| Titanium Alloy is good for the last stage blade of a steam turbine because of its high specific strength and excellent corrosion resistance. Liquid impact erosion is one of the major causes of the turbine blade failures. It is a feasible solution with an erosion resistant coating on the blade surface to prevent from the liquid impact. In present study, a thermodynamic evaluation is carried out by Gibbs function on a Ti-Al-N system. To simulate the temperature field in the molten pool when laser surface alloying. The composite coating of TiN/Ti3Al and TiN/Al is prepared by laser processing, and the effects of laser processing parameters are investigated on phases, phase contents, and the quality, hardness and wear resistance of the coating. This research also focuses on the formation and wear mechanism of the coating. The main contents and conclusions are as follows:(1) Gibbs function is employed in the calculation of the variations of the Gibbs free energy△G in reactions of the Ti-Al-N system when laser alloying with Ti6Al4V substrate to analyze the feasibility of formation of reinforcement phases TiN. The results show that Gibbs free energy from TiN is the lowest, and TiN is the primary generated product in the system, which then forms the composite coating with ceramic reinforcement phase.(2) Considering the thermophysical performance parameters of material, radiation, convection and the latent heat, a3-D finite element model of transient temperature filed is built up for laser surface alloying on Ti6Al4V with presetting of plasma sparing Al coating. ABAQUS is used to analyze the temperature distribution during the laser alloying and the effect of laser technology parameters on temperature fields. The analysis shows that the peak temperature of the sample surface deviates from the center of the laser spot to the backside. In the direction of width, the temperature gradient is zero at the center of the molten pool, and the gradient first rises away from the center, but falls while approaching the edge of the pool. In the direction of depth, the highest gradient exists at the surface, then falls while approaching the bottom. With the increase of the laser power and the decrease of the scanning speed, the peak temperature on the surface of the molten pool, the pool depth and width all increase. The increase of the laser power also leads to the increase of the temperature gradient in both the width direction and the depth direction, while the change of scanning speed exerts no distinctive impacts. The result of the peak temperature of the molten pool from simulation matches the result from estimate formula. The actual depth of the molten pool well matches the calculated depth when the laser power is low and the scanning speed is high.(3) It is the first time that the continuous CO2laser is used in laser nitriding on the surface of Ti6Al4V with presetting of plasma coating to prepare the metal matrix composite coating TiN/Ti3Al. The phase composition, microstructure and formation mechanism of the composite coating are analyzed with different processing conditions. Results show that AlN and TiAl3are converted to and Ti3Al with the increase of laser power. Then, The TiN contents in the composite coating share the same tendency as the thickness and uniformity of the composite coating with the increase of laser power, the decrease of scanning speed, the increase of the nitrogen flow rate. The growing mechanism of the reinforcement phase TiN in the composite coating is dissolution-precipitation. TiN is equiaxed spherical particle at nucleation, then, the microstructure at the bottom of the coating is mainly cellular (columnar) dendrite when solidification. Meanwhile, it is spherical or ellipsoid cell structure in the middle, and cell structure, cellular dendrite and dendrite on the top of the coating. The size of the particles decreases with the increase of laser power, increase of the scanning speed and the decrease of nitrogen flow rate. The formation mechanism of metal matrix composite coating TiN/Ti3Al is as follows:AlN is formed at the surface of the molten pool, TiAl3is formed at the interface of the molten pool and the substrate, and then replacement reaction of [Ti]+AlN'TiN+[Al] takes place and TiN is formed by synthesis reaction. TiN is first precipitated in the solidification process, and Ti3Al is precipitated last.(4) Synchronic powder feeding and gas feeding technique is used for the first time to successfully prepare crack-free metal matrix composite coating of TiN/Al. The phase structure and formation mechanism of the composite coating with different techniques are analyzed. Results show that with increase of the laser power, decrease of the scanning speed and increase of the nitrogen flow rate, the phases of TiN、Al and TiAl3in the composite coating are converted into TiN and Al. The formation of phase is determined by the concentration gradient of the diffused matter, the temperature gradient of the molten pool and the duration of existence. The increase of the laser power, the decrease of the scanning speed and the increase of the nitrogen flow rate result in the increase of TiN concentration in the composite coating. Phases in the middle of the metal matrix composite coating of TiN/Al are TiN and Ti3Al. The formation mechanism of the metal matrix composite coating of TiN/Al is as follows:TiN is formed by synthesis reaction at the surface of the molten pool, then TiN is precipitated first in the process of solidification. TiN then is diffused evenly into Al or Al+TiAl3or Ti3Al.(5) Hardness and wear test is implemented on the laser alloying composite coating and the impact of laser parameters on hardness and wear resistance is recorded and analyzed. The results show that the increase of laser power, the decrease of scanning speed, the increase of nitrogen flow rate lead to the increase in hardness and wear mass loss of the coating. The maximum hardness of TiN/Ti3Al composite coating and TiN/Al composite coating is3times and4times of that of their substrates respectively. The wear resistance of TiN/Ti3Al composite coating and TiN/Al composite coating is4times and6-8times of that of their substrates respectively. The study of the wear mechanism of the substrate of Ti6A14V and the laser alloying composite coating shows that abrasive wear and adhesive wear occur to the substrate, while micro-ploughing and peeling of TiN reinforcement occur to the composite coating. |
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