The Study On Microstructure And Mechnical Property Of Al₂O₃ Ceramic And 310S Stainless Steel Reactive Air Brazed Joints

Due to the excellent properties,such as high strength and hardness,outstanding thermostability and good electrical insulation,Al₂O₃ ceramic is regarded as the most promising structure material for applications in electronics,aerospace nuclear and automotive industries.However,because of Al2O3’s intrinsic brittleness,it is difficult to manufacture large and complex parts.Realizing the reliable connection between Al2O3 ceramic and stainless steel can complement each other’s advantages and extend Al2O3 ceramic application range.A reliable connection between Al₂O₃ ceramics and 310S stainless steel is realized in air atmosphere by reactive air brzaing using Ag-CuO and Ag-CuO+ZrO2 system as brazing filler metal respectively.The wettability and wetting mechanism of brazing filler metal on the base metal were studied.The effects of parameters and ZrO₂ ceramic particle content on interface microstruce,mechanical properties and fracture morphology of Al2O3/310S joint were investigated.The wettability of Ag-xmol%CuO（x=0,2,4,6,8,12）filler metal on 310S stainless steel and Al₂O₃ ceramic at 1050℃/20min was investigated in this paper.The result shows that pure liquid Ag can not wet 310S stainless steel because the MnCr₂O₄ and Cr₂O₃ protective films on the 310S stainless steel have hindered the wet process.The new reaction layer formed by the additive reaction of the CuO in Ag-CuO brazing filler metal with the protective film can be wetted by the filler metal.With the increase of CuO content,the weting interface reaction layer thickened and wetting angle decreased.The reaction layer is complex oxide reaction layer mainly composed of CuCrMnO₄,CuCr₂O₄,CuFe₂O₄,NiFe₂O₄ and small amount of Fe₂O₃.The wetting experiment of Al₂O₃ ceramics shows that the pure liquid Ag can wet the Al₂O₃ ceramic by the reason of it has a high oxygen content and strong erosion effect in the air.the CuO in Ag-CuO filler metal can react with Al2O3 ceramics to produce CuAl2O4 and cause the wetting angle to be further reduced.With the increase of CuO addition,the wetting interface reaction layer gradually thicken and the wetting angle decrease.When CuO exceeds 6.6mol%,the reaction layer thickness will no longer increase and the wetting angle will also become stable due to the hindrance of interface reaction layer on element diffusion.Ag-xmol%CuO（x=2,4,6,8,12）braze filler metal was used to braze Al2O3 ceramic and 310S stainless steel by reactive air brazing.The result showed that the joint interface was mainly consists of three zones:composite oxide reaction layer adjacent to 310S stainless steel and its composition is same as the wetting interface;Ag（s,s）which is containing oxide particles in the middle;the compound oxide reaction layer adjust to Al₂O₃ Ceramic and mainly includes Fe（Al,Cr）₂O₄,MnAl₂O₄,CuFe₂O₄,CuCr₂O₄,NiFe₂O₄ and a small amount of CuAl₂O_4,FeO.The influence of CuO content.Brazing temperature and holding time on the microstructure and mechanical properties of the joint are significant,the maximum shear strength was 44Mpa when the joints were brazed Ag-4mol%CuO filler metal at 1050℃for 15min.The fracture morphology includes ductile fracture and plastic fracture with different proportions.In order to reduce the residual stress and further improve the strength of joints,Ag-4mol%CuO+Ywt%ZrO2（Y=2,4,8,12）composite filler was used to brazed Al₂O₃ ceramic and 310S stainless steel at the optimal parameters.The results show that the addition of ZrO2 ceramic particles can not only play the role of dispersion strengthening to joints,but also reduce the joint residual stress and improve the shear strength of joints.The maximum shear strength of joints reached 53Mpa when brazed with Ag-4mol%CuO+4wt%ZrO₂ at 1050℃for 15min and fractures propagated primarily in Ag（s,s）.