CAREER: In Operando Investigation of Laser Powder-fed Directed Energy Deposition: Process Physics, Microstructure Evolution, and Mechanical Properties

增材制造的颠覆性创新依赖于其对传统制造方法无法或难以制造的复杂形状产品进行大规模定制的能力。然而，加工可重复性差是其广泛应用的主要障碍之一，特别是对于关键的承重金属部件。巨大的可变性，特别是对于激光粉末直接能量沉积，是由于极高的加热和冷却速率，这发生在粉末流与形状变化的液态金属池的随机相互作用过程中，并导致远离平衡和高度动态的现象，这是极具挑战性的研究。该学院早期职业发展（Career）奖支持使用高能高速x射线的研究，以实现对上述瞬态现象的基本理解。将进行不同时间和长度尺度的实验，以建立工艺和特征关系，以预测增材制造零件的质量、可重复性和性能。教育和推广活动旨在促进妇女和代表性不足的少数民族充分参与先进制造业，并发展一支训练有素、多样化和可持续的制造业劳动力队伍。在项目过程中，PI将与中学女生、本科女生和代表性不足的少数族裔合作，分别组织一系列关于增材制造的讲习班和实践活动，参加午餐系列活动，并在为期10周的暑期项目中指导学生。该项目的主要研究目标是通过了解粉末直接能量沉积过程的基本物理原理来改变粉末直接能量沉积。定制的定向能沉积设备将与康奈尔高能同步加速器源的设备相连接，以进行独特的实验。结合同步加速器硬x射线成像技术和具有空前时空分辨率的衍射技术，研究了钛基和镍基合金的粉末流动、熔池动力学、非平衡相变以及孔隙形成。此外，将进行时间分辨x射线测量实验，以研究增材制造固有的循环热处理对微观结构和残余应力演变的影响。将建立成分、工艺参数、工艺特征与所得微观结构、孔隙率和残余应力之间的相关性。这些发现将用于克服增材制造的长期局限性，通过促进等轴凝固来减少可访问的工艺参数范围内的各向异性，ii)通过管理过程中的热量积累来最小化残余应力和变形，iii)最大化工艺效率，同时最小化熔池不连续，iv)构建具有零件性能最佳组合的工艺图。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The disruptive innovation of additive manufacturing relies on its ability of mass customization of complex-shaped products that are impossible or difficult to make by conventional manufacturing methods. However, poor processing repeatability is one of the major obstacles to its widespread use, especially for critical load-bearing metallic components. The huge variability, for laser powder-fed direct energy deposition in particular, is due to extremely high heating and cooling rates, which occur during stochastic interactions of the powder flow stream with a shape-changing liquid metal pool and cause far-from-equilibrium and highly dynamic phenomena that are extremely challenging to study. This Faculty Early Career Development (CAREER) award supports the study of using high-energy, high-speed x-rays to achieve fundamental understanding of the aforementioned transient phenomena. Experiments at different time and length scales will be performed to establish process and characteristic relationships to predict quality, repeatability and properties of additively manufactured parts. The educational and outreach activities aim to promote a full participation of women and underrepresented minorities in advanced manufacturing and to develop a well-trained, diverse and sustainable manufacturing workforce. Over the project course, the PI will work with middle school girls, undergraduate women and underrepresented minorities through organizing workshop series on additive manufacturing with hands-on activities, attending lunch series and mentoring students in a 10-week summer program, respectively.The main research objective of this project is to transform the powder-fed direct energy deposition by understanding the fundamental physics of the process. A custom-built directed energy deposition apparatus will be interfaced with the facility at Cornell High Energy Synchrotron Source to perform unique experiments. Synchrotron hard x-ray imaging and diffraction techniques with unprecedented spatial and temporal resolutions are combined to study the powder flow stream, melt pool dynamics and non-equilibrium phase transformation of titanium and nickel-based alloys, as well as pore formation. Moreover, experiments with time-resolved x-ray measurements will be conducted to investigate the influence of cyclic heat treatments, intrinsic to additive manufacturing, to the microstructure and residual stress evolution. Correlations between compositions, processing parameters, process signatures and the resultant microstructure, porosity and residual stresses will be established. The findings will be used to overcome the long-lasting limitations of additive manufacturing by i) reducing anisotropy within the accessible process parameter range by promoting equiaxed solidification, ii) minimizing residual stresses and distortions by managing the heat accumulation during the process, iii) maximizing the process efficiency while minimizing melt pool discontinuities and iv) constructing process maps with the best combination of part properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.