A study on the hot tearing in AL-1% Sn cast alloys

Abstract

Hot tearing in certain cast metals continues to bother metal casters. A number of casting parameters such as pouring temperature and extent of grain refinement affect the susceptibility to hot tearing. Published literature seems to be incomplete or conflicting in this regard. The present work involves experimental investigations with an objective of determining conditions under which hot tearing can be eliminated. Experiments were conducted on Al-1% Sn alloys. Al-1% Sn is a high-freezing-range alloy known to be extremely prone to hot tear cracking. Effects of pouring temperature, grain refinement, and presence of Fe and Ni have been investigated by making use of a ring type casting with a steel core. Al-5Ti-1B was used as the grain refiner. Optical and scanning electron microscopy (SEM) was used in the study. The results showed that contrary to some published work, grain refinement alone could not eliminate hot tearing. Average hot tear length decreased with increased grain refinement and the minimum achieved was at 5 per cent by weight of Al-5Ti-1B addition. Both Fe and Ni have positive effects in minimizing hot tear formation. However, no correlation could be noted between secondary dendritic arm spacing and hot tearing tendency. SEM studies have shown that with Fe there is an Al-Sn-Fe-rich phase in the form of a leaf-like structure, which is believed to provide anchorage to the adjacent grains and thus overcome hot tear.