Microstructure and Strength of Al2O3 – EN24 Vacuum Brazed Joints

In the present investigation, microstructural features and lap shear strength of vacuum brazed joints of polycrystalline alumina and low alloy steel EN24 were investigated. The joints were brazed in vacuum brazing furnace at 850°C by varying time using Ag26.7Cu4.5Ti (wt%) brazing alloy. The microstructure was characterized by optical microscope and scanning electron microscope (SEM). Qualitative and quantitative phase analysis was carried out from SEM Energy Dispersive Spectroscopy (EDS), Electron Probe Micro Analysis (EPMA) and X-Ray Diffraction (XRD) analysis. The presence of phases TiO₂, (Al,Ti)₂O₃, CuAlO₂, Cu₃(Ti,Al)₃O, Cu₂(Ti,Al)₄O and intermetallics of Cu-Ti were observed in the brazed joint near alumina interface; presence of Cu₃Ti, Ag-Cu hypo- and hypereutectic solid solutions and formation of FeTi and TiFe₂ intermetallics were observed in the brazed joint near En24 interface. The thickness of the reaction layers increased with increase in brazing time. Lap shear strength of the brazed joints increased from 56±16 MPa to 76±14 MPa with increase in brazing time from 15 minutes to 45 minutes.