Mechanical properties of tailor welded Al/Steel blanks made by friction stir welding and the effect of post heat treatment

Abstract

Aluminum (Al) alloy, A6061-T6, plate and stainless steel, SUS304, plate with different thicknesses were butt welded by a friction stir welding (FSW) process to fabricate tailored blanks. The Al plate was thicker than the steel plate. The FSW tool was offset to the Al side and the probe was inserted only into the Al plate. The shoulder of the tool was also plunged only into the Al plate because of the different thicknesses of the plates. The microstructural observation and X-ray CT revealed that small steel fragments were dispersed in Al-side stir zone (SZ) due to the scratching of steel side surface by the probe. The hardness of 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 thermo-mechanically affected zone (TMAZ) in Al side. The tensile failure occurred at TMAZ because of the bonding strength between Al and steel was higher than TMAZ. However, the tensile strength of the dissimilar weld was approximately 38% lower than that of the aluminum alloy base metal due to the softening at TMAZ. Post heat treatment increased the hardness at TMAZ, whilst the tensile strength was still about 33% lower than the base metal.