Fatigue Behavior of Al/Steel Dissimilar Friction Stir Welds and the Effect of Die-Press Forming

6016-T4 aluminum (Al) alloy plate was butt welded to the cold-rolled steel plate by a friction stir welding (FSW) technique, and the fatigue properties of Al/steel dissimilar welds were investigated. Some dissimilar welds were die-press formed to the hat shape, and specimens were sampled from the corners of the hat shape, where severe plastic deformation occurred by the die-press forming. The die-press-formed samples had better tensile strengths and hardness than the as-welded ones because of the work hardening. It indicates that the die-press forming is applicable for the Al/steel dissimilar welds. However, large voids were formed around the steel fragments dispersed into Al matrix by die-press forming. Those voids acted as the fatigue crack nucleation sites, thus reducing the fatigue lives of the die-press-formed specimens. When the bottom side of the welds experienced tension plastic deformation, the fatigue strength was the lowest.