Carbonitriding of Ti-6Al-4V alloy via laser irradiation of pure graphite powder in nitrogen environment

In the study, an innovative carbonitriding process was implemented to increase hardness and hardening depth of Ti-6Al-4V alloy via laser irradiation of pure graphite powder in a nitrogen environment. The formation of the carbonitrided layer on the surface depends on the diffusion of carbon and nitrogen atoms and the kinetics of carbonitriding reaction. Dendritic phase transformations within the hardened layer help in determining the overall depth and hardness of the hardened layer. To characterize the proposed carbonitriding process more fundamentally, samples of an Ti-6Al-4V alloy were self-quenched (SQ), laser-carburized (LC), laser-nitrided (LN), and laser-carbonitrided (LCN) at the same laser irradiation conditions. This was followed by comparing the microstructural evolutions and mechanical properties of the respective samples after laser surface treatments. The hardening characteristics of the respective laser surface treatments were significantly influenced by the reaction compounds and laser energy density. The LCN Ti-6Al-4V alloy was dominated by compounds with mixed structure of carbonitrides and exhibited excellent hardness with increase in hardening depth. The results confirmed that the proposed laser carbonitriding is a feasible and effective process to increase the hardness and hardening depth of Ti-6Al-4V alloy without process-induced cracks.