed in detail in this article. Hydroforming (hydrobulging) has been reported most recently in automobile industry, especially for tube forming and the forming of panel parts [24–27]. The hydro-mechanical deep-drawing process can be divided into the hydrostatic type and the hydrodynamic type on the basis of the feature of the fluid pressure. The hydro-bulging process can also have the two different situations according to the loading. In most cases, hydrobulging is in the hydrostatic state, but there exist some cases, such as explosive forming with fluid as the medium, where the fluid is in the dynamic state. The hydro-mechanical deep-drawing process has different performances during operation. Fig. 1 (a) shows
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Fig. 1. Schemes of the hydromechanical deep-drawing process.
the conventional deep-drawing process, where LDR is somewhere between 2.2 and 2.3 [5], although the theoretical prediction should be about 2.715 [11]. The main reason for the reduction in LDR is due to the friction
between the blank flange and the die. The fluid pressure is used the in hydro-mechanical deep drawing process mainly so that friction can be significantly reduced and so that the clamping (or friction-holding effect) between
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Fig. 1. (Continued)
the punch and the blank can prevent fracture of the blank at the punch corner, thus enabling the LDR value to be increased. However, the fluid can be used in different ways, therefore the LDR values can be increased to different levels. Usually the LDR values can be increased to about 2.7 within one stroke by means of the hydro-mechanical drawing process, whilst the LDR can be increased to 3.2 for the radial hydro-mechanical deep-drawing process in one step. It is reported that the LDR values may be increased to about 6 for the radial-pressure deep-drawing method (two steps). The hydro-mechanical deep-drawing process may be sub-divided into the following different types.
2.1. Deep-drawing process with fluid-assisted blank holding This process is shown in Fig. 1(b) and (c). The
traditional blank holder is replaced by a fluid pressure with a thin rubber diaphragm transferring the pressure to the blank flange, a fluid blank-holding device being required. The friction can be reduced considerably and the LDR can reach 2.3; the productivity is reduced a little in this case. This process is referred to as the hydro-mechanical deep-drawing process, deep drawing with a fluid-pressure-assisted blankholder or hydrostatic deep drawing in [11,12,15,16].
2.2. Hydro-forming process As in Fig. 1(d), when the female die is totally replaced by a fluid pressure, the pressure is transferred by a thin rubber diaphragm between the blank and the fluid. The punch determines the final shape of the workpiece. The fluid pressure in the chamber can be controlled by a device (a valve or a pump). The fluid
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pressure is very important because wrinkles will appear if the pressure is not sufficiently high. In the case of too-high a pressure, the workpiece may be damaged by rupture. This process is referred to as the hydro-form or hydroforming (HF) process by researchers [4,11,28,29]. It is also called flexforming, or internal high-pressure metal forming [28 – 30].
2.3. The typical hydro-mechanical deep drawing process As in the former section, in the hydro-mechanical deep-drawing process when the female die is replaced by fluid pressure and the final shape of the workpiece is determined by the punch, an O-ring is used for preventing the flowout out of the fluid on the flange. The function of the blank holder is almost the same as in the conventional deep drawing [4]. The process is named hydro-mechanical deep drawing, or Hydromec which was the name of the process when German researchers obtained a patent [3,4] for it (see Fig. 1 (e)).
2.4. Hydrodynamic deep drawing process If the fluid below the blank can flow out at a high speed, then the fluid pressure will change quickly with increase of the punch stroke. In such a situation the process is a hydrodynamic deep-drawing one, as can be seen in Fig. 1 (f) and (g). This process shown in Fig. 1 (g) is complex, but it can be used for forming different workpieces, for example a cup with variable thickness can be produced by using a ball die that can rotate freely. A hexagon cup can also be formed [11]. It is unnecessary to control the fluid pressure and a thin rubber diaphragm is not necessary. This process was proposed by the Japanese researchers Kazuga et al. as pressure-lubricated deep-drawing in 1958 and also referred to as aquadraw deep drawing or the daalderop method, in which the thin rubber diaphragm is replaced by a hydrodynamic fluid [20]. This has proven to be cost effective. The process is also referred to as hydromechanical forming [5] or hydro-forming [31,32].
