Porosity Prediction in A356 Wheel Casting
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IT is well known that porosity can act as a stress concentrator and crack initiator in cast components thereby degrading fatigue performance.[1] In the case of cast aluminum alloy wheels, there are two main types of porosity: macro-porosity and micro-porosity; the former often also referred to as shrinkage-based porosity and the latter as hydrogen-based porosity. Macro-porosity occurs in regions in a casting in which a significant volume of liquid is encapsulated and ‘‘cut-off’’ from the supply of liquid needed to compensate for the volume change associated with the liquid-to-solid transformation. In the case of wheels, macro-porosity is commonly seen at the junction between the rim and spoke, where several solidification fronts can interact. The size and distribution of the resulting porosity depend on the volume of liquid that is encapsulated. Liquid encapsulation in a casting can be
P. FAN, S.L. COCKCROFT, D.M. MAIJER, and L. YAO are with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] C. REILLY is with Cast Analytics Inc., Vancouver, BC V5Z 2H3, Canada. A.B. PHILLION is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 4L8, Canada. FLUENT is a trademark of Ansys. Inc., Canonsburg, PA, United States. Manuscript submitted March 3, 2019.
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accurately predicted using computer-based heat transfer simulation models providing the models are capable of accurately describing the heat transfer occurring during casting. The accurate prediction of the size range and distribution of the macro-porosity, however, remains a challenge. In contrast to macro-porosity, micro-porosity refers to pores that are relatively small in scale, usually less than 300 lm.[1] Micro-porosity occurs either due to inadequate compensatory flow in the mushy zone (semi-solid material) at high fractions of solid and/or the exsolution of gas. In the case of the aluminum alloy castings, it is well known that the solubility of hydrogen in the liquid is much higher than in the solid. Aluminum is prone to picking up hydrogen when held in the liquid state particularly under warm and humid conditions. During solidification, hydrogen is rejected from the solid phase into the adjacent liquid. When the hydrogen concentration, or more specifically, the activity of hydrogen in the liquid exceeds its local solubility plus an additional amount to overcome nucleation, a hydrogen bubble will form. Over the last several decades, there has been a significant body of work presented in the literature to understand, characterize, and simulate the formation of hydrogen-based porosity. The reader is referred to a good review of early work by Lee et al.[2] and more recent work can be found in References 3, 4, 5, 6, 7, and 8. One aspect of hydrogen pore formation that has not received much attention is the extent to which its formation is influenced by macro-segregation. Macro-segregation of al
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