The Effect of Equal Channel Angular Pressing on Coarsening Kinetics of AZ80–0.2Y–0.15Ca Alloy in Semisolid State

Abstract

Magnesium (Mg) alloys, as viable candidates for lightweight construction, are widely used in various fields, including the aerospace, rail transportation, and automotive fields, because of their low densities and high specific strengths. [1,2] However, Mg alloys often show extremely low workability at room temperature because of the hexagonal close packed crystal structure of the alloy and limited independent slip systems of this structure. [3] Therefore, magnesium alloys can only be formed by warm or hot working. These conventional hot working processes are inefficient, which restricts their commercial applications. [4] The semisolid forming method can overcome the shortcomings of the low forming efficiency of magnesium alloy and realize mass production. Strain-induced melt activation (SIMA), hot working above the recrystallization temperature with subsequent reheating to the semisolid state, is the most promising method for preparing semisolid billets because of the high production efficiency and low equipment cost of the manufacturing process. [5] The conventional hotdeformation methods such as hot extrusion, rolling, and forging decrease the size of the initial ingots. The strain introduced into the blank is limited and inhomogeneous. It is not conducive to the formation of uniform and fine solid grains during the subsequent partial remelting process. Equal channel angular pressing (ECAP) is a promising deformation method, which has the advantage of producing metal billets containing a large and strain without the variations of shape and dimension. [6] The size and roundness of solid grains are two critical factors for the thixoforming of semisolid billets. Solid grains with low roundness are not conducive to the smooth progress of thixoforming, and coarse solid grains will worsen the mechanical properties of thixotropic parts. [7] Therefore, it is necessary to minimize the size of solid particles and maximize their roundness to obtain excellent thixotropic products. As a matter of fact, the coarsening and spheroidizing processes are controlled by the coarsening dynamics of solid grains during partial remelting. The smaller the coarsening dynamics is, the more favorable for the alloy to maintain fine solid grains after thixoforming. Although a considerable amount of work has been devoted to the preparation of semisolid billets using the SIMA method, the studies have mainly focused on the effect of processing parameters on the semisolid microstructure, including liquid fraction and the size and roundness of solid grains. [8-10] Conversely, there is scanty research on the processing parameters