Fatigue behavior of dissimilar A6061/rolled steel (SS400) friction stir welds

摘要

The dissimilar welds between aluminum (Al) alloy, A6061-T6, and rolled steel, SS400, had been fabricated by a friction stir welding (FSW) technique. The FSW tool was offset to the Al side and the probe was inserted only into Al plate. Based on the microstructural observation, small steel fragments were recognized in Al side due to the scratching of steel side surface by the probe. Subsequently, hardness, tensile and fatigue tests of Al similar and Al/steel dissimilar welds had been conducted. The hardness of stir zone (SZ) in Al side was lower than the base metal due to the dissolution of hardening precipitates during welding, where the lowest hardness located near the interface and thermo-mechanically affected zone (TMAZ) in Al side. The tensile strength of the dissimilar weld was lower than that of the similar one. Static fracture of the dissimilar weld occurred through the interface, where the hardness was the lowest, while through the TMAZ in the similar weld. The dissimilar welds exhibited lower fatigue strengths than the similar ones. The fractographic analysis revealed that fatigue crack initiated at the interface between Al and steel, but propagated through Al alloy near the interface. Based on the fracture mechanics, it was concluded that the lower fatigue strengths of the dissimilar welds could be attributed to the lower fatigue crack initiation resistance.