CAREER: Recrystallization in additive manufactured metallic materials

近净形状的金属增材制造（AM）（也称为3D打印）可能会给汽车、航空航天和能源生产行业带来重大的范式转变。广泛采用金属增材制造的主要挑战是在所有方向上获得可比较和一致的机械性能，如强度和延展性，而不管打印方向如何。该项目探索了打印后热处理的基础科学，以在原子尺度上控制金属的成分和排列，从而在整个3D打印部件中获得均匀的机械性能。高级高倍成像的缺陷发生在原子水平上，被称为位错，揭示了他们如何组织自己，以及他们在促进或阻碍金属内部结构的形成中起什么作用，被称为“颗粒”。这些内部结构从根本上决定了金属的性能，创造更小的无缺陷晶粒的过程被称为再结晶。这项研究为增材制造中的位错和再结晶提供了新的视角，以提高机械性能并延长使用寿命。这项研究还与教育和推广活动交织在一起，旨在提高传统上代表性不足的社区对增材制造的认识和参与。活动包括：在本科和研究生阶段的实验课程中纳入AM，在伊利诺伊大学香槟分校建立一个专注于AM的研究中心，以及持续开发一个免费参与者，以材料为主题的中学女孩夏令营，目标是低收入的少数民族。增材制造（AM）实现了多种材料的近净成形，从而减少了加工需求，同时也减少了浪费。不幸的是，这种工艺通常会产生强烈的各向异性，这限制了作为制造技术的广泛采用。旨在触发再结晶的构建后退火处理是一种简单且经济有效的策略，可以减少任何几何形状的性能各向异性。然而，尽管位错密度很高，但大多数增材制造材料的再结晶非常稀疏，且动力学缓慢；这一观察结果仍有待理解。该项目旨在阐明通过激光粉末床熔合和定向能沉积制备的增材制造金属材料再结晶的基本机制。利用电子显微镜结合原位同步加速器实验，实现了一种实验-数值相关框架，以精确地绘制、数字化和分析AM再结晶过程中的微结构。结果允许对驱动力进行定量测量，以实现专门为增材制造材料量身定制的基于物理的微观结构演变规律。通过这种方法，具有各向同性特性的增材制造材料的变革性印刷和合金设计策略正在产生。研究工作伴随着外展活动，将伊利诺伊州中部的中学、本科、研究生和专业水平的代表性不足的人群暴露在快速发展的AM领域。该项目还将金属AM纳入伊利诺伊大学厄巴纳-香槟分校的本科和研究生课程。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 Near-net-shape, metal additive manufacturing (AM) (also known as 3D printing) may allow for a major paradigm shift in the automotive, aerospace and energy generation industries. The main challenge that remains for broad adoption of metal AM is the obtaining of comparable and consistent mechanical properties such as strength and ductility in all orientations regardless of print direction. This project explores the basic science of thermal treatments after printing to control the composition and arrangement of metals, at the atomic scale, to obtain uniform mechanical properties throughout a 3D printed part. Advanced high magnification imaging of defects that occur at an atomic level, called dislocations, is revealing how they organize themselves and what role they play in encouraging or discouraging the formation of inner structures in metals known as “grains”. These inner structures fundamentally dictate a metal’s performance and the process of creating smaller defect free grains is a process known as recrystallization. This research generates new perspectives on dislocations and recrystallization in AM to increase mechanical performance and enable longer lifespans during service. This research also intertwines with educational and outreach activities aiming to increase awareness of and participation in additive manufacturing among traditionally underrepresented communities. Activities include: incorporation of AM within laboratory courses at the undergraduate and graduate levels, creation of a research center focused on AM at the University of Illinois, Urbana-Champaign, and continuous development of a free-to-participants, materials-themed summer camp for middle-school girls targeting underrepresented minorities of low income. TECHNICAL SUMMARY Additive manufacturing (AM) enables near-net-shaping across a large variety of materials, thereby reducing the need for machining while also reducing waste. Unfortunately, this process generally induces strongly anisotropic properties which limit broad adoption as a manufacturing technique. Post-build annealing treatments aiming to trigger recrystallization are a simple and cost-efficient strategy to reduce property anisotropy in any geometry. Yet, despite a high dislocation density, most AM materials recrystallize surprisingly sparsely and with sluggish kinetics; an observation that still remains to be understood. The project is elucidating the fundamental mechanisms driving recrystallization in AM metallic materials fabricated by laser powder-bed fusion and directed energy deposition. An experimental-numerical correlative framework using electron microscopy combined with in-situ synchrotron experiments is being implemented to accurately map, digitize and analyze AM microstructures during recrystallization. Results are allowing for quantitative measurement of driving forces with a view to enable physics-based microstructure evolution laws tailored specifically to AM materials. With this approach, transformative printing and alloy design strategies for AM materials with isotropic properties are being produced. Research efforts are accompanied by outreach initiatives exposing under-represented populations in central Illinois at the middle school, undergraduate, graduate and professional levels to the rapidly evolving field of AM. This project also enables the incorporation of metal AM into the undergraduate and graduate curriculum at the University of Illinois Urbana-Champaign.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.