Microstructure and tensile behavior of a friction stir processed magnesium alloy

In this work the effect of multi-pass friction stir processing (FSP) followed by warm pressing on an as-extruded ZK60 Mg plate was investigated. The microstructure, texture and resulting mechanical properties are reported here. Multi-pass FSP to partial depths on the top and bottom plate surfaces produced a novel, layered structure with three distinct microstructural zones associated with stirred, transition and core regions. In the stirred zone, FSP, followed by pressing at 200 °C, created a 0.8 μm ultrafine grain size which accounts for ∼55 vol.% of the material. The transition region (∼10 vol.%), showed extensively sheared coarse grains distributed in a matrix of finer grains. However, the core region (∼35 vol.%) showed extensive twinning inside coarse grains in an overall bimodal microstructure reminiscent of extrusion. The processed Mg with a strong basal texture exhibited high yield strength (>300 MPa) and retention of adequate tensile ductility (>10%). The enhancement in mechanical properties of processed Mg is found to be highly influenced by the layered microstructure: UFG grained stirred zone, finer precipitates and strong basal texture.