2.5. The radial-pressure deep-drawing method or the hydro-rim process This process was advanced by Nakamura and Nakagawa (Fig. 1 (i) and (j)) [2,5]. Some other researchers also carried out investigations into this process [33]. The process is a modification of the hydrodynamic deep-drawing process, including the direct method (Fig. 1 (i)) and an indirect method (Fig. 1 (j)). The LDR values can reach 3.2– 3.3 [6] in one step. In [5] a 0.8 mm thick and 30 mm diameter cup was drawn with a DR of 3.3, the pressure being 25.0 MPa.
3. Experimental research into the process
3.1. Progress in process performance and principle in6estigation Buerk [3] reported work on hydro-mechanical drawing in 1967; he used a seal ring between the blank flange and the die. It was concluded that the deformation is a combination of stretching, compressing and bending. An upward bead is usually formed at the free portion of the blank due to the fluid pressure, which increases very rapidly whilst the punch moves down. The upward bead causes radial and tangential stresses in the flange area. The actual drawing takes place over the bead, but not over the drawing radius on the die and the LDR can reach a value of 3.0 or greater. The seal ring is made of a wear-resisting plastic and it is preferred that a copper ring be placed on top of the plastic cord for very high pressure. Buerk also proposed different combinations of the hydro-mechanical drawing, the redrawing (second draw) and the reverse draw (counter draw) processes and also a combination of blanking and drawing or other combinations. Larsen [5] made some investigations into the hydrodynamic process (aquadraw) in 1977. Oil was used as the pressure fluid medium. When the opening pressure (threshold pressure) is reached, the oil is pressed out under the blank flange; the oil flow acts as a lubricant and the friction between the blank flange and the die is a minimum. At the same time, the critical area moves from the bottom of the workpiece to the area around the die radius. The fluid chamber can be pressurized by a pressure pump (called a preliminary pressure pump) before the punch moves down to the blank. If a preliminary pressure pump is provided then no contact between the blank and the die radius exists, the deformation is better and time is saved due to the deformation occurring before reaching the opening pressure. The pressure can be regulated by a valve. The strain distribution, the optimal preliminary pressure and the effect of the die radius on the process were also determined experimentally. Becker and Bensmann [4] reported a modification to the hydromec process in which the seal ring is dispensed with, an undulating die-ring or undulating blank-holder being used instead. This process is in fact a hydrodynamic process (see Fig. 1 (h)). A dual-action die was designed for the predraw and the redrawing process. Nakamura and Nakagawa [2] carried out some investigations into the fracture mechanism of the workpieces during forming and how to suppress the fractures. Their work has contributed a great deal to the applications of the process in Japan. Their work proved that pre-bulging before hydro-mechanical forming can increase the LDR considerably. They proposed the re-
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drawing process, which can perform conventional drawing and hydro-mechanical deep drawing within one stroke, the LDR values reaching 4.9 [5]. Nakamura and Nakagawa [6] recently published a paper reporting hydro-mechanical drawing in detail and systematically. The principles, the process features, the equipment and the applications are introduced and analyzed. Yang et al. [33] made some modifications to the radial-pressure deep-drawing method when forming very long cups with conventional predraw and hydromechanical redrawing process. A separated radial-pressure system was used for the slight difference between the optimal radial pressure and the optimal chamber pressure to obtain a very high drawing ratio and ironing was used after redrawing the cups to secure a good surface quality and fine thickness distribution. The drawing ratio reached 4.46 in their experiments and the quality of the product proved to be very good. Yossifon and Tirosh [13] proposed a new set-up which in principle is a deep-drawing process with fluid-assisted blank holding (see Fig. 1(c)). The set-up looks like a hydroforming set-up, but pressure is exerted only upon the flange, a rigid die and a rigid punch still being in use. Yossifon and Tirosh [17] investigated deep drawing with fluid pressure assisted being blankholder. The process seemed feasible and had some features different from those of conventional deep-drawing and hydro-forming/hydro-mechanical forming, a sound working-pressure path being suggested. Anderson [6,7] performed some experimental investigations into the hydro-mechanical forming behavior of high strength steel workpieces, the relationship between the shape accuracy of the workpieces, the material properties and process parameters being determined by experiment. It is indicated that the accuracy is affected by the residual stresses and the anisotropy of the sheet blanks. The shape accuracy is determined and compared by measuring the radii of workpieces drawn conventionally and hydro-mechanically and the strain distribution is determined by the grid-method and a computer-vision based analysis system. Hydro-mechanical drawing was first put into use at SMG Engineering Germany [6]. SMG Engineering [23] developed hydro-mechanical drawing and referred to it as active hydro-mechanical forming. It includes: (a) pre-bulging; (b) closing of the dies; and (c) increase of the pressure. Large automotive outer panels can be produced by this technology. Large outer panel parts need high stiffness to avoid buckling or vibration during running and thus many measures have been taken in order to enhance the stiffness. The new technology
