Synergistic effect of oscillating laser-Nb foil on microstructure and mechanical properties of TC4/7075 dissimilar joints

Oscillating laser welding of TC4 titanium (Ti) to 7075 aluminum (Al) alloys with assistance of Nb foil was performed in an overlap Ti-on-Al configuration, and Nb foil was determined to improve the metallurgical reactions based on analysis from thermodynamic calculations. The synergistic effect of oscillating laser-Nb foil on microstructure and mechanical properties of TC4/7075 dissimilar joints was investigated. Results show that the reduction of Ti-Al joint properties is mainly due to the weak regions at Ti/Al interface composed of TiAl and TiAl3 compounds, where lattice distortion inside TiAl3 and the dislocations in Al matrix grains adjacent to TiAl3 are detected. A sound Ti/Al dissimilar joint is obtained under the beam rotation of oscillating laser assisted by Nb foil. The joint connection area is increased and interfacial heat input is reduced with the beam oscillation, which arises from the weakened heat transfer in penetration direction and heat absorption of Nb foil, thus it is beneficial to reducing the Al melting amount and preventing the overmixing and mutual diffusion between Ti and Al. At the optimal welding parameters, the maximum tensile-shear property of joints increases from 1663 N to 2131 N, representing a 28.14% enhancement compared with that of non-oscillation mode. Moreover, oscillating laser produces a significant effect on the metallurgical regulation of Nb foil, under which the phase composition at Ti/ Al interface is replaced by Nb-containing Ti-Al compounds with lower brittleness, as well as some new phases like Ti2NbAl and NbAl3. However, when oscillation frequency is high, such as 100Hz, the melting amount of Nb foil is reduced, and the dual-interface joint is formed which is composed of Ti/Nb and Nb/Al interfaces, where the Nb metallurgy regulation is weakened.