An Investigation Of Preparation And Mechanical Properties Of Room-temperature Superplastic Zn-22 Wt.% Al Alloy

In this paper, Zn-22Al alloy ingot with dimension of 400mm×195mm×85mm has been prepared via die casting. A combination of metal heat treatment and plastic processing were applied to process the casting ingot. The microstructure of the Zn-22A1 alloys was characterized by using metallography and electron microscopy. The mechanical properties of the alloys were measured by tensile and compressive testing. The effects of heat treatment and plastic processing on the microstructure and mechanical properties of the alloys were discussed, and the main factors accounted for the achievement of room-temperature superplasticity of the alloys were also analysized. The results show that dendritic segregation, shrinkage cavities and rarefaction occurred in the cast microstructure. In order to obtain room-temperature superplasticity, it's necessary to homogenize the Zn-22Al alloy at 360℃for 48 hours and quench it subsequently. The dendritic segregation was removed after the ingot being homogenized, a structure of equiaxed zinc-rich and aluminum-rich phases was obtained after quenching. Hot rolling with a reduction of 82% in thickness was performed on the homogenized ingot. The microstructure of the alloy which was cooled in air after hot rolling, named RA alloy became more uniform. The RA alloy was solution treated at 360℃for 3 hours and then quenched or furnace cooled, and the products were named RQ alloy and RF alloy, respectively. The RQ alloy has a mixture microstructure containing coarsen grains and fine grains, and the RF alloy shows a lamellar eutectoid structure. Mechanical tests show that the ductility of RF alloy is poor. The flow stress and elongation of the RQ alloy are higher than that of the homogenized ingot. The RA alloy shows good room-temperature superplasticity. The maximum elongation recorded in the tensile testing was 214% under a strain rate of 10-4s-1, and the flow stress was 131MPa. The alloy also possessed excellent compressive forgeability at room-temperature with flow stress of 163 MPa in a compressive strain rate of 10-4s-1. When the RA alloy was solution treated at 360℃for 6 hours and then quenched, its microstructure became more uniform and its room-temperature superplasticity was improved. The tensile flow stress was 77 MPa in a strain rate of 4×10-3s-1, and the elongation was 237%. The compressive flow stress is lower than 131MPa when the compressive strain rate is less than 10-3s-1. The results also implied a positive correlation between elongation and strain rate sensitivity in the room-temperature superplastic Zn-22A1 alloys.