SHS Joining Via Combustion of Ti-Containing Systems

Abstract

The joining of intermetallic parts to the metals (Ti, Al, Cu, Ni) or different intermetallics (e.g. NiAl, TiAl) is important for high-temperature applications. Traditional fusion welding is difficult to apply due to the large thermal expansion mismatch between the intermetallics to be joined and the filler material. Brazing and diffusion bonding are also inefficient, energy-consuming, and must be carried out in vacuum or protective atmosphere. Meanwhile, Self-propagating High-temperature Synthesis (SHS) that is widely used for production of powders, compact materials and parts holds considerable promise as a method of joining various component parts. We report on a new SHS-based approach to joining TiAl and NiAl intermetallics with metal substrates Ti and Ni. The adiabatic combustion temperatures for the systems under study and stoichiometric ratios of green mixtures were determined by thermodynamic calculations. Experiments were performed in the modes of pressure-assisted thermal explosion and combustion wave propagation, with the samples placed between the punches of experimental set up. Investigated were the processes taking place in thermally treated Ti or Ni plates mechanically impregnated (by Mechanically Activation or Vibration Milling) with a mixture of metallic particles. Vibratory treatment of an Al and Ti surfaces in a powder mixture of metals was found to result in destruction of the surface oxide film and formation of extensive physical contacts between the Ni or Ti matrix and the metallic particles mechanically impregnated into the near surface layer of Ni or Ti.
Subsequent thermal treatment was then used to launch chemical reaction yielding intermetallides within the impregnated layer. Performing SHS reactions yielding melted intermetallides in the coatings (or pellets) deposited (or placed) onto the surface of thus treated Ti or Ni plates, one can obtain strong weld joining between solidified SHS products and substrates. This approach can also be used for deposition of coatings with a desired composition onto substrates.