Investigation of nitrogen ionization state and its effect on the nitride layer during fiber laser gas nitriding of Ti-6Al-4V alloy

Abstract

A nitride layer can be formed on the surface of a titanium alloy during laser gas nitriding, to improve the surface hardness and wear resistance of the titanium alloy. Importantly, the nitride layer is influenced by the nitrogen ionization state (nitrogen molecules or nitrogen atoms and ions). No research reports exist on the nitrogen ionization state or the effect on the nitride layer during fiber laser nitriding of titanium alloys. Radiation images that were taken with a high-speed camera and emission spectra that were collected with a fiber spectrometer were used to study the nitrogen ionization state. To study the effect of nitrogen ionization state on the nitride layer, the surface nitrogen and oxygen content were measured with an electro-probe microanalyzer, and the surface morphology of the nitride layer was studied by optical and scanning electron microscopy. The experimental results showed that the laser power density was too low to induce nitrogen ionization during fiber laser gas nitriding of the Ti-6Al-4V alloy. The nitrogen molecules had a lower energy than the nitrogen plasma (nitrogen atoms and ions) and a slower reaction rate with the titanium alloy, which resulted in a low nitrogen content and a few defects in the nitride layer. The relationship between the oxidation and nitriding was competitive; that is, nitrogen and oxygen competed to react with the titanium alloy. As such, a low nitrogen content led to a high oxygen content, and the nitride surface was golden-yellow in color with black markings. • Nitrogen did not ionize during fiber laser gas nitriding of Ti-6Al-4V alloy. • Molecular nitrogen is transferred into the molten pool. • Molecular nitrogen reduces the nitrogen content and minimizes nitride-layer defects. • A low nitrogen content led to a high oxygen content in the nitride layer.