ARTICLE

J. Cent. South Univ. (2019) 26: 1582-1591

DOI: https://doi.org/10.1007/s11771-019-4114-7

Influence of AZ31 sheet treated by cryogenic on punch shearing

HU Zhi-qing(胡志清)^{1, 2}, GUO Chao-fan(郭超凡)³, LI Hong-mei(李洪梅)³

1. Rolling Forging Research Institute, Jilin University, Changchun 130025, China;

2. Key Laboratory of Automobile Materials (Ministry of Education), Jilin University,Changchun 130025, China;

3. College of Materials Science and Engineering, Jilin University, Changchun 130025, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Punch shearing is used to form the part in the material process. Cryogenic treatment (CT) has active effect on local mechanical properties of steel, but it is still uncertain of the influence of CT on the properties of the magnesium alloy during punch shearing. In this work, the influence of AZ31 sheet treated by cryogenic on punch shearing was studied. Microstructures were observed with a ZEISS optical microscope, and mechanical properties, as well as shear properties were tested by tensile testing and punch shearing. The results show that the number of secondary phase increases and a large number of twins appear in the grains after CT. Meanwhile, the ultimate tensile strength (UTS), the ductility, and hardness of AZ31 are improved, while the yield strength (YS) decreases gradually during CT. During punch shearing, the shearing strength decreases, the rollover radius changes insignificantly, and the height of the burr on the edge of the cross section decreases. At the same time, a larger proportion of smooth zone on the cross section has been achieved.

Key words: AZ31 magnesium alloy; cryogenic treatment; mechanical properties; punch shearing

Cite this article as: HU Zhi-qing, GUO Chao-fan, LI Hong-mei. Influence of AZ31 sheet treated by cryogenic on punch shearing [J]. Journal of Central South University, 2019, 26(6): 1582-1591. DOI: https://doi.org/10.1007/s11771- 019-4114-7.

1 Introduction

Magnesium alloy has attracted more and more attention in the fields of consumer electronics, automotive and aerospace due to light density, high specific strength, excellent damping capability, good thermal and electrical conductivity, and superior electromagnetic shielding performance [1-4]. Despite these advantages as mentioned above, there are still poor formability and limited ductility at room temperature because of their hexagonal close-packed (hcp) structure and limited slip systems, making its application as structural components limited in a large extent [5, 6]. At room temperature, basal slip is the predominant contributor to the plastic deformation of magnesium alloys. According the von Mises criterion, at least five independent slip systems in a polycrystalline material are necessary to develop homogeneous plastic deformation, however, magnesium alloys have only two independent slip systems. In order to improve their formability, they need to activate some slide systems and decrease some critical resolved shear stresses. Therefore, it is important to understand microstructure characteristic of magnesium alloys and to improve mechanical properties of Mg alloys as well as predict and control manufactured specimen quality [7-9].

The punch shearing and shearingcomponents have been widely used in the past decades. The machining of punch shearing is frequently encountered in producing novel products in many industries. More than 70% of the features in IT metal parts are shearing features, and some IT metal parts even consist entirely of shearing features [10]. Therefore, the research on the punch shearing process of magnesium alloy becomes extremely important due to the wide application of magnesium alloys. During punch shearing, the quality of shearing components is affected by the punching parameters such as punching speed, clearance, and rounding size. But the quality of shearing components mainly depends on the properties of the material itself including strength, hardness, toughness, etc. Cryogenic treatment (CT) is a supplementary process of traditional heat treatment which has been acknowledged for decades [11]. During CT, the specimens are soaked in the cryogenic liquid (temperature below -130 °C) for a period of time to change the properties of material [12, 13]. CT could improve the cutting properties of tool steels which were earliest reported by GULYAEV [14] in 1937. During the Second World War, scientists found metals show better wear resistance at low temperatures [15]. Afterwards, people began to pay more attention to cryogenic treatment. In the past several years, many literatures had shown that CT has a significant effect on the local mechanical properties of steel including strength, toughness, surface hardness [16-21]. Most recently, researchers began to pay more attention to the effect of cryogenic treatment on magnesium alloys. PRECIADO et al [22] found that grains of AZ91 magnesium alloy were refined and the ductility was improved after CT. LIU et al [23] researched the effects of CT on microstructure evolution and physical changes of AZ91 as-cast magnesium alloy, and the results showed that compressive strength and ductility after CT were significantly improved as well as the secondary phase particles increased, while the lattice constant and resistance decreased. JIANG et al [24] found that CT can change the orientation of grains of AZ31 magnesium alloy, and the tensile strength and hardness were improved after CT. LIU et al [25] found that Mg-1.5Zn-0.15Gd magnesium alloy exhibited a better ductility after CT, and the distribution of twins was more homogeneous. JIANG et al [26] found cryogenic treatment significantly improved the corrosion resistance of Mg-7Y-1.5Nd alloy. GONG et al [27] also found that corrosion resistance of AZ61 magnesium alloy welded joints improved significantly after cryogenic treatment.Although CT has been used to deal with magnesium alloy, the relevant researches on the effect of CT on magnesium alloy have been rare yet.

