Improvements of strength and ductility in aluminum alloy joints via rapid cooling during friction stir welding

Microstructures, tensile properties and strain hardening behavior of a friction stir welded (FSWed) thick AA2219 aluminum alloy under optimized welding parameters and varying cooling conditions (air cooling and water cooling) were investigated with three slices (top, middle and bottom) through the plate thickness. While the yield strength was lower in the FSWed joints than in the base metal, the ultimate tensile strength of the FSWed joints with water cooling reached nearly that of the base metal. In particular, FSW resulted in a significant improvement in the ductility of the alloy due to the presence of recrystallized fine grains with fragmented and uniformly dispersed second-phase particles in the weld nugget zone. Water cooling resulted in both higher strength and ductility, but lower strain hardening capacity than that with air cooling during FSW. Compared with the middle and bottom slices, the top slice had a higher strength, but lower ductility and strain hardening capacity. While stages III and IV hardening occurred after yielding in both base metal and FSWed samples, the FSW led to higher hardening capacity and strain hardening rate and exponent mainly in the middle and bottom slices. The fracture surfaces after FSW exhibited more obvious ductile fracture characteristics with dimples and tearing ridges along with micropores.