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can be used for forming complex parts; the deformation can be increased so that the strain hardening is greater, the stiffness can be increased whilst the quality is improved, weight can be reduced and the costs can be lowered. The active hydro-mechanical forming technology can be applied to all large-surface outer panels: hoods, deck lids, roofs, doors and fenders. In the active hydro-mechanical process, an oil-in-water emulsion is used as the active fluid medium, prebulging is used before deep drawing and then the fluid is pressurized to a particular pressure during pressing. Nakamura, et al. [34] presented techniques used in the hydro-mechanical drawing of automotive body panels. They discussed how to integrate drawing and bending, how to insist on using only one die half (avoiding using the lower die), how to improve the surface quality (eliminating surface torsion) and how to reduce wrinkles. ABB [28,29] reported that fluid deep-drawing (hydroforming) can produce drawn parts such as boxes, cups and parabolic shapes in one step and that hydroforming allows 50% greater DR and 50% less material thinning than for conventional deep-drawing.
3.2. Other progress Tirosh et al. [30] proposed the process hydrostatic ironing, in which the ironing process is assisted by hydrostatic pressure in order to increase the friction between the punch (or mandrel) and the workpiece. The punch force can thus be reduced and even omitted (with a threshold pressure) and the limit ironing ratio (LIR) can be increased considerably. Asakura et al. [36] carried out some investigations on deep drawing utilizing lateral fluid pressure (Fig. 2 (a)), high pressure being exerted upon the perimeter of the blank whilst only a lower punch force is needed. The LDR reached 4 and 6 in two experiments on hard aluminium workpieces. The punch force decreases with the increase of the lateral fluid pressure. When the lateral fluid pressure is sufficiently high, the punch force is close to zero, therefore experiments without a punch were also performed. When the die diameter is within a strict range, the drawing experiments failed due to buckling of the workpieces. It is worth noting that a new fluid-pressure-assisted deep-drawing process was reported recently [35], which is referred to as an HBU process (high-pressure sheet metal forming). In this process, the fluid pressure plays the role of the punch, in contrast to hydro-mechanical forming. Two workpieces are formed in each work-cycle and the female dies determine the final shapes of the workpieces (Fig. 2 (b)). This process is suitable for the forming of complex sheet parts and small lot production. The process time may be a little longer and accurate positioning at the dies may be difficult.
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Fig. 2. Some new developments in hydromechanical forming processes.
4. Theoretical investigation and numerical simulation of the process
4.1. Theoretical in6estigation Larsen [5] proposed an equation for calculating the opening pressure in the chamber to lift the blank free from the die radius in aquadraw: 2 ln b P= 2kt Rd where P is the pressure, k is the yield stress in pure shear, t is the blank thickness, Rd is the die radius and b is the drawing ratio. Ressener and Hora [38] analyzed the hydromec process by the energy-stability criterion and the stresses and the strains occuring throughout the deep drawing being analyzed by iteration. The LDR can be predicted considering the fluid pressure and other forming conditions. Yossifon and Tirosh [12] proposed a concept MDR (maximum drawing ratio) for the hydroforming process, because excessive pressure can cause rupture and insufficient pressure may cause wrinkling (buckling). The pressure-path has a working zone without failure and two critical pressure values can be calculated under different conditions. A good process can be successfully carried out through the pressure passage without touching the two critical lines. The two critical pressure values may be increased and the passage may be widened by reducing the friction or changing other process conditions. The MDR reaches a minimum value between n=0.1 and 0.2, this having been verified by experiments. Rupture occurs on the portion near the die radius (not the punch radius as in conventional drawing) and wrinkles occur at the unsupported portion between the flange and the punch.
