Effects of extrusion parameters on tensile properties of magnesium alloy tubes fabricated via hydrostatic extrusion integrated with circular ECAP

    Effects of extrusion parameters on tensile properties of magnesium alloy tubes fabricated via hydrostatic extrusion integrated with circular ECAP Renshu Yuan, Zhilin Wu, Hongming Cai, Lei Zhao, Xinping Zhang PII: DOI: Reference:

S0264-1275(16)30436-1 doi: 10.1016/j.matdes.2016.03.141 JMADE 1619

To appear in: Received date: Revised date: Accepted date:

30 January 2016 19 March 2016 29 March 2016

Please cite this article as: Renshu Yuan, Zhilin Wu, Hongming Cai, Lei Zhao, Xinping Zhang, Effects of extrusion parameters on tensile properties of magnesium alloy tubes fabricated via hydrostatic extrusion integrated with circular ECAP, (2016), doi: 10.1016/j.matdes.2016.03.141

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ACCEPTED MANUSCRIPT Effects of extrusion parameters on tensile properties of magnesium alloy tubes fabricated via hydrostatic extrusion integrated with circular ECAP

School of Mechanical Engineering, Nanjing University of Science and Technology,

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Renshu Yuan a), Zhilin Wu a,b), Hongming Cai a,b), Lei Zhao a,b), Xinping Zhang c,*)

Nanjing, 210094, China

Institute of Science and Technology, Nanjing University of Science and Technology,

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b)

Nanjing, 210094, China c)

School of Material Science and Engineering, Nanjing University of Science and

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Technology, Nanjing, 210094, China

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Abstract: To meet the need for high strength tubes for various industrial applications, a new method, known as hydrostatic extrusion integrated with circular equal channel angular pressing (HECCAP), was proposed for the fabrication of AZ80 magnesium

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alloy tubes. Small diameter billets could be extruded into tubes of larger diameters

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using this method. The effects of the extrusion ratio (ER) and the conical mandrel angle (CMA) on the microstructure and tensile properties of the AZ80 magnesium alloy tube

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were investigated. The HECCAP-treated alloys exhibited a partial dynamically recrystallized microstructure. More shear zones formed inside of these grains, and a higher recrystallization grain volume fraction were presented in the alloy after HECCAP treatment with a larger ER or CMA values. The tensile yield strength,

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ultimate tensile strength, and elongation percent increased as ER and CMA also increase. The highest ultimate tensile strength, tensile yield strength and elongation were 335 MPa, 308 MPa, and 7.2 %, respectively, at an ER of 2.77 and CMA of 120°. A formula was proposed to calculate the total generated plastic strain after HECCAP, which increased with increasing ER and CMA.

Key words: AZ80 magnesium alloy; Warm-hydrostatic extrusion; Microstructure; Mechanical properties; Extrusion ratio; Conical mandrel angle Corresponding author: X.P. Zhang. Tel. / fax: + 86 25 84303983. E-mail address: [email protected] (X.P. Zhang). Complete postal address: School of Materials Science and Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing 210094, China 1

ACCEPTED MANUSCRIPT 1. Introduction Severe plastic deformation (SPD) techniques can improve the material properties of alloys by refining their grain structure. Such methods are attractive as ultra-fine grained

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(UFG) materials have been shown to exhibit outstanding material properties, such as

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high strength [1]. Most SPD methods, such as equal channel angular pressing (ECAP), high-pressure torsion, and accumulative roll bonding, are suitable for various

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geometries, including bars, rods, plates, and sheets. Despite the need for high strength tubes for various industrial applications, little effort has been made to produce tubular parts using SPD methods.

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Magnesium alloys are increasingly used in the automotive and aerospace industries due to their low density and high specific strength [2]. ECAP is deemed to be the most

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efficient SPD technique for refining the microstructure. When the ECAP technique is integrated into a hydrostatic extrusion die for tubes, the extruded parts will have UFG properties. The high hydrostatic pressure during extrusion will reduce crack defect

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induced by ECAP. These benefits make such a process attractive for commercial use.

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In the present work, a new method, named as hydrostatic extrusion integrated with circular equal channel angular pressing (HECCAP) method, was proposed to fabricate

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AZ80 magnesium alloy tubes. Effects of the extrusion ratio (ER) and conical mandrel angle (CMA) on microstructure and tensile properties of AZ80 magnesium alloy tube were investigated.

