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

非技术性SUMMARYAdditive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零部件。高温激光用于金属合金的添加制造通常会产生高度不均匀的微观组织，从而导致3D打印材料中存在很大的不均匀残余应力。残余应力通常不利于材料的性能或部件的寿命，因此限制了添加剂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打印领域已经得到了广泛的研究，但微观残余应力的来源和控制还没有得到很大的探索。这项合作研究旨在了解和控制添加制造的不锈钢中的微尺度残余应力。不锈钢具有优异的机械性能、耐腐蚀性和抗氧化性，是一种广泛应用于汽车、船舶、飞机、核电站、医疗植入物等领域的主力材料。本研究将通过结合微观结构表征、力学测试和计算建模来研究3D打印微结构对不锈钢微尺度残余应力的影响。所获得的力学见解将用于指导添加剂制造，从而减轻3D打印不锈钢中的微尺度残余应力。研究结果将为今后开发具有定制组织和优异力学性能的添加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，扩大未被充分代表的群体的参与，并丰富课程开发努力，特别是在材料科学和先进制造的跨学科领域。技术总结通过激光粉床融合和激光工程净成形技术进行金属合金的附加制造具有高度局部化的熔化过程、快速的冷却速度和强烈的温度梯度。这些极端的激光打印条件会导致高度非平衡的微结构，从而在添加制造的材料中导致严重的不均匀残余应力。本研究旨在阐明3D-Print不锈钢的添加剂制造方法、非均匀组织和微观残余应力之间的基本关系。该项目由两个主要推力组成。Struts I涉及3D打印、微观结构表征、机械测试以及在大范围打印方案和参数以及相应的各种打印微结构下对不锈钢中残余应力的原位同步x射线测量。推力II包括建立考虑非均匀晶胞结构和亚晶胞结构的微观结构敏感晶体塑性有限元模型。通过实验和模拟相结合的方法，系统地研究了晶间和晶内残余应力对印刷样品力学响应的影响。所获得的力学见解将用于指导打印方案和参数的优化，从而缓解三维打印不锈钢的微尺度残余应力。本文提出的综合实验和模拟方法，对于理解和控制其他添加成形金属合金的残余应力具有普遍的适用性。该项目将邀请高中生和未被充分代表的少数族裔参与研究。这些活动将为他们提供机会，激发他们追求未来先进制造事业的兴趣。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.