Fatigue Crack Growth Behavior of FSWed Joint Joined with a Bobbin Type Tool in Different Aluminum Alloys

Fatigue crack growth (FCG) behavior at weld nugget zone (WNZ) and heat affected zone (HAZ) in friction stir welded (FSWed) joints joined by using a bobbin type tool in 5052, 6N01 and 7N01 aluminum alloys were investigated comparing to the base materials (BM). Constant stress amplitude fatigue crack growth tests were conducted with stress ratio of 0.1. Crack closure behavior was investigated simultaneously during fatigue test with unloading elastic compliance method. The results in this study showed that in near threshold region, FCG resistance in WNZ of FSWed 5052 and 6N01 joints was lower than that in the BM and the HAZ. In contrast, FCG resistance in WNZ of PS Wed 7N01 joint was higher than that in the BM and the HAZ. At high Delta K region, WNZ of FSWed 5052 joint had the similar FCG resistance with the BM and the HAZ. However, WNZ of FSWed 6N01 joint showed lower FCG resistance compared to the BM and the HAZ. FCG resistance in WNZ of FSWed 7N01 joint was the similar with that in the HAZ but was lower than that in the BM. Crack closure behavior was observed in BM and HAZ for all materials tested. In WNZ, crack closure was found in FSWed 7N01 joint but was not found in FSWed 5052 and 6N01 joints. Difference in FCG behavior at different weld region was mainly due to difference in crack closure behavior in FSWed 5052 and 6N01 joints. However, in case of FSWed 7N01 joint, it is considered that effect of microstructure was significant on FCG behavior even at high Delta K region.