Electrically assisted friction stir welding for joining Al 6061 to TRIP 780 steel

An electrically assisted friction stir welding (FSW) system has been developed, which enables a local electrical current field moving with the FSW tool without requiring the tool to be one of the electrodes. The effectiveness of electro-plastic effect, which describes a material softening phenomenon during plastic deformation induced by high density current, was investigated for FSW of dissimilar Al 6061 to TRIP 780 steel. Measured temperature results indicate that the electro-plastic effect could hardly be separated from resistance heating for this process. The relative significance of these two factors depends on welding parameters. Axial welding force can be effectively reduced with application of current and the phenomenon is more pronounced under a lower rotating speed and a smaller tool offset into aluminum. Microstructure analysis has been performed for cross sections perpendicular to weldline both at beginning and middle position of the weld. Enhanced formation of thin layer of intermetallic compounds and micro-interlock features at the Al-Fe interface can be observed in electrically assisted welding conditions, which are believed to be beneficial for joint quality.