【作者】 刘正道；

【导师】 王正东； 张显程；

【作者基本信息】 华东理工大学 ， 机械设计及理论， 2014， 博士

【摘要】 钛合金的高比强度和优异的耐腐蚀性能使它成为制造汽轮机低压末级叶片的理想材料。水蚀是造成汽轮机叶片失效的主要原因之一。在钛合金叶片表面制备耐水蚀保护涂层是解决水蚀问题的有效方法。本文对Ti-A1-N体系材料进行了热力学分析,模拟分析了激光合金化涂层熔池的温度场,制备了TiN/Ti3A1和TiN/A1激光合金化复合涂层,并探讨了激光工艺参数对物相及物相含量、涂层的质量、硬度和耐磨性的影响,研究了涂层的形成机制和磨损机制。本文的主要研究内容有以下几个方面：(1)采用物质吉布斯自由能函数法计算了以Ti6A14V为基体材料,激光合金化过程中Ti-Al-N体系的吉布斯自由能的变化ΔG,分析了TiN增强相形成的可行性。结果表明,TiN在系统中形成的吉布斯自由能最低,是首要生成产物,可形成具有陶瓷增强相的复合涂层。(2)综合考虑材料的物性参数、辐射对流散热、相变潜热等随温度变化的因素,建立了以Ti6A14V为基体的等离子喷涂预置涂层激光合金化连续移动三维温度场有限元模型。利用ABAQUS软件,模拟分析了激光合金化过程中的温度场分布,研究了激光工艺参数对温度场的影响。结果表明,试样表面最高峰值温度位置偏离了光斑中心,处于激光光斑中心的后侧。熔池宽度方向的温度梯度在熔池中心为零,随着远离熔池中心温度梯度先上升,在接近熔池边缘时下降；而在熔池深度方向,温度梯度的最大值在激光合金化表层,成单减变化。随着激光功率的增加、扫描速度的降低,熔池表面的峰值温度、熔池深度和熔池宽度均增加。随着激光功率的提高,熔池宽度方向和深度方向的温度梯度均相应增加,而激光扫描速度对温度梯度的影响不明显。熔池表面最高温度的模拟结果与估算公式计算的结果吻合较好。实际熔池的深度和计算熔池的深度,在较低激光功率和较高扫描速度时吻合较好。(3)首次采用等离子喷涂A1涂层和激光氮化结合的工艺在Ti6A14V表面成功制备了TiN/Ti3Al金属基复合涂层。分析了不同工艺条件下复合涂层的物相组成、显微组织特征及其形成机制。结果表明,激光功率增加时,A1N和TiAl3转变为TiN和Ti3A1。激光功率的增加、扫描速度的降低、氮气流量的增加,都会使复合涂层中TiN的含量增加；同时,激光合金化涂层的厚度和均匀性具有相同的趋势。复合涂层中的增强相TiN的生长机制为溶解-析出机制,TiN形核时为等轴的球状粒子,在凝固过程中,涂层底部的显微组织以胞(柱)状树枝晶为主,涂层的中部为球形的或椭球形的胞状晶体组织,涂层上部呈现的是胞状晶、胞状树枝晶和树枝晶。激光合金化涂层的晶粒的大小随着激光功率的增加、扫描速度的增加及氮气流量的减小而减小。TiN/Ti3Al金属基复合涂层的形成机制：首先在熔池表面化合生成A1N,熔池和基体材料交界面化合生成TiAl3;然后发生置换反应[Ti]+AlN→TiN+[Al]及化合反应生成TiN；最后在凝固过程中TiN先析出,而后析出Ti3A1。(4)首次采用同步送粉、送气的工艺方法在Ti6Al4V表面成功制备了无裂纹的TiN/Al金属基复合涂层。分析了不同工艺条件下复合涂层的物相组成及其形成机制。结果表明,复合涂层的物相由IN、Al和TiAl3转变为TiN和A1。物相的组成由Ti-Al-N体系中的扩散物质浓度梯度、激光熔池的温度梯度及存在时间所决定。随着激光功率的增加、激光扫描速度的降低和氮气流量的增加,复合涂层中TiN含量增加。在TiN/Al金属基复合涂层的中部物相为TiN和Ti3Al。TiN/Al金属基复合涂层的形成机制为在熔池表面化合生成TiN,在凝固过程中TiN首先析出,而后弥散均匀分布于A1或Al+TiAl3、 Ti3Al。(5)对激光合金化复合涂层进行了硬度测试和磨损试验,考察了激光工艺参数对硬度和耐磨性能的影响。试验结果表明,随着激光功率的增加、激光扫描速度的降低、氮气流量的增加,激光合金化复合涂层的硬度增加,涂层的磨损质量失重降低。TiN/Ti3Al和TiN/Al复合涂层的最大硬度分别为基体材料硬度的3倍和4倍。TiN/Ti3Al和TiN/Al复合涂层的耐磨损性能分别为基体材料的4倍和6-8倍。研究还揭示了Ti6Al4V基体材料和激光合金化复合涂层的磨损机制,基体材料磨损为磨粒磨损和粘着磨损。激光合金化复合涂层的磨损主要是磨粒磨损,表现为显微切削和TiN硬质相的剥落。更多还原

【Abstract】 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.更多还原