Controlling and Minimizing Blistering during T6 Heat Treating of Semi-Solid Castings

Surface blistering during T6 heat treating is an artifact that is essentially unique to high pressure casting processes such as semi-solid casting and die casting. It is believed that the blistering originates from subsurface defects present in the castings. When the castings are exposed to elevated temperatures during solution heat treatment, the strength of the aluminum is reduced, and the defects expand to deform the surfaces of the castings. There are three potential sources for the subsurface defects - entrapped air, die lubricant or shot sleeve lubricant.This paper will report on a study to determine the origin of the defects present in the castings that produce the blisters. Along with attempting to separate the influence of air and the two types of lubricants on blister formation, the study will also examine the impact of a number of process parameters on blistering.