Study On Brazing Process And Mechanism Of C/C Composites And Metals With Different Dissolution Behavior

C/C composite is a new type of lightweight heat-resistant material.It has a series of advantages,such as low density,low coefficient of thermal expansion,excellent ablation resistance and so on.It has a broad application prospect in the aerospace field.However,its machinability is poor,so it usually needs to be connected with high temperature resistant metal with good plasticity.Active brazing is an effective method to connect C/C composites and metals.In this paper,Ag-5Cu-1.25Ti-1Al(wt.%)active solder was used to braze two kinds of high temperature metal materials with different dissolution behavior,and the connection mechanism between them and C/C composites was studied.In this paper,two high temperature resistant metals Nb and GH3044 were used for active brazing with C/C composites respectively.The interface structure and mechanical properties of the brazed joint at room temperature and high temperature were investigated by scanning electron microscope(SEM),X-ray diffraction(XRD),energy dispersive spectroscopy(EDS),transmission electron microscope(TEM)and universal testing machine.The results show that the formation of Ti C is the key factor for wetting C/C composites,and the structure of Al-Cu-Ti intermetallic compound attached to particle Cu is formed in the middle of brazing joint.A multi-layer reaction layer structure is formed on one side of the metal base metal.When the brazing temperature is too high and the holding time is too long,C/C composites have defects,resulting in the decline of mechanical properties.The shear strength of C/C composite/Nb joint is the highest at room temperature at 950℃ for 600 s,which is55 MPa,and the shear strength is 35 MPa at 550℃.The optimal connection parameters of C/C composite/GH3044 are 930℃ for 600s.The shear strength of the joint is43 MPa at room temperature and 36 MPa at 550℃.The dissolution behavior of the two base metals is studied.With the increase of brazing temperature and holding time,the dissolution amount of the two base metals increases.In Nb system,there are no obvious defects with the change of dissolution amount,but in GH3044 system,microcracks will occur when the dissolution amount is too low or too much.The calculation model of base metal dissolved thickness at constant temperature is established by mass conservation and Nernst-Brunner equation.The main input variables in the model are brazing temperature,holding time and brazing gap.The calculated dissolved thickness of base metal by mathematical model is in good agreement with the measured dissolved thickness of base metal by orthogonal experiment.When the solder is not completely dissolved or the solder is lost in the actual brazing process,there will be a deviation from the calculation model.This model can effectively predict the dissolution of metal base metal and provide a basis for the optimization of process parameters.