Brazing Technology And Mechanism Of Porous Si₃N₄Ceramic And Invar Alloy Joints

Porous Si3N4ceramics is a new structural and functional composite materialwith alarge quantity of connected and closed holes. It possesses low permittivity,thermal shock resistance and antioxidation, which traditional ceramic doesn’t have.Thus Porous Si3N4ceramics is becoming the ideal wave-transparent materials ofultra-high-speed missile radome. In the process of the radome assembled and used,the radome meterial has to be effectively joined to metal ring, to obtain the product.Because of low coefficient of thermal expansion at the range of room temperature,Invar Alloy is becoming the preferred material of the metal ring. So this paper carrysout the research on joining the porous Si3N4ceramic and Invar Alloy. Theoptimization of filler metal and process parameters is used to control interfacialmicrostructure and improve mechanical properties.The Ag-Cu-Ti filler was introducted into joining the porous Si3N4ceramic andInvar Alloy. In this study, when the brazing temperature was higher, the residualstress at the interface of the joint increased resulting in a lower joint sheat strength.The shear strength first increasde and then decreased with prolonging holding time.When the holding time was shorter, a lower joint shear strength was obtained due toinsufficient reaction on the interface. And when the holding time was too long, thenumber of brittle compounds increased leading to a weakened joint. The highestjoint shear strength was received while the porous Si3N4ceramic and Invar Alloywas brazed with Ag-Cu-Ti filler at1093K for15min.The Ag-Cu-Ti/Ti composite filler was introducted into joining the porous Si3N4ceramic and Invar Alloy. The relationship among Ti foil thickness, brazingparameters and joint microstruction and properties was established. The bondingproperties became weaken because of the residual stress yieled by mismatch ofthermal expansion coefficient between porous Si3N4ceramic and Invar Alloy andthe formation of brittle compounds. In this study, Joint shear strength decreased withelevating brazing temperature, and first increased and then decreased withprolonging holding time. When the thickness of Ti foil was added from20μm to60μm, the joint shear strength first increased and then decreased. The highest shearstrength could be obtained with Ag-Cu-Ti/Ti composite filler under the condition ofbrazing temperature of1093K, holding time of45min and Ti foil thickness of30μm.In addition, microstructure evolution and joining mechanism were revealed bymeans of thermodynamics computation. At ceramic interface, Ti reacted with Si3N4ceramic forming the reaction layer. On the other hand, good metallurgical bondingcould be got at Invar alloy interface. Thus, a good Si3N4/Invar brazed joint was finally obtained.