Research On Joining Of Al₂O₃ Ceramics To Cu And TC4 Alloy

Joining of Al₂O₃ceramics to Cu and to Ti alloys are essential in electronic packaging and aerospace applications.In this study,vacuum brazing of Al₂O₃to Cu and to Ti6Al4V（TC4）alloys have been studied with Ag-26.7Cu-4.5Ti filler and refractory metal foils.The effect of refractory metal foils on the microstructure,mechanical properties and stress relief of joints has been investigated.Joining of Al₂O₃to Cu has been successfully achieved with Ag-26.7Cu-4.5Ti filler and W or Ta foil.The reaction of Ti and Cu in the filler with the Al₂O₃gives rise to the formation of a Cu₃Ti₃O reaction layer between the Al₂O₃and braze layer.The braze layer in the joint with Ag-26.7Cu-4.5Ti filler is mainly composed of Ag-based solid solution（Ag（ss））and Cu-based solid solution（Cu（ss））.For the joints with W or Ta foil,the braze area can be divided into braze layer I,refractory metal layer and brae layer II.Both the filler layers I and II are mainly composed of Ag（ss）and Cu（ss）.For the joint with W foil,the adsorption of Ti at the braze/W interfaces promotes the interfacial bonding between the liquid fillers and W layer,whilst slight diffusion and dissolution of Ta into the fillers take place in the joint with Ta foil.Joining of Al₂O₃to TC4 alloy has been performed with Ag-26.7Cu-4.5Ti filler and Mo or Nb foil.Similar to the Al₂O₃/Cu joining,a Cu₃Ti₃O reaction layer is formed close to the Al₂O₃substrate.Meanwhile,a Ti-rich diffusion layer is formed adjacent to TC4 due to the diffusion of Cu towards the TC4.The braze layer in the joint with Ag-26.7Cu-4.5Ti filler is mainly composed of Ag（ss）and Ti Cu.For the joint with Mo or Nb foil,the braze area consists of braze layer I,refractory metal layer and braze layer II.The braze layer I is mainly composed of Ag（ss）and Cu（ss）,while the braze layer II is mainly composed of Ag（ss）and Ti Cu.Notably,slight diffusion and dissolution of the refractory metal foil into the fillers occur during the joining.The shear strengths of Al₂O₃/Cu joints with W and Ta foils are 32.7 MPa and 37.3MPa,respectively,both higher than that without refractory metal foil（24.9 MPa）.The shear strengths of Al₂O₃/TC4 joints with Mo and Nb foils are 32.4 MPa and 27.9 MPa,respectively,both higher than that without refractory metal foil（23.8 MPa）.These results indicate that the application of refractory metal foils in Al₂O₃/metal joining is beneficial for the joint strength improvement.Introduction of refractory metal foils can improve the stress distribution and reduce the stress concentration in the joint since the coefficients of thermal expansion of refractory metals approach that of Al₂O₃.Furthermore,Ta and Nb foils with good ductility can inhibit the development of microcracks in the joint,thereby contributing to the joint strength promotion.