This study aims to investigate the mechanical and fatigue behavior of additively manufactured metallic materials. Several challenges associated with different metal additive manufacturing (AM) techniques (i.e. laser-powder bed fusion and direct laser deposition) have been addressed experimentally and numerically. Experiments have been carried out to study the effects of process inter-layer time interval – i.e. either building the samples one-at-a-time or multi-at-a-time (in-parallel) – on the microstructural features and mechanical properties of 316L stainless steel samples, fabricated via a direct laser deposition (DLD). Next, the effect of building orientation – i.e. the orientation in which AM parts are built – on microstructure, tensile, and fatigue behaviors of 17-4 PH stainless steel, fabricated via a laser-powder bed fusion (L-PBF) method was investigated. Afterwards, the effect of surface finishing – here, as-built versus machined – on uniaxial fatigue behavior and failure mechanisms of Inconel 718 fabricated via a laser-powder bed fusion technique was sought. The numerical studies, as part of this dissertation, aimed to model the mechanical behavior of AM materials, under monotonic and cyclic loading, based on the observations and findings from the experiments. Despite significant research efforts for optimizing process parameters, achieving a homogenous, defectree AM product – immediately after fabrication – has not yet been fully demonstrated. Thus, one solution for ensuring the adoption of AM materials for application should center on predicting the variations in mechanical behavior of AM parts based on their resultant microstructure. In this regard, an internal state variable (ISV) plasticity-damage model was employed to quantify the damage evolution in DLD 316L SS, under tensile loading, using the microstructural features associated with the manufacturing process. Finally, fatigue behavior of AM parts has been modeled based on the crack-growth concept. Using the FASTRAN code, the fatigue-life of L-PBF Inconel 718 was accurately calculated using the size and shape of process-induced voids in the material. In addition, the maximum valley depth of the surface profile was found to be an appropriate representative of the initial surface flaw for fatigue-life prediction of AM materials in an as-built surface condition.

<p dir="ltr">The increasing use of additive manufacturing technologies such as Laser Powder Bed Fusion (LPBF) has enabled the manufacturing of parts with complex features such as optimized cooling channels. However, due to the layer-by-layer deposition of LPBF requiring an approximation of design intent, cooling channels manufactured by LPBF are affected by surface roughness effects and manufacturing inaccuracies. Consequently, the effect of implementing them on the mechanical properties of parts should be studied to understand their limits of applicability. This study aims to determine the effect of helical cooling channels in LPBF GRCop-42 specimens on their high-cycle fatigue properties. We present monotonic tensile testing and high-cycle fatigue testing results for three specimen types (no channel, straight channel and helical channel) of LPBF GRCop-42 under uniaxial loading, tested at two temperature conditions (room and 500°C). We show that at room temperature, the no channel specimens had the highest fatigue strength, followed by the straight channel and then helical channel specimens. The relative significance of potential causes for the detriment in fatigue life for the straight and helical channeled specimens were quantified using finite element analysis (FEA) and analytical fatigue models (based on Murakami-type defect corrections), and the findings from this analysis were validated by experimental observations from fracture surface analysis. Our results demonstrate that for the straight channel specimens, manufacturing-induced porosity around the channel is relatively a stronger driver for the detriment of fatigue life, compared to surface roughness. For the helical channel specimens, intended to simulate complex cooling channels in real-world applications, the effects of surface roughness combined with multiaxial stress concentrations around the channel were the primary driver for the lesser fatigue life. We anticipate our results will be useful for designers and manufacturers of LPBF components with complex features, and those involved in the potential implementation of LPBF GRCop-42 parts in high-cycle fatigue applications.</p>