Surface carburizing of Ti-6Al-4V alloy by laser melting

Surface carburizing of a Ti-6Al-4V alloy using laser melting has been investigated experimentally, with the aim of increasing surface hardness and hence improving related properties such as wear and erosion resistance. The surface of the material was coated with graphite prior to laser irradiation. Carburizing was achieved by a laser alloying mechanism, which includes melting the substrate and dissolution of the graphite in the liquid state. Two different types of lasers were used: (i) a continuous wave CO2 (CW CO2) laser with a maximum power of 3 kW, and (ii) a pulsating Nd-YAG laser with a maximum power per pulse of 100 W. Optical microscopy, scanning electron microscopy, EDS-analysis, and X-ray diffraction were carried out to analyze the microstructure and identify phases of the carburized layers. The results show that the carburized layers produced by CW CO2 and Nd-YAG lasers are macroscopically homogeneous and have gradient features. The microstructures consisted of TiC crystals in the matrix of alpha'-Ti. The TiC crystals are either in the form of particles or dendrites. The depths to which these layers extend ranged from about 0.2-0.5 mm, depending on the treatment parameters. The volume fraction of the dendrites was found to decrease with increasing laser power or increasing traverse speed. Microhardness has been found to be directly related to the volume fraction and the size of the TiC phase. It increased to a value ranging from 500 to 800 Hv as compared to 350 Hv for the as received substrate. (C) 2009 Elsevier Ltd. All rights reserved.