High-speed indirect extrusion of Mg-Sn-Al-Zn alloy and its influence on microstructure and mechanical properties

This investigation into the effect of indirectly extruding Mg-7Sn-1Al-1Zn (TAZ711) alloy at high exit speeds on the microstructure and tensile properties found no evidence of surface cracking, not even at the maximum speed tested of 27 m/min. This high-speed extrudability is attributed to the relatively high incipient melting temperature of the alloy (535 °C), which results from the formation of a thermally stable Mg2Sn phase. All extruded samples exhibited a completely recrystallized (DRXed) structure consisting of coarse DRXed grains with few particles in combination with relatively fine DRXed grains containing numerous fine precipitates. With an increase in extrusion speed, the total amount of Mg2Sn precipitates decreased and the size and quantity of coarse DRXed grains increased due to a rise in temperature during extrusion. This has the effect of reducing the strength of the extruded alloy through a reduction in precipitation and grain-boundary strengthening, yet the elongation of the extruded alloy remains essentially the same due to a loss of ductility caused by the increase in DRXed grain size.