Hot Tear Nucleation During Solidification of Aluminum and Its Alloys

This article presents a discussion of hot tear formation by first re-interpreting tensile test experimental results for aluminum castings in the literature. Based on the previous findings that a pore should first nucleate to result in a hot tear, the physics of pore formation is reviewed through theoretical fracture pressure calculations. Theoretical fracture pressure of liquid aluminum was calculated to be approximately −3.5 GPa, which is 4 orders of magnitude higher than the tensile strength data reported in the literature. Further calculations showed that it was impossible for a pore to nucleate either homogeneously or heterogeneously in liquid aluminum. The formation of pores and hot tears in aluminum castings can only be explained by inflation of entrained surface oxide films (bifilms) under reduced pressure and/or with dissolved gas, which involves only growth, avoiding any nucleation problem. This mechanism is consistent with tensile test results in the literature as well as physics principles.