Additive Manufacturing of Fatigue Resistant Austenitic Stainless Steels by Establishing the Process-Structure-Property Relationships
Type of DegreePhD Dissertation
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This work presents a multidisciplinary approach (relating microstructure features to mechanical properties) to utilize additive manufacturing as a tool to fabricate superior, fatigue resistant materials. The ultimate goal of the study was to establish the Process-Structure-Property relationships of an additive manufactured 304L stainless steel to determine if and how material can be tailored to enhance mechanical properties by controlling their microstructural grain size, orientations, and crystalline texture during their manufacture. The research centers on a unique ex-situ microstructural characterization technique to inform material selection and fabrication process to avoid the typical failure mechanisms associated with the material. Microstructural traits and their relationships to crack initiation and microstructurally short crack growth of the wrought and additive manufactured material is evaluated by optical and scanning electron microscopy, x-ray diffraction, and electron backscatter diffraction on representative samples as well as fatigue specimens through a systematic ex-situ experimental characterization process. The Process-Structure-Property relationships were established by correlating the resulting microstructural features to the material’s mechanical properties before, during, and after fatigue testing. It was shown that austenitic stainless steels such as 304L possess certain characteristics that allow their fabrication through laser beam powder bed fusion, while minimizing the failure mechanisms associated with these alloys. The fatigue resistance of the additive manufactured material in the stress relieved condition was thus shown to be superior to the conventionally processed wrought material. Traditional solution annealing treatments, however, were shown to be detrimental to the additive manufactured material as they resulted in the crack initiation to shift from microstructural features, such as annealing twin and high angle grain boundaries, to process induced defects. These profound results suggest that by understanding the Process-Structure-Property relationships additive manufacturing can be leveraged to fabricate materials with improved microstructures capable of enhancing their mechanical performance. Results from this study have progressed the understanding of additive manufactured 304L stainless steel by providing (i) the relationships between AM process and post-process conditions and eventual microstructural morphology/orientation, (ii) characterization of the insufficiently-documented fatigue resistance of AM materials, and (iii) a unique experimental approach for advanced understanding of the microstructure-property relationships that has, thus far, been camouflaged by the effects of process-induced defects.