Effects of pin size and of using an interlayer on interface bonding and fatigue strength of AA2024 to Ti6Al4V lap joints made using friction stir welding

A strong effort has continued to optimize friction stir lap welding (FSLW) process and to achieve high strength of dissimilar metal welds. For more than a decade, various studies have shown that Al-to-Ti FSL welds can be strong under quasi-static loading. Recently, Al-to-Ti FSL welds have also been shown to be strong under cyclic loading, due to the unique and thin diffusion bonding layer formed at the interface. In this study, how readily high fatigue strength of AA2024-to-Ti6Al4V FSL welds can be achieved is assessed. First, the effect of increasing the pin size on the fatigue strength of the welds has been evaluated. Second, fatigue testing has been conducted on the welds of which diffusion bonding is more ascertained by the use of an interlayer. It has been found that the fatigue strength of the welds is insensitive to the pin size. This will be shown to be the result of the width of the diffusional bond to be insensitive to the use of a larger pin or whether the pin has penetrated to the bottom plate. Simulation has suggested that stress concentration in locations of lapping ends is not significantly affected by the increase of the metallurgically welded width, explaining that pin size and penetration-dependent weld width do not affect the fatigue strength. It has also been shown that interlayer assisted diffusion bonding has affected insignificantly the fatigue strength of the welds. Thus, the high fatigue strength is insensitive to process variation.