Influence of multi-pass friction stir processing on microstructure and mechanical properties of 7B04-O Al alloy

Three-pass friction stir processing (FSP) with different moving distances of the stirring tool between the two successive passes, 50% diameter of the pin (traditional way) and 50% diameter of the shoulder (novel way), was conducted on 7B04-O Al alloy. The result shows that an improvement in the mechanical properties of the processed zone is accomplished due to grain and second phase particles refinement. The hardness of the multi-pass FSP (M-FSP) sample is about HV 40 higher than that of the base metal. And the tensile strength of the M-FSP specimens is also significantly increased to about 1.4 times that of the base metal. Besides, the weak region of the processed zone is mainly dependent on the moving distance, where it is the previous pass stir zone in the traditional way and the transitional zone in the novel way. Increasing the rotational speed narrows the weak region in the novel way, while it does not in the traditional way.