Strengthening of Al/Cu dissimilar joint due to complicated interface produced by pulsed TIG welding with a constricted nozzle

Abstract

Al/Cu dissimilar joining is a key technology for weight reduction in fabricating high-functional products, however, conventional tungsten inert gas (TIG) welding is difficult to be applied due to a large fusion area. The constricted nozzle equipped inside the conventional TIG torch has been developed and can improve the position accuracy of tungsten electrode and arc plasma characteristics, moreover increase the heat input density. In this study, 0.5 mm-thick Al and Cu dissimilar sheets were butt welded using pulsed TIG welding with the constricted nozzle under two welding speed-current combinations of 50 mm/s-85 A and 100 mm/s-105 A. The 50 mm/s-85 A joint exhibited that Al and Cu mixed each other in the entire weld metal, containing two intermetallic compounds (IMC) of Al2Cu and Al4Cu9 mainly in the Al side. In contrast, both elements hardly mixed each other in the 100 mm/s-105 A weld metal and the interface exhibited hook-like shape at a nearly constant pitch, depending on the welding speed and pulse frequency. Consequently, the formation of IMC layers was limited. Based on the three-dimensional images reconstructed using a serial sectioning technique, the hook-shaped Al/Cu interface was also formed inside the weld metal. The mixed-zone volume per unit weld length of the 100 mm/s-105 A joint was estimated to be about 0.12 mm3/mm, and is much smaller than that of the 50 mm/s-85 A joint (0.97 mm3/mm). The average tensile strength of the 100 mm/s-105 A joint were higher than those of the 50 mm/s-85 A joint, suggesting that the hook-shaped Al/Cu interface together with the reduced mixed zone seems to increase the joint strength.