Mechanisms of extrusion-ratio dependent ultrafine-grain fabrication in AZ31 magnesium alloy by continuous squeeze casting-extrusion

Abstract

Ultrafine-grained AZ31 alloy was successfully fabricated by the novel continuous squeeze casting-extrusion process with different extrusion ratios (ER). The average grain size (AGS) reduced first and then grew as the extrusion ratio increased from 4 to 36. Increasing the extrusion ratio cannot ensure the refinement of AGS. The dynamic recrystallization behavior was successively promoted by the increase of ER. But the average grain size reached the minimum with ER of 25, since the size of recrystallized grains increased in large extrusion ratio. The rise of extrusion temperature contributed to the growth of recrystallized grains. The texture evolution resulted in some extruded rods with different ERs exhibiting distinct mechanical property characteristics, despite similar AGS. The ultrafine-grained AZ31 alloy with ER of 16 presented a strong texture, and the AGS of 1.90 μm is achieved. It displayed a high ultimate tensile strength of 332.08 MPa, yield strength of 221.04 MPa, and elongation of 16.9%. When ER increased to 25, the finest microstructure was obtained with the AGS of 1.85 μm. Its elongation reached maximum of 20.9% due to the weak texture and ultrafine-grained microstructure, although the ultimate tensile strength and yield strength slightly reduced. Microstructure distribution, dynamic recrystallization behavior, and texture evolution were investigated and discussed in this study.