Process Modeling to address Distortion issues resulting from Friction Stir Welding process

Ability to join thin structural components, reduced levels of residual stresses and resulting material behavior of the joints are driving the application of FSW (Friction Stir Welding) process to aerospace, defense and marine applications. While the level of residual stress is less severe compared to conventional welding process, structural distortions they induce dictate critical decisions on FSW process design. Designing the secondary processes to control these distortions to acceptable limits largely depends on accurate means to quantify these residual stresses. Even though the weld process in production environment is well controlled with respect to the shop floor environment, machine controls, fixtures and plate material, variations can still occur in the resulting part with respect to dimensional tolerances leading to rejections. Initial stage of this work is to carryout experiments on representative aluminum alloy plate sections welded under process conditions used on real structures in production, develop and calibrate micro models to represent experimental feedback including distortions as direct measure of residual stresses. Repeatability of the process measurements is also a key metric in designing these FSW weld experiments. These micro models represent basic weld process at the pin region that includes both thermal and thermo-mechanical process history along the weld path. Basic heat flux based thermal models and detailed stir process models at the weld pin are considered in this work. Material aging effects on plate distortions is also captured at this stage. Using calibrated micro models, for computational efficiency residual stress along the weld path are mapped to full scale part to drive the macro model response and capture distortions at the structure level. This modeling approach helps us understand fundamental nature of the FSW induced residual stresses on production parts, identify correct range of process conditions and optimize the secondary processes to mitigate the distortions to acceptable levels.