Friction stir spot welding of SPCC low carbon steel plates at extremely low welding temperature

Abstract

Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50 rpm, which was much lower than that generally used for the conventional FSSW technique. Due to the very low heat input, the welding processes could therefore be completed at a peak welding temperature below 160 °C. As a result, a significantly refined microstructure with an average grain size of about 0.41 μm was formed in the stir zone of the joints and the J1{0–11}<−211> and J2{1–10}<−1–12> shear textures were the dominant components, which are different from the D1{11–2}<111> and D2{−1–12}<111> shear textures formed in the conventional FSSW joints. In addition, no heat affected zone could be detected along the cross-sectional plane of the joints. Although a few void-like non-bonded areas were still observed along the interface between the upper and lower steel plates, the shear tensile loads of the joints increased to about 10.0 kN when welded at a condition of 8 t, 20 rpm and 30 s, and the joints fractured through the plug failure mode.