­­­ Analysis on flow stress of magnesium alloys during high temperature deformation

YANG Ya-qin(杨亚琴), LI Bao-cheng(李保成), ZHANG Zhi-min(张治民)

College of Materials Science and Engineering, North University of China, Taiyuan 030051, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The flow stress of magnesium alloys during hot compression at different temperatures and strain rates was studied by experiments. Materials used were AZ91D alloys in as-cast, homogeneous treatment states, AZ31 and ZK60 alloys in as-cast state. The results show that the thermal simulation curves of different alloys differ from one another at the same deforming condition. The general curves of AZ31 and AZ91D alloys have the character of dynamic recrystallization. There are increase of true stress, drastic falling of true stress and increase of true stress in most curves of ZK60 alloy, while the other curves have the characteristics of dynamic recrystallization. From the analysis the reasonable deforming temperature should be selected from 523 to 673 K for AZ31 and the unhomogenized AZ91D alloy, from 473 to 673 K for the homogenized AZ91D alloy, and it was concluded to be 473 K or 673 K for ZK60 alloy.

Key words: AZ31; AZ91D; ZK60; wrought magnesium alloy; stress; strain

1 Introduction

Magnesium and its alloys are the low density materials used as structural components and have excellent specific strength and stiffness, machinability, dimensional stability, especially their high recycling capability. They are, therefore, very attractive in such applications as automobile, aviation, electronic and communication industry[1]. Since non-basal slip systems can be activated at high temperatures, hot deformation processes have been frequently proposed for magnesium alloys. During the hot deformation, some metallurgical phenomena such as work hardening, dynamic recovery and dynamic recrystallization may occur simultaneously, resulting in grain refinement and reduction of deformation resistance[2]. Studies[3-6] on AZ31 alloy showed that the deformation which occurred by twinning in the low temperature regime (453-513 K), was gradually replaced by dynamic recrystallization (DRX) above 573 K. New DRX grains formed primarily near grain boundaries and twin boundaries. Reduction in flow stress and increase in ductility were observed due to increased DRX above 633 K. ION et al[7] investigated the recrystallization behavior of Mg-0.8%Al for temperatures between 423 K and 623 K. When temperature is lower than 603 K, they concluded that strain is preferentially localized in the vicinity of initial grain boundaries and it was suggested that new grains form from such severely rotated regions.

In this work, the compression mechanisms of the most commonly used types of the Mg-Al-Zn and Mg-Zn-Zr alloys were investigated in detail at temperature between 423 K and 773 K and at strain rates from 10^-3 s^-1 to 5 s^-1. By investigating the properties of the curves and the deformation parameters, the optimized compression parameters were stated for providing the theoretical support to make the deformation of the magnesium alloys.

2 Experimental

Alloys used in this study are commercial AZ31, AZ91D and ZK60 die cast alloys and their compositions are given in Table 1. AZ91D alloy was homogenized before deformation. Homogenization temperature conditions were set as follows. Billets were heated to 628 K and maintained at this temperature for 16 h and then cooled in furnace.

Table 1 Composition of die cast alloys (mass fraction, %)

To find out the flow stress of the different magnesium alloys at various deformation conditions, high temperature compression test was performed in the temperature range of 423-773 K with 323 K interval and strain rates of 10^-3-5 s^-1 by using a Gleeble-1500D testing machine. Cylindrical specimens with a diameter of 10 mm and a height of 15 mm were heated to the desired temperature in the vacuum environment, followed by 5 min holding to ensure thermal equilibrium and compressed to 0.92, and then quenched in water. The deformation temperature was measured by thermocouples welded onto the center of a specimen surface. The deformation strain, temperature and strain rate were automatically controlled and recorded.

