The effects of activating flux containing nanoparticles on the welding microstructure and hardness distribution of titanium alloy were investigated. The fluxes, consisting of NaF with 1 wt-% of either nano-TiO 2 or nanodiamond, effectively yielded refined grain size and deep weld penetration. The hardness of the weldment was close to that of the base metal for 99 wt-% NaF plus 1 wt-% diamond, leading to a desirable uniform mechanical property in the weld design. In the meantime, the transmission electron microscopy images illustrate that the nanoparticles promote b phase coarsening and martensitic a9 phase formation at the b veins.

The distribution of welding residual stresses in titanium alloy plates composed of Ti-6Al-4V (Ti-64) and Ti-4?5Al-3V-2Mo-2Fe (SP700) alloys was measured and calculated under the same heat input. Ti-64 alloy exhibited a higher residual tensile stress than the SP700 alloy due to the low thermal conductivity and thermal diffusivity of the former alloy, in addition to its high thermal expansion coefficient. Diamond like carbon (DLC) thin film deposited on the as welded titanium alloy plates reduced the magnitude of the welding residual stress caused by the cathodic arc physical vapour deposition (PVD) process. The distribution of the measured welding residual stresses agreed with the finite element method (FEM) simulated results very well.