High Strength and Ductility of Additively Manufactured 316L Stainless Steel Explained
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LASER powder bed fusion is a type of laser-based additive manufacturing (AM) process during which metallic parts are fabricated from a 3D computer-aided drawing (CAD) file by selective melting of successive layers of powder using a sharply focused laser beam, e.g., selective laser melting (SLM), direct metal laser melting (DMLM). This ‘‘freeform’’ manufacturing method can produce extremely complex shapes relative to a traditional subtractive manufacturing method (machining).[1] The density, microstructures, and, consequently, the mechanical properties of the additively manufactured material are sensitive to laser processing parameters such as laser power, scanning speed, and scanning strategies. Indeed, it has been reported that
MD. SHAMSUJJOHA, SEAN R. AGNEW, and JAMES M. FITZ-GERALD are with Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Contact e-mail: [email protected] WILLIAM R. MOORE and TABITHA A. NEWMAN are with the Naval Surface Warfare Center Dahlgren Division, Dahlgren, VA 22448. Manuscript submitted September 3, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
with proper selection of laser processing parameters and scanning strategies, it is possible to obtain near fully dense parts.[2–4] The microstructural development in AM materials is complex because of the complicated thermal distribution. Each location in the component first undergoes rapid solidification, and then undergoes repeated reheating and cooling with an additional deposited layer. This complicated thermal history can result in microstructural heterogeneity in the as-built part.[5] 316L stainless steel is one of the most widely investigated materials for AM. This is because 316L has a wide range of applications in marine, biomedical, nuclear reactor, chemical, and petrochemical industries owing to its high strength, good ductility, and high corrosion resistance.[6–8] Several studies have been performed to optimize the laser processing parameters to fabricate near fully dense 316L SS parts.[4,9–11] These studies reported that the average strengths of the optimized parts are higher than the counterpart as-cast, hot-pressed (HP), or hot isostatic-pressed (HIP) 316L stainless steel parts, while the ductility has been reported to be lower.[7,8,12] The microstructure in the as-built 316L stainless steel materials is reported to be a submicron cellular-dendritic solidification structure,
resulting from the high cooling rate inherent to AM process.[8,13,14] Many researchers have attributed the higher strength of the as-built 316L stainless steel to the refined microstructure and high dislocation density. However, a quantitative connection between the dislocation density and the observed strength is yet to be made. Anisotropy of the mechanical properties, especially the ductility, with respect to as-built directions is still a matter of controversy. For example, some researchers[15–18] reported a higher ductility of the built material when the sample is tested perpendicular to the building direction r
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