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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.

Additive Manufacturing (AM) processes are gaining prominence in industry as they can build parts to near-net-shape with minimal postprocessing. Metal laser AM techniques, such as Selective Laser Melting (SLM), offer rapid cooling rates on the order of 10^5-10^6 K/s. This is due to a highly-focused laser heating a microscopic volume in an otherwise lower-temperature environment. Hence, metal laser AM can manufacture novel, out-of-equilibrium microstructures that cannot be produced in near-net-shapes with other processes. It is desirable to optimize feedstocks for metal AM processes to leverage their advantages. One option of optimizing feedstocks is through in-situ alloying, or by using elemental powders. Elemental powders homogenize over the course of multiple laser passes, or intrinsic heat treatments. However, rapid cooling rates prevent the homogenization of a layer when first printed. To investigate the homogenization process, this thesis used synchrotron X-ray Diffraction (sXRD) to track the phase transformations during the SLM of a 14-layer single wall (single-hatch, multilayered) of Ti-1Al-8V-5Fe (Ti-185) from elemental Ti, Fe and an alloyed AlV powders, capturing frames at 250 Hz. Infrared imaging was performed simultaneously on the surface at 1603.5 Hz to observe the temperature changes at the surface. Post-mortem electron microscopy was performed on cross-sections of the wall perpendicular to the scanning direction to observe the changes in the microstructure with respect to the build direction. Specifically, Electron Dispersive X-Ray Spectroscopy and Electron Backscatter Diffraction were performed to observe the alloying elemental distribution and microstructure of the wall with respect to the build direction. The research performed found that in the melted zone, phase transformation times below 50 ms yielded a partially-alloyed microstructure, with regions concentrated and dilute in alloying elements. Partial mixing was diffusion-induced by laser beam heat and the exothermic heat of mixing of Ti-185 from its constituent elements. Further diffusion during reheating cycles yielded an alloyed microstructure. / Thesis / Master of Applied Science (MASc)

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<p> Laser powder bed fusion (LPBF) is a rapidly emerging manufacturing technology capable of producing complex part geometries through the repeated, precise laser melting of metallic powder layers. At present, the process is primarily employed in high-value-added applications which exist in the aerospace, biomedical, and dental industries. As industrial implementation of LPBF has matured, research has focused on established materials for which there are already large bodies of literature and regulatory approval, such as Inconel 718, Inconel 625, Ti-6Al-4V, and 316 stainless steel. However, the rapid solidification process inherent to LPBF leads to vastly different microstructures with improved strength compared to these traditional materials in cast or wrought forms. In general, the high solidification velocity and thermal gradients result in cellular and dendritic solidification structures with finer grain and precipitate sizes than conventionally processed alloys. These microstructure changes warrant the exploration of new alloy systems and reevaluation of historically cast compositions with optimized microstructures, especially considering the tunability of a digitally controlled fabrication process. This work examines laser powder bed fusion of Mar-M 509, a carbide-strengthened cobalt alloy that is typically investment cast directly into complex-shaped components such as nozzle guide vanes (NGVs). NGVs are stationary components in gas turbine engines for propulsion and energy production which require strength under moderate mechanical loading at high temperatures. Investment cast microstructures have porosity defects in slower-cooled regions due to lack of liquid feed to interdendritic regions. As-printed, the cellular and dendritic Mar-M 509 LPBF microstructures lead to the formation of continuous, fiber-like, eutectic carbide structures in the intercellular and interdendritic regions, which limit macroscopic ductility. Thermo-Calc is used for calculation of phase diagrams (CALPHAD) to estimate the equilibrium transformation temperatures of MC, M23C6, and M7C3-type carbides, which informs design of isothermal heat treatments to engineer microstructures with enhanced ductility over the as-printed or cast versions of Mar-M 509 while maintaining tensile strength. Scanning and transmission electron microscopy reveals the composition and distribution of carbide phases as a function of heat treatment temperature. Lastly, heat treatment recommendations for nozzle guide vanes are made.  </p>

