Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling

This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5 m/min to 7.5 m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg₁₇Al₁₂ precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5 m/min, the tensile and compressive yield strengths improve significantly by 51 MPa and 114 MPa, respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5 m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5 m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.