Microstructure, shear strength and failure mechanism of TZM/graphite joints bonded by a SPS pressureless brazing technique

This study developed a spark plasma sintering (SPS) pressureless brazing technique to join TZM alloy and graphite, using a self-designed mold with Ti foil as the interlayer. The effect of brazing temperature on the microstructure and shear strength of the TZM/graphite joints as well as the interfacial failure mechanism were investigated. At the optimal brazing temperature of 1580 °C, the joint interface exhibited no detectable defects such as cracks or un-welded regions, with the microstructure consisted of island-like TiC, plate-like TiC, finger-like TiC coexisted with (βTi, Mo) solid solution, and continuous (βTi, Mo) solid solution. The shear strength of TZM/graphite joint brazed at 1580 °C reached a maximum of 55.34 MPa, with fracture predominantly occurred at the bonding interface between graphite and plate-like TiC. The island-like TiC, formed by the reaction of the partially melted Ti diffusing into graphite pores, promoted crack deflection or even crack arrest, thereby enhancing the interfacial strength. Fracture analysis revealed a mixture of intergranular fracture along the granular TiC and transgranular cleavage fracture within island-like TiC. This study demonstrates the feasibility of SPS pressureless brazing for fabricating high-strength TZM/graphite joint with favourable microstructural characteristics.