In this work, the effects of CT on the microstructures and mechanical properties of AZ31 were studied. Meanwhile, punch shearing tests were employed at room temperature to assess the formability properties of AZ31 magnesium alloys after CT.

2 Experimental material and methods

The commercial AZ31 sheet with the thickness of 1 mm was used in this work, and its chemical composition is shown in Table 1.

Table 1 Chemical composition of AZ31 magnesium alloy (mass fraction, %)

Firstly, the material was annealed at 200 °C for 2 h to eliminate the residual stress and unify the differences of commercial material. In order to study the effects of ACT on the properties of AZ31, the tensile specimens of 60 mm×8 mm×1 mm, metallographic specimens of 1 mm×1 mm×1 mm, and punch shearing specimens with size of 35 mm×20 mm×1 mm were prepared by a wire- cutting machine tool. Then these specimens were soaked in liquid nitrogen for different cryogenic time. The specimens were divided into five groups according to cryogenic time, and every group is designated as shown in Table 2.

Table 2 Cryogenic treatment conditions for tested alloy

The size of the tensile specimen is shown in Figure 1. The tensile test was performed by DNS-100 electronic universal testing machine at room temperature with a tensile speed of 2 mm/min followed ISO 6892-1:2009 MOD metal materials tensile test standards. The Vickers hardness test was performed by 1600-5122 VD MICROMET 5104 hardness test system with a load of 25 N and a holding time of 10 s.

Figure 1 Profile and size of tensile specimens (Unit: mm)

In order to investigate the effects of ACT on the microstructure, the specimens before and after CT were prepared, and then the surface of specimens was polished by standard metallographic techniques. Then the specimens were etched for 6-8 s in a solution of 5 g picric acid, 5 mL glacial acetic acid, 10 mL distilled water and 100 mL absolute ethanol. The microstructure of the alloy was observed by ZEISS optical microscope.

In this work, punch shearing test was performed by KD-20A precision electronic servo pressure machine, the punch diameter is 5 mm, the die diameter is 5.07 mm and the speed of punch shearing is 5 mm/s. The punch shearing principle, specimens and profile of section are shown in Figure 2. Before the experiment, the sheet was inserted into the slot on the die so as to be fixed, and three experiments were performed under each condition to ensure the reliability of experimental results. The fracture sections were observed by ZEISS microscope to assess the section quality. The formula of shearing strength is as follows:

                           (1)

                      (2)

where P is shearing force; t is the thickness of the sheet; r_punch is the punch radius; r_die is the die radius.

3 Results and discussion

3.1 Microstructure evolution

The microstructures of AZ31 magnesium alloy under different treatments are shown in Figure 3. It can be seen from Figure 3(a) that the grains of alloy untreated are coarse and the uniform distribution of grains is poor, furthermore, there are not obviously secondary phases. After AT, as shown in Figure 3(b), it can be seen that some grains are refined obviously, and the phase structure of the alloy mainly composes of fine grains and some coarse original grains, and the average size of the grains is about 20 μm. Meanwhile, the number of secondary phase increases slightly, and it mainly distributes in the grain boundaries. According to literature description [28], secondary phase have been recognized as Mg₁₇Al₁₂, which has a strengthen effect on the material properties. After ACT1, as shown in Figure 3(c), it is observed that the number of secondary phase further increases, and there are some twins in the grains. Then with the increase of cryogenic time, it can be seen from Figures 3(d)-(f) that more twins can be observed obviously, JIANG et al [24] also confirmed that the number of twins increased further after cryogenic treatment, but no evidence is found in the literature to explain this phenomenon. Meanwhile, the secondary phases become much finer, and the distribution is much wider in the organization. During CT, due to the sharp drop of temperature, on the one hand, the solid solubility of Al in the magnesium matrix is reduced, on the other hand, a large internal stress is generated inside the material due to changes in material volume. The interaction of two aspects results in the secondary phase precipitating from dislocations and other defects, furthermore, the atom diffusion rate is slow and the secondary phase cannot continue to grow under low temperature conditions, thus the secondary phase has a finely dispersed state distribution.

