Brazing parameters determine the degradation and mechanical behaviour of alumina/titanium brazed joints

The main aims of the present study are simultaneously to relate the brazing parameters with the correspondent interfacial microstructure, the resultant mechanical properties and the electrochemical degradation behaviour of commercially pure titanium/alumina brazed joints. A filler metal on the Ag-26.5Cu-3Ti system has been used. Three different brazing temperatures (850, 900 and 950°C) and three holding times (0.3, 1.2, 2.4 ks) were tested, in order to understand the influence of each combination of brazing temperature holding times, over the final microstructure and properties of the joints. The mechanical properties of the M/C joints were assessed on the basis of bond strength tests carried out using a shear solicitation scheme. The fracture surfaces were studied morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The degradation behaviour of the M/C joints was assessed by means of electrochemical techniques. It was found that a brazing temperature of 850°C and a holding time of 2.4 ks, produces the best results in terms of bond strength, 130 ± 16 MPa. The mechanical properties obtained could be explained on the basis of the different compounds identified on the fracture surfaces by XRD. On the other hand, a brazing temperature of 950°C produces the best results in terms of corrosion rates (lower corrosion current), 3.44 ± 0.73 A cm^–2. However, the joints produced at 850°C using a holding time of 1.2 ks present the best compromise between mechanical properties and degradation behaviour, 122 ± 12 MPa and 7.59 ± 1.47 A cm^–2 respectively. The role of Ti diffusion from the metallic Ti to the Al₂O₃ surface is fundamental in terms of the final value achieved for the M/C bond strength. On the contrary, the Ti distribution along the brazed interface does not seem to play any relevant role in the M/C joints electrochemical performance.