Friction Stir Processing and Welding of Wrought and Cast Aluminum Alloys: Process Optimization and Thermal Modeling

Friction stir welding and processing (FSW/FSP) are solid-state techniques widely used for joining, localized microstructure/property modifications, and repairing in the aerospace sector.  Understanding their effects on microstructure and static and dynamic properties is critical in design for structural integrity.  In this study, four aluminum alloy systems (wrought 6061 and cast A356, 319, and A390) were friction stir processed using various processing parameters in both as-fabricated and pre-weld heat treated conditions.  The effects of processing and heat treatment on the resulting microstructures, hardness/micro-hardness, and tensile properties were systematically investigated and mechanistically correlated to changes in grain size, characteristic phases, and strengthening precipitates.  Tensile tests were performed in room temperature air both along and across the processing zones. Optimum processing parameter domains that provide both defect-free welds (evaluated using a new quality index) and good mechanical properties were determined for each alloy and associated to the thermal history of the process. A novel 2D heat transfer model was developed to predict weld temperature distributions and their evolution during processing. The results of these studies will be presented and discussed, together with recommendations for design and materials/process optimization.