Effect of Nb or Ta Interlayer on Microstructure and Mechanical Properties of Graphite/Ti6Al4V Alloy Joints

Abstract

Graphite/Ti6Al4V (TC4) joining has been conducted with Cu–Ti–Ni filler and refractory metal (Nb or Ta) interlayer. Microstructural characterizations reveal that defect‐free interfaces are obtained among the refractory metal interlayer, fillers, and substrates, suggesting favorable interfacial bonding in the joints. The diffusion and dissolution of Nb or Ta atoms into the molten fillers occur during joining, leading to the formation of (Ti, R) solid solution ([Ti, R]ss, R = Nb, or Ta) in the filler layers and thickness decrease in the interlayer. Two filler layers adjacent to the interlayer primarily consist of Ti–Cu, Ti–Cu–Ni intermetallics, and (Ti, R)ss. A Ti‐rich diffusion layer and a thin TiC‐containing layer exist near TC4 and graphite substrates, respectively. The joints brazed with Nb and Ta interlayers exhibit shear strength of 2 1.0 ± 2.7 MPa and 20.2 ± 3.1 MPa, respectively, both higher than those without interlayer, implying that the introduction of refractory metal interlayer may benefit the joint stress relaxation by its intermediate coefficient of thermal expansion and high ductility. The joint strength with Nb interlayer is slightly higher than that with Ta interlayer, mainly attributed to the relatively lower elastic modules and yield strength of Nb.

Transient liquid phase bonding of graphite to Ti6Al4V alloy

“…The XRD result of the bonding zone in the graphite/TC4 joint with Cu foil shown in Figure 1(i) confirms the formation of TiCu and Ti 2 Cu intermetallics, which is consistent with the FESEM and EDS results. Ma et al [1] also claimed that TiCu and Ti 2 Cu were formed in the filler zone for graphite/TC4 brazing with Cu-Ti-Ni filler and Nb interlayer.…”

“…Joining of graphite to Ti alloy is essential in thermal management and subatomic particle detection applications [1, 2]. However, the joining is challenging due to the physical/chemical mismatch between the graphite and Ti alloy.…”

Joining of graphite to Mo using Ti/Cu and Ti/Cu/Nb/Ti/Cu foils

“…One of the stress relief methods for graphite/metal joints is introduction of a refractory metal foil. Ma et al [8] applied Ta or Nb foil in Cu-Ti-Ni filler for joining graphite/Ti6Al4V to alleviate the joint residual stress. Zhong et al [9] introduced a Mo foil to relax the residual stress of graphite/Cu joint brazed using Ti-based filler.…”