Microstructure evolution and effect on mechanical property in AZ80 Mg alloy during thermal processing

WANG Qiang(王 强), ZHANG Zhi-min(张治民), LI Bao-cheng(李保成), LI Xu-bin(李旭斌)

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

Received 20 April 2006; accepted 30 June 2006

Abstract: The microstructure and mechanical properties of AZ80 alloy were investigated during thermal processing. The samples of 4 mm in thickness machined from cast ingot were compressed at 300 ℃ with a thickness reduction of 75% and cooled in the water to room temperature. Then ageing(T5) and solution+ageing (T6) treatments were employed respectively. The results show that mechanical properties are significantly improved after thermal processing than those of as-cast AZ80 alloy due to grain refinement and discontinuous precipitates. The heat treatment has significant influence on microstructural evolution for sample formed at moderate temperature. Microstructural evaluation indicates that the β-phase increases because of sufficient solution and the alloy is strengthened evidently.

Key words: AZ80 Mg alloy; β-phase; mechanical property; thermal processing

1 Introduction

Mg alloys are promising structural light metals because their low densities, good recyclical potential and abundant resources. However, the lower mechanical properties of common cast magnesium components restrict the use of Mg alloy to light duty[1]. Hence, further improvement in mechanical properties will be necessary in order to expand the use of Mg alloy. It is well accepted that grain refinement may realize high strength and high ductility of Mg alloy [2, 3].

Several methods based on severe plastic deforma- tion (SPD) for grain refinement in Mg alloy have been proposed over the last five years in order to obtain microstructure with improved mechanical behavior [4, 5]. The processing methods such as equal channel extrusion(ECAE), accumulative roll bonding(ARB) or high pressure torsion(HPT) have been extensively studied [6-8]. Grain refinement of AZ31 alloy was achieved previously through warm compression forming, resulting in improved mechanical properties[9]. However, the microstructure evolution of Mg alloy during thermal processing is still largely unknown. In particular, the effect of parameters such as deformation temperature, deformation extent, heat treatment, initial grain and others on the microstructure development needs to be investigated.

In this study, the microstructure and mechanical properties of AZ80 alloy prepared by different methods were investigated. The aim is to explore the possibility of applying warm forming for grain refinement of AZ80 alloy, as well as to investigate the effect of heat treatment on properties of AZ80 alloy.

2 Experimental

The alloy used in the present investigation was a commercial Mg-Al-Zn alloy, AZ80. The chemical composition of the alloy is shown in Table 1. The material was produced by electromagnetic casting method. The billets had a diameter of 290 mm and a length of 150 mm, which has been machined in order to remove the effect of the surface layer of the ingot (hereafter, denoted as-cast material) and homogenized before deformation. Homogenization temperature conditions were set as follows: billets were heated to 355 ℃ and maintained at this temperature for 16 h and then

Table 1 Chemical composition of AZ80 billet (mass fraction, %)

cooled in furnace.

The compression samples were machined from cast billet of d 290 mm×150 mm along axis direction. The length and width of sample were 150 mm and 15 mm, respectively. The thickness of the sample was 4 mm. The trials were performed on the 3 150 kN vertical extrusion press with an initial compressing speed of 8mm/s. The test set is represented in Fig.1. Test set and sample were heated to appointed temperature together in an electric furnace and soaked 20 min. The samples were compressed into 1 mm in thickness at 300 ℃ and then cooled in the water to room temperature. Ageing treatment ((177±5)℃, 16 h, T5) and solution+ageing ((420±5)℃, 10 h+(177±5)℃, 16 h, T6) were carried out respectively.

Fig.1 Schematic test set for compression

All samples were machined into standard flat tensile specimens perpendicular to the compression direction with a gauge section of 35 mm×10 mm×1 mm. Tension tests were performed at ambient temperature on WAW-Y500A according to the specifications GB/T228-2002.The mechanical properties of as-cast and homogenized AZ80 alloy were tested too in order to estimate the mechanical properties. The microstructure of the billet and compressed sample on the section cut parallel to the compression direction were observed using image MAT A1 optical microscopy (OM). Sample preparation for OM consisted on grinding on SiC paper with increasing finer grits, mechanical polishing and final polishing using colloidal silica. The etching solution was 2% acetic acid-2% nitric acid-15% distilled water mixture for OM observation.

3 Results and discussion

3.1 Mechanical properties of AZ80 alloy

Table 2 lists the mechanical properties of AZ80 alloy prepared by different processing methods, and properties of as-cast AZ80 materials are also given for comparison. It can be found the ultimate tensile strength decreases and elongation increases appreciably after homogenization for AZ80 alloy. By thermal processing, the mechanical properties are significantly improved than those of as-cast materials. The most important reason for the mechanical properties improvement is grain refinement caused by compression. The heat treatment has significant influence on mechanical properties of AZ80 alloy. The ultimate tensile strength made from T5 heat-treatment is a little lower than that of as-compressed materials. But after T6 heat-treatment, the ultimate tensile strength is much higher under the same compression condition, and increases to 330 MPa. However, elongation of AZ80 alloy prepared by thermal processing doesn’t improve than that of as-cast materials. Elongation is a little lower instead after T5 treatment. This is possibly related to tension direction perpendicular to the metal flow direction during compression.

