Microstructure and mechanical properties of friction stir welded ultralight Mg-14Li-1Al alloy

Abstract

In this paper, commercially cold-rolled ultralight Mg-14Li-1Al (LA141) alloy plates having a thickness of 2 mm were successfully joined by friction stir welding (FSW). Compared with the traditional H13 steel tool, it can easily produce sound joints at low heat input condition using WC-Co cemented carbide tool. The grain size of the joints exhibited a clear inverse proportional relationship with the Zener-Hollomon ( Z ) parameter as ln d = 8.41–0.32 ln Z . The grain refinement mechanism was attributed to the combined effect of continuous dynamic recrystallization and geometric dynamic recrystallization. Ultrafine grained joint with a mean grain size of 1.3 μm can be achieved at extremely low heat input condition of 600 rpm and 200 mm/min. The joint had the highest strength of 184 MPa and a welding efficiency of 95.8%. The strengthening mechanism of the joint was considered as the synergic effect of grain boundaries, dislocations, and nanoscale MgAlLi 2 precipitations. A small amount of stacking faults and deformation twins also contributed to the strength improvement. The grain boundary strengthening was considered as the dominant strengthening mechanism. This study demonstrated that the LA141 alloy exhibited a good weldability when it was subjected to the FSW technology. It also provided a practical guidance for the application of FSW LA141 alloy in the field of lightweight manufacturing. • Ultralight LA141 alloy plates of 2 mm in thickness were successfully joined by FSW. • It was easier to obtain a sound joint using WC-Co cemented carbide tool. • Ultrafine grained joint with a mean grain size of 1.3 μm was achieved. • The dominant grain refinement mechanism was attributed to continuous dynamic recrystallization. • The FSW LA141 alloy joint exhibited a highest welding efficiency of 95.8%.