Improved strength of Mg alloy extruded at high speed with artificial cooling

The application of artificial cooling to the high speed extrusion (13.5 m/min) of Mg-7Sn-1Al-1Zn alloy has been found to dramatically reduce the size of recrystallized (DRXed) grains, while also increasing the number of fine Mg₂Sn precipitates. This is attributed to a decrease in the temperature of the deformation zone, and has the effect of improving the strength of the extruded alloy. The increase in yield strength (YS) and ultimate tensile strength (UTS) is enhanced by increasing the feed rate of the cooling water due mainly to a decrease in the DRXed grain size. Significantly, this increase in strength does not come at the expense of a loss of elongation (EL), as the formation of twins that act as a crack initiation site is suppressed during tensile deformation by the grain refinement. As a result, high-strength TAZ711 Mg alloy (210 MPa YS, 303 MPa UTS, and 10.2% EL) can be obtained even with high-speed extrusion by using artificial cooling.