Role of Fe2Al5 in fracture of novel dissimilar aluminum-steel resistance spot welds using multi-ring domed electrodes

In a comparative study of resistance spot welded (RSW) 1.2 mm AA6022 joined to 2.0 mm high strength low alloy (HSLA) steel and 1.2 mm AA6022 joined to 2.0 mm low carbon steel (LCS) it was found that quasi-static tensile tests and mini shear tests exhibited similar intermetallic compound (IMC) strength but different load capacity, i.e., nominal shear strength in tensile shear tests. This phenomenon was related to the Fe2Al5 structure and properties at the Al-steel interface which were unique for each stack-up. The thicker Fe2Al5 layer in 1.2 mm AA6022-2.0 mm LCS was comprised of fewer but larger grains which contributed to a larger difference between the Young's modulus of FeAl3 and Fe2Al5 than in 1.2 mm AA6022-2.0 mm HSLA. The Fe2Al5 in the AA6022-LCS stack-up was more brittle and rigid with greater nanohardness values and smaller variation in elastic modulus that may have contributed to lower overall joint strength with large variation. In addition, AA6022-LCS welds produced with non-optimized and optimized weld schedules produced different IMC shear strengths which was attributed to the different mismatch in Young modulus values that accentuated the differences in shear stresses at the FeAl3/Fe2Al5 interface.