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황태우 · 한상욱 · 오일영 · 문영훈
질산용액 및 질소가스 분위기에서 Ti-6Al-4V 합금의 레이저 표면질화 특성분석
Received: 11 April 2019;  Accepted: 3 June 2019.  Published online: 5 June 2019.
Laser nitriding is a surface engineering technique that is being used on titanium alloys to improve their surface hardness and wear resistance. It is an attractive process because of its simplicity and the possibility of producing hard layers of substantial depth with minimum effect to the bulk of the material. The current study characterized and compared laser surface nitriding of Ti-6Al-4V alloys in a nitric acid solution and in a nitrogen gas atmosphere. The melt pool shape, microstructure, hardness, and cracking susceptibility of the nitride layers processed at various nitrogen concentrations were investigated over a wide range of processing variables. The results show that the hardness of the laser nitrided layer increases with increases in both the concentration of nitrogen and the energy density of the laser. The hardness levels and hardened areas were significantly different because nitrogen has a different diffusivity in the nitric acid solution compared to a nitrogen gas atmosphere. When nitriding in a nitrogen atmosphere, the more energetic thermal reaction and accelerated surface vaporization cause rougher nitrided surfaces. The laser surface nitrided in a nitric acid solution significantly enhanced the hardening efficiency and hardness level without processinduced cracks. The results of this study will be useful in process design for the laser nitriding of Ti-6Al-4V alloy, which significantly enhances the surface performance of laser-processed parts.
Keywords: laser surface nitriding, Ti-6Al-4V alloy, nitric acid, nitrogen, hardness, roughness
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