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

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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.