2. Materials and methods

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2.1 Material

Direct chill (DC) cast alloy with an average composition of Mg-8.5% Al-0.49% Zn-0.21% Mn (AZ80) was received in the form of cylindrical ingots with dimensions of Ø 70 mm × 400 mm. 2.2 Hydrostatic extrusion experiment A schematic of the HECCAP extrusion die is shown in Fig. 1. The diameter of the billet is the same as the maximum outer diameter of the mould core. There were two ECAP channels between the extrusion container and the mould core. The billets were extruded in two passes with a channel angle (φ). The corner angles (ψ) of these passes were 0°. The working procedure of the hydrostatic extrusion starts by lading a billet into the container, then sealing the container with the ram and filling it with castor oil, after which the ram is then advanced into the container. This pressurizes the chamber forcing the billet through the die. The extrusion speed is approximately 0.2 mm/s. 2

ACCEPTED MANUSCRIPT As informed by previous research on the effects of heat treatment on microstructures and mechanical properties of Mg AZ80 alloys [3-6], the AZ80 alloy ingots with a diameter of 70 mm were homogenized at 415 °C for 32 h, and then machined into

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billets with a diameter of 60 mm. These billets were heated in a furnace at 290 °C for 1

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h and then extruded to tubes with an outer diameter of 70 mm and an inner diameter dependent on the ER.

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Due to the limitations of the sealing material, the pre-heat temperature of the extrusion container was set to 220 °C. Due to limitations in the extrusion force of the hydrostatic extruder, the ERs were set as 1.25, 1.92 and 2.77, while the conical mandrel angle

2.3 Tensile test at room temperature

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CMA values were fixed at 105° and 120°. Experiments were performed in triplicate.

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Dog-bone-shaped flat specimens with a 6 mm gauge length and a 2 mm gauge width were used in the tensile tests. The tensile test specimens were sectioned along the extrusion axis. The dimensions of the specimens and the test methods used in this study

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are identical to those documented previously [7, 8]. Placements of the tensile specimen

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and microstructure specimen are shown in Fig. 2. Tensile yield strength was determined by the offset method as described in ASTM: E9-89a, with an offset of 0.2%.

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2.4 Microstructural characterization N side (radial direction) and S side (extrusion direction) samples (see Fig. 2) were prepared for microscopy analysis by polishing them with various grades of SiC paper (with increasing fineness) and with 1 µm diamond paste in the last step. The samples

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were then ultrasonically cleaned in pure ethanol. In order to make the grain structure easily visible, the polished samples were etched in a solution of 100 ml ethanol, 6 g picric acid, 5 ml acetic acid, and 10 ml water. Microstructural and composition characterization were carried out using optical microscopy (OM), scanning electron microscopy (SEM, FEI Quanta 250 FEG) and energy disperse spectroscopy (EDS). The phases of the alloy were confirmed by X-ray diffractometry (XRD, Bruker D8 X) using Cu Kα radiation. 3. Results and Discussion 3.1 Homogenized and HECCAP-treated microstructures An example of an AZ80 tube prepared using the HECCAP method is shown in Fig. 3. It can be seen that the resulting surface is quite smooth (the average roughness Ra ≤ 3.0 μm). After cutting off the stub bar, the variation in the wall thickness over the length of the tube was ≤0.3 mm. 3

ACCEPTED MANUSCRIPT The radial-forward extrusion method is a method of fabricated a larger diameter tube from a small diameter billet [9-10]. In this work, small diameter billets were extruded into tubes of larger diameters using the HECCAP method. The major difference

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between the radial-forward extrusion method and the HECCAP method is back

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pressure. Back pressure (BP) is generated during the HECCAP due to castor oil while there is no back pressure during the radial-forward extrusion. The application of a BP

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ensures the homogeneity of the workpiece processed and leads to an increased rate of continuous dynamic recrystallization (CDRX) [11]. Due to the hydrostatic pressure conditions, the BP is advantageously able to reduce or completely eliminate dead zones,

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which contributes to an increase in the magnitude and homogeneity of the imposed strain along the cross section of the deformed workpiece. Additional benefits include

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significant reductions in tensile stress areas, which can initiate the development of cracks or fractures [12]. Hence, the HECCAP method can improve the homogeneity of the workpiece, refine the microstructure of the workpiece, and decrease the defects.

