Factors affecting the ductility and microstructure of Ti-6Al-6V-2Sn titanium alloy weldments

The purpose of the investigation was to study the effect of material thickness, weld heat input and post weld heat treatment on the tensile properties and microstructure of Ti-6Al-6V-2Sn Titanium, and obtain a practical weld/thermal treatment cycle. The objectives were accomplished by electron beam and gas tungsten arc welding 0.060, 0.125 and 0.250 inch thick material and then heat treating at various temperatures and times. Electron beam and gas tungsten arc welding enabled an approximate 4:1 ratio of heat inputs to be utilized. Resultant welded and heat treated material was tensile tested and metallographically examined. Heat treatments involved both air and furnace cooling from temperatures as high as 1550ºF. Post weld heat treatments involving air cooling resulted in partial solution treatment and yield strengths as low as 50% of the as-received properties. Elongation values were also lower than those achieved from heat treatments involving furnace cooling. The effect was considerably increased with increasing heat treatment temperature. The most practical post weld heat treatment of 1400ºF for 4 hours and furnace cool resulted in strengths that were 95-100% of the as-received properties. Elongation values varied from as low as 2% in the as-welded condition to 7-12% following this post weld heat treatment. The microstructure revealed a fusion zone with considerable alpha plate growth compared with the typical as-welded coarse transformed beta and the structures approach equilibrium quantities of the alpha and beta phases and equilibrium composition of the individual phases. Increasing material thickness and decreasing weld heat input had little effect on tensile strength, however ductility was increased by 2-3% elongation --Abstract, pages ii-iii