Publications

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Mechanical testing was conducted to collect the data needed to build a Xue-Wierzbicki (XW) fracture model for PH13-8 Mo H950 stainless steel (PH 13-8 SS). This model is intended for use in structural analysis of this material between room temperature and -40° C. Tests were performed on four different specimen geometries such that a range of stress states were characterized at room temperature and -40° C. Tensile tests on R5 tensile specimens were also performed to assess material anisotropy. Fracture toughness test were also conducted. The fracture toughness of this material at -40° C was 68% of the room-temperature value. Material strength generally increased with decreasing temperature while the opposite trend was observed for ductility. These trends were most pronounced for specimens with the largest stress triaxialities.

The mechanical properties of additively manufactured metals tend to show high variability, due largely to the stochastic nature of defect formation during the printing process. This study seeks to understand how automated high throughput testing can be utilized to understand the variable nature of additively manufactured metals at different print conditions, and to allow for statistically meaningful analysis. This is demonstrated by analyzing how different processing parameters, including laser power, scan velocity, and scan pattern, influence the tensile behavior of additively manufactured stainless steel 316L utilizing a newly developed automated test methodology. Microstructural characterization through computed tomography and electron backscatter diffraction is used to understand some of the observed trends in mechanical behavior. Specifically, grain size and morphology are shown to depend on processing parameters and influence the observed mechanical behavior. In the current study, laser-powder bed fusion, also known as selective laser melting or direct metal laser sintering, is shown to produce 316L over a wide processing range without substantial detrimental effect on the tensile properties. Ultimate tensile strengths above 600 MPa, which are greater than that for typical wrought annealed 316L with similar grain sizes, and elongations to failure greater than 40% were observed. It is demonstrated that this process has little sensitivity to minor intentional or unintentional variations in laser velocity and power.

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