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Typical microstructures and XRD results of the homogenized AZ80 Mg alloy are

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shown in Fig. 4. The grey features in Fig. 4 (a) and the black features in Figs. 4 (b)-(c) was α phase Mg. Eutectic structures (formed during non-equilibrium freezing) were

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present on the boundary of this phase. The black features in Fig. 4 (a) and the grey features in Figs. 4 (b)-(c) correspond to Mg17Al12 (II), which had a lamellar structure. The small undissolved particles located in the grains were Mg17Al12. It was verified by the EDS results, as listed in Table 1. The mean grain size and the non-base matrix

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phase (eutectic structure, Mg17Al12 (II), and Mg17Al12) contents of the homogenized specimen were 75 ± 8 µm and 9 ± 3%, respectively. Fig. 5 shows the microstructures of the HECCAP-treated AZ80 Mg alloy. A large amount of new fine grains appeared at the initial grain boundary, due to dynamically recrystallization. Refine eutectic and Mg17Al12(II) also appeared along the grain boundary. The AZ80 Mg alloy exhibited a partial dynamically recrystallized (DRXed) microstructure. The partial DRXed microstructure was most likely due to the relatively low extrusion temperature (290 °C) and slow extrusion speed used (approximately 0.2 mm/s), which are known to reduce the DRX fraction [13]. Furthermore, there were no bands of stringers parallel to the extrusion direction present in any specimens. The shear force appeared when the billet moved through the two ECAP channels, leading to shear zones formed inside of the grains, as shown in Fig. 5 (b). 4

ACCEPTED MANUSCRIPT Large particles (diameter > 1 µm) can act as nucleation sites for DRX during hot deformation ( also known as particle-stimulated nucleation) [14-17], and although the larger Mg17Al12 particles (2–15 µm, see Fig. 5) were partially broken during extrusion,

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their size remains greater than 1µm, as shown in Fig. 5 (C). We therefore believe that

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these particles acted as heterogeneous sites for nucleation of recrystallization by generating local inhomogeneity in the strain energy during extrusion, resulting in an

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increase in the DRX fraction of the HECCAP-treated alloy.

The difference between the microstructure at the N side and the S side were the DRX fraction, the grain size and shape. The DRX fraction at the S side was larger than that

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at the N side, while the grain size and the ratio of length to width of the grain of the former were smaller than these of the latter.

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3.2 Effects of extrusion ratio on the microstructure and tensile properties of HECCAP-treated AZ80 Mg alloy

Typical microstrcutures of the HECCAP-treated AZ80 Mg alloy with different ERs

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are shown in Fig. 6. More shear zones formed inside of the grains, and a higher DRX

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fraction were appeared in the AZ80 Mg alloy treated using HECCAP at a larger ER value. For example, the DRX fraction increased from 43% to 50% and 75% when the

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ER increased from 1.25 to 1.92 and 2.77. The expression for the volume fraction of a dynamically recrystallized grain Xd obtained by the classical treatment of grain boundary nucleation kinetics proposed by Cahn [18] is given by Equations (1) and (2):

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(1) (2)

where ε is the plastic strain, εp is the peak strain, a1 ,a2 , n1 are the material constants,

 is strain rate, n is the stress exponent, Q is the activation energy for the deformation, R is the gas constant, 8.314 J/mol, and T is absolute temperature. As εp increases with  (see Eq. (2)), it prevents the occurrence of DRX. When the extrusion speed is held constant, the strain rate increases as ER increases. Accordingly, the εp needed to cause DRX of specimens extruded at a lower ER is lower than that at a larger ER value, which prevents the occurrence of DRX when the specimen is extruded at a larger ER. However, a larger ER leads to a higher strain, which promotes DRX when the specimen is extruded at a larger ER. In addition, a larger ER 5

ACCEPTED MANUSCRIPT improves the temperature increase of the billet during HECCAP. Higher deforming temperatures reduce εp, which also promotes DRX when the specimen is extruded at a larger ER. In this work, a larger ER increases the DRX fraction.

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The effects of ER on the tensile properties of HECCAP-treated AZ80 Mg alloy tubes

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are shown in Fig. 7. The tensile yield strength, ultimate tensile strength, and elongation percent all increased as ER increased. The highest ultimate tensile strength, tensile

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yield strength and elongation were 335 MPa, 308 MPa, and 7.2 %, respectively, at an ER of 2.77 and CMA of 120 °.

The tensile properties of the HECCAP-treated AZ80 Mg alloy tubes were not lower

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than those of the SPDed AZ80 Mg alloy. For example, Sepahi-Boroujeni et al the as-cast through the expansion equal channel angular extrusion (Exp-ECAE) operation.

