Research On Brazing Sapphire And Titanium Alloy With Graphene-enhanced Composite Solde

As an optical material,sapphire is widely used in satellite,fighter,missile and submarine aviation fields.Ti-6Al-4V titanium alloy(TC4)has high specific strength,good heat resistance and corrosion resistance,and has been widely used in aviation.Therefore,the sapphire and the metal can be connected to obtain both excellent performance of the component,so as to maximize the excellent performance of sapphire.However,there are great differences in chemical and physical properties between sapphire and titanium alloy,especially the difference in thermal expansion coefficient.During the brazing process,the brazing joint will generate great residual stress,which greatly weakens the mechanical properties of the joint.Therefore,the GNSs-AgCuTi composite filler metal was prepared by mechanical ball milling,and the brazing connection between sapphire and titanium alloy(TC4)was realized under vacuum conditions.The effects of GNSs on melting point,wettability,interface structure and mechanical properties of AgCuTi solder were studied.At the same time,the influence of brazing process parameters on the structure and performance of joints is discussed.Finally,the interface behavior of GNSs in welded joints is analyzed.The GNSs-AgCuTi composite filler metal was prepared by mechanical ball milling method.When the graphene content in the composite filler metal is 0.5 wt.%,the minimum melting point of the composite filler metal is 777.4℃.Wetting experiments showed that the wettability of the composite filler metal was improved by adding an appropriate amount of graphene.When the graphene content increased to 0.3 wt.%,the wettability of the composite filler metal was the best.However,when the graphene content exceeded 0.3 wt.%,the dispersion of graphene in the composite filler metal was not uniform,resulting in poor wettability of the composite filler metal.Typical interface organization of TC4 and sapphire brazed joint is:TC4/Ti2Cu/TiCu/Ti3Cu4/Ag(s.s)/TiC+GNSs/TiCu/Ag(s.s)/Ti3Cu3O/sapphire.When the brazing temperature is 850℃ and the holding time is 10min,the maximum shear strength of the brazed joint is 28.5MPa.When the temperature is low and the holding time is short,the thin Ti3Cu3O compound is easy to fail under a certain load.When the temperature is further increased and the holding time is prolonged,Ti3Cu3O compound with a certain thickness can relieve some residual stress of the joint,thus reducing the risk of joint failure.However,when the brazing temperature is too high and the holding time is too long,the Ti-Cu intermetallic compound is fully grown,resulting in a decrease in joint strength.The results of fracture analysis show that the interface reaction layer with a certain thickness can improve the plasticity of the joint and the performance of the joint.Brazing titanium alloy and sapphire with composite filler with different graphene content.When the graphene content is 0.1 wt.%,the maximum strength of the brazed joint is 29.1 MPa.When the content of graphene is low,the brazed weld is distributed in a large number of Ti-Cu phase,resulting in a low shear strength of the joint.With the increase of graphene content,the growth of Ti-Cu phase was inhibited,and the shear strength of the joint was improved.When the graphene content is too much,the Ti3Cu3O interface layer reaction is discontinuous,and the sapphire side and the filler metal crack occurs,which reduces the joint strength.The C atom at the edge of graphene has high activity,and it is easy to react with Ti atom to form TiC.Particle TiC can be dispersed in the joint structure more evenly,thus playing a role of grain refinement.