Achieving High Strength and High Ductility in Friction Stir-Processed Cast Magnesium Alloy
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MAGNESIUM alloys are attracting great interest from transportation industries for structural applications because of their high strength-to-weight ratio.[1,2] However, low ductility and room temperature formability limit the processing and application.[3] As a result, the main fabrication route of magnesium alloys remains casting.[1,2] Casting defects such as microscopic shrinkage porosity and inclusions adversely affect the strength and ductility of magnesium alloy. It is expected that the application will be expanded if the strength and ductility of magnesium alloys can be enhanced. Grain refinement has been a good option for enhancing the strength of magnesium alloys. Several severe plastic deformation (SPD) tools, such as equal channel angular extrusion/pressing,[4,5] high-pressure torsion,[6,7] accumulated roll bonding,[8] alternate biaxial reverse corrugated pressing,[9] differential speed rolling,[10,11] and friction stir processing (FSP)[12–14] have demonstrated capability in refining grain structure of magnesium alloys. However, grain refinement-induced strength enhancement is generally accompanied by the
WEI YUAN, Researcher, is with the Department of Materials Science & Engineering, Missouri University of Science and Technology, Rolla, MO 65409, and also with the Research & Development Division, Hitachi America Ltd., 34500 Grand River Avenue, Farmington Hills, MI 48335. SUSHANTA K. PANIGRAHI, Assistant Professor, is with the Department of Materials Science & Engineering, Missouri University of Science and Technology, and also with the Indian Institute of Technology Madras, Chennai 600036, India. RAJIV S. MISHRA, Professor, is with the Department of Materials Science & Engineering, Missouri University of Science and Technology, and also with the Center for Friction Stir Processing, Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203. Contact e-mail: [email protected] Manuscript submitted September 9, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
reduction in ductility, especially when the grain size is refined to an ultrafine regime, mainly because of the reduced work hardening.[15–17] Texture, which is always observed in magnesium alloys during intense shear deformation, plays an important role on room-temperature deformation behavior in addition to grain size as a result of limited independent slip systems available. Texture modification of wrought AZ magnesium alloys by SPD has shown promising room-temperature ductility because of the formation of a strong basal texture, which orientated the easy basal slip plane to the preferred orientation; however, the tensile yield strength was very low.[18–25] The special basal texture generation during SPD also led to the anisotropy in mechanical properties, especially in yield strength, which is directly related to the activation of deformation slip or twinning systems.[14,20,26] It is desirable to enhance strength as well as ductility in magnesium alloys. Recently, the FSP of cast alloys has shown effective elimination
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