An investigation of microstructure and mechanical properties of as-cast Zn–22Al alloy during homogenizing and equal channel angular pressing

Abstract

In this study, the effect of homogenization heat treatment and equal channel angular pressing process (ECAP) on microstructure and mechanical properties were explored in Zn–22Al alloy. It was observed that homogenization caused a significant part of the structure to transform to equiaxed grains from an initially lamellar structure. Also, with increasing time of homogenization, a homogeneous structure with no coarse phases were created. Average grain sizes of 48, 2.5, 1.2 and 1.1 μm were respectively achieved after homogenization for 4, 8, 12 and 24 h at 375 °C followed by water quenching. By removal of the lamellar structure during homogenization, improved superplastic behavior was observed. By increasing the time of homogenization, a decrease in microhardness took place which can be due to the loss of hard dendritic phases in the microstructure. On the other hand, grain size of 0.44 μm was achieved after 8 passes of ambient temperature ECAP process which was found to be quite effective in forming the ultra-fine grained (UFG) and equiaxed microstructure leading to further improvement in superplastic behavior. By increasing the number of ECAP passes, the failure elongation increased. On the other hand, the microhardness curve showed a work softening which is associated with the dynamic recovery/recrystallization of η phase after ECAP process.

Keywords

• Zn–22Al alloy
• Superplasticity
• Homogenizing heat treatment
• Equal channel angular pressing process (ECAP)