Yossifon and Tirosh [13] applied the energy method to the analysis of the buckling (wrinkles) phenomena and proposed a formulation. The buckling mode (with a number of wrinkles) can be predicted under particular pressure and strain and the general solution can be obtained under particular conditions. The number of wrinkles can vary under different pressures and usually increases with the increase of the fluid pressure. The lowest fluid pressure suppressing buckling was predicted and compared with the results from experiments, which established the validity of the formulation. In fact, this formulation demonstrates the relationship between the current blank geometry (b/t), the flange width (1− a/b) and the minimum fluid pressure. Tirosh and Konvalina [14] reported the LDR values for the hydrodynamic process using limit analysis in plasticity theory coupled with the flow analysis of viscous, non-inertial fluid. The LDR values are theoretically determined by the upper-bound and lower-bound methods. The stress in the flange floating are presented and the effect of die curvature on the LDR values, as well as the maximum effective cup height, are analyzed. Shirizly et al. [15] reported their work on limit analysis of the deep-drawing process with fluid-pressure blank-holding. The upper-and lower-bound results were obtained, with special emphasis on the geometry of the die profile. The blank is divided into five zones. Yossifon and Tirosh [16] analyzed the hydroforming process, with particular attention being paid to tensile rupture. The bank is divided into three portions, the stress and deformation are predicted and the working path between the rupture and the wrinkles is investigated. Hsu and Hsieh [32] analyzed the rupture and wrinkle loci in hemisphere punch hydroforming processes (in fact hydro-mechanical drawing). The blank was divided
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into three zones and the stress and the internal pressure were obtained so that the diagram of pressure – punch stroke could be formed on the basis of calculations. Bay and Malberg et al. [8,9] proposed the mastercurve concept (the hydromec process). A diagram showing the relative drawing-in and the actual drawing ratio can be determined by experiment or calculation under a constant fluid pressure (hydromec) and blankholder force and the feasibility of the hydro-mechanical drawing process with the same conditions can also be determined by this diagram. When the drawing-in curve intersects with the master curve, the workpiece will be subjected to rupture; if not, the process will be successfully carried out. Using this method, the hydro-mechanical forming of workpieces with complex shapes (cylindrical, conical or combined shapes) can be predicted. It is also predicted that the LDR value increases with the increase in the fluid pressure, due to the friction increase between the punch and the deformed workpiece with increased pressure. A paper published by Zhang and Li [10] presented both an analytical method and a computer program that can predict the process feasibilities according to the analytical calculated data and the experimental loading conditions. Gelin and Delassus [18] presented a theoretical calculation of the pressure under the blank holder for the aquadraw process. The stress in the flange and the thickness of the lubricant film are calculated. The velocity of the flange in the radial direction and the influence of the cavity volume on the process are demonstrated. They also introduced some other proposals for measuring the cavity pressure in Ref. [19], where they pointed out some disagreements with the experimental results in Ref. [18]. Some of their experimental results gave the pressure cavity evolution, which is dependent on the material behavior, the blank thickness, the die entrance radius and the drawing ratio. The cavity pressure and the blank-holding pressure are estimated theoretically and the winding angle around the die entrance radius is given. A new mechanical model was recently proposed by them [20] to determine the fluid pressure in the die cavity and the blank-holding pressure. It was discovered that the blank-holding pressure has little effect on the cavity pressure and therefore, a new model is set up by the equilibrium of the blank-holder. The blankholder pressure is low near to the blank edge. The model was proven to be better than previous ones when using aquadraw to draw stainless- and mild-steel workpieces. A finite-difference method analysis was made by El-Domiaty and Shabara [39] for the radial-pressure deep-drawing process. According to their analysis, the LDR can be increased by utilizing the radial pressure. A higher value of P/k (P, radial pressure; k, shear yield stress) can improve the LDR values, wich can rise to 2.86 for steel sheets and 3.5 for aluminium.
