Unter den verschiedenen additiven Fertigungsverfahren stellt das selektive Laserschmelzen (SLM) eine optimale Technologie für die Herstellung von metallischen Bauteilen mit komplexen Geometrien und hervorragenden Eigenschaften dar. SLM-Bauteile werden Schicht für Schicht mit hochenergetischen Laserstrahlen hergestellt, was das SLM flexibler als konventionelle Produktionstechnologien wie das Gießen macht. Die beim SLM auftretenden schnellen Aufheiz-/Kühlraten können zu deutlich unterschiedlichen Gefügen im Vergleich zu herkömmlichen Herstellungsverfahren führen. Die beim SLM entstehenden Hochtemperaturgradienten können sich weiterhin positiv auf die Gefügeentstehung (Phasenbildung, Morphologie, …) und damit auf die mechanischen Eigenschaften der SLM-Bauteile auswirken. Darüber hinaus können die mit SLM gefertigten Teile mit der Notwendigkeit einer minimalen Nachbearbeitung in den Einsatz genommen werden. Bisher wurden mehrere Studien zu den Parametern: Optimierung oder Verarbeitung von Verbundwerkstoffen mit fehlerfreien Teilen durchgeführt Die Scanstrategie hat dabei einen besonders großen Einfluss bei der Materialbearbeitung durch die additive Fertigung. Die Optimierung der Scanstrategie ist daher von zentraler Bedeutung für die Synthese von Materialien mit verbesserten physikalischen und mechanischen Eigenschaften. Diese Arbeit untersucht die Wirkung von vier verschiedenen Scanning-Strategien auf das Gefüge und das mechanische Verhalten von 316L Edelstahl, synthetisiert durch selektives Laserschmelzen (SLM). Die Ergebnisse deuten darauf hin, dass die Scanstrategie einen vernachlässigbaren Einfluss auf die Phasenbildung und die Art des Gefüges hat, die während der SLM-Verarbeitung entsteht: Austenit ist die einzige Phase, die sich bildet, und alle Proben weisen eine zelluläre Morphologie auf. Die Scanstrategie beeinflusst jedoch erheblich die charakteristische Größe von Zellen und Körnern, die wiederum der Hauptfaktor für die Festigkeit unter Zugbelastung zu sein scheint. Andererseits haben Eigenspannungen offenbar keinen Einfluss auf die quasi-statischen mechanischen Eigenschaften der Proben. Das mit einem Streifenmuster mit Konturstrategie hergestellte Material weist das feinste Gefüge und die beste Kombination mechanischer Eigenschaften auf: Streckgrenze und Bruchdehnung liegen bei 550 MPa und 1010 MPa und die plastische Verformung bei über 50 %. Ein weiterer wichtiger Aspekt für die Anwendung des mittels SLM synthetisierten 316L-Stahls ist seine thermische Stabilität. Daher wurde der Einfluss des Glühens bei verschiedenen Temperaturen (573, 873, 1273, 1373 und 1673 K) auf die Stabilität der Phasen, der Zusammensetzung und des Gefüges des 316L-Edelstahls untersucht, der unter Verwendung des Streifenmuster mit Konturstrategie hergestellt wurde. Darüber hinaus wurden die durch die Wärmebehandlung induzierten Veränderungen genutzt, um die entsprechenden Variationen der mechanischen Eigenschaften der Proben unter Zugbelastung zu verstehen. Das Glühen hat keinen Einfluss auf die Phasenbildung: Bei allen hier untersuchten Proben wird ein einphasiger Austenit beobachtet. Darüber hinaus ändert das Glühen nicht die zufällige kristallographische Orientierung, die im Material nach der Synthese beobachtet wird. Das komplexe zelluläre Gefüge mit feinen Subkornstrukturen, die für die as-SLM-Proben im Ausgangszustand charakteristisch sind, ist bis zu 873 K stabil. Die Zellgröße nimmt mit steigender Glühtemperatur zu, bis das zelluläre Gefüge bei hohen Temperaturen nicht mehr beobachtet werden kann (T ≥ 1273 K). Die Festigkeit der Proben nimmt mit steigender Glühtemperatur durch die mikrostrukturelle Vergröberung ab. Die ausgezeichnete Kombination von Festigkeit und Duktilität des Materials im Ausgangszustand ist auf das komplexe zelluläre Gefüge und die Subkörner sowie die Fehlausrichtung zwischen Körnern, Zellen, Zellwänden und Subkörnern zurückzuführen. Mit dem Ziel, das mechanische Verhalten des 316L-Stahls weiter zu verbessern, wird der Einfluss harter Partikel einer zweiten Phase auf das Gefüge und die damit verbundenen mechanischen Eigenschaften untersucht. Dazu wurde mittels SLM ein Verbund aus einer 316L-Stahlmatrix und 5 Vol.