Friction stir spot welding of low-carbon steel using an assembly-embedded rod tool

•The AER tool can quickly increase the rate of the rise in temperature.•The depth of TMAZ is about 6mm for the AER tool at a dwell time of 100s.•The failure load for the AER tool is 1.7 times greater than that for the plain tool.•Using the AER tool to lap weld 3mm thick upper plate can achieve high lap strength.•The shear strength increases along with the depth of the stir zone. Friction stir spot welding (FSSW) has been conducted on a low carbon steel (SS400) plate pair with different thicknesses using the assembly-embedded rod (AER) and plain tools under a downward force of 8kN, a rotating speed of 1200rpm and a dwell time of 100s. The temperature at 2mm below the center of the stir surface rapidly increases to about 900°C using the AER tool, but it increases to about 510°C using the plain tool in primary dwell time. The rate of the rise in temperature using the AER tool is 1.7 times that using the pain tool. The depth of TMAZ using the AER tool is about 6mm; it is 2 times that using the plain tool. When the thickness of the upper plate is less than 3mm, the failure load using the AER tool is about 35kN. This is about 1.7 times higher than that using the plain tool. The failure load for a 4mm thick upper plate using the AER tool is still greater than that for a thinner plate using the plain tool. The strong bonding mechanism using the AER tool could be explained by diffusion reaction at a high interface temperature with a deep TMAZ.