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The maximal ultimate tensile strength is 298 MPa [19]. Chen et al reported the UTS, YS and elongation to fracture of the repetitive upsetting-extrusion AZ80 magnesium alloy are 337 MPa, 246MPa and 24.6%, respectively [20]. Cao et al reported the

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ultimate tensile strength and elongation of the hot flow formed AZ80 magnesium alloy

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were 308 MPa and 9.8%, respectively [21]. The tensile strength of the AZ80 Mg alloy could be attributed to the combined effects of

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several strengthening mechanisms: (1) grain boundary strengthening by ultra-fine fully DRXed grains, (2) strain hardening, (3) precipitation strengthening by fine Mg17Al12 precipitates, and (4) solid-solution strengthening by dissolved Zn. The tensile -

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elongation of extruded Mg alloys is strongly related to their DRX fraction [22]; that is, -

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{1011} contraction and {1011}-{1012} double twinning are more easily generated in large unDRXed grains during tensile deformation along the elongation direction at room temperature, which in turn provides initiation sites for micro-cracking that degrade the ductility [23-25]. Thus, the higher tensile elongation of the AZ80 Mg HECCAP-treated at a larger ER was most likely a result of its greater DRX fraction, or more specifically the relative lack of large unDRXed grains. In addition, a larger ER allowed for greater strain hardening, which also improved the tensile strength of AZ80 Mg treated using HECCAP at a larger ER. 3.3 Effects of conical mandrel angle on the microstructural and tensile properties of HECCAP-treated AZ80 Mg alloys Typical microstrcutures of AZ80 Mg alloy HECCAP-treated with different CMAs are shown in Fig. 8. More shear zones formed inside of the grains and a higher DRX 6

ACCEPTED MANUSCRIPT fraction appeared in the AZ80 Mg alloy HECCAP-treated with a larger CMA. For example, the DRX fraction increased from 31 % to 43 % when CMA increased from 105° to 120°.

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The effects of CMA on the tensile properties of HECCAP-treated AZ80 Mg alloy

elongation percent increased as the CMA was increased.

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tubes are shown in Fig. 9. The tensile yield strength, ultimate tensile strength, and

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The flow pattern and strain-stress states of HECCAP are different from those of conventional ECAP. In conventional ECAP, the strain state can be considered as simple shear, while in HECCAP there are some additional radial and circumferential resulting from

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tensile and compressive strains. To calculate the accumulated strain

conventional ECAP with channel angles φ and corner angles ψ, the following Equation

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[3] can be used:

(3)

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During the first pass, the flow pattern and the strain-stress state of the material near the

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container wall is different from that of the material near the mould core. Hence, the average accumulated strain during the first pass is set as the product of λ and the

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corresponding value of conventional ECAP. λ is a function of CMA and φ and 0 < λ < 1. It should be noted that the method for accurately determining a value for λ merits further investigation.

applied to the alloy during HECCAP processing can be

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The total accumulated strain

calculated using Equation (4), derived from common engineering plasticity formulae and the geometry values shown in Fig. 1: (4)

In this work, ψ is 0°, so

can be written using Equation (5): (5)

The relationship between the CMA γ (0 < γ < π) and φ defined using Equation (6): (6) That is,

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where 0 < γ < π/4. increased with the values of CMA and ER. As a consequence, the effects

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Hence,

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of CMA on the microstructure and tensile properties were the same as those of ER. 4. Conclusions

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Small diameter billets were extruded into tubes with a larger diameter using a hydrostatic extrusion process integrated with circular equal channel angular pressing method.

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These HECCAP-treated AZ80 alloys exhibited a partial dynamically recrystallized microstructure, and shear zones were formed inside of the grains.

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More shear zones formed inside of the grains and higher DRX fractions appeared in the AZ80 Mg alloy HECCAP-treated at a larger extrusion ratio or a larger conical mandrel angle, which led to a higher tensile yield strength, ultimate tensile strength, and

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elongation. The highest ultimate tensile strength, tensile yield strength and elongation

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were 335 MPa, 308 MPa, and 7.2 %, respectively, when the extrusion ratio was 2.77 and the conical mandrel angle was 120°.