3 Results and discussion

3.1 Analysis of flow curves of AZ91D

Typical curves of AZ91D alloy in as-cast, homogeneous treatment states obtained from the hot compression deformation are shown in Fig.1. Fig.1(a) and Fig.1(b) display the effect of the temperature for a given strain rate (0.01 s^-1), whereas the effect of the temperature for a given strain rate (1 s^-1) is shown in Fig.1(c) and Fig.1(d). Similar trends are observed in the general flow stress curves under all deformation conditions. Strain hardening occurs in the first step of deformation, leading to a peak stress followed by important softening. With decreasing strain rate or increasing temperature, the strain hardening effect becomes weakened, while the degree of strain softening becomes notable. As a result, the peak stress varies according to the processing parameters, so does the peak strain. Under a constant strain rate, the peak stress and the peak strain increase with decreasing temperature. Under the same temperature, the peak stress and the peak strain increase with increasing stain rate.

Fig.1 Flow stress—strain curves of AZ91D alloy in different states at various temperatures: (a) 0.01 s^-1, as-cast;  (b) 0.01 s^-1, homogeneous treatment;  (c) 1 s^-1, as-cast;  (d) 1 s^-1, homogeneous treatment

For the low strain rate (0.01 s^-1) conditions, the flow stress of the alloy after homogenization is lower than that of the as-cast at the low temperature (473 K) and the difference of the stress decreases with raising temperature. The slope of the strain hardening curve of the alloy after homogenization is smaller than that of the as-cast state. For the high strain rate (1 s^-1) conditions, the curves are smoother and the flow stress of the alloy as-cast is lower than that of the homogenized alloy at the high temperature (573-623 K). In a word, under the low strain rate conditions, the homogenization treatment may improve the formability of AZ91D alloy at the lower temperature, whereas only little improvement is observed for the samples tested at the higher temperature. For the high strain rate, the formability of AZ91D alloy homogenized is worse a little than that of the as-cast sample under high temperature conditions.

It can be concluded that the plastic formability of AZ91D annealed is better than that of the unhomogenized alloy under the low temperature (473 K) with the low strain rates (0.001-0.01 s^-1), but is similar to each other under the other deformed conditions. For the low temperature conditions(473 K) there are tree-like crystal segregation and much reticular γ-Mg₁₇Al₁₂ in the microstructure of the as-cast AZ91D, both of which are of disadvantage to plastic forming, while after homogenization the segregation of the tree-like crystal eliminates basically and the amount of γ phases decreases, the appearance and the distribution of which change to be beneficial to plastic forming[8-9]. For the high temperature conditions (573-673 K), the disadvantage of the tree-like crystal segregation and the block of the γ-Mg₁₇Al₁₂ of the as-cast AZ91D microstructure decrease and the grains become coarser. This has little improvement to the formability after homogenization [10-11]. From the analysis the reasonable deforming temperature should be selected from 523 K to 673 K for the unhomogenized AZ91D alloy, from 473 K to 673 K for the homogenized AZ91D alloy.

3.2 Comparison of flow curves of AZ91D and AZ31

Fig.2 shows the stress—strain curves of the AZ31 alloy under different deformation temperatures at 0.1 s^-1. As the thermal simulation curves of AZ31 show a flow softening whose mechanism is dynamic recrystallization except some curves show a fluctuation mode especially under 473 K, it can generally be concluded that dynamic recrystallization is responsible for the high temperature deformation mechanism of AZ31 alloy[12]. Therefore, the deformation temperatures of AZ31 should be selected from 523 K to 673 K.

The general characteristics of the flow stress curves of AZ31 and AZ91D are similar under all deformation conditions. The flow stress increases to a peak (initial

