An A Priori Hot-Tearing Indicator Applied to Die-Cast Magnesium-Rare Earth Alloys
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INTRODUCTION
HOT tearing is one of the most detrimental defects in castings, as the cracks compromise a cast component’s structural integrity, can lead to a loss of pressure tightness, and can act as stress raisers aiding in the propagation of fatigue cracks or catastrophic failure. While hot tearing can be reduced by optimizing the design of the cast part in order to reduce areas of high stress intensity and to allow for more effective feeding of solidification shrinkage,[1] some alloys are intrinsically prone to hot tearing. Therefore, it is of critical importance to alloy design if the hot-tearing susceptibility of a potential alloy can be predicted. The phenomenon of hot tearing is very complex as it is influenced by intrinsic alloy properties and process parameters. These factors include the solidification path,[2–5] wettability or interfacial energy of the solid phase by the liquid phase,[6] orientation of neighboring solid grains,[7] the viscosity of the liquid phase,[8] plastic deformation of the solid phase,[9–11] grain morphology features such as grain size and the dendrite arm spacing,[4,5,12–15] and cooling rates and casting speed or
MARK A. EASTON, formally Professor with the Department of Materials Engineering, CAST Co-Operative Research Centre, Monash University, Building 69, Clayton, VIC 3800, Australia, is now Deputy Head of School (Manufacturing and Materials), with the School of Aeorospace, Mechanical and Manufacturing Engineering, PO Box 71 Bundoora, VIC 3083, Australia. Contact e-mail: mark.easton@rmit. edu.au MARK A. GIBSON, Senior Principal Research Scientist, is with the CSIRO Process Science and Engineering, Clayton, VIC 3169, Australia. SUMING ZHU, Research Fellow, is with the Department of Materials Engineering, CAST Co-Operative Research Centre, Monash University. TREVOR B. ABBOTT, Director R&D, is with the Magontec Ltd., Level 8 139 Macquarie Street, Sydney, NSW 2000, Australia. Manuscript submitted June 10, 2013. Article published online April 1, 2014 3586—VOLUME 45A, JULY 2014
regime.[9,16,17] This means that comprehensive hot-tearing models are by nature very complex, as they need to involve a thermal solidification model, which then generate a stress/strain/strain rate response in a casting during solidification.[1,17–21] In turn, this needs to be coupled to a feeding model where the remaining liquid in a casting is required to feed through the mushy zone to accommodate the solidification shrinkage. Then, an essential criterion for tear initiation must be considered[22,23] and the point at which grain coalescence occurs identified.[24–26] While substantial progress has been made to develop comprehensive models, it is realized that for the purposes of alloy design, these models are more complex than is often necessary. Evidence of this is that even now, researchers are still using the much simpler hot-tearing models, such as the Clyne–Davies model of hot tearing to explain many results,[27–31] despite it being clear that it is unable to explain the hot-tearing response in many situations.[3
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