Investigations of Process Optimization, Microstructure Modification and Material Performance Improvement in High Deposition-rate Laser Metal Deposition of Inconel 718

实现大型金属构件的短交货期集成设计与制造是现代制造业的重要发展趋势之一，而这正是激光金属沉积（LMD）的关键发展方向之一。IN718是一种典型的沉淀强化镍基高温合金，广泛应用于航空发动机工业，本课题主要研究IN718的高沉积速率LMD （HDR-LMD）。与常规LMD（沉积速率~0.5kg/h）相比，HDR-LMD的目标是每小时沉积数公斤。一方面，对材料性能不利的孔隙、开裂等材料缺陷的发生频率增加。激光强度分布、粉末流场分布以及粉末特性和气体环境对材料缺陷有很大影响。另一方面，沉积的材料具有较低的强度和较高的塑性，这被认为是由于Nb偏析的规模和程度较低的结果。因此，利用微结构特性来获得优越的材料性能应该是可能的。因此，我们提出以下工作。首先，为了有效、合理地利用激光能量，建立激光强度分布与粉末流的匹配关系，在此基础上研制HDR-LMD喷嘴。研究了粉体特性对材料缺陷的影响，提出了hdr - lmd用粉体的质量标准。然后，基于过程监控（高速相机和热像仪）和金相分析，对过程进行建模，并将材料的微观结构与工艺参数相关联。最后，通过调整工艺参数来改变微观组织，并开发相应的热处理工艺来处理沉积态材料，以提高材料的性能，使部分具有颗粒状形貌的高温Laves相作为第二强化相。这可能是提高IN718使用温度的一个显著创新。

It is one of the most important developing trends of the modern manufacturing industry to realize the integrated designand fabrication of large-scale metal components with short delivery time, which is exactly one of the key developmentdirections of Laser Metal Deposition (LMD). This project focuses on the High Deposition-Rate LMD (HDR-LMD) ofInconel 718 (IN718) which is a typical precipitation strengthened nickel based super-alloy and widely used in the inaircraft engine industries. Compared with conventional LMD (deposition rate ~0.5kg/h), the aim in HDR-LMD is thedeposition of several kilograms per hour. On one hand, material defects, such as porosity and cracking which aredetrimental for material performance, occur more often. The laser intensity distribution and the powder streamdistribution as well as the powder characteristics and gas environment have strong effects on material defects. On theother hand, the as-deposited material has a lower strength and a higher ductility, which is believed to be a result of thelower scale and degree of Nb segregation. Thus, it should be possible to achieve superior material performance bytaking advantage of the microstructure characteristics. Therefore, we propose the following work. Initially, in order touse the laser energy efficiently and suitably, the matching between laser intensity distribution and powder stream willbe established, based on which a HDR-LMD nozzle will be developed/adapted. Afterwards, the effects of powdercharacteristics on material defects will be investigated, and furthermore the quality specifications for powder used forHDR-LMD will be proposed. Then, based on process monitoring (high speed camera & thermal camera) andmetallographic analysis the process will be modeled, and the material microstructures will be correlated with processparameters. Finally, the microstructure will be modified by adjusting process parameters, and moreover the adaptedheat treatment process will be developed to treat the as-deposited material to improve material performance, whichkeeps partially the high temperature Laves phases with granular morphology as a second strengthening phase. Thismay be a remarkable innovation to improve the service temperature for IN718.