EXAMINING THE MECHANISMS AND KINETICS OF TRANSIENT LIQUID PHASE BONDING OF TITANIUM ALLOYS USING COPPER AND NICKEL FILLER METALS
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Copper and nickel were examined as potential interlayers in transient liquid phase brazing (TLPB) of commercially pure titanium (CP-Ti) and Ti-6Al-4V. Experimental parameters were set such that the braze temperature was above intermetallic formation (1050°C) and was examined for all systems for the purpose of consistency. In addition, a foil width of 50 μm and a heating and cooling rate of 20 Kpm was used throughout experimentation. Using high temperature thermal analysis techniques (DSC, DIL), as well as microstructural observations (SEM, Optical Microscopy), the efficacy of these interlayers was monitored for levels and mechanisms of dissolution, rates of isothermal solidification, and the time to homogenization. Dissolution in both titanium systems requires the development and evolution of a diffusion couple between Ti-Cu and Ti-Ni as dictated by the binary phase diagram. This involves the growth of intermetallic compound (IMC) layers and progresses with incipient melting at the lowest melting intermetallic. In copper systems this is found on the Cu-rich side whereas in the nickel systems it occurs on the Ti-rich side. For copper systems this led to a preferential dissolution of copper foil with minimal solute uptake. In nickel systems the IMC layer is slowly consumed by the liquid which delays homogenization. Overall gap widening is largest in Ni systems since the solubility of titanium in the liquid phase is higher, but this was found to be mitigated by a higher Ni solute uptake when compared to copper. These trends were also observed in Ti-6Al-4V samples. Isothermal solidification studies showed that the rate of isothermal solidification is faster in nickel than in copper and faster in CP-Ti in comparison to Ti-6Al-4V. These trends can be explained by the apparent diffusivity of solute in these systems which was calculated. The higher rate of isothermal solidification led to shorter isothermal solidification times compared to copper-based joints. DSC analysis was able to produce 2WISZ curves with the capability of predicting complete isothermal solidification time for any starting gap, or foil, size. In addition, the time to homogenization can be calculated directly from these curves.