Effect of High Temperature Plasma Based Ion Implantation on Niobium Welded Joint

Niobium (Nb) and its alloys have been considered excellent for applications in aerospace components submitted to high temperatures, essentially in rocket propulsion. A relevant aspect in the processing of refractory metals is the formation of oxide layer, which can degrade the mechanical properties of the material. In the case of Nb, the formation of niobium nitrides (NbxNx) on the material surface can act as a barrier to oxygen diffusion into the material. In the present work, Nb sheets (thicknesses of 3.0 mm and 1.6 mm, respectively) were joined by a Yb:fiber laser welding system. Bead-on-plate joint conditions were maintained constants, with laser average power of 1500 W, welding speed of 6.0 m/min. Helium with a flow rate of 45 l/min was used as shielding gas. After, nitrogen ions were implanted into welded niobium sheets by high temperature plasma based ion implantation (HTPBII) in order to produce niobium nitrides on the material surface. The HTPBII was performed in the temperature of 1000° C with negative pulses of 10 kV/20µs/500 Hz during 1 h. Composition of the welded sample submitted to HTPBII were evaluated by X-ray diffraction. It can be observed a significant number of Nb2N peaks on surface material as consequence of nitrogen implantation. Due to heating of the substrate during the HTPBII process, the recrystallization of the sample is also observed after the treatment. In fact, the niobium presents a recrystallization temperature of the order of 1050° C for a deformation of 90%. This means that the microstructure changes in the material bulk can improve, or at least modify the mechanical strength of material. Further studies will associate these results to mechanical properties of untreated weld and sample submitted to post-weld by HTPBII.