2014 Volume 55 Issue 2 Pages 371-377
The effect of friction stir processing followed by an aging heat treatment on an extruded α/β dual-phase Mg–8.5Li–2.8Al–1.1Zn (LAZ931) alloy is investigated. The aim of present study is to explore its microstructural characteristic and tensile mechanical properties. The friction stir process reduces the extruded texture and causes a significant grain size refining effect on the coarse Mg-rich α-phase and Li-rich β-phase grains. An apparent decrease in the volume fraction of α-phase is confirmed, and it is resulted from the dissolution and solid solution of the α-phase within the β-phase matrix. The α/β-phase grain size refining and solid solution strengthening effects by the friction stir process can not only improve the microhardness within the stir zone, but also enhance the tensile strength of extruded dual-phase LAZ931-F alloy. With performing an aging heat treatment to friction stirred LAZ931-FSP alloy at 150°C, the microhardness is further increased with an age hardening effect by the precipitation of α-phase and metastable θ-MgLi2Al precipitate in the stir zone. However, the coarsening of precipitated α-phase at β-phase grain boundaries and the phase decomposition of metastable θ-MgLi2Al into a AlLi compound reduces the precipitation strengthening effect to the tensile strength of aged LAZ931-FSP/A alloy. Regardless of being aged state or not, applying a tensile force perpendicular to the stir processing direction causes a decrease in the tensile strength and elongation to friction stirred LAZ931-FSP alloys.