Effect of post-weld heat treatment on microstructure and hydrogen embrittlement of stir zone of friction stir welded 2xxx aluminum-alloy joints

Stir zone was weakest constituent of friction stir welded (FSW) aluminum-alloy joints, however, hydrogen embrittlement on the isolated stir zone has not been clarified. Here, effect of post-weld heat treatment (PWHT) on the microstructure and hydrogen embrittlement of the stir zone of FSW 2xxx aluminum-alloy joints is investigated by electrochemical hydrogen charging, tensile tests and advanced micro-characterization methods. Samples of stir zones extracted from the as-welded FSW joint and PWHT FSW joint are labeled sample A and sample B, respectively. Pristine sample A contains very few θ precipitates. PWHT causes abundant θ′ phase re-precipitation in sample B, following a (0 0 1)_θ′//(0 01 )_α-Al and [1 0 0]_θ′//[1 0 0]_α-Al orientation relationship. After PWHT, the tensile strength of stir zone could be increased by 9%, accompanied by the decrease in the plasticity, which is attributed to the dislocation movement obstruction and dislocation accumulation caused by θ′ phases. Notably, hydrogen embrittlement sensitivity index by tensile strength was 13.9% for sample B, much higher than that for sample A because of θ′ phase re-precipitation in the stir zone. Abundant θ′ precipitates in sample B obstruct dislocation movement, cause dislocation pile-ups, and function as hydrogen-trap sites enabling hydrogen-atom accumulation, leading to significant hydrogen embrittlement failure triggered by hydrogen-enhanced local plasticity.