Diffusion bonding of Ni3Al-based alloy using a Ni interlayer

Abstract

Ni3Al-based alloys were direct diffusion bonded to themselves at 950–1100 °C for 10–60min under a pressure of 20 MPa. The effects of the joining parameters on shear strength and interface bonding ratio of the direct diffusion bonded joints were studied in detail. The maximum joint strength achieved was 689 MPa, when the joint was bonded at 1100 °C for 60min under 20 MPa, corresponding to an interface bonding ratio of 95%. However, the fracture surface of the bonded joint was characterized mainly by cleavage fracture, indicating that the fracture mode was brittle rupture. The formation mechanism of the direct diffusion bonded joint and the coarsening phenomenon of γ’ precipitates in the Ni3Al-based alloy were revealed. To inhibit the coarsening of γ’ caused by high bonding temperature, a Ni interlayer was introduced. When bonded at 1050 °C with a 30 μm thick Ni foil, the pure Ni interlayer completely vanished because of interdiffusion with the Ni3Al-based alloy; however, the performance of the joint was still inferior to the base metal because of the heterogeneity in the microstructure of the diffusion zone. Therefore, the fracture occurred mainly at the diffusion zone. When bonded at 1050 °C with a 3 μm thick electroplated Ni coating, the microstructure of the diffusion zone was consistent with that of the base metal because of sufficient interdiffusion. The shear strength basically reached the level of the direct diffusion bonded joint at 1100 °C. The fracture morphology changed from cleavage fracture to alternating dimples and facets after introducing the electroplated Ni coating.