Parametric investigation in an open atmosphere laser nitriding process of titanium aiming to minimize cracks

Abstract

Open-atmosphere laser nitriding is a new technique for the formation of nitride layers on Ti surfaces in air. To form a crack-free nitride layer, the influence of processing parameters involving the beam-focusing condition and beam-scanning speed was carefully investigated. Ti surface irradiation using a defocused pulsed laser beam enhances energy transfer from the generated laser-induced plasma (LIP), enlarges the surface melting pool, and results in the formation of a thicker nitride layer compared to irradiation by an ideally focused laser beam. In addition, a lower scanning speed was linked to an increase in the number of repetitions of the pulsed beam, facilitating the penetration of the surrounding gases, mainly N, into the melt pool, thereby increasing the N content of the layer. The layer thickness and crystallographic phase of the nitride layer directly influenced the degree of crack formation; specifically, thinner layers mainly comprising TiN_0.3 exhibit significantly fewer cracks. By adjusting the beam parameters, nitride layers with different properties can be deposited on Ti surfaces, thereby broadening the range of Ti industrial applications.