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

近年来，新的固态连接技术如搅拌摩擦焊（FSW）和线性摩擦焊（LFW）的出现为工艺开发开辟了新的途径，特别是在工业界非常感兴趣的净成形制造领域。 在这两个过程中，摩擦热软化了要连接的表面，锻造压力的应用使界面区域塑化并加固接头。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以及开发更高效、更高性能部件的最佳途径之一。然而，3D打印组件的主要缺点之一是它们的铸造结构。这些结构的固有特征是不均匀的微观结构和化学元素的不均匀分布（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，然后使其变形以达到最终形状。这种方法具有很大的优势，将不均匀的铸造组织转化为更各向同性和均匀的组织（锻造组织）。这种数据生成将显著扩大3D打印技术的应用，使其能够应用于大量因使用条件而无法接受铸造结构的部件。因此，从科学的角度来看，了解3D打印材料微观结构演变的基本机制作为热变形工艺参数的函数是非常重要的。本研究计划是在这种背景下定义的，其目的是研究3D打印镍基高温合金的热变形行为，以期开发使用线性摩擦焊接技术制造3D打印叶片到锻造盘的知识库，这是航空航天工业中的现代制造技术。为此，将使用光学、扫描（EBSD）和TEM研究微观结构的演变。 基于这些结果和在以前的研究中开发的专业知识，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。 该模型还将包括相变动力学，再结晶，微观织构变化，和第二相沉淀在每种材料和它们的相互影响。有限元软件FORGE NXT将用于评估模型的有效性。最后，焊后热处理将被应用，其对接头的机械性能，包括疲劳寿命的影响将被检查。 在今后五年内，将培养两名博士和两名硕士，沿着还有5名本科生。*

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.***