Grain Refinement of AZ31 Magnesium Alloy by Titanium and Low-Frequency Electromagnetic Casting

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magnesium alloys for structural components in the automotive industry is attractive due to their excellent specific strength and low density. Highpressure die casting is currently the dominant process for magnesium automotive applications, but wrought magnesium alloys, i.e., extrusions and sheet products, are receiving increasing attention from academia and industries due to their greater potential for vehicle mass reduction and performance improvement.[1,2] However, the cost of wrought magnesium products remains high due to the poor formability and low productivity associated with current magnesium extrusions and sheet products, and their formability and productivity are largely dependent on the quality of direct-chill (DC) casting billets.[3] Presently, there are several quality issues with magnesium alloy billets, such as macrosegregation, inclusions, porosity, coarse and asymmetric grains, and surface defects (liquation burl, stratification, and microcracks). YINGXIN WANG, Doctoral Student, XIAOQIN ZENG, Associate Professor, and WENJIANG DING, Director, are with the National Engineering Research Center of Light Alloys Net Forming, Shanghai Jiao Tong University, Shanghai 200030, People’s Republic of China. Contact e-mail: [email protected] ALAN A. LUO, Staff Research Engineer, and ANIL K. SACHDEV, Group Manager, are with the General Motors Research and Development Center, Warren, MI 48090-9055, USA. Manuscript submitted November 27, 2006. Article published online June 26, 2007. 1358—VOLUME 38A, JUNE 2007

It is well known that a finer grain structure reduces the size of defects such as microporosity and secondary particles; therefore, grain refinement is an important technique for improving the quality of magnesium alloy billets. A variety of methods, such as superheating, agitation, and the addition of particles and solute elements, have been developed to refine the magnesium alloys.[4–6] The grain-refinement effects of the Al-4Ti-5B master alloy[4] and Sr as a solute element[6] in AZ31 alloy have been studied. It was found that it was very difficult to achieve dissolution of the Al-4Ti-5B master alloy, and the Sr additions to AZ31 led to the formation of acicular phase, which was detrimental to mechanical properties. Low-frequency electromagnetic casting (LFEC) processing is very effective in refining billet grain size, eliminating the macrosegregation, improving the temperature distribution during solidification, and enhancing the casting velocity. The LFEC process has been used to fabricate high-quality Al-Zn-Mg-Cu,[7] AZ80,[8] and ZK60[9] alloy billets. There are few publications that discuss the effect of Ti additions on grain refinement of magnesium alloys, although Ti is an effective grain refiner of aluminum alloys.[10] Xue et al.[11] reported the grain-refinement effect of Al3Ti on pure Mg and AZ31 alloy. It is well known that Al3Ti is unstable at a melt temperature above 700 C;[10] Ti as a solute element has an important role in the grain refinement of pure Mg and the AZ31 alloy. The current study aims to investigate th