Application of Flash Welding to a Titanium Aluminide Alloy—Microstructural Evaluations

Alpha-two titanium aluminides represent strong candidates for replacing many conventional titanium and nickel-base superalloys for intermediate temperature applications. One potential application of these alloys is turbine engine rings. Nonrotating rings of this type are typically manufactured by flash butt welding. The performance of welds in this alloy is known to be strongly affected by the weld microstructure. Welding processes that result in very slow cooling rates yield relatively coarse Widmanstatten-type microstructure(s), which generally yields acceptable weld performance. Processes that result in intermediate cooling rates, however, result in acicular alpha-two martensite microstructures. These microstructures have very little ductility and lead to reduced weld performance. Finally, for processes where the cooling rate is very rapid, the weld microstructure is a retained ordered beta phase, which apparently results in improved weld properties. In this paper, microstructures of some representative flash welds on T-14Al-21Nb were examined. In addition, flash welding conditions were varied to examine the effects of initial die opening and upset distance. In general, it appears that all the welds included in this study contain the ordered HCP martensite in the weld zone. However, both the scale of the microstructure and the weld hardness seem to depend heavily on the particular set of processing conditions used. These results were then used to estimate the relative cooling rates in these welds, and understand the effects of these processing conditions on the developing microstructures.