Improving Mechanical Properties and Grain Refinement Mechanism of a High-Speed Extruded Zn-Free Mg-5.2Al-0.37Mn (Mass%) Alloy Using Large Extrusion Ratio

A commercial Mg-5.2Al-0.37Mn alloy (mass%, AM50) alloy was extruded using different extrusion speeds (3.5 m/min and 8.7 m/min) and ratios (20 and 50). The effect of the extrusion conditions on the mechanical properties and microstructures were evaluated. When the extrusion ratio of 20 was used, the extrusion at the high speed of 8.7 m/min resulted in grain coarsening. The 0.2% proof stresses in tension and compression (TPS and CPS) decreased to 187 MPa and 137 MPa, respectively, causing a large anisotropy (CPS/TPS = 0.73). In contrast, using the extrusion ratio of 50 was effective in refining grain structures even after the extrusion at the high speed, which contributed to the TPS of 208 MPa, CPS of 171 MPa, and an improved anisotropy of CPS/TPS = 0.82. Finite element modeling revealed that the extrusion at the high speed resulted in a substantial increase in temperature, which led to coarsening of grain structures. However, the extrusion ratio of 50 was effective in introducing large plastic strain, and a large density of dislocations was introduced by an enhanced activity of pyramidal slip, contributing to a grain refinement despite a large increase in temperature.