Study On Diffusion Bonding Process And Mechanical Properties Of Titanium Alloy Based On Molecular Dynamics

The titanium alloy diffusion bonding structure is widely used in aviation manufacturing because of its advantages of good welding quality and small stress deformation.The diffusion bonding process parameters determine the diffusion bonding quality of the titanium alloy,which determine the mechanical properties of the diffusion joint.In the thesis,molecular dynamics simulation is used to study the diffusion bonding process of titanium alloy,in order to provide theoretical support for the optimization of titanium alloy diffusion bonding process parameters and prediction of mechanical properties of titanium alloy connections.Firstly,the molecular dynamics analysis method of titanium alloy diffusion bonding was established based on the basic principle of molecular dynamics.And the random replacement method was used to obtain the initial TC4(Ti-6Al-4V)configuration of corresponding atomic percentage.Meanwhile,the process of phase transition of metastable phase was simulated with the combination of EAM potential and Morse potential which was carried out to simulate the precipitation process of new? phase from the metastable ? phase.According to the change of lattice structure during phase transition,the mechanism of phase transition was analyzed,and the stable TC4 titanium alloy biphase model was obtained as well.Secondly,based on the above model results,the Nose-Hoover temperature control method and Parrinello-Rahman pressure control method were applied to simulate the diffusion bonding process of TC4 titanium alloy under various process parameters.The atom concentration distribution under different conditions were obtained by using Matlab programming,the change rule of diffusion connection width with process parameters was also analyzed.The model with different scales and contact types was considered to study the effect of roughness on the closure of interface holes.The mean square deviation function(MSD)was used to get the diffusion coefficient of Ti,Al and V.Meanwhile,the Arrehenius equation of diffusion coefficient under different holding temperatures was established.The rationality of the simulation results were preliminarily verified by coMParison with the related experiments and simulated data.Finally,the tensile deformation behavior of TC4 titanium alloy diffusion joint was simulated in order to obtain the stress-strain curves under different technological parameters.Combined with Joseph Femandus empirical formula,the molecular dynamics prediction model of mechanical properties and process parameters was established.According to the atomic structure changes and the distribution of fracture zones during the process of tensile deformation,the micro fracture mechanism of TC4 titanium alloy diffusion joint was analyzed.