Microstructural mechanisms and mechanical behavior of friction-stir-welded Mg alloy laminate joints

Achieving reliable welding of multi-layered magnesium laminates is pivotal for advancing lightweight structural applications in energy-efficient transportation. However, the welding process and resultant joint properties of such complex laminates are issues of concern in material processing. This study aims to investigate the generic scientific principles governing the weldability and mechanical behavior of multi-layered Mg laminates using friction stir welding as a case study. An AZ31/Mg-6Zn-1Mn-2Gd/AZ31 laminate was successfully welded at 1000 rpm (rotation speed)-100 mm/min (welding speed) but not at 800 rpm-100 mm/min. The microstructural evolution, deformation behavior, and mechanical properties of the laminate joints were systematically investigated. The results indicated that both the AZ31 and Mg-6Zn-1Mn-2Gd layers in the laminate joint presented an approximately symmetrical texture distribution from the advancing side to the retreating side, with the texture intensity of the AZ31 layer higher than that of the Mg-6Zn-1Mn-2Gd layer. Tensile testing revealed strain localization, with mechanical properties (yield strength: 119 MPa, ultimate tensile strength: 211 MPa, elongation: 6.1 %) between single-material AZ31 and Mg-6Zn-1Mn-2Gd joints but worse than those of the initial laminate. The laminate joint ultimately fractured near the nugget zone interface on the advancing side. This was attributed to the fluctuation in the Schmid factor for basal slip and extension twinning, which were determined by the texture distribution. The current investigation provides insights into the correlation among the process parameters, microstructural evolution, and mechanical performance of Mg alloy laminate joints.