| Titanium alloy is being paid more and more attention in the industry because of its good properties such as high specific strength, good corrosion resistance. However, titanium alloy is limited in the application of engineering structure because of poor thermal conductivity, low hardness, and poor wear resistance and so on. Therefore, it has become a popular research focus to improve surface hardness and wear resistance of titanium alloy by various surface engineering technologies, In this paper, Ni-based wear-resistance coating was fabricated at the surface of Ti6A14V alloy by laser cladding technolgy using a Ni-C mixing powder. Further, to gain the microstructure and properties of the coating, the specimens were analyzed by super depth metallographic microscope, scanning electron microscopy (SEM) with a genesis type X-ray energy dispersive spectrometer (EDX) attachment, X-ray diffractometer (XRD), microhardness tester and friction abrasion testing machine. The main researeh contents and conclusions are listed as follows:(1) The TiC reinforcement Ni-based composite coating was produced by laser cladding when the Ni-C mixing powder was pre-presetted on the surface of Ti6A14V alloy. The reinforcement phase of the coating is TiC particles, and the matrix phase of coating is Ni3Ti phase and Ti2Ni phase. The microstructure of the cross section of the coating show that TiC phase was synthesized in laser-melted pool because carbon atoms of replaced powder layer and titanium atoms of the substrate was uniformly mixed in laser-melted pool by convection mass transfer. From top to bottom at the cross-section of the laser clad coating, TiC particles content rise, and TiC particles size is fine, TiC particles change from dendrite crystal to cellular crystal. These characteristic of microstructure agrees to the profile of microhardness at the cross-section of the laser clad coating.(2) Laser power and laser scanning speed play a key role for the width, depth and quality of single single laser clad channel. When laser power rise, the width and depth of single laser clad channel is larger, surface microhardness is higher. When laser power is constant, the energy input and lifetime of the laser-melted pool was determined by the scanning speed. In this research, the optimized laser process for a continuous wave CO2 laser as follows:spot size 4 mm, laser power 2.4 kW, scan speed 5 mm/s. In addition, TiC particle which change from dendrite crystal to cellular crystal depends on the C content of the Ni+C mixing powder.(3) The cooling process analysis of the laser-melted pool indicates that the sequence of crystallization in laser-melted pool with the decrease of temperature is TiC, Ni3Ti, eutectic Ni3Ti and NiTi. And then NiTi decompose into TijNi and Ni3Ti.(4) The patterns which are composites of TiC particles indicated that the convection in the laser-melted pool played a key role not noly for the composition homogenization of the laser-melted pool but also for the growth, morphology, and distribution of TiC phase. The microstructure and the distribution of chemical composition in the first half of the rapid solidification laser-melted pool also confirm that the convection is very important for the TiC growth.(5) The results of numerical simulation based on fluent software imply that the maximum temperature and the maximum velocity of the laser melted-pool are almost changeless after 0.2 second under the laser radiation. And the increasing of the laser melted-pool in geometric is slowing. The maximum temperature locates at the centre of the pool surface. The maximum velocity locates near the edge of the pool. The non-planar free sufece of laser melted-pool is the important reason for the simulation pool is biger than the experiment pool in width.(6) TiC particle is fine in the overlap zone of the overlap coating. According to the distribution of TiC particles, the sequence of overlapping is able to be recognized at the heat overlap coating. The bonding line of the coating and the substrate is peak shape at the overlap zone, and the angle of the peak shape line increase with the increasing of overlapping rate.(7)The result of the ball on disc dry sliding wear testing showed that the wear mechanisms of in situ TiC reinforcement Ni-based composite coating were abrasive wear and adhesive wear. The wear mechanism of Ti6A14V alloy is abrasive wear with typical ploughing wearing. In addition, with the increasing of carbon content, the loss mass of the coating reduces and the wear resistance rise. |
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