The analysis reveals that there is an excellent strengthening effect for AZ80 alloy with hcp structure. The maximum tensile strength values around 330 MPa are achieved after thermal processing. The increment of the tensile strength is above 1 time compared to 140-155 MPa of as-cast AZ80 Mg alloy. The improved mechanical properties of common wrought magnesium components can expand the use of Mg alloy to light duty.

Table 2 Mechanical properties of AZ80 alloy prepared by different methods

3.2 Microstructure of AZ80 alloy

Fig.2(a) shows the microstructure of as-cast AZ80 alloy. It mainly consists of α(Mg) and β(Mg₁₇Al₁₂) phases, with a few AlMgMnFe intermetallic compounds dispersing on α(Mg). The average grain size of α(Mg) is about 100 μm and similar to the grain size of the reference alloy. After homogenization, a majority of Mg₁₇Al₁₂ particle congregated at grain boundaries disperses. Dendritic segregation is eliminated basically. Although grains become a little coarser, the microstr- ucture is of advantage to plastic forming, as can be seen in Fig.2 (b).

Fig.2 Microstructures of AZ80 alloy cast (a) and after homog- enization (b)

Fig.3 shows the microstructures of AZ80 alloy prepared by different methods. It can be seen that after compressing at 300 ℃, both α(Mg) and Mg₁₇Al₁₂ have undergone a grain refinement greatly, as shown in Fig.3 (a), although still some large initial grains remain present. The grain size of α(Mg) is 15-30 μm, which is much smaller than that of as-cast materials. The severe plastic deformation (SPD) provided by compression can also break the coarse intermetallic phases into fragments, thus Mg₁₇Al₁₂ phases are refined too. The alloy is streng- thened by grain refinement evidently.

After artificial ageing, much more Mg₁₇Al₁₂ phases are not found, as shown in Fig.3 (b). It shows that precipitation strengthening effect weakens because of insufficient solution. The tensile strength doesn’t change and has decreasing trend. However, significant microstructural changes occur: much more Mg₁₇Al₁₂ phases are seen after solution and ageing treatment. It is clearly apparent in Fig.3(c) that supersaturation is obtain- ed because of sufficient solution decomposed and precipitated during subsequent artificial ageing. The alloy is strengthened by precipitation strengthening except for grain refinement.

Fig. 3 Microstructures of AZ80 alloy F (a), T5 (b) and T6 (c)

3.3 Discussion

The present results suggest that compressing at moderate temperature can be also utilized as an effective processing method to refine grain size in an AZ80 alloy. MUKAI et al [10] suggested that significant grain refinement can also be achieved using rather traditional methods, such as extrusion of the as-cast material, when the Zener-Hollomon parameter (Z=exp(Q/RT)) is high, i.e., using high strain rates or low temperatures. Larger reductions (higher strain rates) and low temperatures result in higher values of Zener-Hollomon parameter during warm compression forming. Warm forming not only can produce the largest amount of deformation through the smallest compression passes without causing material failure, also an excellent strengthening effect that the increments of the tensile strength about 40%-50% is achieved for as-cast AZ31 alloy [9].

For AZ80 alloy which contains much alloy content, precipitation and solution strengthening are strengthen-

ing mechanism except for fine-grain strengthening. This is different from AZ31 magnesium alloy too. The effect of aging temperature on precipitates in AZ80 magnesium alloy was studied [11,12]. At lower aged temperature, the discontinuous precipitation occurred is lamellar structure parallel or perpendicular to the basal plane of matrix, nucleated at grain boundaries and grew into grains. The grain boundaries moved into the grains with the growth of these discontinuous precipitates. The alloy was strengthened by the discontinuous precipitates evidently.

The analysis reveals that magnesium components fabricated by warm forming exhibit higher ductility and strength than the die-cast components. The wrought magnesium alloys are considered as potential candidates to substitute aluminium parts in future auto-mobiles.

The difference in grain size and discontinuous precipitates might explain among other factors the difference in mechanical properties. A more detailed microstructure study would be necessary to better understand the influence of processing conditions on strengthening effect. Moreover, investigation on effect of the deformation temperature and heat treatment temperature on the microstructure and mechanical properties would be necessary to optimize processing parameters.

4 Conclusions

1) The tensile strength about 285 MPa can be attained under a thickness reduction of 75% at 300 ℃. The results suggest that warm forming not only can refine grain, also produce the largest amount of deforma- tion through the smallest compression passes.

2) The heat treatment has significant influence on microstructure evolution for sample formed at moderate temperature. The tensile strength doesn’t improve by T5 heat-treatment. But after T6 heat treatment, the tensile strength increases to 330 MPa.

3) The increment of the tensile strength is above 1 times compared to 140-155 Mpa of as-cast AZ80 Mg

alloy. Fine-grain strengthening and precipitation streng- thening play an important role on realizing high strength of AZ80 Mg alloy. The improved mechanical properties can expand the use of Mg alloy to light duty.

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(Edited by LI Xiang-qun)

Foundation item: Project (50405026) supported by the National Natural Science Foundation of China; Project(20051022) supported by the Department of

Science and Technology of Shanxi Province, China

Corresponding author: WANG Qiang; Tel: +86-351-3921398; Fax: +86-351-3921778; E-mail: ncustwangq@nuc.edu.cn