Statistical and Metallurgical Analysis of Dissimilar Friction Stir Spot Welded Aluminum/Copper Metals

In this paper, friction stir spot welding of 2-mm-thick AA 1060 Al to T2 Cu is conducted. The effects of dwell time, tool rotational speed and shoulder plunge depth on tensile shear failure load (TSFL) are investigated. A quadratic regression model is proposed to fit the relationship between TSFL of the joint and the welding process parameters. The analysis-of-variance results show that the rotational speed is the most significant factor affecting the TSFL. Joint fabricated at the low rotational speed is weak-bonded by discontinuous intermetallic layers due to the insufficient heat input. Increasing the rotational speed can improve the TSFL by promoting intermetallics formation at the interface, while an excessive increase in rotational speed results in formation of microcrack at the interface, which seriously compromises the tensile property of the joint. Shoulder plunge depth, tool rotational speed and dwell time of 0.1 mm, 2371 r/min and 4.96 s, respectively, were identified as the optimal combination of welding process parameters. Results of microstructure characterization show that the joint produced at the optimized process parameters is well metallurgically bonded by a 1.9-μm-thick laminated intermetallics layer which comprises CuAl and CuAl₂ sublayers. Result of fracture path reveals that the fracture firstly extends along the interfacial CuAl₂ layer by a brittle mode and then propagates through the aluminum matrix in the stir zone by a ductile mode.