Collaborative Research: Fundamental Investigation of Microscale Residual Stresses in Additively Manufactured Stainless Steel

增材制造，也称为3D打印，是一种颠覆性技术，用于制造汽车，航空航天，国防，生物医学和其他行业的工程部件。用于金属合金增材制造的高温激光束通常会产生高度不均匀的微观结构，从而导致3D打印材料中存在较大的不均匀残余应力。残余应力通常对材料的性能或部件的寿命有害，因此限制了增材制造在工程应用中的广泛采用。虽然宏观残余应力在金属3D打印领域已得到广泛研究，但微观残余应力的来源和控制在很大程度上尚未探索。这项合作研究旨在了解和控制增材制造不锈钢中的微尺度残余应力。由于其优异的机械性能，耐腐蚀性和抗氧化性的组合，不锈钢是一种广泛应用的主力材料，如汽车，船舶，飞机，核电站，医疗植入物等。该研究将通过整合显微结构表征，机械测试，和计算机建模。所获得的机理见解将用于指导增材制造，以减轻3D打印不锈钢中的微尺度残余应力。这项研究的结果将为未来开发具有定制微观结构和优异机械性能的增材制造金属材料奠定坚实的基础。该项目将通过多学科方法促进教学、培训和学习，扩大代表性不足的群体的参与，丰富课程开发工作，特别是在材料科学和先进制造的跨学科领域。技术概述通过激光粉末床融合和激光工程净成形技术进行金属合金的增材制造具有高度局部化的熔化过程，快速冷却速率，和强烈的温度梯度这些极端的激光打印条件导致高度不平衡的微观结构，导致增材制造材料中严重不均匀的残余应力。该研究旨在阐明3D打印不锈钢中的增材制造方法，异质微观结构和微尺度残余应力之间的基本关系。该项目包括两个主要目标。Thrust I涉及3D打印、微观结构表征、机械测试和不锈钢中残余应力的原位同步加速器X射线测量，用于大范围的打印方案和参数，以及相应的各种打印微观结构。推力二涉及微观结构敏感的晶体塑性有限元模型，占异质晶粒结构和亚晶粒凝固细胞结构的发展。将通过实验和模拟相结合，系统地研究晶间和晶内残余应力对印刷样品力学响应的影响。所获得的机理见解将用于指导打印方案和参数的优化，从而减轻3D打印不锈钢中的微尺度残余应力。开发的集成实验和建模方法通常适用于理解和控制其他增材制造金属合金中的残余应力。该项目将吸引高中生和代表性不足的少数民族参与研究。这些活动将提供机会，激发他们在追求未来的职业生涯在先进制造业的兴趣。这个奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。

NONTECHNICAL SUMMARYAdditive manufacturing, also called 3D printing, is a disruptive technology for the manufacture of engineering components in automotive, aerospace, defense, biomedical and other industries. The high-temperature laser beam used for additive manufacturing of metal alloys usually produces highly heterogeneous microstructures that result in large inhomogeneous residual stresses in 3D-printed materials. Residual stresses are generally detrimental to the performance of a material or the life of a component, thus limiting the wide adoption of additive manufacturing in engineering applications. While the macroscale residual stresses have been widely studied in the field of metal 3D printing, the origin and control of the microscale residual stresses remain largely unexplored. This collaborative research aims to understand and control the microscale residual stresses in additively manufactured stainless steel. Due to its excellent combination of mechanical properties, corrosion, and oxidation resistance, stainless steel is a workhorse material used in a wide range of applications such as cars, ships, airplanes, nuclear power plants, medical implants, etc. The research will investigate the effects of 3D-printed microstructures on the resultant microscale residual stresses in stainless steel by integrating microstructural characterization, mechanical testing, and computational modeling. Mechanistic insights gained will be applied to guide additive manufacturing, so as to mitigate the microscale residual stresses in 3D-printed stainless steel. Results from this research will lay a solid foundation for future development of additively manufactured metallic materials with tailored microstructures and outstanding mechanical performance. The project will promote teaching, training, and learning through multi-discipline approaches, broaden the participation of underrepresented groups, and enrich curriculum development efforts, particularly in the interdisciplinary areas of materials science and advanced manufacturing.TECHNICAL SUMMARYAdditive manufacturing of metal alloys via laser powder bed fusion and laser engineered net shaping technologies features highly localized melting processes, fast cooling rates, and strong temperature gradients. These extreme laser-printing conditions result in highly nonequilibrium microstructures that lead to severely inhomogeneous residual stresses in additively manufactured materials. The research aims to elucidate the fundamental relationships between the additive manufacturing methods, heterogeneous microstructures and microscale residual stresses in 3D-printed stainless steel. The project consists of two major thrusts. Thrust I involves 3D printing, microstructural characterization, mechanical testing and in situ synchrotron x-ray measurements of residual stresses in stainless steel for a large range of printing schemes and parameters, and accordingly a variety of printed microstructures. Thrust II involves the development of microstructure-sensitive crystal plasticity finite element models that account for the heterogeneous grain structures and sub-grain solidification cell structures. The impact of both intergranular and intragranular residual stresses on the mechanical responses of printed samples will be systematically studied by combining experiments and simulations. Mechanistic insights gained will be applied to guide the optimization of printing schemes and parameters, so as to alleviate the microscale residual stresses in 3D-printed stainless steel. The integrated experimental and modeling approach developed is generally applicable to understand and control the residual stresses in other additively manufactured metal alloys. The project will engage high school students and underrepresented minorities for research. These activities will provide opportunities to inspire their interest in pursuing future career in advanced manufacturing.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.