Impact of homogenization heat treatment on the high temperature deformation behavior of cast AZ31B magnesium alloy

Uniaxial isothermal compression tests were conducted on cylindrical specimens extracted from a DC cast AZ31B ingot with and without a homogenization treatment in the temperature range of 300 degrees C-500 degrees C, at strain rates of 10(-3) s(-1)-1.0 s-(1). As-cast specimens (without a homogenization treatment) exhibited cracking at a strain rate of 1.0 s(-1) for deformation temperatures of 400 degrees C and above. The cracking is believed to be associated with incipient melting of the gamma-Mg17Al12 phase. Homogenization of the specimens at 450 degrees C for 5 h improved the hot workability and no cracking was observed under the investigated conditions. Deformation of the cast-homogenized material was successfully conducted at 500 degrees C and 1.0 s(-1), to a final strain of 1.0. The volume fraction of dynamically recrystallized (DRX) grains was observed to increase with strain, but the DRX grain size exhibited little to no change. The DRX grain size however did increase with deformation temperature and with decreasing strain rate. Increasing the deformation temperature from 350 degrees C to 500 degrees C increased the maximum allowable strain before cracks were observed at the surface. Additional specimens were homogenized at temperatures of 400 degrees C or 500 degrees C, for times of 3 h or 5 h, followed by a water quench or air cooling. Under all circumstances, incipient melting of the gamma-Mg17Al12 phase was no longer observed upon heating using differential scanning calorimetry (DSC). Furthermore, homogenization at a temperature of 350 degrees C for up to 8 h resulted in partial dissolution of the gamma-Mg17Al12 phase, and an increase in incipient melting onset temperature was observed by DSC.