Microstructure evolution and mechanical properties of pulse high current diffusion bonding γ-TiAl alloy to Ti2AlNb alloy

The development of lightweight high-temperature materials and their processing methods are receiving increasing attention as the need for improved performance and energy efficiency in aero-engines increase. In this study, the gamma-TiAl alloy was successfully soundly bonded to Ti2AlNb alloy by pulsed high current (PHC) diffusion welding. The effect of different high level (HL)/low level (LL) of composite pulse current on the microstructure and mechanical properties of the joints have been studied. The representative interface microstructure of PHC-950 degrees C TiAl/Ti2AlNb joint was gamma-TiAl substrate/diffusion bonding zone/Ti2AlNb impacted zone/Ti2AlNb sub-strate. The diffusion bonding zone was composed of continuous equiaxed alpha 2 grains and a small amount of B2 and O phases. The alpha 2 phase in the diffusion bonding zone was mainly transformed from the gamma phase in gamma-TiAl substrate and the B2 and O phase in Ti2AlNb substrate. The composite pulse current has a significant promoting effect on the diffusion of elements and joint formation. The diffusion coefficients of DAl and DNb in hot-pressing (HP, without current effect) diffusion welding joint at interface were 2 x 10-14 m2/s and 0.92 x 10-14 m2/s, respectively. In the representative PHC-950 degrees C joint with HL/LL = 12/2 cycles (single cycle duration 3.3 ms), diffusion coefficients of DAl-12/2, DNb-12/2 at interface were 19.62 x 10-14 m2/s, 13.34 x 10-14 m2/s, respectively. Increasing the duration of LL would weaken the current effect. The current waveform parameters were tuned to HL/LL = 12/4 and 12/6 cycles, the diffusion coefficients of DAl-12/4, DNb-12/4 DAl-12/6 and DNb-12/6 were reduced to 15.69 x 10-14 m2/s, 12.49 x 10-14 m2/s, 15.31 x 10-14 m2/s and 10.99 x 10-14 m2/s, respectively. The PHC-950 degrees C joints have the highest shear strength of 257.3 MPa at HL/LL = 12/2 cycles, reaching 83.3% of the base material strength. The HP-950 degrees C TiAl/Ti2AlNb joint has a shear strength of 230.5 MPa, reaching 74.6% of the base material strength. The shear strength of PHC-950 degrees C joints at HL/LL = 12/4 and 12/6 reduced to 205.3 MPa, 195.8 MPa due to insufficient element diffusion and damages the bonding quality. This work provides a strategy that contributes to reduce the thermal damage of base metal and improve the joint forming efficiency.