5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

A mathematical model has been established to coupling predict the formation of inverse segregation and porosity for up-vertical unidirectional solidification of Al-Cu alloys. Based on the analysis on the redistribution behaviours of both gas element and the alloying elements in the mushy zone. The model first investigates the volume change during solidification intensively, including the specific mass variation and the solidification shrinkage both during the growth of primary phase and eutectic reaction, to derive the back flow needed. Then, the fraction of porosity is obtained by combing the effects of hydrogen segregation and the pressure depression associated with the feeding liquid flow through a porous media and being solved with Darcy’s equation. Consequently, both the feeding flow and the well known 'local solute redistribution equation' are modified with the presence of porosity. Finally, the solute distribution is obtained by coupling a finite difference solidification model with the modified segregation model. Numerical results show that the overall back-flow, which is needed to compensate the volume shrinkage, decreases with an increasing initially dissolved hydrogen concentration. As a result, the inverse segregation, being induced mainly by back-flow which is generally rich in solute, decreases with an increasing amount of porosity. The calculating results are also compared with the experimental results reported in literatures, which shows rather good agreement. The model can be used to predict inverse segregation with the presence of porosity in vertically unidirectionally solidified Al-Cu alloys.