Dissimilar Joining of Low-Carbon Steel to 6061-T6 Aluminum Alloy by Novel Method of Pinless Friction Stir Spot Welding-Brazing

Abstract: In this research, AA6061-T6 aluminum alloy sheets were successfully lap-joined to St14 low-carbon steel sheets through the friction stir spot welding-brazing method with a pinless tool in the presence of a zinc interlayer. The effects of rotating speed and dwell time on the metallurgical structure and mechanical properties were investigated. It was observed that placing the pinless tool on the steel side melts the zinc interlayer and aluminum base metal and leads to forming different microstructures on both sides of the joint. The achievements demonstrated that the joints had a sound shape, and the maximum tensile-shear failure load (TSFL) obtained for all the joints was more than 4.2 kN. The specimen produced at the rotating speed of 1200 rpm and dwell time of 15 s represented a maximum TSFL and fracture energy of 7.4 kN and 35 J, respectively. Moreover, increasing the intermetallic layer thickness to almost 25 μm improved the TSFL. However, any further thickness growth caused an intense drop in the TSFL. The intermetallic layer, composed of brittle FeAl and FeAl phases, was formed near the Al-Zn alloy interface with a great amount of Al, and this culminated in an expansion of the joint surface. Furthermore, fracture surface analysis indicated the occurrence of a fracture at the interface. Some porosities with an average diameter of 50 to 255 μm were observed for the sample having the maximum heat input at the fracture surface of the aluminum side, which was probably due to the zinc volatilization causing a decrease in the mechanical properties. Graphical Abstract: [Figure not available: see fulltext.].