% CeO2-Partikeln hergestellt. Die SLM-Parameter, die zu einer fehlerfreien 316L-Matrix führen, sind für die Herstellung von 316L/CeO2-Verbundproben nicht geeignet. Hochdichte Verbundproben können jedoch durch sorgfältige Einstellung der Laserscangeschwindigkeit unter Beibehaltung der anderen Parameter prozessiert werden. Die Zugabe der CeO2-Verstärkung verändert die Phasenbildung nicht, beeinflusst aber das Gefüge des Verbundwerkstoffs, welches im Vergleich zum partikelfreien 316L-Material deutlich verfeinert ist. Das verfeinerte Gefüge bewirkt eine signifikante Verstärkung im Verbund, ohne die plastische Verformung zu beeinträchtigen. Die Analyse des Einflusses einer zweiten Phase wird fortgesetzt, indem untersucht wird, wie TiB2-Partikel das Gefüge und die mechanischen Eigenschaften eines 316L-Edelstahls beeinflussen, der durch selektives Laserschmelzen hergestellt wird. Das für die unverstärkte 316L-Matrix charakteristische komplexe zelluläre Gefüge mit feinen Subkörnern ist in allen Proben zu finden. Die Zugabe der TiB2-Partikel reduziert die Größe der Körner und Zellen erheblich. Darüber hinaus sind die TiB2-Partikel in der 316L-Matrix homogen dispergiert und bilden kreisförmige Ausscheidungen mit einer Größe von etwa 50-100 nm entlang der Korngrenzen. Diese mikrostrukturellen Merkmale führen zu einer signifikanten Verfestigung im Vergleich zu den unverstärkten 316L-Proben. Diese Ergebnisse belegen, dass SLM erfolgreich zur Synthese von Verbundwerkstoffen aus dem Edelstahl 316L mit herausragenden mechanischen Eigenschaften im Vergleich zu einer unverstärkten 316L-Stahlmatrix eingesetzt werden kann. Dies könnte dazu beitragen, den Einsatz von SLM bei der Herstellung von Stahlmatrix-Verbundwerkstoffen für die Automobilindustrie, die Luft- und Raumfahrt und zahlreiche andere Anwendungen zu erweitern. / Among the different additive manufacturing processes, selective laser melting (SLM) represents an optimal choice for the fabrication of metallic components with complex geometries and superior properties. SLM parts are built layer-by-layer using high-energy laser beams, making SLM more flexible than conventional processing techniques, like casting. The fast heating/cooling rates occurring during SLM can result in remarkably different microstructures compared with conventional manufacturing processes. The high-temperature gradients characterising SLM can also have a positive effect on the microstructures and, in turn, on the mechanical properties of the SLM parts. Additionally, the SLM parts can be put into use with the necessity of minimal post-processing treatments. To date, a number of studies have been devoted to the parameters optimization or processing of composite materials with defect-free parts. The scanning strategy is one of the most influential parameters in materials processing by additive manufacturing. Optimization of the scanning strategy is thus of primary importance for the synthesis of materials with enhanced physical and mechanical properties. Accordingly, this thesis examines the effect of four different scanning strategies on the microstructure and mechanical behaviour of 316L stainless steel synthesized by selective laser melting (SLM). The results indicate that the scanning strategy has negligible influence on phase formation and the type of microstructure established during SLM processing: austenite is the only phase formed and all specimens display a cellular morphology. The scanning strategy, however, considerably affects the characteristic size of cells and grains that, in turn, appears to be the main factor determining the strength under tensile loading. On the other hand, residual stresses apparently have no influence on the quasi-static mechanical properties of the samples. The material fabricated using a stripe with contour strategy displays the finest microstructure and the best combination of mechanical properties: yield strength and ultimate tensile strength are about 550 and 1010 MPa and plastic deformation exceeds 50 %. Another important aspect for the application of 316L steel synthesized by SLM is its thermal stability. Therefore, the influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by using the stripe with contour strategy has been investigated. Moreover, the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains. With the aim of further improving the mechanical behaviour of 316L steel, this works examines the effect of hard second-phase particles on microstructure and related mechanical properties. For this, a composite consisting of a 316L steel matrix and 5 vol.