Mechanical Properties and Microstructure Analysis of High Power Ultrasonic Spot Welded AA2139-TiAl6V4 Dissimilar Joints

Dissimilar welding of aluminum alloy to titanium alloy is problematic due to the large difference in melting point of the two materials and the formation of potentially brittle intermetallic phase at the interface. High power ultrasonic spot welding is a low energy input solid-state welding technology that has potential capacity to solve this problem. In this study, AA2139 and TiAl6V4 dissimilar alloys were successfully welded by high power ultrasonic spot welding. The effects of welding time and natural aging on peak load and fracture energy were investigated. It was demonstrated that in ultrasonic welding, the temperature and time are such low and short that a brittle reaction layer does not form at the interface. Indeed, even using high resolution microscopy, no reaction layer could be detected until the weld was subjected to a prolonged post-weld heat treatment. The maximum joint strength and failure energy measured therefore greatly exceed those observed in Al-Fe and Al-Mg spot joints made using the same technique, in which a brittle reaction layer forms rapidly. The hardness profile of AA2139 side after welding was studied, demonstrating that the heat introduced by the welding process leads to some softening due to its effect on the strengthening precipitates in the alloy. This study has shown that high powered ultrasonic welding has potential as an attractive method for making joints between aerospace aluminum and titanium alloys with good mechanical properties.