Microstructure and mechanical properties of AZ31 Mg alloy processed by high ratio extrusion

CHEN Yong-jun(陈勇军), WANG Qu-dong(王渠东), LIN Jin-bao(林金宝),

ZHANG Lu-jun(张陆军), ZHAI Chun-quan(翟春泉)

National Engineering Research Center for Light Alloy Net Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The microstructure and mechanical properties of AZ31 Mg alloy processed by high ratio extrusion (HRE) were investigated. General extrusion with extrusion ratio of 7 and high ratio extrusion with extrusion ratio 100 were contrastively conducted at 250, 300 and 350 ℃. The results show that HRE process may be applied successfully to AZ31 Mg alloy at temperatures of 250, 300 and 350 ℃ and this leads to obvious grain refinement during HRE process. The strength of HRE process is improved obviously compared with that of general extrusion. The grain refining mechanism of HRE process was also discussed. The current results imply that the simple high ratio extrusion method might be a feasible and effective processing means for refining the microstructure and improving the mechanical properties of AZ31 Mg alloy.

Key words: AZ31 Mg alloy; high ratio extrusion; grain refinement; mechanical properties

1 Introduction

Magnesium alloys are being increasingly used in the electronics, automobile and aerospace industries due to their low density, high specific strength, excellent machinability and good electromagnetic shielding characteristics[1-2]. Nevertheless, as a consequence of their hcp structure, they generally present the limited ductility and strength at room temperature. It is well known that the mechanical properties of polycrystal at room temperature can be improved through grain refinement, as illustrated by the Hall-Petch relation- ship[3]. Severe plastic deformation(SPD) has the ability of intensive grain refinement and can directly make the microstructure of materials transform to the range of submicrotructure or nanostructure, which is thought to be a promising method to fabricate bulk nanometals and ultra-fined materials[4]. Recently, it has been reported that the room temperature mechanical properties of magnesium and magnesium alloy can be improved by severe plastic deformation, such as equal channel angular pressing (ECAP)[5-6] and accumulative roll bonding (ARB)[7]. However, there has been a little attempt to improve the room temperature mechanical properties of Mg-based alloys through the use of simple high ratio extrusion (HRE) although the extrusion technology is very important for processing magnesium alloys because of its technical and economic advantage in the production of structure components[8]. In the present study the grain refinement was conducted and the subsequent mechanical properties of AZ31 Mg alloy at room temperature were investigated.

2 Experimental

The HRE process was as follows: firstly, the d92 mm AZ31 (Mg-3.091%-1.023%Zn-0.421%Mn) ingots were held for 30 min at 400 ℃, then the ingots were compressed to d118 mm at 350-400 ℃ twice. Secondly, the compressed AZ31 alloys were annealed at 400 ℃ for 1 h. Followed the annealing treatments, they were extruded into bars from d118 mm to d11.8 mm using a high extrusion ratio 100:1 at 250, 300 and 350 ℃ operated at extrusion speed of 20 mm/min. For comparison, the general extrusion with extrusion ratio of 7 was carried out under the same condition. For each separate extrusion, the material and die were coated with a lubricant of graphite powder. The extruded tensile samples were machined from as-extruded materials. The microstructure observation of the as-extruded specimen was done by optical microscopy and the mean grain size was measured by the linear intercept method. The hardness was measured by Chinese 69-1 hardness tester, the load applied was 300 N and the test period was 30 s. The fracture surfaces were investigated by scanning electron microscopy (SEM).

3 Results and discussion

3.1 Microstructure

Fig.1 shows the optical microstructures of cross sections of AZ31 Mg alloy after extrusion ratio of 100 at different temperatures. Whatever the temperature, the recrystallization leads to the development of fine equiaxed and homogeneous grains. The mean grain sizes after high ratio extrusion at 250, 300 and 350 ℃ are 4, 5 and 8 mm, respectively. The measurement shows, therefore, that HRE is effective on reducing the grain size of AZ31 Mg alloy. The systematic observation in the front, middle and back portions of the HRE extruded bars reveals that almost all grains are reasonably equiaxed and homogeneously distributed along the bar as well as in the cross section at extrusion temperatures of 250, 300 and 350 ℃. And in this work, high ratio extrusion at temperature of 300 ℃ shows the best uniform microstructure. Meanwhile, with increasing the extrusion temperature, the grain size trends to increase, which may be reasoned that it is easy for grain growth at higher temperature. The development of fine grain may be explained as follows: 1) Large true strain (about 4.6) is obtained by HRE with extrusion ratio of 100, which is more than the total true strain of 4 passes ECAE. It is expected that a large amount of dislocations and severe grain boundary distortion are involved in the HRE processing, which cause a large driving force for dynamic recrystallization[9], thus high recrystallization speed is obtained. 2) During extrusion process, intersection, distortion and collision between the adjacent grains are unavoidable, thus the fine grain may be divided into more fine grain by large shear in which intense strain occurs.

