A Critical Conception of Hot-Tearing Susceptibility: Shape Casting with Wrought Aluminum Alloys

Instrumented constrained rod casting method and the controlled diffusion solidification (CDS) process were used to study the high susceptibility of the aluminum alloy AA 7068 to hot tearing. A precise analysis of the measured hot-tearing curves is provided considering the casting design and different feeding mechanisms. Scanning electron microscopy was used to study the hot-tear surfaces. It was found that the solidification of the eutectic liquid at the film stage and the phenomenon of solid feeding strongly affects the hot-tear formation. The early formation of an absorbed eutectic layer on the primary phase due to low rate of back-diffusion is proposed to explain the loss of ductility at the film stage. The layer can serve as an efficient substrate for the eutectic nucleation and growth; hence, its formation can advance the eutectic solidification. The isolation of liquid eutectic by its partial solidification results in development of solid-feeding stresses breaking the solid eutectic and provides the required tearing initiators. By increasing the back-diffusion rate, the CDS process avoids formation of the adsorbed solid layer which postpones eutectic solidification thereby mitigating the hot-tearing susceptibility.