% CeO2 particles was fabricated by SLM. The SLM parameters leading to a defect-free 316L matrix are not suitable for the production of 316L/CeO2 composite specimens. However, highly-dense composite samples can be synthesized by carefully adjusting the laser scanning speed, while keeping the other parameters constant. The addition of the CeO2 reinforcement does not alter phase formation, but it affects the microstructure of the composite, which is significantly refined compared with the unreinforced 316L material. The refined microstructure induces significant strengthening in the composite without deteriorating the plastic deformation. The analysis of the effect of a second phase is continued by investigating how TiB2 particles influence the microstructure and mechanical properties of a 316L stainless steel synthesized by selective laser melting. The complex cellular microstructure with fine subgrains characteristic of the unreinforced 316L matrix is found in all samples. The addition of the TiB2 particles reduces significantly the sizes of the grains and cells. Furthermore, the TiB2 particles are homogeneously dispersed in the 316L matrix and they form circular precipitates with sizes around 50-100 nm along the grain boundaries. These microstructural features induce significant strengthening compared with the unreinforced 316L specimens. These findings prove that SLM can be successfully used to synthesize 316L stainless steel matrix composites with overall superior mechanical properties in comparison with the unreinforced 316L steel matrix. This might help to extend the use of SLM to fabricate steel matrix composites for automotive, aerospace and numerous other applications.

The use of selective laser melting for AlSi10Mg has been gaining a lot of popularity, but unfortunately, there are a great deal of issues surrounding internal porosity. Hot isostatic pressing (HIP) has been used in many instances alongside a standard T6 treatment to reduce porosity, but that typically involves water quenching. The application for this project is meant for the satellite industry, which has tight dimensional tolerances and as such, water quenching is not adequate. Currently, annealing for a stress relief treatment is the only post- processing measure that does not involve water quenching. This project studied a novel direct HIP approach, whereby an argon quench is used after solution annealing. Three different cooling rates were studied within the DHIP process (DHIP-L=50°C/min, DHIP- M=200°C/min, and DHIP-H=400°C/min) and compared to specimens that were stress relieved (SR). Uniaxial tensile testing revealed that the strength and ductility of DHIP-H outperformed the SR condition. The true stress/strain results showed that all DHIP conditions had a superior true strain and true stress at fracture. All DHIP conditions and SR showed evidence of void growth and coalescence. SR fracture is driven through crack initiation, while the DHIP conditions fracture is driven through localized necking. In-situ tensile tests via scanning electron microscopy coupled with μ-DIC revealed that the DHIP conditions feature damage due to particle fracture, while the SR condition experiences strain localization along the interface of Si particles and the α-Al phase. In-situ tensile testing via XCT studied a comparative analysis between DHIP-M and SR and revealed that DHIP-M experiences more void growth and nucleation than the SR condition. / Thesis / Master of Applied Science (MASc)

Optimal print parameters for additively manufacturing 316L stainless steel using selective laser melting (SLM) at Cal Poly had previously been identified. In order to further support the viability of the current settings, tensile material characteristics were needed. Furthermore, reliable performance of the as-printed material had to be demonstrated. Any influence on the static performance of parts in the as-printed condition inherent to the SLM manufacturing process itself needed to be identified. Tensile testing was conducted to determine the properties of material in the as-printed condition. So as to have confidence in the experimental results, other investigations were also conducted to validate previous assumptions. Stereological relative density measurements showed that the as-printed material exhibited relative density in excess of 99%. Optical dimensional analysis found that the as-printed tensile specimens met ASTM E8 dimensional requirements in 14 out of 15 parts inspected. Baseline tensile tests indicated that the yield stress of the as-printed material is 24% higher than a cold-rolled alternative, while still achieving comparable ductility. The location of a tensile specimen on the build plate during the print was not found to have a significant effect on its mechanical properties. Theoretical behavior of notched tensile specimens based on finite element models matched experimental behavior in the actual specimens. Unique fracture behavior was found in both the unnotched reference and the most severe notch after microscopic inspection, and a root cause was proposed. Finally, extrapolating from previous studies and observing that experimental results matched theoretical models, it was determined that features inherent to SLM parts were not detrimental to the static performance of the as-printed material.

