Research On Crack Resistance Of Vacuum Brazed Joint Of Al₂O₃ Ceramic And Austenitic Stainless Steel

The dissimilar materials bonding of Alumina ceramic and 304 stainless steel combined the benefits of ceramic and metal,having a significant potential for aerospace and electronical applications;But these two materials have large differences about physical and chemical properties,which lead to cracks of joint and poor wetting,Especially in the large ceramic/metal components,the cracking situation is much more serious,These defects severely limited the application and popularization of ceramic and metal composite components.In this paper,In order to study the cracks of Vacuum Brazed Joint of alumina Ceramic and Austenitic Stainless Steel,active brazing of alumina ceramic to 304 Stainless Steel was carried out using Ag-Cu-Ti brazing alloy.The cracks path of brazed joint were analyzed.The reaction product were analysed with EDS and XRD technology.Also,the effect of brazing parameters on microstructure and the cracks of alumina/stainless steel joint was investigated.The formation mechanism of the crack of Al₂O₃/SS304 joint was expounded.Finally,the method of adding Mo slice was used to reduce the joint stress,and the influence of the interlayer was studyed on the microstructure and crack of the joint.The crack of Alumina ceramic and 304 stainless steel brazed joint was successfully eliminated.And the connection mechanism of Alumina ceramic / 304 stainless steel joint was analysed.The research results are follows:Under the premise of good forming of brazing seam,there is cracking phenomenon in Alumina ceramic/304 stainless steel brazing joints.The cracks in the brazing joints of dissimilar materials between Alumina ceramic and stainless steel,performs for two fracture paths of mixed fracture and ceramic fracture.When the brazing temperature is high?above 850??,the holding time is long?more than 20min?and the cooling rate is fast?5?/min?,the joint has a large thermal stress,leading to the cracks occurred in ceramic firstly,and then the cracks extend along the ceramic under stress.When the brazing temperature was in the range of 810?-830? and the holding time was 5-20 min,the thermal stress of the joint was relatively more smaller.At this time,the interface is more affected by the microstructure of the joint,and when the brazing joint has a large amount of intermetallic compounds,the cracks are firstly produced in the brazing seam,and then the cracks are drilled into the ceramic for expansion.304 Stainless steel and Alumina ceramic were brazed with Ag-Cu-Ti filler metal,and the interfacial microstructure of 304 Stainless steel/Ag-Cu-Ti/ Alumina ceramic is as follows: Alumina ceramic/ TiO+Ti3Al/ Ag?s,s?+ Cu?s,s?+Fe2Ti/ 304 Stainless steel.With the increasing of brazing temperature,prolonging the holding time or thickening of the filler metal layer,the number and the size of Fe2 Ti intermetallic compounds increased,and the reaction layer of the ceramic interface increased,So the joint cracks becamed more serious.The heat treatment process at 600? for 30 min and cooling in the furnace effectively reduced the joint cracks and the joint stress,but it can't eliminate cracks of joint,the heat treatment process had no significant effect on the joint microstructure.Decreasing the cooling rate can inhibit the crack of the joint to a certain extent,but it can not be eliminated,and with the decreasing of the cooling rate,the amount of intermetallic compound in the joint increased and thickness of the reaction layer of ceramic interface increased.The addition of the Mo intermediate layer reduced the joint thermal stress.As the thickness of the Mo intermediate layer increases,the joint crack is further suppressed.When the brazing temperature was 820? for 10 min and the intermediate layer was0.2mm,there is no crack in the brazing joint.The addition of the Mo intermediate layer can hinder the diffusion of Fe element in stainless steel to the ceramic side to a certain extent.In the brazing seam which near the stainless steel side,there is a large number of Fe2 Ti intermetallic compound aggregate,and only a small amount of Fe2 Ti intermetallic compound particles in the brazing seam which near the Alumina ceramic side.The changing of the Mo intermediate layer did not have a significant impact on the joint microstructure.