A study of the weldability of alpha-2 titanium aluminide

The weldability of Ti-24.5Al-12.5Nb-1.5Mo (at%) alpha-2 titanium aluminide was assessed using autogenous electron beam welding as well as gas tungsten arc welding (GTAW) processes. Evaluation methods of as-welded specimens and specimens subjected to a postweld heat treatment (PWHT) consisted of microstructural characterization, mechanical property testing and test specimen fractography. Microstructural characterization was performed on parent material (as-received, heat treated and PWHT conditions) and welded samples (as-welded and w/PWHT conditions). Metallographic techniques used for microstructural characterization consisted of optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical property evaluation of test samples (as-welded and w/PWHT) and parent material consisted of microhardness testing, room and elevated temperature tensile testing and room temperature fracture toughness testing. Failure analysis was performed using SEM techniques. A two-dimensional finite element model representing a cross-section normal to the welding direction was developed for each weld to derive continuous cooling behavior of the welds. The temperature versus time histories determined from these analyses were used to predict the resulting microstructure throughout the welded coupons.; It was determined that crack-free welds were achievable with the EB welding process when process parameters were properly chosen. Solid state cracking was noted in the fusion zone when pre-heating procedures were not used. The cracking was completely eliminated when specimens were EB welded using 370C(700F) pre-heat. Although, crack-free welds were obtained using 370C(700F) pre-heat, however, the microstructures and the mechanical properties of the as welded and PWHT samples were poor.; After EB welds were produced, GTA welding was conducted. In this effort, the high temperature heat treatment of 1070C(1960F) was applied to welding. In addition, a high preheat temperature of 760C(1400F) was used during welding. This was done to reduce the postweld cooling rate so that secondary alpha-2 could form in the weld zone upon cooling. This would eliminate the need for a high temperature postweld heat treatment to transform the beta phase which is retained upon rapid cooling. This process was successful in allowing the beta phase to transform in the fusion zone and the heat affected zone (HAZ) during postweld cooling. The tensile properties show that the strength of these welds was very high compared to the EB welds. This is reflective of very fine lath size. The ductility of the GTA weld was higher than as-welded EB weld but lower than the heat treated EB welds. It is likely that the ductility of the GTA weld could have been increased by a slower postweld cooling rate which also would have resulted in a larger lath size and lower strength.; On the basis of overall evaluation, it was concluded that the Ti-24.5Al-12.5Nb-1.5Mo had satisfactory weldability with the EB and GTAW welding processes. If proper parameters and the weld HAZ cooling rates are maintained, crack-free welds with parent metal tensile properties could be obtained.