基于动态调控原位ＸＲＤ激光沉积制造显微组织/应力演化规律研究

Laser direct metal deposition (LDMD) is an advanced additive manufacturing (AM) technology, which is also called laser deposition manufacturing (LDM). It has tremendous potential for producing high-value, complex, individually customized parts and also can be used to repair and rebuild worn or damaged components by forming the wear and corrosion resistance coatings. LDM is a process by layer-by-layer deposition directly from a 3-D digital model. In this process, the solidification characteristics for metal-based materials are very different from those formed under the equilibrium condition. This is due to the rapid cooling speed and the periodic thermal recycling inducing the obvious change of the microstructure and stress. Therefore, it is necessary to study the microstructure and stress evolution behavior during the LDM heating and solidification process. At the present, the study of the phase transformation and stress evolution are mainly based on the solidification microstructure analysis under the room temperature according to the equilibrium phase diagram speculation. In this project, we propose a new idea to in situ study of the phase transformation and stress evolution during the laser manufacturing process. The phase transformation and the stress evolution can be identified directly from the characteristic lines change of the X-ray diffraction spectrum. We chose the extensively used Ti6Al4V alloy. The mechanism of phase transformation and stress evolution was investigated in LDM by in situ XRD and TEM and EBSD characterization. Finally, we explore a new method to change the phase transformation behavior by using an auxiliary heat source. By adjusting the processing parameters, such as laser power, scanning speed and input speed of powder, the microstructure can be controlled on real time directly. The study of this project will be very important for not only the theoretical significance of LDM material but the engineering application.

高性能金属构件激光增材制造是当前先进制造领域的国际前沿和竞争热点，其工艺特点决定的快速凝固和周期性热循环导致成形零件物相组成以及显微结构和力学性能发生显著变化。因此，揭示激光加热材料凝固组织显微结构/应力演化机理，探索原位动态调控微观组织演变是本领域当前尚未解决而又亟待解决的基础科学问题。我们提出一种全新的思路：基于X射线衍射的实验方法，在线研究凝固相结构在激光沉积过程中动态转变行为，以广泛应用的Ti6Al4V为对象，结合透射电子衍射技术、EBSD技术等结构表征手段，阐明该过程中Ti6Al4V物相转变、应力发展机理。并且探索添加辅助热源通过原位XRD实时监控相变及应力变化，调节辅助热源参数以实时调控激光沉积凝固组织显微结构演变和应力行为，为激光直接制造材料可控沉积提供一种全新的方法。本项目的研究不仅具有重要的理论意义，也具有深远的工程应用背景。

本项目以激光增材制造为背景，以航空结构材料钛合金为研究材料，系统研究了激光增材制造Ti-6Al-4V合金经快速加热和快速凝固、反复经受热循环作用后的微观组织分布、成分分布、物相分布、织构取向、缺陷形态及大小，并对激光增材制造钛合金构件的力学性能进行了系统研究，包括：力学性能随位置和方向取向性，力学性能随温度和应力变化的变化规律，微观缺陷对力学性能的影响规律，室温和高温拉伸断裂过程中裂纹形核位置、扩展方式以及断裂机理，室温和高温拉伸变形过程中织构取向关系、微观组织演变关系等。研究取得的重要结果有：1、激光增材制造Ti-6Al-4V合金的力学性沿激光扫描方向和激光堆积方向呈各向异性，沿扫描方向的拉伸强度为993MPa，屈服强度为923MPa，延伸率8%，断面收缩率10%，沿堆积方向拉伸强度为957MPa，屈服强度为868MPa，延伸率14%，断面收缩率34%，力学性能各向异性的原因是由于钛合金材料本身结构的原因和激光增材制造反复堆积垂直散热的原因，激光增材制造Ti-6Al-4V合金的拉伸强度和屈服强度高于锻件退火态的强度，延伸率和断面收缩率高于锻件固溶+时效后的延伸率。2、激光增材制造Ti-6Al-4V合金在400℃时的拉伸强度为657MPa，屈服强度为532MPa，延伸率18%，断面收缩率60%。 随温度升高，拉伸强度、屈服强度降低，延伸率、断面收缩率增大，且高温时的各项力学性能指标均高于锻件时的指标。3、激光增材制造Ti-6Al-4V合金拉伸断裂时裂纹首先沿缺陷位置处发起，然后沿β晶界传播。这些研究结果对促进激光增材制造钛合金的广泛应用及提升我国航空业的发展具有重要的意义。