Vacuum Brazing Ti-15-3 with a TiNiNb Braze Alloy

Among all types of brazing fillers, Ti-based fillers show satisfactory joint strengths in brazing titanium alloys. However, the major concern in using such fillers is the formation of Cu/Ni/Ti intermetallic compound(s) in the joint. In this study, a Ti-15-3 alloy was vacuum brazed with a clad Ti-35Ni-25Nb foil. The brazed zone consisted of a Ti2Ni intermetallic compound in a (beta-Ti,Nb)-rich matrix for specimen brazing at 1000 degrees C/600 s. Raising brazing temperature and time resulted in the Ti2Ni dissolving into the (beta-Ti,Nb)-rich matrix. For the specimen brazing at 1100 degrees C/600s, Ti2Ni could only be observed at the grain boundaries of the (beta-Ti,Nb)-rich matrix. After further raising it to 1200 degrees C/600 s, the Ti2Ni intermetallic compound was all dissolved into the (beta-Ti,Nb)-rich phase. The average shear strength was significantly raised from 140 (1000 degrees C/600 s) to 620 MPa (1100 degrees C/3600 s). Crack initiation/propagation in the brittle Ti2Ni compound with the cleavage fractograph were changed into the Ti-15-3 base metal with a ductile dimple fractograph. The advantage of using Nb in the TiNiNb filler foil was its ability to stabilize beta-Ti, and most of the Ni in the braze alloy was dissolved into the beta-Ti matrix. The brazed joint could be free of any intermetallic phases with a proper brazing cycle applied, and the joint was suitable for a few harsh applications, e.g., repeated stresses and impact loadings.