International audienceCompared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The "model chain" is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a "material by design approach" from process to performances

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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 J 2 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 dispersoı¨ds, 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.

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