Effects of TiZn3 and TiZn16 components on the microstructure and mechanical performance of Ti-6Al-4 V alloy joints formed via ultrasonic assisted brazing using pre-galvanized workpieces.

While the use of a Zn interlayer has been demonstrated to reduce the temperature required for joining easily oxidized metal alloys in atmospheric environments, the effects of reactions between the titanium alloy workpieces and the Zn interlayer on the mechanical performance of the finished joints are poorly understood. The present work addresses this issue by evaluating the chemical compositions at the interfaces of pre-galvanized Ti-6Al-4 V alloys joined at 420 °C in an atmospheric environment by ultrasonic-assisted brazing, and relating the observed compositions to the mechanical performances of the joints. The Ti-6Al-4 V alloy workpieces are first wetted by pure Zn using an ultrasonic assisted hot dip galvanizing (U-HDG). The obtained ultrasonic excitations are demonstrated to destroy the oxide film on the surfaces of the Ti-6Al-4 V workpieces and promote reactions between Ti and Zn at the interfaces. The plating of Zn on the workpiece surfaces is demonstrated to be realized by the formation of intermetallic compounds (IMCs) comprising a uniform TiZn3 layer in contact with the Ti-6Al-4 V surface, followed by a mixed TiZn3 + TiZn16 layer and a η-Zn layer at the outer surface. Application of the ultrasonic-assisted brazing process is demonstrated to maintain uniform TiZn3 layers next to the Ti-6Al-4 V surfaces, while the concentration of the TiZn16 phase near the midpoint of the joints increases with increasing ultrasonic treatment time (UST) from 5 s to 20 s, and the corresponding concentration of the η-Zn phase decreases. The results of mechanical testing demonstrate that the shear strength of the joint obtained with a TiZn3 layer thickness of 8-12 µm and a UST of 10 s is 210 MPa, which is 3.55 times greater than that obtained for joints processed without pre-galvanization.