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Bayoumi [40] carried out a theoretical analysis and some experiments using brass, steel and annealed aluminium under different process conditions (different friction and punch roughness) to investigate the LDR values. The effects of material properties (n and R values) on the process are discussed. Hyun and Cho [41] proposed a method using an artificial neural network to predict the forming-pressure curve for hydroforming processes based on the upperbound analysis by Tirosh et al. [16]. The effects proved to be very satisfactory.
4.2. Numerical simulation Computer simulation has been used in the research and design of hydro-mechanical deep drawing process. Computer simulation has developed quickly in recent years and compared to experimental methods computer simulation has the advantages of low cost, short time, more information and well-illustrating graphics. Therefore, it is now widely used in metal forming. A finite-element analysis of the hydro-mechanical process (hydro-forming —in fact an aquadraw process) was reported recently [31]. The forming of an oil pan upper of an automotive engine was simulated using an explicit non-linear finite-element code on a Pentium-90 PC. Shell elements were used for simulating the blank, the punch, the die and the blank holder. The fluid-pressure path and the blank holding force path were determined by the simulation in order to obtain a successful process. The blank size and the draw-bead location could also be determined by the simulation, but the draw-bead shape and size could not be predicted using this code. Yang et al. [31] performed FEM simulations of the separate radial-pressure drawing of the long cups. A rigid–plastic FEM code employing Hill’s anisotropic theory was used. The numerical simulations predicted the optimal chamber pressure, the optimal radial pressure and the thickness distribution; the simulations can help in judging whether the process can be carried out successfully. FEM analysis of the axisymetrical deep drawing process was also performed by these authors [33]. Gelin and Delassus [18,19] performed FEM simulations of the aquadraw process of stainless steel and low-carbon steel workpieces. In the numerical analysis, a 3D explicit elastic–plastic code was used and the forming process with fluid pressure was simulated to predict the distribution of the thickness. In future, the fluid pressures in the chamber and beneath the flange are expected to be coupled with the deformation. FEM analyses of the hydroforming (hydrobulging) of tubular and complex automobile parts have been reported recently. Wu et al. [25] used the LS-DYNA3D code to analyse the hydroforming of complex tubular
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parts, the results proving good and the code being suitable for the analysis and design of the process. Berg et al. [26] developed a new shell element by the explicit FEM code to simulate the hydroforming of automobile parts. An FE analysis [42] was made of a flanging operation using a fluid cell press. This is the flanging process of sheet material by rubber die forming, which is somewhat similar to hydroforming. The fluid pressure is transferred through some layers of rubber membrane to the workpiece; the rubber plays the role of the punch, the blank is forced to contact the die and thus, flanging is performed. This analysis was carried out using the LS-DYNA3D code and the wrinkles on the flange were predicted in the simulation. An FE analysis was also arried out of the HBU process [37]; the 3D FEM code INDEED (developed in Germany) was used in the simulation of this process and the tool components and the O-ring seal behavior were simulated by a FEM code named MARC. The contact area and the wrinkles were predicted by the FEM simulation. Asakura et al. [43] performed a numerical simulation of the deformation process of punchless drawing with lateral fluid pressure using the matrix method. The deformation behavior of the blank and the contact behavior on the die shoulder under various tool conditions were analysed and the optimum die radius for the punchless drawing was determined theoretically. The matrix method is the early form of the finite-element method, the present authors using this method in their research around 1983.
