Effect of flow of nitrogen side-feed on microstructure and crystal orientation in laser melting nitridation

Abstract

We examined the effects of a nitrogen side-feed on the surface nitriding of a Ti plate. Surface nitriding of the Ti plate was performed by fiber laser irradiation, thus producing a hardened TiN layer. Two different nitrided areas (rough part and smooth part) could be formed on the surface of the Ti plate by changing the flow of the nitrogen. The roughness, microstructure, chemical composition, and phase composition of the two different nitride areas were analyzed with 3D-SEM, OM, XRD, EPMA, and EBSD. The cross-sectional microstructure, as observed, can be clearly divided into three parts: a modified layer which includes a film and a dendrite, a heat-affected zone (HAZ), and a substrate. In particular, the EBSD analysis shows that the crystal orientation of the smooth part is highly consistent, the preferential growth direction of the crystal is the main and the crystal grain size is large. The crystal orientation of the rough part is low in consistency, in addition to the preferential growth direction, other directions can also grow sufficiently, and the crystal grain size is small. The hardness test was performed on the cross-section of the nitride areas, revealing that the hardness of rough part and smooth part was 8 times that of the substrate.