Study On Diffusion Bonding Process And Joint Properties Of Ni₃Al-based Alloy

As a new generation of high-temperature structural materials,Ni₃Al-based alloys have excellent high-temperature strength,oxidation resistance and hot corrosion resistance.Compared with Ni-based alloy,Ni₃Al-based alloys have higher service temperature and lower density,which makes them have potential application value in combustion chamber,guide blade and turbine blade in the aero-engines.The realization of Ni₃Al-based alloy self-bonding and the exploration of a suitable joining process are of great importance to broaden the application of Ni₃Al-based alloy.Therefore,two bonding methods,including diffusion bonding and transient liquid phase（TLP）diffusion bonding,were used to join the Ni₃Al-based alloy in this project.For the diffusion bonding process,the Ni3Al-based alloys were direct diffusion bonded to themselves with and without Ni interlayers;In the process of transient liquid phase diffusion bonding,BNi-2 interlayer and Ni/Ti composite foils were respectively used for joining Ni₃Al-based alloys.After bonding,the microstructure and mechanical properties of the bonded joint were analyzed to explore the influence of process parameters on the microstructure and mechanical properties of the joint.Moreover,the formation mechanism of the joint was analyzed.A maximum shear strength of 689MPa and an interface bonding ratio of 95%were achieved for the Ni₃Al-based alloy joint that was direct diffusion bonded at 1100°C for60min under the pressure of 20MPa.Under this process condition,coarsening of theγ’precipitates in the joint were occurred.The coarsening ofγ’precipitate in the bonded joint was alleviated by the introduction of the Ni interlayer.When bonded at 1050°C with a 3μm electroplated Ni coating,the shear strength of the joint basically reached the level of the direct diffusion bonded joint bonded at 1100°C.Moreover,the degree of coarsening of theγ’precipitates was inhibited.In order to completely eliminate the coarsening of theγ’precipitates during diffusion bonding,the Ni₃Al-based alloy was joined by TLP with the BNi-2 interlayer.With the increase of process parameters,the range of diffusion affected zone increased,and the amount of B compound precipitations in the isothermal solidification zone decreased gradually.Shear strength reached the maximum value of 712MPa at 1100℃-2h.The presence of B compound precipitations in diffusion affected zone and isothermal solidification zone was eliminated by post bond heat treatment,and the shear strength of the bonded joint was further increased to 838MPa,and the fracture mainly occurred in Ni₃Al-based alloy substrate.In order to prevent the formation of precipitations containing MPD elements in the joint,the Ni/Ti composite foils were used for TLP bonding of the Ni₃Al-based alloy.With the increase of process parameters,the amount of Ti Ni₃phase decreased while that ofγ′-Ni₃（Al,Ti）phase increased due to the accelerated isothermal solidification rate.The shear strength of the joint first increased with the increase of bonding parameters and then tend to be stable.The maximum shear strength was 828MPa at1220℃-60min,and the fracture mainly occurred in the base metal.The shear strength of the joint tested at 650℃and 800℃was 625MPa and 533MPa,respectively.