激光立体成形GH4169合金的再结晶机理研究

Nickel based superalloy fabricated by Laser Solid Forming (LSF) performs worse fatigue property due to the coarse columnar grains and the simple interface structure. Because no further hot working process is available to the LSF metal parts, it's difficult to achieve the refinement of its grain structure. However, that recrystallization occurred in LSF GH4169 superalloy during its post heat treatment were attributed to the residual stress existed, and the grains were observed to be refined to a certain extent. This gives a new way to refine the grains of LSF nickel based superalloy. This program detailedly studies the fundamental topics about the recrystallization processing of LSF GH4169 superalloy based on the results of prior experiments. Using the methods of experimental measuring and numerical simulation, the distribution and the level of residual stress will be analyzed aimed to declare the driving force of the recrystallization and to handle its regularity. In order to achieve the homogeneous distribution of recrystallized grains, the homogeneously nucleation conditions will be found out through the systematical investigation of the nucleation sites, the quantity of nucleation cores and the mechanics of nucleation. The crystal orientation and the interface structure of grains before and after recrystallization heat treatment will be studied for the law of the random orientation and the proper distribution of larger angle grain boundary. All these will lead to the optimization of recrystallized grain structure via the adjustments of LSF parameters and post heat treatment processing. The results of this program can provide experimental and theoretical evidences for the promotion of the application of LSF technology and the enrichment of the rapid forming theory.

激光立体成形镍基高温合金较低的疲劳性能主要是由其粗大的柱状晶组织及其简单的界面结构造成的。因不能经过后续热加工，激光立体成形后零件的组织细化受到一定限制。借助成形中产生的残余应力诱导发生再结晶，可达到细化晶粒的目的，对再结晶过程的控制至关重要。本项目在前期研究的基础上，对激光立体成形GH4169合金再结晶过程中的一些基本问题展开研究。使用实验测量和计算机模拟的方法对微观区域残余应力的大小和分布规律的研究，把握再结晶的驱动力因素，从再结晶源头上把握再结晶规律；系统分析再结晶形核位置、形核数量和形核机制，找到再结晶均匀形核的条件，实现再结晶晶粒的均匀分布；深入研究再结晶前后材料的晶体取向和界面结构演化规律，实现再结晶晶粒晶体取向的随机分布和大角度晶界合理分布；从成形工艺参数和热处理工艺调整来优化再结晶组织。研究结果可为激光立体成形技术的推广以及快速成形理论的丰富提供实验和理论依据。

由于激光立体成形技术所制备的零件往往具有粗大柱状晶组织，在涡轮盘等零件的激光快速成形中的应用受到一定的限制，实现激光立体成形零件组织的细化和均匀化具有重要意义。粗大的柱状晶组织及简单的界面结构导致激光立体成形GH4169 合金的疲劳性能较低，不能满足工程应用的需要。因不能后续热加工，激光立体成形后零件的组织细化及材料性能提高受到限制。项目借助镍基高温合金激光立体成形后材料内部存在的残余应力，通过再结晶的方法实现晶粒组织细化，有望成为涡轮盘等对晶粒组织有要求的关键发动机零件激光立体成形后组织调控的有效手段。.项目采用残余应力显微硬度压痕法测量了激光立体成形GH4169合金微观区域残余应力测量，揭示了搭接率等工艺参数对微观区域残余应力的影响规律，发现搭接率的增加可以一定程度上增加合金内部残余应力数值，且可以减小残余应力在道间和道内的差别；进行了激光立体成形GH4169合金再结晶形核机理分析，揭示了晶界弓出机制、亚晶合并机制和孪晶机制在合金再结晶过程中的作用。发现了孪晶形核机制在激光立体成形GH4169合金晶粒细化中的重要作用，且合金再结晶形核的位置也受激光立体成形工艺和后续热处理工艺影响，增加搭接率和增加热处理升温速率可以促使再结晶形核从道间区域优先形核变为道间和道内均匀形核；比较了激光功率、扫描速度、道间搭接率和后续热处理对激光立体成形GH4169合金再结晶过程的影响，揭示了不同条件下合金再结晶组织演化规律，发现合金的再结晶过程受搭接率的影响大于激光功率和扫描速率的影响，增加搭接率可以显著调高合金再结晶后晶粒尺寸均匀性。提高热处理温度可以加快激光立体成形GH4169合金再结晶的完成，而采用较高的升温速率，可以获得更加细小和均匀的再结晶组织，且再结晶晶粒的细化速率随升温速率的增大而增大；提出了利用电磁搅拌实现激光立体成形GH4169合金组织调控的方法，进行了电磁搅拌在GH4169合金激光立体成形中的应用研究，发现电磁搅拌技术可以对沉积态材料中枝晶间共晶Laves相形貌和体积分数产生影响，且对合金残余应力分布和再结晶组织产生影响。.在项目支持下，项目组在国内外期刊共发表相关学术论文9篇，其中SCI收录5篇，EI收录8篇，英文论文5篇，中文核心论文4篇。参加国内学术会议5次，发表会议报告5次。培养硕士研究生5人，其中毕业2人，在读3人。