Effect of post-weld heat treatment on mechanical properties and fatigue crack growth behaviour of friction stir welded 7075-T651 Al alloy

The effect of post weld heat-treatments (PWHT) on the mechanical and fatigue crack growth of friction stir welded 7075-T651 aluminum alloy was investigated. The PWHT of natural aging (NA), artificial aging at 190 °C for 24 h (HT1), and 423 °C for 12 h (HT2) resulted in significant improvement in the mechanical properties of the welded joint as compared to the base metal. The highest improvement in mechanical strength is seen in HT1 post heat treatment followed by NA and HT2 post heat treatments. The ductility of welded joint for PWHT at 190 °C with a soaking period of 24 h is about 70 % of the base metal. Crack growth tests at constant amplitude loading (CAL) and single tensile overload (OL) with an overload ratio of 1.8 were conducted at a stress ratio of 0.1. A difference in the crack growth rates after application of overload is seen in all PWHT conditions. Different heat treatments did not change the growth behaviour in base metal in both the loading cases. However, a slight difference in crack growth rates is observed due to the application of overload. The crack growth acceleration followed by delayed retardation is observed due to the application of overload. The post weld heat treatment enhances the retardation effect due to tensile overload and the degree of enhancement depends on the PWHT.