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A formula was proposed to calculate the total generated plastic strain for HECCAP, which was found to increase with both the extrusion ratio and conical mandrel angle. Acknowledges

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This work was supported by the Major National Science and Technology Projects, No. ZX201204010101; the Fundamental Research Funds for the Central Universities，No. 30920140112008; Jiangsu Qinglan Project; Jiangsu National Natural Science Foundation, No. BK20151489. References [1] Zhu R, Wu YJ, Ji WQ, Wang JT. Cyclic softening of ultrafine-grained AZ31 magnesium alloy processed by equal-channel angular pressing. Mater Lett 2011; 65: 3593-6. [2] Zhang XP, Feng SF, Hong XT, Liu JQ. Orientation-related specimen thickness effects on mechanical properties of hot extruded AZ31B magnesium alloy. Mater Des 2013; 46:256-63. [3] Yakubtsov IA, Diak BJ, Sager CA, Bhattacharya B, MacDonald WD, Niewczas M. Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy 8

ACCEPTED MANUSCRIPT at room temperature. Mater Sci Eng A 2008; 496:247-55. [4] Zhao DG, Wang ZQ, Zuo M, Geng HR, Effects of heat treatment on microstructure and mechanical properties of extruded AZ80 magnesium alloy. Mater Des 2014;

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ACCEPTED MANUSCRIPT AgIn addition on deformation behavior, microstructure and texture in Mg–Al–Zn alloy during single pass warm rolling. Mater Des 2015; 81(15)11-20 [17] Zhang J, Fang C, Yuan FQ, Liu CP. A comparative analysis of constitutive

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ACCEPTED MANUSCRIPT Figures captions Fig. 1 Schematic illustration of hydrostatic extrusion integrated with circular equal channel angular pressing process

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Fig. 2 Placements of tensile specimen and microstructure specimen

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Fig. 3 Magnesium tube prepared by hydrostatic extrusion process integrated with circular equal channel angular pressing. (a) Whole tube, and (b) longitudinal section

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Fig. 4 Microstructures and XRD result of AZ80 Mg alloy homogenized at 415 °C for 32h. (a)optical microscope, (b) and (c) SEM, and (d) XRD patterns Fig. 5 Typical microstrcutures of HECCAP-treated AZ80 Mg alloy. (a)OM of N side, (b)

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OM of S side, (c) SEM of S side, and (D) SEM of DRXed regions Fig. 6 Effects of ER on the microstructure. (a) 1.25, (b) 1.92, and (c) 2.77. CMA was

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120°

Fig. 7 Effects of ER on tensile properties of HECCAP-treated AZ80 Mg alloy Fig. 8 Microstructure of AZ80 Mg alloy HECCAP-treated with an ER of 1.25 and a

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CMA of 105°

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Fig. 9 Effects of CMA on tensile properties of HECCAP-treated AZ80 Mg alloy

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Fig. 1 Schematic illustration of hydrostatic extrusion integrated with circular equal channel angular pressing process

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Fig. 2 Placements of tensile specimen and microstructure specimen

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Fig. 3 Magnesium tube prepared by hydrostatic extrusion process integrated with circular equal channel angular pressing. (a) Whole tube, and (b) longitudinal section

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Fig. 5 Typical microstrcutures of HECCAP-treated AZ80 Mg alloy. (a)OM of N side,

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(b) OM of S side, (c) SEM of S side, and (D) SEM of DRXed regions

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Fig. 6 Effects of ER on the microstructure. (a) 1.25, (b) 1.92, and (c) 2.77. CMA was

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HECCAP-treated AZ80 Mg alloy tube at 220 C 340 CMA : 120  Ultimate tensile strength

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Elongation percentage

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Fig. 7 Effects of ER on tensile properties of HECCAP-treated AZ80 Mg alloy

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CMA of 105°

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Fig. 8 Microstructure of AZ80 Mg alloy HECCAP-treated with an ER of 1.25 and a

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Strength, MPa

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HECCAPed AZ80 Mg alloy tube at 220 C ER was 2.77, tensile yield strength ER was 2.77,ultimate tensile strength ER was 1.92, tensile yield strength ER was 1.92,ultimate tensile strength ER was 2.77, elongation percentage ER was 1.92, elongation percentage

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Fig. 9 Effects of CMA on tensile properties of HECCAP-treated AZ80 Mg alloy

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ACCEPTED MANUSCRIPT Table caption

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Table 1 EDS results of the locations shown in Figs. 4 (b)- (c)

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Table 1 EDS results of the locations shown in Figs. 4 (b)- (c) Location Mg, wt. % Al, wt. % A 92.14 7.86 B 93.20 6.80 C 79.61 20.39

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights [1] Hydrostatic extrusion integrated with circular ECAP was proposed.

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[2] High properties AZ80 Mg alloy tube was prepared by HECCAP.

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[3] Small diameter billet could be extruded into tube of larger diameter using the method.

[4]UTS, TYS and EP of the tubes were 335 MPa, 308 MPa, and 7.2 %, respectively.

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[5]A formula was proposed to calculate the total generated plastic strain for

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HECCAP.

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