Additively manufactured (AM) 316L and 17-4PH stainless steel parts, concretely made by laser powder bed fusion (L-PBF), are characterized and micro-mechanical properties of those steels are analyzed. This study also explored and extended to proton irradiation and small-scale mechanical testing of those materials, to investigate how irradiation affects microstructural evolution and thus mechanical properties at the surface level, which could be detrimental in the long term in nuclear applications. In-depth anisotropy analysis of L-PBF 316L stainless steel parts with the variations of volumetric energy density, a combined study of nanoindentation with EBSD (electron backscatter diffraction) mapping is shown to be an alternative methodology for enriching qualification protocols. Each grain with a different crystallographic orientation was mapped successfully by proper indentation properties. <122> and <111> oriented grains displayed higher than average indentation modulus and hardness whereas, <001>, <101>, and <210> oriented grains were found to be weaker in terms of indentation properties. Based on an extensive nanoindentation study, L-PBF 17-4 PH stainless steels are found to be very sensitive to high load rates and irradiation further escalates that sensitivity, especially after a 0.25 s-1 strain rate. 3D porosity measurement via X-ray microscope ensures L-PBF stainless steel parts are of more than 99.7% density and could be promising for many industrial applications. High percentages of increment of nanohardness, maximum theoretical shear strength, and yield strength were observed due to proton irradiation of 5 um damage depth on the surface of 17-4 PH steel parts. Small-scale mechanical testing of irradiated AM nuclear stainless steels such as 17-4 PH was carried out and investigated by micro-compression of FIB fabricated pillars of different sizes of diameter. Irradiated 17-4 PH materials have never been investigated by this kind of testing procedure to asses the stress-strain characteristics of micro-scale volumes and to explore the structure-property relationship. Both as-built and irradiated AM 17-4 PH micropillars exhibited step-ups in the early stage of load-displacement curves with a varying number of slip bands intermittently formed throughout the pillar volume while compressed by the uniaxial load. As for the radiation-damaged zone, micropillars displayed lesser slip bands compared to as-built parts as irradiation damage creates an obstacle to dislocations movement and hence hardening. It requires higher loads to initiate plastic deformation as dislocation must overcome irradiation-induced obstacles for the slip to occur and localization of strain without increasing the load for a certain amount of time during the test. Proton irradiation effects on the compressive mechanical properties of AM 17-4 PH stainless steel parts depending on the volumetric energy density (VED) used during the parts' fabrication process. On as-built parts, compressive yield strength varied from 107.27 MPa to 150.70 MPa and it was in the range of 133.43 MPa to 244.57 MPa under irradiated conditions. All 2 μm pillars were fabricated as their height falls within the radiation damage depth of 5 μm. It was expected to generate the highest yield strength and tensile strength due to the radiation hardening effect as discussed earlier. Yield and tensile strength were found to be the highest as expected as of 244.57 MPa and 375.08 MPa in irradiated 17-4 PH sample 1 (VED = 54.76 J/mm3). Samples with lower VED exhibited better micro-mechanical compressive responses than higher VED AM 17-4 PH parts in both as-built and irradiated conditions.

Additive Manufacturing (AM), commonly known as 3D printing, represents manufacturing technology that creates objects layer by layer based on 3D model data. AM technologies have capabilities that provide engineers with new design opportunities outside the constraints of traditional subtractive manufacturing. These capabilities of AM have made it attractive for manufacturing components in the space industry., where parts are often bespoke and complex. In particular, Laser Powder Bed Fusion (LPBF) has attracted attention due to its ability to produce components with the part properties required for space applications. Additionally, the precision of the laser enables the production of innovative near-net shape and low-weight part designs.  However, due to the powdered metal material, the LPBF process is categorised with rough surfaces in the as-built state. The extent and effect of surface roughness are closely linked to geometrical design variables, including build orientation, overhangs, support structure, and build parameters; hence the more intricate the design, the more difficult the removal of this roughness. Consequently, the as-built surface for most applications is too rough and could adversely affect proprieties, i.e., fatigue. Hence, practical Design for AM (DfAM) supports should be developed that understand how design factors, such as surface roughness, will impact a part’s performance. This thesis therefore presents literature reviews on research related to LPBF surface roughness and design support, exploring the trends in managing surface roughness and investigations on the characteristics of design support. Additionally, through a space industry case study, a proposed process involving additive manufacturing design artefacts (AMDAs) is considered to investigate and describe the relationship between design, surface roughness, and performance. The review found that, in general, research focuses on the relationship between surface roughness and LPBF build parameters, material properties, or post-processing. There is very little support for design engineers to consider how surface roughness from an AM process affects the final product (less than 1% of the review articles). In investigating surface roughness, the AMDA process identified characteristics that impact roughness levels and geometric adherence to part design. Additionally, twelve characteristics of design support were identified and considered to review the AMDA process. The process aided the evaluation of design uncertainties and provided indications of part performance. However, iterations of the process can be required to clarify product-specific design uncertainties. Though, the designer obtains a better understanding of their design and the AM process with each iteration. The inclusion of the requirement to set evaluation criteria for artefacts was recommended to develop the AMDA process as design support.

