Equal channel angular pressing of magnesium alloy AZ31
Introduction
As the lightest structural material of engineering significance Mg alloys have attracted considerable recent attention [1], [2]. However, Mg alloys exhibit poor formability and possess only moderate strength compared to Al alloys. One of the promising methods to increase ductility and strength in Mg is through microstructural refinement. ECAP has been shown to be effective in refining grains in various Mg alloys with improved ductility, strength and superplasticity [3], [4], [5], [6], [7], [8], [9], [10], [11]. In most investigations, however, the processing temperature was at or above 200 °C to avoid cracking [8], [10]. Processing at such temperatures resulted in not only complete recrystallisation [4] but also grain growth [3]. In this investigation, a back pressure was applied to enable an AZ31 alloy to be ECA deformed at as low as 100 °C with significantly refined grains.
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Experimental material and procedures
AZ31 was used with the following nominal composition: Mg–3Al–1Zn–0.3Mn (wt.%). The as-cast billets were extruded at 380–400 °C to rods of 19 mm in diameter which were either used directly or machined by milling into specimens of 8 mm × 8 mm in cross-section dimensions for ECAP.
ECAP dies with an angle of 90° were used. The die was preheated to and stabilised at the testing temperature before a lubricated ECAP specimen was inserted into the entrance channel. It was held for 15 min before pressing. For
Microstructures
The as-extruded materials had average sizes of ∼15–22 μm. The microstructures after ECAP at 200 °C for 2 and 4 passes are shown in Fig. 1. Although many grains were already significantly refined after only 2 passes, the grain structure was not homogeneous with very fine grains of 1–2 μm as well as coarse grains of greater than 10 μm. After 4 passes, the microstructure became visibly uniform with an average grain size of ∼3 μm. Deformation up to 8 passes further refined the grains to an average size
Discussion
The AZ31 alloy was successfully ECA deformed for 8 passes at temperatures as low as 100 °C with the application of a moderate back pressure (50 MPa). Without the back pressure, AZ31 and other Mg alloys easily crack below ∼200 °C, as demonstrated by many [8], [10]. A back pressure established a compressive hydrostatic pressure and this suppressed the nucleation and growth of cracks. Back pressures were also shown to be necessary in ECAP of an Al–5 wt.% Fe alloy which was otherwise difficult to
Summary
- (1)
AZ31 was successfully ECA deformed for up to 8 passes at temperatures as low as 100 °C with the application of a back pressure.
- (2)
The grains were significantly refined to much finer than 1 μm after 4 passes at 150 °C and 4 passes at 100 °C. The microstructure was initially not uniform with a “bimodal” grain size distribution but became increasingly homogeneous with further ECAP passes.
- (3)
HV increased with decreasing grain size. The dependence of hardness on grain sizes appears to be in line with that
Acknowledgments
We are grateful to Prof. Shuisheng Xie of the General Research Institute for Nonferrous Metals in Beijing for providing experimental materials. This project was supported partially by the Australian Research Council.
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