The Formation Mechanism Of Titanium/Steel Resistance Brazing Joints Using Metal Powder As Transition Layer

Titanium alloy and stainless steel composite component has comprehensive advantages of two materials and good economic benefit,there is a wide range of applications in the aerospace industry and important areas of the national economy.However,because of the differences in the physical properties of titanium and stainless steel,and Fe-Ti intermetallic compound is easily generated during welding,leading to the deterioration of joints performance.This is the basic problem that should be dealed with for the reliable welding of most different kinds of metal.Aiming at the problem,a process method of resistance brazing technology for titanium and stainless steel using the metal powder as the transition layer was attempted.The influence of metal powder interlayer's category and technological parameters on the microstructure,mechanical properties and atomic diffusion of the joint was searched.Analyzing the metallurgical reaction principle of metal powder in the titanium and stainless steel joints.The main work and conclusions are as follows:(1)A process method of resistance brazing technology for titanium and stainless steel using the metal powder as the transition layer and smearing Heat-Resistant Structural adhesive as antioxidation measure was explored.According to the compatibility of metal elements,Cr?Ni?Nb and V were determined as object of investigationbased on the parent metal,five kinds of joints were designed.Accoeding to the test of mechanical properties and XRD?SEM microstructure analysis,effectiveness of adding elements for joint was explored.It was shown by the result that Cr?Nb and V can be the adding elements for the welding of titanium and stainless steel,this elements can increased the shear strength of joints,and the number of Fe-Ti?Cu-Ti intermetallic compounds was decreased obviously.A mass of Ti solid solution is formed in joints,it increased the performance of joints obviously.It was shown by the fracture analysis that the fracture of joint is ductile and intercrystalline mixed fracture model.(2)A process method of resistance brazing technology for titanium and stainless steel using the metal powder as the transition layer was explored.The optimum technological parameters were confirmed by the orthogonal experiment.And the influence of welding current,welding time,tip pressure,addition of powder,powder particle size on the microstructure and mechanical properties was searched.It was shown that the reliable joints were acquired when the parameters match well,bigger or lower parameter is harmful for joints.The shear strength of joints could relatively higher be 226.18 MPa when the welding current is 3.0KA,welding time is 2.0s,tip pressure is 0.40 MPa,additive amount of Cr is 0.03 g,additive amount of Nb and V is 0.04 g.The influence of metal powder as transition layer on the titanium/steel lap-welding joints atomic diffusion was studied.It was shown by analysis that the influence of Cr powder and Nb,V mixed powder on atomic diffusion behavior is different.Cr powder reacts less and keeps the particle shape to obstruct the diffusion of Ti and Fe;Nb and V mixed powder reacts more and the unreacted powder still obstruct the atomic diffusion.It can be insured that the remainder metal powder can obstruct the diffusion of Ti and Fe and enough powder can involved into reaction using reasonable additive amount of powder.(3)The behavior of matel powder metallurgy in the process of joint formation was discussed.When metal powder as the transition layer,powder particle can be regard as atom cluster.In the process of brazing,metal powder was coated by liquid brazing filler metal,the atoms on the surface break away from atom cluster into liquid brazing metal,and react with Ti,Fe and atoms of brazing metal Ag?Cu?Zn in liquid brazing metal.Metal powder not only obstruct the diffusion of Ti and Fe,but also change the the metallurgical reaction interface of joint.Metal powders react in a stripping mode.It changed the distribution of new phases and avoided the concentration of intermetallic compound.Because of the dispersed distribution of new phase,it increased the shear strength of joints obviously.The transition region on the side of titanium is weak area in joints because of more Cu-Ti intermetallic compound.Therefore,choosing the brazing metal which can reduces the number of intermetallic compound and appropriate homogeneous metal powder is a worth exploring solutions.