Fig.2 Flow stress—strain curves of AZ31 alloy in compression at 0.1 s^-1

strain hardening) and then decreases to a steady state. Such flow stress behaviors are typical characteristics of hot working that is accompanied by dynamic recrystallization[13-14]. Though AZ31 and AZ91D both have the characteristic of dynamic recrystallization, there are still some distinction between them as follows. The thermal simulation curves of AZ31 and AZ91D have different forms. There are obvious indent wave of the curves of the AZ31 alloy while the curves of the AZ91D alloy appear smoothly. The peak stress of AZ31 and AZ91D alloys are different. The peak stress of AZ91D is higher than that of AZ31 alloy under the same deformation conditions. The difference is the highest at 200 ℃ while the smallest at 400 ℃. There is no peak stress observed for AZ31 alloy at the higher temperature (623 K and 673 K). The slope of the strain hardening curve and the true stress of AZ91D alloy are larger than that of AZ31 alloy. The flow stress curves of AZ91D change severely except at 523 K, which demonstrates that the formability of AZ91D alloy is worse than AZ31 alloy at elevated temperatures, mainly because the content of Al in AZ91D is higher than 8%, which leads the γ-Mg₁₇Al₁₂ to appear in discontinuous reticular shape along the boundaries.

3.3 Comparison of flow curves of AZ91D and ZK60

Fig.3(a) shows that under a constant strain rate, the peak stress increases with decreasing temperature. Fig.3(b) shows that under the same temperature, the peak stress increases with increasing strain rate. Some curves have the characteristic of dynamic recrystallization. For example, at 1 s^-1 and 473 K the material can be deformed successively, while most curves of ZK60 have obvious characteristic that around 0.2 in strain the stress reaches the peak and declines rapidly afterwards and lands the

Fig.3 Flow stress—strain curves of ZK60 alloy in compression at various deformation conditions: (a) 0.01 s^-1; (b) 473 K

lowest. The declining of the curve illustrates that the test specimen has been destroyed and crackle can be found in the test specimens correspondingly.

The curves have complex properties at the various deformation temperatures as the secondary phase and the base material have various influences under the various conditions. The secondary phase is little so there is a small amount of grain boundary in ZK60 under the lower temperature and there are more secondary phases separated out with increasing deformation temperature, so the plasticity is lowered with adding grain boundary. Though the grains separated out grow obviously and they have bad effect on the plasticity, ZK60 alloy still has better plasticity at high temperature since non-basal slip systems of the alloy can be activated at high temperature (higher than the recrystallization temperature). It can be concluded from Fig.3(b) that not only the flow stress rises up, but also the declining of the peak stress becomes more notable with increasing strain rate to ZK60 magnesium alloy. This is mainly because there are more grain boundaries in ZK60, and the linear defects gather more easily around the boundaries with increasing strain rate, which produces the critical stress concentration. So the material has ruptured as there is not enough time to cause the dynamic recrystallization with the higher strain rate. It can be concluded that there are two suitable deformation temperature ranges, one is the low temperature(473 K) and the other is the high temperature(673 K)[15].

The results from these tests demonstrate that thermal simulation curves of AZ91D and ZK60 have different forms and the plastic formability at high temperatures of AZ91D alloy is better than that of ZK60 alloy, mainly because there are different effects of the second-phase on dynamic recrystallization. There are more grain boundaries in ZK60 magnesium alloy because of the generous eutectic element in ZK60 as-cast microstructure, which mainly includes rough tree-like crystal and more unfixed appearance of the secondary Mg-Zn phases including lamellar, pearl, etc, which are of disadvantage to the plastic forming[16-17]. In AZ91D there is little effect of the Mg₁₇Al₁₂ phase on dynamic recrystallization as the Mg₁₇Al₁₂ phase dissolves into the matrix material in the forming process at high temperature.

4 Conclusions

1) It can be concluded that the plastic formability of AZ91D annealed is better than that of the unhomogenized alloy at the low temperature and the low strain rates, but is similar to each other under the other deformed conditions.

2) AZ31 and AZ91D both have the characteristic of dynamic recrystallization and there are still some distinctions between their flow stress curves. The formability of AZ91D alloy is worse than that of AZ31 alloy at elevated temperatures.

3) The plastic formability of AZ91D alloy at high temperatures is better than that of ZK60 alloy.

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(Edited by YANG Hua)

Corresponding author: YANG Ya-qin; Tel: +86-351-3923956; E-mail: yangyaqin@nuc.edu.cn