Multiscale Analysis of the Strength and Ductility of AA 6056 Aluminum Friction Stir Welds

The number of parameters affecting the friction stir welding process, the subsequent forming operations, and the structural integrity is very large: chemical composition of the two welded materials, welding parameters and thermal history, initial microstructure, flow properties of each alloy, etc. A multiscale analysis based on macro- and micromechanical tests has been conducted in order to determine and quantify the phenomena controlling the mechanical properties of joints made by welding AA 6056 Al alloys in a T4 or T78 state and to construct a predictive model for plasticity and fracture. Small tensile test samples were machined inside the various zones of the welds and parallel to the welding direction to identify the local plastic and fracture properties. Macrotensile tests using samples machined transverse to the welding direction and strain maps obtained by digital image correlation (DIC) provided information about the overall strength, plastic strain localization, and fracture of the joint. Three-dimensional (3-D) finite element (FE) analysis of the deformation of the welded samples loaded transverse to the welding line based on J₂ flow plasticity theory and on the parameters identified on the small test samples was used to quantify the effects of the geometrical, microstructural, and mechanical factors affecting the plastic flow localization process and the evolution of the constraint in the weak zone, which controls the damage rate. Uniform plastic flow is controlled not only by the yield strength mismatch between the weak zone and its surrounding but also by the strain hardening mismatch, both related to the precipitation of the Q phase. The ductility was addressed using a micromechanics-based damage model. A key ingredient of the model was to account for both large primary voids nucleated early on intermetallic particles and small secondary voids nucleated on dispersoïds, which have a first-order effect on the fracture of the AA 6056 Al alloy. The model is shown to capture very well the drop of the overall ductility in the welded joints.