The effect of grain refinement on the formation of casting defects in alloy 356 castings

One of the main reasons for rejection of wheel castings is the formation of shrinkage defects at the spoke-rim junction. Industrial trials show that the severity of defects is accentuated by the addition of grain refiner beyond an optimum level. Subsequent laboratory castings also found that the addition of grain refiners beyond this optimum level increased the size of the defect at hot spots in a casting. This is explained by a decrease in the size of interdendritic channels during feeding when the grain size is reduced. Therefore feeding becomes restricted earlier in grain refined castings and thus a larger amount of shrinkage needs to be accommodated in the hot spot. It was also observed that when plentiful fresh oxide films are present then the nucleation of porosity is favoured. However, when fresh oxide films have not been generated, then external shrinkage (slumping of the casting surface) is more likely to occur.