Effect of FSW process parameters on microstructure and mechanical properties of the dissimilar AA2024 Al alloy and 304 stainless steel joints

The effect of friction stir welding (FSW) parameters on microstructure and mechanical properties of the dissimilar AA2024 Al alloy and 304 stainless steel joints was elucidated. The results showed that tool offset had a crucial effect on the properties. The optimum condition was obtained at a rotational speed of 750 rpm, a traverse speed of 65 mm/min, and a zero tool offset. At positive offsets and lower heat inputs, tunnel and interfacial gap defects were formed. At negative offsets and higher heat inputs, thick intermetallics compounds (IMCs) were formed. However, at optimum condition, sound and serrated interface, composite like the structure of stir zone (SZ), and thin IMCs around steel particles in Al matrix caused higher strength and ductility with a dimple like a fracture mode. During FSW, a high-strained ultra-fine grained (UFG) layer with a thickness of 5 μm is formed in the interfacial area, which causes micro cracks poor in iron. Aluminum fills these micro cracks, and then large steel particles dispatch towards the SZ. Finally, IMCs form around the dispatched steel particles concurrent with their further fragmentation in SZ. Thus, a composite structure composed of dynamically recrystallized aluminum matrix and UFG steel particles surrounded by IMCs forms in SZ.