Laser nitriding

In the present work the influence of the nitrogen concentration of the gas atmosphere in the range of 0-25 % on the microstructure, hardness and cavitation erosion behavior of laser nitrided cp-Ti and Ti-6Al-4V has been investigated. The following conclusions are drawn. 1. The nature, volume fraction and distribution of the phases formed during laser nitriding are determined by the chemical composition of the alloy and by the nitrogen concentration of the gas atmosphere. Laser nitrided cp-Ti exhibits massive martensite structure and acicular martensite structure for nitrogen concentrations below and above 7 %, respectively. In contrast, the microstructure of laser nitrided Ti-6A1-4V changes from acicular martensite to a fine mixture of alpha-Ti and beta-Ti grains if the nitrogen concentration exceeds 5 %. For higher nitrogen concentrations in both Ti alloys the formation of globular and plate-shaped TiN3 phase is observed. 2. The increase in hardness in the laser nitrided layers of both Ti alloys is mainly attributed to solid solution hardening of nitrogen dissolved in the Ti lattice. 3. The cavitation erosion resistance of both Ti alloys can be distinctly improved by laser nitriding. Optimal cavitation erosion resistance is already achieved by laser nitriding under gas atmospheres containing relatively little nitrogen (9-13 %).