| Titanium（Ti） alloys with extraordinary combination of high specific strength and excellent corrosion resistance are leading candidate materials in various engineering fields including aerospace and nuclear power generation industries. However, despite its property priorities, the applications of Ti alloys are considerably constricted primarily owing to its high fabrication cost. Exploiting novel low cost joining techniques specific to both Ti alloy/Ti alloy and Ti/dissimilar materials joining couples is a viable route to reduce the cost of Ti. In the current study, efforts have been made to overcome the difficult in farbrication of Ti-based filler for brazing Ti alloy by developing transient liquid phase（TLP） bondng technique using pure Ni, Cu and Cu-Ti-Cu composite interlayers. Meanwhile, partial transient liquid phase（PTLP） bonding procedure using Nb core interlayer specific to Ti alloy/dissimilar joining couple was developed as well, in an attempt to eradicate the detrimental effect of brittle interfacial reaction products on the mechanical performances of the joint.Brazing of CP-Ti using Ag-28 Cu and Ti-Zr-Cu-Ni amorphous fillers was conducted to investigate the evolution of interfacial microstructure and mechanical properties of brazed Ti joint. It was found that the brazed CP-Ti joint using Ag-28 Cu filler was dominated by Ti-Cu intermetallic compounds（IMCs）. The amount of Ti-Cu IMCs increased with increasing brazing time. Such IMCs deteriorate the strength of the joints. In the case of brazing CP-Ti using Ti-Zr-Cu-Ni amorphous filler at 900 °C, IMCs including Ti2 Cu and Ti2 Ni were generated within the joint. Upon prolonged soaking time, the Cu and Ni were progressively diluted and complete homogenization was achieved at 60 min, leading to the formation of IMCs free joint. The homogenized joint fractured in the base metal during tensile test, indicating that the joint exhibit mechanical properties superior to that of the CP-Ti substrate.In oder to overcme the difficult in fabricating amorphous Ti-based filler, TLP bonding of Ti64 using pure Ni and Cu interlayer was developed. It was found that eutectic liquid can be generated when the Ti64/Ni/Ti64 assemblly was heated to 960 °C. Upon subsequent annealing at 930 °C for 20 min, homogenized joint with base metal level mechanical properties was obtained. In the case of TLP bonding Ti64 using Cu interlayer, complete wetting of the faying surfaces can be achieved at 930 °C. Upon isothermal solidification and homogenization, solid state transformatin between TiCu, Ti2 Cu and β-Ti occurred, utimately resulted in homogenized joints consisting of lamellar Ti2 Cu distributing along α-Ti boundaries. The homogenized joints exhibited base metal leval strength with samples fractured in the substrate during tensile tests. In an attempt to ease the requirement in external pressure in TLP bonding using pure Cu interlayer, cladded Cu-Ti-Cu interlayer was designed. It was found that complete melting of cladded Cu-Ti-Cu interlayer and wetting of the sbstrate can be achieved at 930 °C without external loading. In the Cu-Ti-Cu case, the TLP bonded joints exhibited interfacial evolution procedure similar to that of the Cu interlayer case. However, owing to enhanced interaction between the completely melted interlayer, the homogenization was considerably accelerated in the cladded Cu-Ti-Cu case.Diffusion bonding of Ti64 and 304 SS using Nb/Cu interlayer was carried out. It was found that the Nb/Cu interlayer can effectively avoid the formation of IMCs between the substrates, leading to the formation of（Ti, Nb） and（Fe, Cu） solid solutions（s.s） at the Ti64/Nb and Cu/304 SS interfaces respectively. Using a consumable Ti intelayer at the Nb/Cu interface was demonstrated to be a feasible approach to overcome the immiscibility of Nb/Cu system. At the initial stage, the consumable Ti interlayer reacted with both Nb and Cu interlayer, leading to the formation of（Nb,Ti） s.s and a series of Ti-Cu IMCs at the Nb/Ti and Ti/Cu interfaces respectively. Upon prolonged holding time, Ti at the Nb/Cu interface was progressively diluted to the point that it is insufficient to form stable IMCs. Utimately, IMCs free joint consisting of（Ti, Nb） s.s/Nb/Nb-Cu-Ti s.s/Cu/（Cu, Fe） s.s was obtained when bonding at 850 °C for 60 min. Upon tensile loading, the homogenized joint fractured in the Cu interlayer in a ductile manner and a maximum tensile strength of 540 MPa was obtained, indicating that the brittleness of the joint was completely eradicated. Bonding temperature was found to have significant effect on the interfacial evolution. When the bonding temperature exceeded the Ti-Cu eutectic point, the Nb-Ti-Cu turned into a TLP bonding, leading to considerably enhanced homogenization of Ti between Nb/Cu.Inspired by the interfacial evolution of the Nb-Ti-Cu system, a PTLP bonding technique specfic to Ti64-Cu dissimilar joining couple was designed. Cu/Ti foils were cladded on Nb interlayer to fabricate a Cu/Ti/Nb/Ti/Cu multi-interlayer system. Upon heating up to 910 °C, Cu/Ti foil at both side of the Nb interlayer melted via eutectic reaction and wetting the Ti64 and Cu base metals respectively. During subsequent isothermal solidification and homogenization, dilution of Cu and Ti occurred at Ti64/Nb and Nb/Cu interfaces respectively. Utimately, Ti64/Cu joint consisting of Ti-Cu eutectoid structure/（Ti, Nb） s.s/Nb/（Nb-Cu-Ti） s.s was obtained when bonding for 60 min. Samples bonded for 60 min fractured in the Cu substrate during tensile test, indicating that the joint strength is superior to that of the Cu substrate.Joining CP-Ti to Inconel 625 via PTLP using the combination of amorphous Ti-Zr-Cu-Ni filler/Nb interlayer/AgCuTi active filler was conducted. With increased duration,（Ti, Nb） s.s were obtained at the CP-Ti/Nb interface by diluting Cu and Ni in the Ti-Zr-Cu-Ni filler. At the Nb/AgCuTi interface, diffusion of Ti in the active filler toward Nb resulted in robust metallurgical bonding, while bonding at the AgCuTi/Inconel 625 interface was achieved by the formation of（Cu, Ni） s.s. In such a maner, IMCs free joint was obtained by a simple PTLP route with thermal cycle similar to conventional brazing. Tensile test results demonstrated that the joint exhibited mechanical properties comparable with that of the CP-Ti base metal. |
