Castings performed under reduced pressure and atmosphere were used to investigate the formation of pores in lost foam casting (LFC) of the AZ91H Mg alloy, and the results are compared with the results of a previous work on the A 356 Al alloy.
In LFC, although the pouring temperature of the AZ91H alloy melt was higher than that of the A356 alloy, the amount of porosity in the AZ91H alloy after solidification was much lower than that of the A356 alloy. The lower porosity was caused by the extra hydrogen solubility. The pore formation mechanism of the AZ91H alloy in LFC was similar to that of the A356 alloy but the critical temperature for generating a different mechanism of pore formation is higher in the A356 alloy by as much as about 323 K. The mold evacuation promotes lower porosity and shrinkage defects due to the easy removal of polystyrene products. However, the exceeded vacuum degree severely entraps polymer pyrolysis products, thereby leaving large pores after solidification.