Study On Low Temperature Diffusion Bonding Process And Mechanism Of Surface Nanostructured TC4 Titanium Alloy

Titanium alloys have low density,high specific strength and specific stiffness,good corrosion resistance,and excellent mechanical properties under high temperature service conditions.They are widely used in aerospace,automotive lightweight,and clinical medicine.Conventional titanium alloy welding has the disadvantages of high welding temperature and large deformation after welding,which will seriously affect the yield of finished products for products with higher dimensional accuracy requirements.To this end,in this paper,the surface nanofine layer is prepared by the SMAT method to achieve low temperature diffusion bonding,which reduces the deformation of the diffusion welding process.Firstly,the effect of SMAT process parameters on surface nanocrystallization was characterized.The effect of surface nanocrystallization on the surface properties of TC4 titanium alloy was investigated.The mechanism of surface nanocrystallization of TC4 titanium alloy prepared by SAMT method was analyzed and the best process parameters of the SMAT method was determined.The results show that the diffraction peak of TC4 titanium alloy is broadened after SMAT;when the treatment frequency is 20 Hz and the treatment time is 2h,the surface layer grain reaches 30 nm or less,and a 100 nm thick nano-fine layer is obtained on the surface of TC4 titanium alloy.The internal grain size is around 20 nm to 50 nm.Compared with the sample without SMAT,the surface roughness value did not change much,the range of variation was 0.1~0.2?m,and the nanoindentation hardness and microhardness of the surface layer increased.The surface nano-treated samples were subjected to low-temperature diffusion bonding experiments.The effects of surface nano-processing parameters on the microstructure of the joints were investigated.The influence of diffusion bonding process parameters on the microstructure and properties of the nano-jointed joints on the base metal was investigated.The optimal low temperature diffusion bonding process parameters for surface nanosized samples.The results show that the surface diffusion efficiency of TC4 titanium alloy is improved after the surface nano-treatment,and the joint welding rate is improved.It has the best at the diffusion connection temperature of 750°C,diffusion connection time of 60 min and diffusion connection pressure of 9Mpa.The joint of mechanical properties has a shear strength of 477 MPa.The influence of the nano-layering on the microstructure and properties of the diffusion joints was investigated,and the mechanical properties of the joints of the base materials were compared.The results show that the maximum shear strength of the joint obtained at diffusion bonding temperature of 750°C,diffusion connection time of 30 min and diffusion connection pressure of 7MPa is 332 MPa.