Research On Method And Mechanism Of Semisolid Pressure Reaction Brazing For 70% SiC_p/Al Composites

70%SiCp/Al composites are widely used in aerospace,weaponry and resonance prevention et al areas.However,the composites have very poor weldability due to the great difference in physical and chemical properties of the matrix alloy and the reinforcement.In this dissertation,a semisolid pressure reaction brazing method was put forward,applicable filler metal for the semisolid pressure reaction brazing of 70%SiCp/Al composites was developed,the best welding parameter were matched,reliable composites welding joints were achieved,the behaviors of Cu and Ti elements during the brazing were analyzed,the mechanism how Cu or Ti improves the wettability of the filler metal was explored,and the interface metallurgical bonding mechanism for the semisolid pressure reaction brazing of 70%SiCp/Al composites was revealed.Semisolid pressure reaction brazing is a pressure brazing method with powder filler metal,whose essence is to form weld metal and metallurgical bond with the base metal by pressure liquid phase reaction sintering.The whole process of semisolid pressure reaction brazing can be microscopically divided into three stages:cohere;sintered neck growth,liquid phase formation and chemical reaction;closed pore spheroidization and reduction.It can be macroscopically divided into several stages:liquid formation and particle rearrangement,the second liquid formation and pores filling,solid solution-precipitation,and solid skeleton formation.The major drivers of the metallurgical process are chemical reaction,the reductions of total surface free energy of the powder and lattice distortion energy.The Cu-contained filler metal Al-8.5Si-15Cu-4Ni-1.5Mg is suitable for the semisolid pressure reaction brazing of 70%SiCp/Al composites,whose solid-liquid coexistence temperature range is 573～6330C and wetting angle on the surface of composites is 85°.The shear strength of the joint semisolid pressure reaction brazed with this filler metal under 580℃,1.5h and 6MPa is 75.3MPa.An improved Ti-contained filler metal Al-1.9Si-0.1Mg-35Ti-15Cu-20In is also suitable for the semisolid pressure reaction brazing of 70%SiCp/Al composites,whose wetting angle on the surface of composites is 19°.The shear strength of the joint semisolid pressure reaction brazed with this filler metal under 600℃,1.5h and 6MPa is 90.2MPa.When the composites are semisolid pressure reaction brazed with Cu-contained filler metal,Al-Si-Cu and Al-Cu eutectic liquid phases form,which can reduce the melting point and improve the fluidity of the filler metal;Cu element reacts with the Al matrix of the composite to form CuAl2 intermetallic compound,which can improve the wettability of the filler metal on the surface of the composite to some extent;Moreover,Al and Cu element of the filler metal can react to form CuAl2 compound,which promotes the metallurgical bonding of weld metal and a small amount of tiny dispersive CuAl2 compound can enhance the weld and improve the property of the joint.When the composites are semisolid pressure reaction brazed with Ti-contained filler metal,Ti,Si and A1 elements of the filler metal react to form block and ring Ti7Al5Si12 phase,which can promote the metallurgical bonding of weld metal.Ti element can react with the exposed SiC particle of the composite to generate discontinuous Ti7Al5Si12 compound,which adhere to the SiC surface.The formation of Ti7Al5Si12 on the one hand translates the wetting behavior to reaction wetting,which improves the wettability of the filler metal,on the other hand changes the "SiC/metal" interface into "SiC/Ti7Al5Si12/metal"interface,which improves the interface bonding force and the mechanical property of the joint.The composites were surface titanizing treated before welding and their surfaces were covered by a continuous Ti7Al5Si12 compound after the treatment.After polishing and cleaning,SiC particles were still covered by Ti7Al5Si12compound,while the Al matrixes were exposed.In the semisolid pressure reaction brazing joints,not only the"SiC/metal"weak interfaces were replaced by“SiC/Ti7Al5Si12/metal" interface,but also retained the strong bonding interface of "Al matrix/metal".The mechanical properties of the joints were obviously improved.Finally,the shear strength of the 70%SiCp/Al composites semisolid pressure reaction brazed joint was 131.3MPa.