This thesis investigates the differences in the design process when developing a product for additive manufacturing (AM) compared to traditional manufacturing methods, such as CNC machining. In recent years, additive manufacturing (AM), including metal-based laser powder bed fusion (L-PBF), has gained popularity, leading to increased adoption by companies. The design process for AM, particularly in the context of metals, differs compared to for traditional manufacturing methods. L-PBF, being a method based on highly concentrated laser beam fusion, offers a higher level of design freedom, enabling the creation of intricate shapes, internal structures, and varying wall thicknesses. In contrast, traditional manufacturing methods based on subtractive processes impose limitations on design possibilities due to tooling and machining constraints. Adapting to L-PBF requires designers to reconsider, re-think and redesign parts specifically for AM, taking into account factors suchas cost, knowledge requirements and build volume limitations. The application of L-PBF extends to various industries, including aerospace and performance automobiles. Designing for L-PBF opens up new possibilities for product development by leveraging the advantages of AM, such as design flexibility and topology optimization. Topology optimization allows for the creation of lightweight components while maintaining structural integrity. However, transitioning from traditional manufacturing to L-PBF presents challenges, requiring designers to navigate the unique considerations and constraints associated with AM. This research aims to enhance the understanding of the design process for AM, with a specific focus on L-PBF, and its implications for product development. By exploring the differences between AM and traditional manufacturing methods, this study contributes to the broader adoption and effective implementation of AM technologies in various manufacturing sectors. / Detta arbete undersöker skillnaderna i designprocessen vid utveckling av produkter för additive manufacturing (AM) jämfört med traditionella tillverkningsmetoder, såsom CNC bearbetning. På senare år har additiv tillverkning (AM), inklusive Laser Powder Bed Fusion (L-PBF), blivit populärt och allt fler företag använder sig av tekniken. Designprocessen för AM, skiljer sig jämnfört med för traditionella tillverkningsmetoder. L-PBF erbjuder en hög grad av designfrihet och möjliggör avancerade former, interna strukturer och varierande väggtjocklekar. I kontrast begränsar traditionella tillverkningsmetoder, som bygger på subtraktiva processer, designmöjligheterna på grund av verktygs- och bearbetningsbegränsningar. Att anpassa sig till L-PBF kräver att designers omprövar och omdesignar delar specifikt för AM och tar hänsyn till faktorer som kostnad, kunskapskrav och begränsningar i byggvolymen. Användningen av L-PBF sträcker sig till olika branscher, inklusive luft- och rymdindustrin samt prestandabilar. Att designa för L-PBF öppnar upp nya möjligheter för produktutveckling genom att utnyttja fördelarna med AM, såsom designflexibilitet och topologioptimering. Topologioptimering möjliggör skapandet av lätta komponenter samtidigt som den strukturella integriteten bibehålls. Övergången från traditionell tillverkning till L-PBF innebär dock utmaningar och kräver att designers hanterar de unika övervägandena och begränsningarna som är förknippade med AM. Denna forskning syftar till att förbättra förståelsen för designprocessen för AM, med särskilt fokus på L-PBF, och dess implikationer för produktutveckling. Genom att utforska skillnaderna mellan AM och traditionella tillverkningsmetoder bidrar denna studie till en bredare användning och effektiv implementering av AM-teknologier inom olika tillverkningssektorer.