Figure 2 Schematic diagrams:

Figure 3 Microstructure of AZ31 magnesium alloy under different conditions:

3.2 Changes in mechanical properties

Due to the changes in the microstructure of material as mentioned above, the changes in the mechanical properties of material after ACT are focused on. The comparison results of the specimens under different treatments are shown in Figure 4. It can be seen from Figure 4(a) that the stress values are almost consistent with each other, but the strain values from AT to ACT4 increase significantly with increasing cryogenic time. Meanwhile, there are also some differences among the UTS as shown in Figure 4(b). With the increase of cryogenic time, the UTS increases and its change conforms to fitting equation as follow: σ_b=0.305 h+ 225. Where σ_b is UTS, h is treatment time.

Figure 4 Parameters of mechanical properties after ACT:

Likewise, it can be seen from Figure 4(c) that the YS is biggest after annealing treatment (AT), then with the increase of cryogenic time, the YS decreases gradually, and the distribution of YS is characterized by the formula σ_0.2=161.9+5.6×0.71^h. Where σ_0.2 is YS, h is different treatment time. Furthermore, the ductility with different cryogenic time is compared as shown in Figure 4(d), and it can be seen that with the increase of the ACT time, the ductility increases slowly. But the difference is small after ACT and expressed by the formula δ=0.26-0.03×0.83^h. The relative hardness is tested as shown in Figure 4(e), and it is indicated that with ACT stepped in, the hardness increases, and with the increase of ACT time, the increasing trend of the hardness is slowed, and becomes horizontal gradually.

From Figure 4, it is indicated that ACT can improve the UTS and ductility of AZ31. The reasons are that there are some secondary phases generated in the grains during ACT, which has a significant obstacle role for the slip of dislocation and then results in the increase of UTS. Secondly, the orientation of some grains is changed due to the generation of twins, which can activate slip system [24] and stimulate the further sliding of the crystal, thus result in the plastic deformation ability improved. The interaction of two aspects during ACT leads to the changes of the properties of AZ31 magnesium alloy. In the meantime, the hardness increases gradually with ACT stepped in, the main reason is that Mg17Al12 generates, increases, and distributes uniformly [29].

3.3 Change of punch shearing properties

3.3.1 Change of shearing strength

Figure 5 shows the shearing strength-punch displacement curves and the ultimate shearing strength under different conditions. It can be seen from Figure 5(a) that from touch to separation between the punch and sheet, the punch’s displacement approximately equals to 0.47 mm after AT which is smaller than those after ACT. The main reason is that the ductility improved after ACT makes the displacement of punch enlarged. Meanwhile, the ultimate shearing strength is shown in Figure 5(b). It can be seen that the ultimate shearing strength after AT gets to 83.9 MPa, and after ACT1, the ultimate shearing strength begins to decrease and arrives at 77.8 MPa. Then with the increase of cryogenic time, the ultimate shearing strength arrives at 79.2, 79.8 and 78.4 MPa after ACT2, ACT3 and ACT4, respectively. It can be known that the ultimate shearing strength after ACT is always lower than those after AT, but the difference of the ultimate shearing strength among the specimens with different cryogenic time is small, from 0.6 to 2 MPa, which is considered negligible. So it can be concluded that with ACT stepped in, the shearing strength is decreased and the ductility is improved.

Figure 5 Shearing strength-punch displacement curve (a) and distribution of ultimate shearing strengths (b) of AZ31 under different cryogenic conditions

3.3.2 Change of rollover

The partial rollover and the measuring method of rollover radius are shown in Figures 6(a) and (b), and the rollover radii under different cryogenic conditions are shown in Figure 7. It can be seen from Figure 7 that the average value of rollover radii of the specimens after AT is about 294 μm,and after ACT1, the average value of the rollover radii is about 305 μm. Then with the increase of cryogenic time, the average value of the rollover radii reaches about 311, 323 and 301 μm after ACT2, ACT3 and ACT4, respectively. It is indicated that the rollover radius increases slightly after ACT, but the increasing trend of rollover radius is unremarkable.

