Effect of combination of gas oxynitriding and selective laser treatment on structure, phase composition and wear resistance of high-strength titanium alloy

Abstract

This study reports on the synergistic effect of oxynitriding (ON) and selective laser treatment (LT) on the microstructure, phase composition, and tribological behavior of BT22 titanium alloy (Ti-5Al-5Mo-5 V-1Cr-1Fe). ON was performed according to a standard heat treatment regime, with nitrogen continuously supplied to the furnace. The oxygen, instead of nitrogen, was pumped in at the final stage of the process. The samples were selectively laser-treated to reach 900 °C on the surface, just above the temperature of α-β phase transition: one batch before and one after ON. Their properties were compared with an oxynitrided sample and an untreated sample. The X-ray diffraction (XRD) analysis, electron backscatter diffraction (EBSD) indicate a phase shift towards a more α-phase and the formation of titanium oxides and nitrides. Transmission electron microscopy (TEM) studies demonstrated the formation of a compound layer consisting of nitrides modified by oxygen and phase changes in the diffusion layer. According to a metallographic analysis, this innovative approach applies two effects to the alloy: one changes the structure of the diffusion layer, and the other alters the phase composition of the TiNO layers. The combination of ON and LT was the most beneficial for improving the BT22 titanium alloy’s wear resistance.