5. Equipment and devices Ordinary presses can be used, either single or double acting. Hydraulic presses are the most suitable because of the constant drawing speed and independently separate control of the motion [3]. Mechanical eccentric presses (crank-driven straightsided press, e.g. with 2-point slide suspension) were tested for the hydro-mechanical drawing process and it is established that the mechanical press can also be used. The punch force undergoes a slight increase if the punch speed is increased compared with the process on a hydraulic press. The testing machine can work at a speed of from 24 to 41 strokes per minute [4]. Specialized equipment and devices for the hydro-mechanical deep drawing process have been devised at AP&T Sweden and at some other companies. The AP&T hydraulic presses [1,46,47] use welded bodies and frame structures and have three or four cylinders, i.e. the principal cylinder, the triple cylinder, the cusion cylinder and a fourth cylinder have a comparatively high pressure, which can perform some additional
working operations, such as blank-holding and blankrejection. The AP&T hydraulic presses can be used in hydro-mechanical deep drawing, as well as in other metal forming processes. The AP&T hydro-mechanical forming devices are independent of the hydraulic presses and can be installed on different AP&T hydraulic presses. These devices have their independent hydraulic systems, which can be used for supplying the fluid, for pressurizing, for pressure-keeping, for pressure unloading and for oil filtering. The hydraulic pressure can be adjusted or programme-controlled with the stroke and can change from 20 to 120 MPa. Three types of AP&T hydro-mechanical forming devices are supplied: (a) the numerical control type, which has independent power system and oil-filtering system, in which the pressure can be adjusted continuously or controlled with the movement of the cylinder, these types of devices being suitable for the forming of complex sheet workpieces; (b) the pressure-limit type, which has a pressure-restraining circuit, where the peak pressure is present by a pressure-restraining valve and can be adjusted manually; and (c) the lifting pressure type, in which the device has a lifting-pressure cylinder at the device bottom, the upper or down movement of the cylinder changing the chamber volume so that the pressure can be changed, this type being suitable for the forming of small complex workpieces. Specialized equipment has also been devised in Amitio Press Technical Center, Japan [2]. A conventional hydraulic press with double-action oil cylinders was first used in Japan. The press was equipped with an ejector, a liquid supplying unit to the die chamber, an automatic hydraulic counter-pressure control unit and an automatic blank-holding force-control device. Many of these items of equipment can control the pressure freely. A single-action press with a cusion cylinder that is simple and convenient for hydro-mechanical forming has also been on the Japanese market since 1983. About 50–60 hydro-mechanical forming machines had been installed in Japan up to 1987 of which about a half were in production whilst others were in research institutes. No seal ring is provided for these machines. A special press for active hydro-mechanical forming is also being developed at SMG Engineering, this being a double-action press equipped with hydromec facilities [23], consisting of a hydromec unit and a quick die change system. SMG intended to supply presses that can perform active hydro-mechanical forming, hydroforming and die try-out for conventional deep-drawing. Lagan Press Sweden (a member of ABB Group) [28] supplies equipment for hydro-mechanical drawing. ABB Metallurgy supplies Qintus presses for hydroforming up to a pressure of 200 MPa [28]. Qintus fluid-cell presses are available in different sizes with maximum forming pressures of 100, 140 and 200 MPa,
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all parts exposed to high stresses being wound with prestress wire. When the pressures exceed 100 MPa, the axial forces in the press tray are absorbed by a prestressed frame.
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The hydro-mechanical deep-drawing process has found many applications in industry because of its obvious features in process, product quality and economical aspects. The applications are reported mostly in Japan. Some applications are summarized in the following.
produced by this process, e.g. a fender trim, of 0.8 mm thickness, 650 mm length and 420 mm width was produced by this process. Nakamura et al. [34] reported the automotive body parts produced at Toyota Motor Corporation by hydro-mechanical drawing (referred to as liquid-pressure press-forming in their paper). Eleven major parts were shown, including fenders, quarter tails and door inners. As demanded by the diversifying market, personalized vehicles can be produced inexpensively and of good quality, by the commercialisation of the hydro-mechanical deep-drawing process. (d) Kitchen utensils and electrical household applications. Applications in this field began earlier, but less detailed descriptions have been published.
