激光直接金属沉积中激光-粉末相互作用的模拟研究

发布时间：2018-10-12 17:16

【摘要】：更高的推力和效率和更长的寿命是对航空发动机的性能提出的更高要求,是推动航空航天事业日益蓬勃发展的动力。利用定向凝固方法得到单晶涡轮和导向叶片几乎已经成为所有商用和军用生产先进发动机的首选。但是在加工和服役过程中叶片难免出现损伤,造成损失,所以修复破损叶片就成为待攻破的难关。而激光金属外延成型技术则可以攻破此类难关,其由激光直接金属沉积和定向凝固技术形成。控制成型中的一些参数,比如凝固速度等,可获得从基板处外延生长的定向凝固组织。此技术可以完美修复破损的叶片,包括恢复几何形状和尺寸精度,并且不影响其性能。在激光直接金属沉积(Laser Direct Metal Deposition,LDMD)过程中,粉末束到达基材前,会与激光束发生短时间的相互作用,激光束受到衰减,其强度减小,分布受到改变。为此,采用三维瞬态模型模拟了同轴LDMD过程中激光束在基材附近处穿过汇聚了的粉末云时受到的衰减,使用钛金属粉末。采用射线追踪方法,计算激光束部分被吸收和部分被散射后在粒子系统各个边界上的光强分布,边界包括入射表面,侧表面,以及出射表面。考虑了激光波长,粒子尺寸和粒子数对激光衰减过程的影响。模拟结果显示具有均匀空间分布的浓度为0.0156g/ml的钛粉末云对激光的吸收率为8.2%。同时激光束受到粉末云的衰减过程受波长影响较小,但衰减随粒子数增加而增强。而且,波长为1.06μm的光束在LDMD中激光与粉末相互作用区域中和在粒子悬浮液中具有相似的传播规律,粒子悬浮液可以被当作是该作用区域的立体放大,这样可以将作用区域单独拿出来研究,这就提供了一种简单直接的方法,用来测量粉末云的光学性能,而不仅仅是在理论上建立模型进行分析。虽然到达熔池的粉末带来的能量相对基体吸收的能量很小,但是考虑到熔覆层性能的精确计算取决于能量守恒,所以模拟研究粉末云对激光的能量吸收是有必要的。另一方面,建立全新的三维瞬态模型模拟了同轴LDMD过程中预加热导致的粒子温度分布。粉末粒子受激光束照射,粒子吸收能量,温度升高。粒子到达熔池的路径不同,形成的最终温度不同,这可能影响熔池中的温度分布,甚至影响加工过程中成型件内部温度场的积累。结果显示,随着粉粒的下落,温度逐渐升高。并且,在900W的激光照射下,钛粉粒产生熔化现象。
[Abstract]:Higher thrust, higher efficiency and longer life are the higher requirements for the performance of aero-engines, and are the driving force to promote the development of aerospace industry. Using directional solidification method to obtain single crystal turbine and guide blade has become the first choice for all commercial and military advanced engines. However, in the process of processing and service, the blade is inevitably damaged, resulting in losses, so repairing the damaged blade becomes a difficult problem to be broken. Laser metal epitaxial forming technology can overcome these difficulties, which is formed by laser direct metal deposition and directional solidification. Some parameters, such as solidification speed, can be used to obtain directional solidification structure grown from substrate. This technique can perfect repair damaged blade, including restoring geometric shape and dimensional accuracy without affecting its performance. In the process of laser direct metal deposition (Laser Direct Metal Deposition,LDMD), the powder beam will interact with the laser beam for a short time before reaching the substrate, the laser beam will be attenuated, its intensity will decrease and its distribution will be changed. For this reason, the attenuation of laser beam passing through the converged powder cloud near the substrate during the coaxial LDMD process was simulated by a three-dimensional transient model, and the titanium powder was used. The ray tracing method is used to calculate the intensity distribution of the laser beam on each boundary of the particle system after the laser beam is partially absorbed and partially scattered. The boundary includes the incident surface, the side surface and the outgoing surface. The effects of laser wavelength, particle size and particle number on laser attenuation are considered. The simulation results show that the laser absorption rate of titanium powder cloud with uniform spatial distribution of 0.0156g/ml concentration is 8.2. At the same time, the attenuation process of laser beam by the powder cloud is less affected by the wavelength, but the attenuation increases with the increase of the number of particles. Moreover, the beam with a wavelength of 1.06 渭 m has a similar propagation pattern in the laser powder interaction region in LDMD and in the particle suspension, and the particle suspension can be regarded as the stereoscopic amplification of the action region. In this way, the action region can be studied separately, which provides a simple and direct method to measure the optical properties of the powder cloud, rather than just establishing a theoretical model for analysis. Although the energy of the powder reaching the molten pool is very small relative to the energy absorbed by the matrix, considering that the accurate calculation of the performance of the cladding layer depends on the conservation of energy, it is necessary to simulate the energy absorption of the powder cloud to the laser. On the other hand, a new 3D transient model is established to simulate the particle temperature distribution caused by preheating in the coaxial LDMD process. When the powder particles are irradiated by laser beam, the particle absorbs energy and raises the temperature. Different paths of particles to the molten pool lead to different final temperatures, which may affect the temperature distribution in the molten pool and even the accumulation of the temperature field inside the molded parts in the process of processing. The results showed that the temperature increased with the drop of powder particles. In addition, titanium particles melt under 900 W laser irradiation.
【学位授予单位】：北京工业大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG665

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