Grain coarsening in semi-solid state and tensile mechanical properties of thixoformed AZ91D-RE

For thixoforming to be possible, the microstructure of the starting material must be non-dendritic, which can be obtained by the strain induced melt activation (SIMA) route. Based on the SIMA route, as-cast AZ91D alloy with the addition of yttrium was deformed by cyclic closed-die forging (CCDF). Microstructure evolution of CCDF formed AZ91D-RE alloy during partial remelting were investigated. Furthermore, the mechanical properties of thixoformed AZ91D-RE magnesium alloy components were also studied. The results showed that prolonged holding time resulted in grain coarsening and the improvement in degree of spheroidization. The coarsening behaviour of solid grains in the semi-solid state obeyed Ostwald ripening mechanism. The coarsening rate constant of CCDF formed AZ91D-RE during partial remelting was 324 um3/s at 550 °C. The value of yield strength, ultimate tensile strength and elongation to fracture of four-pass CCDF formed AZ91D-RE magnesium alloy were 214.9, 290.5 MPa and 14%, respectively. Then the four-pass CCDF formed alloys were used for thixoforming. After holding at 550 °C for 5 min, the values of yield strength, ultimate tensile strength and elongation to fracture of thixoformed component were 189.6 MPa, 274.6 MPa and 12%, respectively. However, prolonged holding time led to remarkable decrease in mechanical properties of thixoformed components.