Effect of Surface Modification on Cumulative Tensile Ductility of AZ31 Magnesium Sheet
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MG alloys, as the lightest structural metallic materials available commercially in large scale, are finding increased application in automotive industry. However, their application as rolled sheet materials for conventional stamping have been limited by their poor room temperature formability compared to Al alloys and steel. Severe plastic deformation (or SPD) processes such as equi-channel angular pressing (ECAP), differential speed rolling (DSR), and friction stir processing (FSP) have been shown to impart ultrafine grain structure and a more randomized crystallographic texture to magnesium sheet surface.[1–6] Also, resulting microstructural and textural modifications have been shown to improve their ultimate tensile strength and tensile ductility. The above processes are still quite limiting in terms of room temperature ductility for conventional stamping, cost-effective production, and MAHDI HABIBNEJAD-KORAYEM, Research Scientist and Advanced Engineering Metallurgist, is with the Advanced Engineering Group, Research and Development Center, Stackpole International, Mississauga, ON, L6J7Y2, Canada, and also with the Department of Mechanical Engineering, McMaster University, Hamilton, ON, L8S4L8, Canada. Contact e-mail: [email protected] MUKESH K. JAIN, Professor, is with the Department of Mechanical Engineering, McMaster University. RAJA. K. MISHRA, Technical Fellow, is with the General Motors Research and Development Center, Warren, MI 48090. Manuscript submitted September 29, 2015. Article published online September 12, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
required commercial blank sizes for automotive applications. Several studies have been conducted in the literature to modify the microstructure of the near surface layers using simpler and more cost-effective processes that require less metal working for ultra-fine surface grain development. In these studies, ultrafine or nano-sized grains result from large sliding loads on the surface.[7,8] However, such studies have not attempted to link the microstructure directly to improvement in mechanical properties. More conventional surface modification processes such as wire brushing (WB),[9–11] ultrasonic and air blast shot peening (SP),[12–14] and surface mechanical attrition treatment (SMAT)[15] may have certain advantages for automotive stamping applications over the other methods noted above. Specifically, wire brushing, often employed for surface polishing and rust removal, has been shown to refine the surface layer grain size of various ferrous and non-ferrous materials such as steel, Al, Cu, Pb,[9] and Mg over much larger depths.[10] However, many of these studies have focused simply on development of nanosized surface grains from dynamic recrystallization and not on their role in improving uniaxial tensile ductility.[9–15] Relatively little attention has been paid to the role of annealing on surface layer microstructure and crystallographic texture development, both of which are linked to uniaxial tensile ductility. Also, no effort has been made to subj
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