Multipass friction stir welding of dissimilar Al–Si and Al–Mg cast aluminum alloys

In this study, dissimilar cast Al–Si and Al–Mg alloys were joined together using friction stir welding (FSW) technique, employing different levels of heat input and various numbers of FSW passes. For this purpose, two sets of welds were prepared with either low or high heat inputs. Each sets consisted of three welds with one, two, or three FSW passes. The microstructure of the welds was examined using optical and scanning electron microscopes on the weld cross section. The phases in the stirred zone (SZ) were identified using x-ray diffraction method. Hardness profiles and tensile properties were examined to understand joint efficiency. Based on the microstructural analysis results, three distinct regions were identified in the SZ of all welds, including the deformed Al–Mg base metal (region 1), the deformed Al–Si base metal (region 2), and the region with interdiffusion of the two base metals (region 3). Increasing the number of FSW passes increases the fraction of region 3 in the SZ, indicating enhanced stirring and interdiffusion of the two base metals. Following these microstructural changes in the SZ, the welds exhibited a significant improvement in strength and ductility with an increasing number of FSW passes and heat input. The weld with three FSW passes and higher heat input demonstrated the highest strength and ductility, reaching the strength of 187 MPa and ductility of 8.3 % which corresponded to 92 % of the strength and 106 % of the ductility of the Al–Si (the weaker) base metal, respectively. It was concluded that an increase in the number of FSW passes, altered the bonding mechanism from predominantly mechanical interlocking in single-pass welds to robust metallurgical bonding in three-passes welds.