Determination of Optimal Weld Parameter for Joining Titanium Alloys by Gas Tungsten Arc Welding using Taguchi Method

The optimal parameters for joining two different titanium alloys were determined by the Taguchi method and applied in similar and dissimilar joining of conventional Ti-6Al-4V and newly developed Ti-3Al-2.5V alloys. The microstructures of the two alloys and their mechanical properties were comparatively evaluated at the optimal parameters. The Ti-6Al-4V alloy showed a larger back bead width than that of Ti-3Al-2.5V under similar heat input, because of its lower thermal conductivity and higher specific heat capacity. The welded zone of Ti-3Al-2.5V contained a retained beta phase, which was absent in that of Ti-6Al-4V. This indicates that the transformation of the Ti-3Al-2.5V weld metal starts above the martensite temperature, while it starts below the martensite temperature for Ti-6Al-4V. The failure of the welded specimen occurred in the base metal for both the titanium alloys, which indicates the superior weld quality. However, the welded Ti-6Al-4V showed superior tensile strength to that of the Ti-3Al-2.5V weld under optimal conditions, owing the high beta phase fraction in its base metal. Meanwhile, it showed inferior ductility to that of Ti-3Al-2.5V because of its coarser beta phase. Key words: Titanium alloys, Taguchi method, Optimal parameters, Microstructure, Mechanical properties