3.2 Mechanical properties

Figs.2 and 3 show the influence of extrusion temperature on mechanical properties of AZ31 Mg alloy at extrusion ratio of 100 and 7, respectively. It is evident from these curves that there is a significant increase in strength of AZ31 Mg alloy after high ratio extrusion. For example, the ultimate strength increases from 288.03, 254.75 and 258.65 MPa at general extrusion to 308.08, 311.74 and 300.26 MPa at high ratio extrusion and is improved by 7%, 22% and 16% after extrusion temper- atures of 250, 300 and 350 ℃, respectively. Whatever  the temperature, the elongation of high ratio extrusion is improved to above 18.5%. As can be seen in Fig.2, It is interesting to note that the ultimate strength and yield strength of HRE extruded AZ31 Mg alloys have the same tendency to change with increasing extrusion temperature, namely, they increase firstly and then decrease a little, which corresponds with the grain size shown in Fig.1 that the mean grain sizes increase with increasing extrusion temperature. Because high ratio extrusion at 300 ℃ shows the best uniform micro- structure, the ultimate strength and yield strength of HRE extruded at 300 ℃ reach the maximum values in this experiment.

Fig.1 Optical microstructures of AZ31 with extrusion ratio of 100 at different temperatures: (a) 250 ℃; (b) 300 ℃, and (c) 350 ℃

Fig.2 Influence of extrusion temperature on mechanical properties of AZ31 Mg alloy with extrusion ratio of 100

Fig.3 Influence of extrusion temperature on mechanical    properties of AZ31 Mg alloy with extrusion ratio of 7

The plot of hardness vs extrusion temperature is shown in Fig.4 for AZ31 Mg alloy with extrusion ratio of 100 and 7, respectively.

Fig.4 demonstrates that the hardness of HRE extruded Mg alloy is a significant improvement compared with ingot with hardness of 46.5 and is a little higher than that of the general extrusion with extrusion ratio of 7. For high ratio extrusion, the hardness of AZ31 Mg alloy extruded at 250 and 300 ℃ has similar hardness of 66, and then the hardness decreases a little with increasing extrusion temperature, which corresponds with the curves of strength in Fig.4. Compared with the general extrusion, there is obvious improvement in strength and hardness. This is because the strength is increased through grain refinement and the change of hardness corresponds with the strength. The improved ductility is obtained because more grains contribute to the macroscopic deformation and the stress concentrations are accordingly reduced and spread over a wider area[5].

Fig.4 Influence of extrusion temperature on hardness of AZ31 Mg alloy with extrusion ratio 100

3.3 tensile fractograghs

Fig.5 shows SEM images of the tensile fractograghs of HRE extruded AZ31 Mg alloy after extrusion ratio of 100 at extrusion temperatures of 250, 300 and 350 ℃. As can be seen in Fig.5, the fracture surfaces are mainly made up the tearing edges and micro-cavities. The micro-cavities distribute among the tearing edges. Therefore, HRE extruded AZ31 Mg alloy exhibits good plastic deformation before fracture. The comparison of Figs.5(a)-(c) reveals that more tearing edges can be seen in Figs.5(a) and (c), the micro-cavities in Fig.5(a) distribute evenly among the tearing edges with mean size of 5 mm. While the micro-cavities in Fig.5(b) distribute unevenly with the transverse size ranged from 4 to 15 mm, which corresponds to the change of elongation shown in Fig.2.

Fig.5 Tensile fractograghs(SEM) of AZ31 Mg alloy with extrusion ratio of 100 at different temperatures: (a) 250 ℃; (b) 300 ℃; (c) 350 ℃

4 Conclusions

High ratio extrusion with extrusion ratio of 100 was successfully applied to AZ31 Mg alloy at extrusion temperatures of 250, 300, and 350 ℃. This leads to both grain refinement due to grain break and the occurrence of recrystallization during high ratio extrusion process and obvious improvement in strength and hardness compared with those of general extrusion. The current results imply that the simple high ratio extrusion method might be a feasible and effective processing means for improving the mechanical properties of hcp metal at room temperature.

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(Edited by CHEN Wei-ping)

Foundation item: Project (50674067) supported by the National Natural Science Foundation of China

Corresponding author: WANG Qu-dong; Tel: +86-21-62933139; E-mail: wangqudong@sjtu.edu.cn