Evaluation of Cooling Rate Effects on the Mechanical Properties of Die Cast Magnesium Alloy AM60

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INTRODUCTION

DUE to their excellent strength-to-weight ratio, magnesium alloys are good candidates for reduction of mass in transportation applications.[1] High-pressure die casting (HPDC) is an important manufacturing process to fabricate complex Mg-alloy components in the automotive industry.[2] Wider application of die cast magnesium alloy is mitigated by variations in the mechanical properties of die cast magnesium alloys, particularly in large, complex cast components typical in the automotive industry. The mechanical properties of HPDC depend fundamentally upon microstructural features, such as porosity, skin thickness, and bimodal distribution of grain size through the casting thickness.[2–8] Previous investigations have shown that the yield strength and, to a lesser extent, elongation are controlled by the skin thickness in the case of HPDC Mg-Al alloy. Therefore, the prediction of the skin thickness in different locations of complex components enables designers to take full opportunities afforded by die cast magnesium alloys. The literature[2–11] confirms that HPDC produces unique microstructures due to a high solidification rate POUYA SHARIFI, Ph.D. Candidate, HOOMAN BAGHAEE ANARAKI and ARINDOM BANERJEE, Master Students, YING FAN, Research Engineer, and JEFFREY WOOD, Associate Professor, are with the Department of Mechanical and Materials Engineering, Western University, London, ON, Canada. Contact e-mail: psharif2@ uwo.ca KUMAR SADAYAPPAN, Research Scientist, is with CanmetMATERIALS, Hamilton, ON, Canada. Manuscript submitted March 6, 2016. Article published online August 10, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

near the mold wall as compared to the interior. As a result, it is observed that there are two well-differentiated regions in the microstructure of HPDC: the skin region and the core (interior) region. The skin region is characterized by a finer grained microstructure and higher integrity, whereas the core region contains a coarser grained microstructure, a higher percentage of defects and externally solidified grains (ESGs) which depend on the thermal and flow properties in the shot chamber.[5,9,10] Various investigations[4,5,10] have shown that the skin region has higher hardness and local yield strength as compared to the core region due to the finer microstructure and higher amount of dispersed intermetallic phase in the skin region. Some studies[5,10] have also shown that the yield strength of die cast magnesium alloy is controlled by the skin region. Therefore, in order to predict mechanical properties, it is critical to determine the skin thickness or area fraction of that region.[4,6] Skin thickness of magnesium die castings has been defined using the micro-hardness profile across the casting thickness,[4,5] the location of the pore band through the casting thickness,[2] porosity-free casting layer,[6,7] the change in the grain size from the skin to the core region,[4,9–11] the onset of ESGs and the areal percentage of eutectic phase.[4,10] As discussed, the distinction between the skin

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