Effect of mold coating on thermal fatigue in Al permanent mold casting

The influence of mold coating and gray iron matrix on initiation and growth of thermal fatigue cracks in permanent mold casting of aluminum was investigated. In particular, the relative effects of the two major sources of thermal strains-the start-up/shut-down cycle and the individual casting cycle-on thermal fatigue cracking were studied. Crack initiation appeared to be most related to the large thermal strains introduced during start-up and/or shutdown. Relatively little effect of mold coating or matrix structure on the number of these cycles required to initiate a crack was found. The rate of growth of the cracks was affected by the total number of individual casting cycles and, in turn, by the mold coating that was used. Insulating coatings reduced the temperature differences within the mold during each casting cycle, thus reducing the thermal strains. Smaller strains resulted in slower crack growth rates.