Effects of Extrusion Speed on the Microstructure and Mechanical Properties of Mg–9Al–0.8Zn–0.9Ca–0.6Y–0.5MM Alloy

Abstract: The effects of extrusion speed on the microstructure and tensile properties of a recently developed Mg–9Al–0.8Zn–0.9Ca–0.6Y–0.5MM (AZXWMM91100) alloy are investigated by extruding at various ram speeds of 1, 4, 7, 10, and 13 mm/s. Direct extrusion results reveal that numerous small edge cracks form in the sheets extruded at ram speeds between 4 and 10 mm/s, whereas severe hot cracking occurs during extrusion at 13 mm/s. All extruded sheets show a fully recrystallized grain structure containing second-phase particles aligned along the extrusion direction. The average size of the recrystallized grains gradually increases with the increasing ram speed, because a higher extrusion speed generates more deformation heat. However, the size, morphology, amount, and distribution of the second-phase particles are nearly identical in all the extruded sheets. As the ram speed increases from 1 to 10 mm/s, the tensile yield strength of the extruded material decreases from 205 to 125 MPa, which is attributed to the decrease in the grain-boundary hardening effect caused by the grain coarsening. The tensile elongation increases from 13.0% at 1 mm/s to 15.1% at 4 mm/s, and then greatly decreases to 2.9% at 10 mm/s. The drastic ductility degradation of the sheets extruded at the ram speeds larger than 7 mm/s is due to the formation of relatively coarse internal cracks in the material during extrusion. Graphic Abstract: [Figure not available: see fulltext.]