Directional Solidification and Microsegregation in a Magnesium-Aluminum-Calcium Alloy
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UCTION
MAGNESIUM alloys are attractive for transportation applications, since their high specific strength and low density enables light weighting.[1,2] Conventional Mg-Al-Zn (AZ) and Mg-Al-Mn (AM) series of alloys offer good strength and ductility at room temperature, combined with good corrosion resistance and excellent castability. However, their high-temperature strength and creep resistance do not meet the challenging requirements of many automotive powertrain components, such as engine blocks or automatic transmission cases, where the operating temperature can be as high as 448 K (175 C).[2,3] Recently, a number of new alloys, Mg-AlRE (AE, with expensive rare earth elements), Mg-Al-Ca (AX), and Mg-Al-Sr (AJ), were developed to provide improved high-temperature strength and creep resistance. AE44 (Mg-4Al-4RE) is used in the first-inindustry engine cradle application at General Motors, and AJ62 (Mg-6Al-2Sr) is used in the BMW composite Mg/Al engine block production. Mg-Al-Ca alloys are considered a lower cost alternative to the AE alloys and offer better castability than the AJ alloys.[4–6] There has been extensive research on the phase equilibria and microstructure of AX alloys in recent
XINGWEI ZHENG, Postdoctoral Candidate, and JIE DONG, Associate Professor, are with the National Engineering Research Centre of Light Alloy Net Forming, Shanghai Jiaotong University, Shanghai 200240, People’s Republic of China. ALAN A. LUO, GM Technical Fellow, and RICHARD A. WALDO, Staff Researcher, are with the General Motors Global Research and Development Center, Warren, MI 48090-9055, USA. Contact e-mail: [email protected] CHUAN ZHANG, Materials Scientist, is with CompuTherm LLC, Madison, WI 53706, USA. Manuscript submitted October 11, 2011. Article published online May 24, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
publications.[7–16] Various thermodynamic models have been developed for the Mg-Al-Ca system,[7–11] and the microstructure of many Mg-Al-Ca alloys has been fully characterized.[12–16] However, there is no work reported on the solidification behavior of the Mg-Al-Ca alloys under different casting conditions (cooling rates). The objective of the present study is to investigate the solidification behavior and microsegregation of a ternary Mg-Al-Ca alloy under controlled directional solidification conditions, which will enable us to predict the microstructure and mechanical properties of this alloy system for automotive powertrain applications. Since most of the AX alloys being developed contain about 3 to 6 pct Al and 2 to 5 pct Ca, a composition of Mg-4Al4Ca (AX44) was selected for this study. All compositions are in weight percent unless otherwise stated. Computational thermodynamics was used to determine the phase equilibria in this alloy, which was validated with careful directional solidification experiments.
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EXPERIMENTAL PROCEDURE
A. Alloy Preparation The AX44 alloy was prepared in a mild steel crucible with a bottom opening for casting (discharging) the melt into a steel mold. About 550 g of pure magnesiu
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