Effect of Interlayer Composition on Microstructure and Properties of Titanium/Aluminum TLP Bond

Abstract

The transient liquid phase bonding of titanium and aluminum alloys was investigated, in which the interlayer was a 30-µm-thick brass foil and pure silver foil. The optimized variables used in this work are bonding temperature and time of 600°C and 30 min, respectively. The bonding temperature of 600°C (higher than both Al–Ag and Al–Cu eutectic temperatures) was selected to ensure melting. In both cases, an interesting intergranular eutectic-like phase was observed along the Al2024 grain boundaries. The evaluation of the interfaces of joints via optical microscopy, scanning electron microscopy, line scan, and X-ray diffraction demonstrated that a high temperature in the transient liquid phase process leads to the diffusion of alloying elements to form the molten eutectic. By holding the joint at the bonding temperature, the diffusion of alloying elements resumes, and the liquid interlayer solidifies isothermally because of the changes in the composition of the melted interlayer. The formation of Al2Cu, MgZn2, and TiMg3Zn15 phases was confirmed in the specimen made with brass foil as the interlayer. The interface width is decreased significantly from 8 µm to 2 µm by using silver foil instead of Cu–Zn foil as the interlayer. It is suggested that applying silver interlayer was successful in controlling the diffusion between two base metals and resulted in the formation of Ag2Al and AgMg phases on the fracture surface. This joint gave a higher shear strength of 154.7 MPa. The significant reduction in the strength of the bond due to using Cu–Zn foil as an interlayer (70.2 MPa) can be attributed to aggregation and growth of the brittle intermetallic compounds (IMCs) in the vicinity of the joint interface.