| TiC reinforced titanium matrix composites(TMCs)has excellent properties such as high strength,high elastic modulus,good corrosion resistance,high temperature strength and high temperature creep resistance,which is applied in aerospace,petrochemical industry and some civilian industries areas.However,due to the brittleness of TMCs,it is difficult to deform.However,welding is proved to be an effective method to realize the complex structure and meet the needs of various working environments.Therefore,it makes sense to study the welding behaviors of TMCs.Compared with other welding methods,brazing has the characteristics of low cost,simple technology and high welding strength which make it the most widely connection methods for TMCs.In the present work,the parent material(Ti-6Al-3Sn-3.5Zr-0.4Mo-0.75Nb-0.35Si+5Vol%TiC)was brazed at different brazing temperature and holding time with Ti-28 Ni eutectic filler metal.The microstructure,the type of precipitated phase and mechanical properties of the joint were systematically studied by using optical microscope,SEM,EDS,XRD and mechanical testing machine.It shows that the brazing seam is mainly composed of three phases: Ti(s,s),Ti2 Ni and TiC.During the process of brazing,a strong diffusion reaction between the base material and the brazing material was carried out.The elements in the brazing seam include Ti,Ni,C,Al and other elements,which were homogeneously distributed in the brazing seam.With the increase of temperature or holding time,more filler metal was squeezed out of the brazing seam under the applied loads,resulting in the reduce of Ti2 Ni phase amount.Also,the width of the brazing seam increased gradually due to the further reaction between the filler metal and TMCs Moreover,with the increase of holding time,the size of TiC in the brazing seam increased,which can weak the dispersion strengthening of TiC particles,leading to the decrease of joint strength..Shear strength testing of TMCs brazing joints under different parameters was carried out at room temperature.The results have shown that all of the samples have a relative high strength.At the holding time of 10 min,with the increasing of brazing temperature,Ti2 Ni phase in the joint was decreased and the microstructures were more uniform,which is beneficial to promoting the strength.However,at the brazing temperature of 1080℃,a large number of coarse lamellar α structures were found in the joint,which is detrimental to the joint strength.Thus,with the increasing of brazing temperature,the joint strength increased first and then decreased.When the brazing temperature was 1010℃,the amount of Ti2 Ni in the joint decreased with the increasing of holding time,accompanied with the size of TiC phase increased.In addition,the microstructure in of the joint was smaller and more uniform when the holding time 20 min.So,the shear strength increases first and then decreases with the increase of holding time at the brazing temperature of 1010℃.When the brazing temperature was 1060℃,the strength of the joint decreases a little with the prolongation of the holding time.The strength of joint was influenced by dispersion strengthening,fine-grained strengthening,solid solution strengthening of elements,pinning effect of TiC and the content of Ti2 Ni phase,among which the content of Ti2 Ni phase plays a crucial role.By comparing the content of Ti2 Ni phase in the brazing seam,it was found that the strength of the joint increases first and then decreases with the increase of Ti2 Ni content.The joint shear fracture morphology has shown a large number of cleavage steps,dimples,tearing edges and secondary cracks,which demonstrated the quasi cleavage fracture mode of joints. |
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