The role of phase transformation in electron-beam welding of TiAl-based alloys

The weldability of two TiAl-based alloys, Ti-45Al-2Nb-2Mn and Ti-48Al-2Nb-2Mn, was investigated with the electron-beam welding process. It was found that the alloys were susceptible to solid-state cracking due to high thermally induced stresses and, more significantly, to the intrinsic brittleness of the microstructures. This work correlated the quality of the TiAl welds, made using different sets of welding parameters which gave rise to different cooling rates, to the microstructures that developed during welding. It was found that the welds were crack-free if the weld cooling rates were such that decomposition of the high-temperature α phase in the weld was not suppressed. It was shown that the Ti-48Al-based alloy was less susceptible to the solid-state cracking and, thus, was more weldable than the Ti-45Al-based alloy because the α phase in the alloy with a higher aluminum content could decompose more readily. A continuous cooling transformation (CCT) diagram is suggested to be used as an appropriate reference for the selection of welding parameters which induce suitable microstructures in the welds and result in crack-free welds.