Mechanical Behavior of Surface Nitrided and Heat-Treated Laser Welded Ti-6Al-4V

Abstract

The welding of the titanium alloy Ti-6Al-4V has been reported to be difficult because of the intrinsic reactivity with oxygen and also because the weld usually produces an unbalanced α/β structure. This contribution deals with a heat treatment of the laser welded Ti-6Al-4V alloy in a CVD nitriding chamber. The heat treatment aims to obtain a fusion zone structure similar to the base material. An additional advantage of the method is creating a hard nitride layer at the surfaces of the piece, increasing the hardness. The CVD treatment at 850°C per 2 hours proved to be efficient to transform the as-welded martensitic structure to a biphase α/β diffusion controlled structure. Although the fusion zone was soft after the treatment (330 HV), the surface is hard attaining about 750 HV. The tensile strength and ductility after welding were very similar to the base material. However, the yield strength decreased from 1030 MPa to 880 MPa and the uniform strain was reduced from 8 to 1% after the CVD treatment. The fatigue behavior was quite different depending on the testing conditions. For the rotating bending condition, the higher fatigue limits, around 400 MPa, were observed for the CVD-treated weld coupon. However, the fatigue behavior in uniaxial conditions was very similar to the as-welded condition and the CVD-treated weld, attaining the fatigue limits at 250 MPa.