Abstract
An investigation has been carried out. under controlled solidification conditions, regarding the manner in which composition, melt superheat and grain structure interact to determine the hot-tearing susceptibilities of Al- Cu alloys in the range 0·15 wt-% Cu. The results demonstrate that the influence of both superheat and grain structure relates to the way in which these variables affect the number of potential sites of crack origin. This number then has to he considered in conjunction with the mechanisms that can operate to withstand hot tearing which are themselves dependent on metal composition. Under the experimental conditions employed, cracking, as detected by measurement of electrical resistance only ever occurred in alloys containing less than 7 wt-% Cu. The actual composition range within which cracking was observed was dependent on grain structure and superheat. Increasing superheat increased the susceptibility to tearing of an alloy of fixed composition and hence the composition range over which cracks were readily detectable and, also, displaced to a lower copper value the composition at which the maximum susceptibility was reached on a susceptibility v. composition plot. The importance of grain structure increased with increasing superheat. At low superheats susceptibility was negligibly affected by variation in the cast grain structure. However, at high superheats, particularly in alloys in which interdendritic fluid flow is the dominant mechanism of crack prevention, refinement of the grain structure led to a reduction in both cracking susceptibility and the composition range in which cracks were detectable.
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