CAREER: Understanding multiscale sintering kinetics and microstructural evolution in binder-based metal additive manufacturing

非技术概述：利用热量将粉末转化为多孔固体而不完全熔化的过程是一种被称为“固态烧结”（SSS）的制造方法。这项技术构成了许多重要的金属和陶瓷材料工业工艺的支柱，如粉末冶金（P/M）、金属注射成型（MIM）和场辅助烧结（FAST）。总的来说，SSS平台是有利的，因为它们不需要高温来完全熔化金属或陶瓷粉末，这意味着通过烧结制造零件所需的能量比其他标准加工方法少。然而，烧结仍然存在长期的挑战，包括孔隙率、收缩和其他最终形状的变化等问题，这些问题由于发生在微米和纳米尺度上的事件而无法很好地预测。本研究项目利用高倍率x射线实验和高倍率电子显微镜，通过对粉末表面和烧结粉末内部结构的实时成像，揭示了多种长度尺度下烧结的基本机理。这些最先进的方法提供了关于缺陷形成方式和内部结构如何变化的新知识。本文以粘合剂喷射3D打印（BJ3DP）为例，因为BJ3DP对汽车行业具有重要意义，但研究结果有助于为各种SSS应用提供新的设计策略。该项目还促进了本科生和研究生在先进制造方面的培训，以解决美国目前日益扩大的技能差距。这是通过让学生接触最先进的表征工具，让他们参与制造研究，并让他们与汽车行业的工业合作伙伴一起获得专业发展机会来实现的。此外，该项目还通过招募和指导来自弱势群体的研究生，以及与密歇根大学自然历史博物馆合作，为K-12年级的学生提供实践社区外展活动，扩大了参与范围。技术概述：固态烧结（SSS）促进了金属和陶瓷材料的高效生产，然而，由于缺乏对促进内部微观结构发展的主要机制的基本认识，长期存在的挑战仍然存在。由于烧结的整体驱动力（界面表面能的降低）可以通过至少六种不同的潜在竞争机制来促进，因此对SSS的强有力理解在很大程度上受到缺乏原位数据的阻碍。本研究采用新型原位x射线计算机断层扫描（XCT）和高能衍射显微镜（HEDM），以微米级分辨率直接成像新型粘合剂喷射3D打印（BJ3DP）中SSS过程中的3D颗粒和内部微观结构演变。粘结剂喷射被用作一个范例，因为它在汽车工业中具有很强的实施潜力。结果将与电子显微镜相结合，以了解SSS过程中的主要扩散机制，以及它们如何受到工艺、原料和/或材料因素的影响。研究结果被用于测试BJ3DP致密化和晶粒生长的基本假设，并作为基于物理模型的校准数据。总的来说，这种新知识可以增强对各种SSS工艺的微观结构演变的预测，这些工艺具有很强的工业相关性，如粉末冶金、金属注射成型和现场辅助烧结。通过为本科生和研究生提供研究经验，并为汽车行业的学生提供专业发展机会，采用综合教育模块来缩小美国目前日益扩大的制造技能差距。教育和推广活动包括积极招募和培训未被充分代表的少数民族和女性，并与密歇根大学自然历史博物馆合作，为K-12年级的学生提供方便的实践活动，以激发他们对先进制造业和STEM的兴趣。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。

NON-TECHNICAL SUMMARY: The process of using heat to turn powder into a porous solid without fully melting said powder is a manufacturing approach known as “solid-state sintering” (SSS). This technology forms the backbone of many important industrial processes for metal and ceramic materials such as powder metallurgy (P/M), metal injection molding (MIM), and field-assisted sintering (FAST). Overall, SSS platforms are advantageous because they do not require high temperatures to fully melt metal or ceramic powders which mean less energy is required to manufacture parts by sintering than by other standard processing methods. However, long-standing challenges still exist for sintering which include issues such as porosity, shrinkage and other changes in final shape that aren’t easily predicted due to events that occur at the scale of micro- and nanometers which aren’t well understood. This research project is revealing fundamental sintering mechanisms across multiple length scales by directly imaging powder surfaces and the internal structure of powders undergoing sintering in real-time using high powered x-ray experiments and high magnification electron microscopes. These state-of-the-art approaches are providing new knowledge on the way defects form and how internal structures change. Binder jet 3D printing (BJ3DP) is used as an example process since BJ3DP is of significant interest to the automotive industry but research findings contribute to new design strategies for a variety of SSS applications. This project also facilitates the training of undergraduate and graduate students in advanced manufacturing to address the current and growing skills gap in the U.S. This is being accomplished by exposing students to state-of-the-art characterization tools, engaging them in manufacturing research and involving them in professional development opportunities with industrial partners in the automotive industry. Additionally, this project is broadening participation through the recruitment and mentoring of graduate students hailing from underrepresented populations and hand-on community outreach events for students in grades K-12 in collaboration with the University of Michigan Museum of Natural History.TECHNICAL SUMMARY: Solid-state sintering (SSS) facilitates the efficient production of metal and ceramic materials, however, long-standing challenges remain due to a lack of fundamental insight of the dominant mechanisms facilitating internal microstructural development. Since the overall driving force for sintering (reduction of interfacial surface energy) can be facilitated by at least six different and potentially competing mechanisms, a robust understanding of SSS has been largely stymied by the absence of in-situ data. This research is employing novel in-situ x-ray computed tomography (XCT) and high energy diffraction microscopy (HEDM) to directly image 3D particle and internal microstructural evolution with micron-scale resolution during SSS in novel binder jet 3D printing (BJ3DP). Binder jet is used as an exemplar due to its strong potential for implementation in the automotive industry. Results are being combined with electron microscopy to understand the primary diffusion mechanisms at work during SSS, and how they are influenced by process, feedstock, and/or material factors. Findings are being used to test fundamental hypotheses on densification and grain-growth in BJ3DP and serve as calibration data for physics-based models. Overall, this new knowledge is enabling enhanced prediction of microstructure evolution applicable to a variety of SSS processes with strong industrial relevance such as powder metallurgy, metal injection molding, and field-assisted sintering. Integrated educational modules are being employed to reduce the currently growing manufacturing skills gap in the U.S. through research experiences for undergraduate and graduate students and professional development opportunities for students with the automotive industry. Educational and outreach activities include actively recruiting and training underrepresented minorities and women and providing accessible, hands-on activities for students in K-12 in collaboration with the U-M Natural History Museum to inspire interest in advanced manufacturing and STEM.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.