Microstructures of dissimilar friction stir welded joints between 2024-T4 aluminum alloy and Al/Mg2Si metal matrix cast composite

In this research, microstructures and hardness profiles across the interface of friction stir welded joints between monolithic 2024-T4 aluminum alloy (AA) and Al/Mg2Si metal matrix cast composite (MMC) and effects of one and two welding passes on these parameters were investigated. Results showed that there were complicated patterns of materials flow in the stir zone (SZ) especially in samples welded in two passes. This region was free of defects like voids and tunnel defects in samples welded in one pass. But some defects were formed in samples welded in two passes. Stirring caused fragmentation and rearrangement of Mg2Si large particles in the stir zone and thermo mechanically affected zone (TMAZ) of MMC, respectively. Eutectic phase has formed on the grain boundaries of 2024 AA in thermo mechanically affected zone due to high heat input to the joint area during welding. In samples welded in one pass, hardness increased from the base metal to the stir zone on the MMC side, reached a maximum in the middle of the joint interface and decreased at about 2 mm away from the joint interface center on 2024 AA side. But, hardness variation in the sample welded in two passes was complicated and there was alternating decrease and increase in hardness value at the joint interface.