Aluminum/steel joints made by an alternative friction stir spot welding process

The effect of the rotational speed and dwell time on the joint interface microstructure and tensile-shear strength of friction stir spot welded Al-5083 aluminum/St-12 steel alloy sheets was investigated. Joining of the sheets was performed using an alternative friction stir spot welding (FSSW) process in which the welding tool tip did not penetrate into the lower steel sheet. Rotational speeds of 900 and 1100 rpm were applied in association with the dwell times of 5 to 15 s to weld the samples. Thermal history was recorded during the joining process. Interfacial microstructure, formation of intermetallic compounds (IMCs) at the joint interface and the fracture locations were studied using stereo, optical and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The used alternative process was successful to join the sheets. For both of the applied rotational speeds, there were optimum dwell times (10 and 12 s for the rotational speeds of 1100 and 900 rpm, respectively) to reach the maximum failure loads. The increasing trend of the strength as a function of the dwell time was related to the formation of a thin intermetallic (IM) layer at the joint interface. The decreasing trend was attributed to the formation of a relatively thick IM layer at the joint interface as well as the grain growth of aluminum at the exit-hole periphery where the final fracture occurred. The IM reaction layer thickness of 2.3 μm was found to be a critical thickness. Compared to the rotational speed of 1100 rpm, stronger joints were achieved by application of 900 rpm rotational speed.