激光熔覆镍基单晶合金过程中晶体生长和组织分布的研究

发布时间：2019-07-05 12:32

【摘要】：镍基单晶高温合金因其优异的高温抗蠕变和高温疲劳性能被广泛的用作现代燃气轮机的高压涡轮叶片的材料。镍基单晶涡轮叶片的材料和制造费用高昂,其使用寿命受热疲劳裂纹、叶尖质量缺失、裂纹、表面磨损等诸多缺陷的影响。镍基单晶涡轮叶片的更新费用占了燃气轮机维护费用的的很大部分。通过检测和修复受损的叶片,从而延长其使用寿命,不仅可以降低燃气轮机的维护费用,而且节约了大量昂贵的合金材料。激光送粉熔覆技术作为一种高效的修复技术已经被用于多晶合金叶片的叶尖净成型修复。对于镍基单晶合金叶片,只有保持修复区域内为与基材组织一致的单晶组织,才能确保修复后的单晶叶片机械性能不会降低,实现成功修复。目前,镍基单晶涡轮叶片主要由熔模铸造方法制造。熔模铸造方法具有生产周期长、费用高昂、较高的失败率和容易产生铸造缺陷等缺点,导致单晶叶片制造费用高昂。激光送粉熔覆技术结合CAD、CAM和反馈控制等技术后可以直接成型各种材料的部件。因此,激光送粉熔覆技术不仅可用于单晶涡轮叶片的修复,还提供了直接快速成型单晶涡轮叶片的可行性。而采用激光技术直接净成型单晶叶片的关键就是实现多层多道激光熔覆过程中单晶组织的连续性生长。然而激光送粉熔覆单晶合金过程中,熔池内晶体生长受到诸多因素的影响,非常难以控制。因此,研究激光送粉熔覆镍基单晶合金过程中的传输现象和单晶晶体生长机理和组织分布非常有意义,也有助于单晶叶片修复和制造技术的飞发展。为了更好地理解激光送粉熔覆单晶合金过程中的传输现象,本研究中建立了一个三维瞬态数值模型。该数值模型定量研究激光-粉末相互作用、热传导、熔化、凝固、液态金属流场、重熔和搭接等诸多物理详细,并分析讨论了激光功率、扫描速度和送粉率等工艺参数对传输现象的影响。在建立的三维数值模型的基础上,一个新的晶体生长模型被建立并耦合进之前的三维模型来计算熔池凝固过程中凝固界面上晶体生长行为和熔覆层内微观组织的分布。新耦合的数值模型计算了工艺参数如激光功率、扫描速度、送粉率、同轴喷嘴倾角、基材晶体方向等对晶体生长和微观组织分布的影响,并与实验结果进行对照分析。针对单晶叶片的叶尖修复,数值优化了激光薄壁熔覆过程中保持单晶组织连续生长的工艺参数。在此基础上,进一步研究而来激光多层多道熔覆过程中的晶体生长和组织分布,并分析了搭接率和扫描路径对晶体生长和微观组织分布的影响。在数值模拟结果的基础上,通过MATLAB构建了一个数学模型来计算单晶组织连续生长的工艺窗口。于此同时,通过数值模拟和实验结果分析了多层多道激光熔覆单晶合金过程中裂纹出现的机理,并设计了辅助工艺方法控制裂纹的出现。最后,激光修复和制造单晶叶片的可行性进行了评估探讨。
[Abstract]:The nickel-based single-crystal high-temperature alloy is widely used as a material for high-pressure turbine blades of modern gas turbines because of its excellent high-temperature creep and high-temperature fatigue properties. The material and manufacturing cost of the nickel-based single-crystal turbine blade are high, and the service life of the nickel-based single-crystal turbine blade is high, and the service life of the nickel-based single-crystal turbine blade is affected by various defects such as fatigue crack, loss of tip quality, The cost of the nickel-based single-crystal turbine blade is a significant part of the maintenance of the gas turbine. By detecting and repairing the damaged blades, the service life of the damaged blades can be prolonged, the maintenance cost of the gas turbine can be reduced, and a large amount of expensive alloy materials are saved. Laser powder-feeding and cladding technology has been used as a high-efficiency repair technology for blade tip net-forming and repair of polycrystalline alloy blades. In that nickel-based single-crystal alloy blade, only the single-crystal tissue which is consistent with the substrate is kept in the repair area, so that the mechanical property of the repaired single-crystal blade can not be reduced, and the successful repair can be realized. At present, the nickel-based single-crystal turbine blade is mainly manufactured by the investment casting method. The investment casting method has the defects of long production period, high cost, high failure rate and easy production of casting defects, and the like, and the manufacturing cost of the single crystal blade is high. Laser powder-feeding and cladding technology can be used to directly form parts of various materials, such as CAD, CAM and feedback control. Therefore, the laser powder feeding and cladding technology can not only be used for the repair of single crystal turbine blades, but also provides the feasibility of directly and rapidly forming single crystal turbine blades. And the key of using the laser technology to directly clean the single crystal blade is to realize the continuous growth of the single crystal tissue in the multi-layer multi-channel laser cladding process. However, in the process of laser-feeding and cladding of single crystal alloy, the crystal growth in the molten pool is affected by many factors, and it is very difficult to control. Therefore, it is of great significance to study the transmission phenomenon and the mechanism of single crystal growth and the distribution of the structure in the process of the laser-feeding and cladding of the Ni-based single crystal alloy, which can also contribute to the development of the single-crystal blade repair and manufacturing technology. In order to better understand the transmission of laser powder in the process of single crystal alloy, a three-dimensional transient numerical model is established in this study. The numerical model is used to quantitatively study the laser-powder interaction, heat conduction, melting, solidification, liquid metal flow field, remelting and lapping. The effects of laser power, scanning speed and powder feeding rate on the transmission are also discussed. On the basis of the established three-dimensional numerical model, a new crystal growth model was established and coupled into the previous three-dimensional model to calculate the crystal growth behavior and the distribution of the microstructure in the cladding layer during the solidification of the molten pool. The effects of process parameters such as laser power, scanning speed, powder feeding rate, coaxial nozzle inclination angle and substrate crystal orientation on the crystal growth and microstructure distribution are calculated and compared with the experimental results. Aiming at the tip repair of single crystal blade, the process parameters of continuous growth of single crystal are optimized in the process of laser thin-wall cladding. On this basis, the crystal growth and tissue distribution in the laser multi-layer multi-channel cladding process are further studied, and the effects of the lapping rate and the scanning path on the crystal growth and the microstructure distribution are also analyzed. On the basis of the numerical simulation results, a mathematical model is constructed to calculate the process window of the continuous growth of the single crystal tissue. At the same time, through the numerical simulation and the experimental results, the mechanism of the crack in the process of multi-layer multi-channel laser cladding single crystal alloy is analyzed, and the auxiliary process method is designed to control the appearance of the crack. Finally, the feasibility of laser repair and the manufacture of single crystal blade is discussed.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG665;TG132.3

【相似文献】