Figure 6 Schematic diagrams of cross-section and rollover of corrective specimens:

Figure 7 Average rollover radius

3.3.3 Change of smooth zone

The height of smooth zone is used to evaluate the quality of fracture sections. The fracture sections of AZ31 sheet under different ACT are shown in Figure 8. It can be observed that ACT can change the height of smooth zone significantly. After AT, the height of smooth zone is about 389 μm as shown in Figure 8(a), and then after ACT1, the height of smooth zone gets to about 421 μm as shown in Figure 8(b). According to ACT2, ACT3 and ACT4, the heights of smooth zone are about 579, 603 and 490 μm, respectively, as shown from Figures 8(c)-(e). It is concluded that the height of smooth zone is smallest after AT, and then with the increase of cryogenic time, the smooth zone increases. But when the cryogenic time exceeds 18 h, the increasing trend of the smooth zone decreases.

3.3.4 Change of burrs

The burrs of specimens under different ACT are shown in Figures 9. It can be seen that no matter AT or ACT, the burrs always exist and the height of burrs is different under different ACT. Through the comparison of results as shown from Figures 9(a) to (e), the burrs formed after AT are higher than those after ACT.

In order to further study the effect of cryogenic time on the height of the burrs, the burrs around the edge of hole measured based on different directions from 0°to 315°are shown in Figure 10(a), and the results are shown in Figures 10(b) and (c). It can be seen from Figure 10(b) that the heights of burrs are reduced after ACT, and the heights of the burrs have some obvious differences in different directions, such as the heights of burrs in the direction of 45°, 135°, 225° and 315° are slightly higher than others. Meanwhile, the average height of burrs is shown in Figure 10(c), and it can be seen from Figure 10(c) that the average height of burrs after ACT shows a decreasing trend compared with the specimens after AT. The average height of the burrs after AT is about 78 μm, and after ACT1, the height of burrs is reduced by about 20 μm and 26% is removed. Then with the increase of ACT time, the drop of the height of the burrs continuously arises, but the drop decreases gradually. When ACT time exceeds 18 h, it can be seen from Figure 10(c) that with the increase of cryogenic time, the changes of the heights of burrs tend to a line. So it can be concluded that the changes of the burr's height subject to the formula as shown in Figure 10(c), and the effect of ACT on the burr height is gradually reduced with the increase of cryogenic time.

Figure 8 Fracture section of AZ31 under different cryogenic conditions:

Figure 9 Macro-section and local micro- burrs of AZ31 magnesium alloy under different cryogenic conditions:

4 Conclusions

The effects of ACT on the microstructure, mechanical properties and shear properties of AZ31 magnesium alloy are investigated based on the experiments, and the conclusions are drawn asfollows.

1) The number of secondary phase generates and increases at different ACT time. Meanwhile, it is observed that there are many twins in the grains after ACT.

2) The mechanical properties of the material after ACT are changed, the UTS and ductility of AZ31 are improved, and the YS decreases gradually, as well as the hardness increases gradually with ACT stepped in.

Figure 10

3) ACT has a positive effect on improving the formability of punch shearing. The shearing strength decreases after ACT, the changes of rollover radii are unremarkable, the smooth zone of the cross-section is improved significantly, and the heights of burrs decrease after ACT.

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(Edited by FANG Jing-hua)

中文导读

深冷处理对AZ31板冲剪的影响

摘要：冲剪成形作为一种材料加工方法，经常用于加工一些成品零件。深冷处理在改善钢的局部力学性能上有显著效果，但有关深冷处理对镁合金冲剪性能影响的研究较少。本文通过金相组织分析、拉伸、硬度测试和冲剪实验等，分析了深冷处理对AZ31板冲剪性能的影响。结果表明：深冷处理后组织中第二相数量增加，晶粒中出现大量孪晶。同时，AZ31镁合金的抗拉强度，延展性和硬度均得到改善，而屈服强度在深冷处理后有所降低。此外，经深冷处理后的AZ31镁合金板在冲剪过程中，剪切强度降低，圆角半径变化不明显，横截面边缘的毛刺高度降低，光亮带所占比例明显增加。

关键词：AZ31镁合金；深冷处理；机械性能；冲剪

Foundation item: Projects(51275201, 51311130129) supported by the National Natural Science of China; Project(20140204062GX) supported by the Jilin Key Scientific and Technological Project, China

Received date: 2018-05-30; Accepted date: 2018-10-30

Corresponding author: HU Zhi-qing, PhD, Professor; Tel: +86-18604421257; E-mail: zqhu@jlu.edu.cn; ORCID: 0000-0002-9692-0826