Forming Characteristics and Analysis of Nitrided Layers During the Laser Nitriding Titanium Alloy

Ti-6Al-4V alloy was nitrided in nitrogen atmosphere using diode laser, in order to study the influence of line energy, laser power and scanning speed on the surface morphology, molten pool geometry, width, penetration depth and the forming coefficient of the nitrided layers. The surface and cross-section morphologies of the nitrided layers were taken by optical and scanning electron microscopy, and the hardness was measured by microhardness tester. The results show that the surface morphology are composed of the smooth and rough surface. When the surface temperature of the molten pool is low, increasing the interaction time has little effect on the surface morphology. With the increase of laser power under the same line energy, the melting depth and width of the nitrided layer increase, and the forming coefficient decreases. As the scanning speed decreases under the same laser power, the penetration depth and width increase, and the forming coefficient reduces. When the forming coefficient is very small, the nitrided layers will produce longitudinally distributed cracks. The convection form and the strength result in the formation of a hemispherical interface, a finger interface, a W-shaped interface and so on; besides, multiple inflections happen in the slope of the boundaries of molten pool.