Study On The Diffusion Bonding Of High Temperature Ti Alloy And TiAl Alloy Treated By Surface Nanocrystallization

Titanium-aluminum alloy has light specific gravity,high specific strength,good heat resistance,good creep resistance and oxidation resistance,and is the first choice of the next-generation aerospace material.At present,titanium alloys have excellent comprehensive performance and are widely used in the aerospace field.Researching the jointing between titanium alloys and titanium-aluminum alloys will facilitate the wide application of titanium-aluminum alloys in the aerospace field.The conventional diffusion bonding between titanium alloy and titanium aluminum requires a relatively high temperature and a long time.In order to further improve the diffusion bonding process,TC11 titanium alloy and TiAl were surface nanostructured by high energy shot peening,and the effects of surface nanocrystallization on the microstructure and properties of joints at different connection times,temperatures,and pressures were systematically studied,and the mechanism of joint formation was further analyzed.The layers are thermodynamically and kinetically calculated.In this paper,the surface integrity and microstructure of TC11 titanium alloy and TiAl alloy after surface nanocrystallization are studied.The experimental results show that the surface morphology of TC11 titanium alloy and titanium aluminum alloy after shot peening becomes uneven,the surface roughness increases sharply,and the integrity of the surface is seriously damaged.The surface of titanium alloy with shot peening has severe plastic deformation.The surface grain morphology has been resolved more than 5 minutes after shot peening.When the shot peening time reaches 10 min,a layer of random nanocrystalline with grain size of 20-50 nm is obtained on the surface of titanium alloy..Titanium-aluminum alloy undergoes shot peening and the surface laminae undergo bending plastic deformation.The TC11 titanium alloy and TiAl with surface nanocrystallization were diffusion bonded under different connection processes and the post-weld heat treatment was conducted on the joint with a low welding rate.The experimental results show that the surface nanocrystallization on the surface of TC11 titanium alloy can promote the diffusion reaction between titanium alloy and titanium alloy.Under the same connection conditions,the thickness of the reaction layer of the sample with surface nanocrystallization is significantly larger than that of the sample without surface nanocrystallization.Joints with optimum mechanical properties are obtained at lower temperatures and shorter times in the sample with surface nanocrystallization.Surface nanocrystallization can accelerate the closure of voids during subsequent heat treatment.The thickness of the interfacial reaction layer determines the mechanical properties and the fracture mode of the joint.When the interfacial reaction layer thickness is 1.7 ?m,the shear strength of the joint reaches the maximum of 280 MPa.For the simultaneous surface nanocrystallization of titanium alloys and titanium aluminum alloy diffusion bonding requires higher pressure to get completely jointed joints.The formation process of TC11 titanium alloy and TiAl joint is divided into physical contact,diffusion reaction and reaction layer increasing.After theoretical analysis,the surface nanocrystallization treatment promotes the increase of the reaction layer by reducing the element diffusion activation energy.The free energy of the diffusion reaction becomes negative,indicating that it can proceed spontaneously.The results of the reaction layer growth kinetics show that the surface nanocrystallization treatment can reduce the diffusion activation energy of the reaction layer growth.