Investigation On The Process And Mechanism Of Microstructure Of Silicon Carbide/446 Stainless Steel Reactive Air Brazing Joint

As a typical high-temperature ceramic,silicon carbide had excellent mechanical properties,extraordinary ability of anti-oxidation of high temperature and chemical stability.Due to its high hardness and brittleness,however,it was difficult to manufacture large-sized and complex-structure products by conventional machining methods,which limited the excellent high-temperature performance of silicon carbide.446 stainless steel was a typical high-chromium ferritic stainless steel,with outstanding high temperature oxidation resistance and preeminent corrosion resistance,which had good plasticity and ductility,making it easy to process and join.The ceramic/stainless steel composite component obtained by joining silicon carbide with 446 stainless steel combined the superiorities of both ceramics and stainless steels,which had great potential in industrial production.Currently,widely-used conventional bonding methods?such as diffusion bonding,active metal brazing?were complicated in processing equipment and technologies and the joints were not resistant to high-temperature and did not have anti-oxidation capability.As a new bonding method,reactive air brazing was only utilized for the joining of oxides and perovskite ceramics,no report had been founded about characteristics of this bonding method of non-oxide ceramics so far.In this project,reactive air brazing was used to achieve the reliable bonding of non-oxide ceramics—silicon carbide—and 446 stainless steel successfully for the first time in high-temperature air atmosphere.In addition,evolution mechanism of organization and mechanical properties of joints were analyzed utilizing of scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffraction?XRD?and shear strength testing.Firstly,a layer of uniformly dense amorphous silica was prepared on the surface of silicon carbide by high temperature air oxidation aimed to improve the wettability of the silver-based brazing filler metal on the ceramic surface.When Ag-CuO brazing system was used for reactive air brazing of silicon carbide ceramics,it was found that Ag-CuO did not wetting ceramics well and could not form stable joints.Silicon carbide ceramics were brazed successfully using Ag-V₂O₅ filler system.The interface morphology,fracture morphology and phase composition of typical brazed joints were analyzed by SEM/EDS,XRD and transmission electron microscope?TEM?.The effects of ceramic oxidation time,brazing filler composition and brazing process on the interface morphology and shear strength at room temperature of joints were studied,the best process parameters were determined which were as follows:brazing filler composition was Ag-8mol.%V₂O₅,1050°C/30 min.During the brazing process,vanadium pentoxide oxidizes silicon carbide to silicon dioxide,which's thickness reached 5-8?m.The surface of the oxide layer remained an extremely thin amorphous C layer.Ag and amorphous C were stably combined in some way,ensuring the reliable brazing between Ag-based brazing filler metal and silicon carbide ceramics.The interface structure of the brazing joints was:SiC/SiO2/amorphous C/Ag+VO₂+V₂O₅+V_xO_y/amorphous C/SiO₂/SiC.The maximum shear strength at room temperature of the joints was 35 MPa.Reliable brazing joints between silicon carbide and 446 stainless steel were obtained using Ag-V₂O₅ brazing filler system.Then,the interface microstructure of joints was analyzed.Based on the background,the effects of filler composition and brazing processing parameters on interface microstructure morphology and shear strength at room temperature were investigated.The optimum processing parameters were determined as follws:Ag-8mol.%V₂O₅,1000°C/30 min.Under the circumstances,the maximum shear strength at room temperature of the joints reached 18 MPa.During the brazing process,V₂O₅ reacted with silicon carbide to form silicon dioxide and other vanadium oxides sunch as VO₂.The brazing seam was formed of Ag+VO₂ as the matrix.Part of low-valence vanadium oxides?such as V₂O₃?tended to accumulate on the surface of stainless steel and formed spinel phase—?Fe,Cr,V?₂O₃—with other metal oxides which could avoid excessive oxidation of 446 stainless steel.Another part of vanadium oxides was prone to gather together near the oxide layer on the ceramic surface continuously,which could be explained by lower surface energy between the oxides.The interface structure of the reactive air brazed joints of silicon carbide and 446 stainless steel was:446SS/?Fe,Cr,V?₂O₃/Ag+VO₂/amorphous C/SiO₂/SiC.Due to large coefficient of linear expansion of the Ag-based fillers,large thermal stress was formed between brazed joint and parent matrixs during the post-weld cooling process,which seriously affected mechanical properties of joints.The composite filler systems?Ag-V₂O₅-LiAlSiO₄?were built by adding negative expansion coefficient phase-LiAlSiO₄-to strengthen fillers,and the silicon carbide ceramics and 446 stainless steel were reactive air brazed by composite brazing filler metals.The mechanical properties of the joints were dramatically improved.The shear strength at room temperature of 446 stainless steel/SiC ceramic joint obtained at 1050°C/30 min using Ag-8mol.%V₂O₅-5wt.%LiAlSiO₄ composite fillers was 25 MPa,which was 39%higher than that of non-added reinforced phase.The hardness of the brazing joint was 145 HV,which increased70 HV.The deformation resistance of the brazed joints was significantly enhanced.Finally,based on above analysis,the mechanism of interface evolution of composite reactive air brazing joints consisited of 446 stainless steel and pre-oxidized silicon carbide ceramics was proposed.