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UBC Theses and Dissertations

Hydrogen pore formation in an Al-Si alloy – an examination of the role of hydrogen macrosegregation Fan, Pan


Porosity related defects are one of the leading causes of cast rejection in casting industry since they are detrimental to the fatigue performance of the cast components, primarily when the pores are located in an area of high cyclic stress or located near the surface. Many efforts have been made to develop computational models that predict pore formation. However, one aspect of microporosity formation that has previously not been considered in detail is the effect of macrosegregation of hydrogen. Towards this goal, the user-defined scalars and corresponding user-defined functions were developed to account for hydrogen macrosegregation during solidification of low pressure die casting (LPDC) of A356 aluminum alloy wheels. Numerical simulation of the LPDC process has been implemented within the commercial CFD software package, FLUENT 16.0. The model has been validated against temperature and microstructural data taken from a commercially cast wheel. The amount of species (silicon and hydrogen) segregation in the wheel has been shown to be significant in the rim/spoke junction. The output data from the FLUENT model were then fed into the in-house microporosity model to predict pore size distribution at discrete locations. The in-house microporosity model has been updated to incorporate the effects of hydrogen macrosegregation, cooling rate and local pressure drop on pore size distribution in the wheel cast. The microporosity model used a Gaussian function of hydrogen supersaturation in the melt to simulate nucleation site distribution and assumed pore growth was controlled by hydrogen diffusion process and besides, the model took into account pore growth associated with liquid encapsulation at a high solid fraction. The samples from a cast wheel have been analyzed using X-rayed microtomography to provide basic validation to the microporosity model. The predicted results showed that the evolution in pressure has the dominant effect on pore growth, but only under conditions where pores have nucleated prior to the abrupt pressure drop. Otherwise, the cooling rate appears to have the dominant effect. The model prediction shows pore size increases with decreasing pressured drop and cooling rate.

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