Research On Process And Properties Of Indirect Diffusion Bonding Of TiAl To Ni-Based Alloy

With highly attractive properties of low density, high melting temperature, good elevated-temperature strength, good oxidation resistance and excellent creep properties, TiAl-based alloys have an excellent potential to become one of the most important high-temperature structural materials. Therefore, TiAl-based alloys are considered as potential replacements for Ni-based superalloys in aircraft airframes and turbin engines which can significantly lighten the weight of aircraft and improve work efficiency. Reliable joining of TiAl and Ni-based alloys (GH99) is crucial for this application. In this paper, pure Ti interlayer, Ti/Nb composite interlayer and Ti/Nb/Ni composite interlayer are used to diffuse bonding TiAl and Ni-based alloy. The interfacial microstructures of the joints obtained by using different interlayers and bonding parameters were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy (OM) and energy disperse spectroscopy (EDS), and their shear strength were determined by shear test. By contrasting investigation, the formation and evolution of interfacial microstructures of the joints obtained by using different interlayers were revealed and the models of diffusion bonding were built. The consistent relationship among interlayers, diffusion bonding parameters, interfacial microstructures and mechanical properties was clarified, and the cracking mechanism was analyzed.According to the investigation, the typical interfacial microstructure of the joint obtained by using pure Ti interlayer was GH99/(Ni, Cr)ss/rich Ti-(Ni,Cr)_ss/TiNi/Ti₂Ni/α-Ti +Ti₂Ni/Ti(Al)_ss/Al₃NiTi₂/TiAl. In this experiment system, the optimum bonding parameter was obtained: bonding temperature T=900℃, holding time t=30min and bonding pressure P=20MPa. The highest shear strength of the joint was 260.7MPa. Ti2Ni layer was the weak zone because it decreased the joint strength.The typical interfacial microstructure of the joint obtained by using Ti/Nb composite interlayer wasGH99/(Ni,Cr)_ss/Ni₃Nb/Ni₆Nb₇/Nb/(Ti,Nb)_ss/α-Ti+(Ti,Nb)_ss/Ti₃Al/TiAl. In this experiment system, the optimum bonding parameter was obtained: bonding temperature T=900℃, holding time t=30min and bonding pressure P=20MPa. The highest shear strength of the joint was 273.8MPa. The joint was fractured in the interface of Ni3Nb and Nb. Further increasing the bonding temperature and holding time, the joint was fractured in the Ni6Nb7 reaction layer, and the shear strength of the joint decreased. By using Ti/Nb composite interlayer, although the formation of Ti₂Ni reaction layer was avoided, the joint had great residual stress, so the shear strength of the joint had less improvement than by using Ti interlayer.The typical interfacial microstructure of the joint obtained by using Ti/Nb/Ni composite interlayer was GH99/Ni/Ni₃Nb/Ni₆Nb₇/(Ti,Nb)_ss/α-Ti+(Ti,Nb)_ss/Ti₃Al/TiAl. In this experiment system, the optimum bonding parameter was obtained: bonding temperature T=900℃, holding time t=60min and bonding pressure P=20MPa. The highest shear strength of the joint was 314.4MPa. The joint was fractured in the Ti3Al reaction layer. The shear strength of joint obtained by using Ti/Nb/Ni composite interlayer had been improved.TiAl and Ni-based alloy can be successfully diffusion bonded by using Ti interlayer, Ti/Nb composite interlayer and Ti/Nb/Ni composite interlayer. When using the Ti/Nb/Ni composite interlayer, Ni foil can effectively alleviate the residual stress of the joint, especially the residual stress at the side of Ni-based alloy, the fracture location of the joint changed from the interface of GH99/Nb by using Ti/Nb composite interlayer to the interface of Ti/TiAl by using Ti/Nb/Ni composite interlayer, and the shear strength of the joint had significant improvements. So the highest shear strength of the joint was obtained by using Ti/Nb/Ni composite interlayer.