Spot friction stir welding of low carbon steel plates preheated by high frequency induction

• High frequency induction can be used as a preheating resource during the spot FSW.
• Preheating shows little influence on the microstructure in the stir zone.
• Preheating increases fraction of high angel boundary in TMAZ.
• Frictional heat can be reduced and can be compensated by high frequency preheating.
• Sound weld can be obtained at lower load and rotation speed by using preheating.

In this study, high frequency induction heating assisted spot friction stir welding was applied to 1.6 mm thick S12C low carbon steel plates. With the same welding parameter including an applied load of 2500 kg, rotation speed of 800 rpm and dwell time of 2 s, the average grain size in the stir zone slightly increased from 12.9 μm for the welds without preheating to 14.8 μm when 10 s preheating was used. However, larger joint interface was formed within the stir zone of the welds with preheating and therefore the bonding strength can be significantly increased. As a result, the shear tensile load of the joint increased from 8 kN to12.4 kN with preheating and the joint fractured through the plug failure mode rather than interfacial failure mode. It was revealed that the frictional heat generated between the rotating tool and the work-piece can be reduced to obtain sound welds by means of high frequency induction preheating.