Microstructural development during equal channel angular pressing of as-cast AZ91 alloy

Abstract

Equal channel angular pressing (ECAP) was successfully applied to a typical as-cast AZ91 alloy with very coarse initial grains without the need for a prior thermomechanical treatment. A two-step ECAP tests were carried out in which the first pass was conducted at 623K to overcome cracking, and the next passes were performed at lower temperature to reduce grain growth. In order to evaluate the influence of ECAP temperature on the evolution of microstructure and mechanical properties, ECAP processing was carried out at different temperatures of 593K, 563K, and 533K at the second step. According to microstructural investigations, simultaneous occurrence of dynamic recrystallization (DRX) and dynamic precipitation during this two-step ECAP processing led to a noticeable grain refinement and formation of a homogeneous microstructure. It was demonstrated that lower ECAP temperatures accelerate dynamic precipitation and DRX resulting in more rapid formation of fine and uniform microstructures. The tensile properties of ECAPed samples were evaluated at room temperature, and it was proved that ECAP processing can simultaneously improve both ductility and strength of the alloy. It has been indicated that a combination of DRX ratio, and dynamic precipitation during ECAP plays a decisive role in improving the mechanical properties.