Microstructure, crystallographic texture and mechanical behaviour of friction stir processed Mg-Zn-Ca-Zr alloy ZKX50

Abstract

The feasibility of applying friction stir processing (FSP) to locally homogenise and to refine the microstructure of a recently developed Mg-Zn-Ca(-Zr) alloy, ZKX50, has been examined. The application of a single pass of FSP resulted in dissolution of coarse intermetallic particles at dendrite boundaries and grain refinement, from an average grain size of ~60µm in the as-cast state to ~1.2µm. The microstructure in the processed zone exhibited banding as well as a variation in grain size across the stir zone (SZ). A second, fully overlapping FSP pass at a lower rotational speed, with the same or the opposite rotational direction, caused further microstructural refinement down to submicron levels and a more homogeneous grain size distribution. A strong basal fiber texture developed as a result of the first FSP pass; application of a second pass enhanced the crystallographic texture induced by the first one. Single-pass FSP resulted in doubling the microhardness within the SZ relative to that of the as-cast material, however, the hardness distribution within the SZ was non-uniform, in agreement with the banded microstructure observed. The application of a second pass brought about a further increase in microhardness, reflecting the more pronounced grain refinement observed. Tensile tests with loading along the processing direction did not result in an enhancement of the tensile strength over that of the base material, whilst the ductility as measured by the tensile elongation was doubled. Tensile tests on notched specimens with load applied in the transverse direction returned higher strength values. This anisotropy in tensile properties was explained in terms of the evolution of micro-texture.