6.1. Applications in Japan
6.2. Applications in other countries
More than 300 kinds of workpieces were formed by the hydro-mechanical forming process in Japan up to 1987 [2,5]. The materials cover mild steel sheets, aluminium sheets, titanium alloys and stainless steel sheets, with a thickness of 0.2 – 3.2 mm. High-strength steel sheets and mild-steel sheets coated with Teflon (a kind of sandwich sheet) have also been used. The sizes range from 30×30×30 mm to 1200 ×1000 × 250 mm. The application fields are mostly related to reflectors of lighting equipment, aircraft parts and automobile parts. Nakamura and Nakagawa [6] reported many examples of automobile body panels formed by hydro-mechanical deep drawing presses in a recent paper. (a) Reflectors of lighting equipment. These parts are usually made of aluminium sheet and have rather complicated shapes and large sizes. They usually need 6 forming steps by the conventional spinning process, but can be formed in one stroke by the hydro-mechanical forming process. In addition, the inner surface quality and the thickness distribution is much better. The die cost is low, which is suitable for small-lot production. The production rate of a automobile light reflector is 8 pieces per minute and one machine can produce 200000 pieces ever month. (b) Aircraft parts. These are subjected to high safety standards, thus local thinning should be avoided. The hydro-mechanical deep-drawing process can reduce the friction between the die and the blank-holder, so that the thickness distribution becomes much better. Some aluminium aircraft parts have been formed by this process instead of by the spinning process. A lip skin, with a thickness of 1.6 mm and a diameter of 1150 mm, was produced with a thinning of 12% in Japan. (c) Automobile parts. Because of the fewer forming steps and the omission of the female dies, some automobile parts have been formed by the hydro-mechanical deep drawing process. In addition to the manufacture of light reflectors for automobiles, some panel parts (autobody) of automobiles have also been
Conical hollow bodies such as reflectors were reported to be formed by hydro-mechanical process at the beginning of 1967 [3]. One draw is sufficient to form a conical hollow workpiece in the hydro-mechanical process, whereas five draws and a sizing are needed for conventional deep drawing (see Fig. 3). Recently, an oil pan upper was reported to be produced in the United States [31], being part of an automotive engine. A motor housing was formed by hydro-mechanical deep drawing process in 1977 [5] (DR= 2.76). In China, tractor oil-pans were manufactured by hydro-mechanical drawing in the 1970s [10]. In the early 1980s, stainless-steel cups with a depth of 200 mm were formed at a factory in Guangzhou. Some specialized presses have been devised in China. In addition to the materials used in Japan, some other materials such as pure copper and brass have also been tried out in other countries [4]. Drewes et al. [44] dealt with the potential of reducing car weight by using the hydro-mechanical deep-drawing or hydroforming process. Hydroforming is reported mainly to be used in automotive industry and tube forming industry. It is reported [45] that a Dutch steel company and a German automotive firm have been implemented using the hydroforming process to produce automotive parts such as exhaust manifolds and engine cradles, at an annual rate of 1.5 million pieces. The hydroformed parts can be very complex. They are lighter, cheaper and of high quality, particularly when high-strength steel is used. Tubular automobile parts are also reported to be hydroformed in some countries [25,26]. Hydroforming is reported to have been used by General Motors and Magna International Plant in Aalen, Germany, to produce automotive parts such as pipes [30]. A sixteen-hundred metric ton Siempelkamp hydroforming press has been introduced, which is said to be the first HF press built especially for automotive components. Huber and Bauer GmbH is reported to be
6. Applications in industry
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Fig. 3. Drawing of conical or parabolic hollow bodies.
a German-based manufacturer of high-pressure hydroforming equipment. Johannisson [28,29] introduced the applications of hydroforming in automotive industry. Hydroforming can be used for producing automotive deep-drawn parts, which makes small-scale production profitable; the main reason being that it requires only one rigid tool half. Flexforming is an unbeatable process for producing prototype parts. The rigid tool half can be made from inexpensive materials. Parts made from blanks of different thickness and materials can be produced using the same tools. The fluid-cell process allows several parts to be formed in the same press cycle. Draw clearance between tools is unnecessary. A car engine cradle arm and a car front fender made by this technology have been presented. Hydroforming can reduce tool costs (only one tool half), production cycle tool costs can be reduced by 75% and lead time reduced to 60% and the process is especially suitable for forming sheet metal parts of prototypes and small scale series production [29]. BMW, VOLVO, Mercedes-Benz and Lebranchu S.A. Paris are all equipped with quintus presses to carry out hydroforming production. The production of a truck body part, a rear fender and an excavator door has been introduced. Other users of flexforming in the automotive industry are Saab, GM, Audi, Ford, Peugeot, Citroen and Renault. The largest press in the world in terms of press force — 150000 t — began operation in Benz in early 1990.
7. Conclusions The deep drawing processes assisted with fluid pressure has been summarized in this article. The process principles and the process features and the development and research progress have been introduced in detail. It is believed that hydro-mechanical deep-drawing processes have a prosperous future in the sheet metal forming industry, but further work must be carried out in their process control and their applications.
Acknowledgements The authors are very grateful to M.R. Jensen for supplying materials.
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