Understanding Microstructure Evolution and Control During Hot Deformation: Application to Solid State Joining of High Strength Alloys

近年来，搅拌摩擦焊(FSW)和线性摩擦焊(LFW)等新型固态连接技术的出现为工艺发展开辟了新的途径，特别是在工业非常感兴趣的净成形制造领域。在这两种工艺中，摩擦热使要连接的表面变软，而锻造压力使界面区域塑化并加固连接。金属材料的3D打印已被证明是改进设计、降低高附加值产品制造成本和开发更高效、更高性能部件的最佳方法之一。然而，3D打印组件的主要缺点之一是其铸造结构。这些结构的本质特征是微观结构不均匀，化学元素分布不均匀(微观和宏观偏析)。最近推出的一种创新方法是将3D打印部件用作预制件，然后将其变形以达到最终形状。这种方法有很大的优势，可以将不均匀的铸态组织转变为更各向同性和更均匀的组织(锻造组织)。这样的数据生成将把3D打印技术的应用扩展到大量由于使用条件而不能接受铸造组织的部件。因此，从科学的角度来看，了解3D打印材料组织随热变形工艺参数变化的基本机制是非常重要的。本研究项目就是在这种背景下提出的，目的是研究三维打印镍基高温合金的热变形行为，旨在开发利用航空航天现代制造技术--线性摩擦焊接技术制造三维打印叶片到锻造圆盘的知识库。根据这些结果和在以前的研究中积累的专门知识，将开发一个微观结构模型，该模型将考虑到两种材料的微观结构之间的相互作用。该模型还将包括每种材料的相变、再结晶、微观织构变化和第二相析出的动力学及其相互影响。有限元软件Forge NXT将用于评估模型的有效性。最后，进行焊后热处理，研究其对接头力学性能包括疲劳寿命的影响。两名博士和两名硕士以及5名本科生将在未来五年内接受培训。*

In recent years, the advent of new solid state joining technologies such as Friction Stir Welding (FSW) and Linear friction welding (LFW) have opened new avenues in process development, particularly, in the areas of net shape manufacturing which are of very high interest to industry. In both processes, frictional heat softens the surfaces to be joined and the application of a forge pressure plasticizes the interfacial region and consolidates the joint.****3D printing of metallic materials has been proved to be one of the best ways to improve design, reduce manufacturing cost of high value added products and develop more efficient and higher performance components. However, one of the major short comings of 3D printed components is their cast structure. These structures are inherently characterized by heterogeneous microstructures and non-uniform distribution of chemical elements (micro and macro segregation). One of the innovative approaches, introduced recently, has been to use the 3D printed part as a preform and then deform it to reach the final shape. This approach has the great advantage to convert the inhomogeneous cast structure to a more isotropic and homogeneous one (wrought structure). Such data generation will significantly expand the application of 3D printing technology to a large number of components where cast structures can't be accepted because of service conditions.****From a scientific point of view, it is therefore, very important to understand the fundamental mechanisms governing microstructure evolution of 3D printed materials as a function of hot deformation process parameters. The present research program has been defined in this context and has for objective to study the hot deformation behavior of 3D printed Ni base superalloys with the view of developing the knowledge base for the manufacturing of 3D printed blades to forged disks using the Linear Friction Welding technique which is a modern manufacturing technology in the aerospace industry.*******To this end, the evolution of the microstructure will be studied using optical, scanning (EBSD) and TEM. Based on these results and the developed expertise during the previous studies a microstructural model will be developed that would take into account the mutual interactions between the microstructures of the two materials. The model will also include the kinetics of phase transformation, recrystallization, microtexture changes, and second phase precipitation in each material and their mutual influences. The FEM software FORGE NXT will be used to assess the validity of the model. Finally, post weld heat treatment will be applied and its influence on mechanical properties of the joint including fatigue life will be examined. Two PhD and two MSc along with 5 undergraduate students will be trained during the next five years.***