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No description available.

Ultrafast lasers are valuable research and manufacturing tools. The ultrashort pulse duration is comparable to electron-lattice relaxation times, yielding unique interactions with matter, particularly nonlinear absorption, melting, and ablation. The field of ultrafast laser manufacturing is rapidly evolving with advances in related laser technologies. The applications of ultrashort pulse lasers in additive manufacturing aim to fill gaps left by conventional techniques especially on the nano- and micro-scale. Concurrently, uptake of ultrafast fiber lasers for micromachining has increased, and may replace the Ti:Sapphire laser as the ultrafast laser of choice. Both additive and subtractive manufacturing are accomplished with ultrafast lasers which presents the possibility of hybrid, all-in-one devices using a single laser source. As one such combination of laser techniques, ultrashort pulse surface modification of additively manufactured metals is an area of limited investigation. This thesis aims to address the ever-changing landscape of ultrafast laser manufacturing by 1) reviewing ultrafast laser additive manufacturing techniques and recent advancements 2) comparing the design, operation, and micromachining potential of a commercial ultrafast Ti:Sapphire and ultrafast fiber laser, and 3) investigating femtosecond ablation of as-printed additively manufactured Ti-6Al-4V at a range of parameters to test the feasibility of surface feature control. Ultrafast laser additive manufacturing is still in its infancy with mostly niche applications. The ultrafast fiber laser architecture is found to deliver a platform that is easier to operate and maintain and has superior micromachining throughput relative to Ti:Sapphire lasers. In our experimental work, five main surface morphologies are obtained by femtosecond ablation of a rough Ti-6Al-4V surface: laser-induced periodic surface structures (LIPSS), undulating grooves, micro-ripples, grooves, and micro-cavities. Transitions between ablation regimes and evolutions of the surface under increasing pulse energy and number of pulses are observed. These patterns allow for control over the surface geometry without the need for post-printing polishing. / Thesis / Master of Applied Science (MASc) / Ultrafast pulsed lasers of <10 picoseconds pulse duration are commonly used to modify, melt, or ablate materials. As an important research and manufacturing tool, ultrafast lasers and techniques have seen great change in the past two decades. Additive manufacturing has emerged as an area in which ultrafast lasers are becoming increasingly prevalent. To make sense of this continuously evolving landscape, this thesis 1) reviews ultrafast laser additive manufacturing techniques, applications, and advances towards industrial use and commercialisation, 2) compares the setup, operability, and characteristics for two ultrafast laser designs, and 3) investigates the surfaces produced by ultrafast laser irradiation of an additively manufactured titanium alloy part. The surface morphologies that are produced are categorised into five main patterns: laser-induced periodic surface structures, undulating grooves, micro-ripples, grooves, and micro-cavities. Each is a distinct pattern that may allow for tuning of the surface properties with respect to the wettability and biocompatibility.

Industry 4.0 will pave the way to a new age of advanced manufacturing. Additive manufacturing (AM) is one of the leading sectors of the upcoming industrial revolution. The key advantage of AM is its ability to generate lightweight, robust, and complex shapes. AM can also customize the microstructure and mechanical properties of the components according to the selected technique and process parameters. AM of metals using selective laser melting (SLM) could significantly impact a variety of critical applications. SLM is the most common technique of processing high strength Aluminum alloys. SLM of these alloys promises to enhance the performance of lightweight critical components used in various aerospace and automotive applications such as metallic optics and optomechanical components. However, the surface and inside defects of the as-built parts present an obstacle to product quality requirements. Consequently, the post-processing of SLM produced Al alloy parts is an essential step for homogenizing their microstructure and reducing as-built defects. In the current research, various studies assess the optimal process mapping for high-quality SLM parts and the post-processing treatment of Al alloy parts. Ultra-precision machining with single point diamond turning or diamond micro fly-milling is also investigated for the as-built and post-processed Al parts to satisfy the optical mirror’s surface finish requirements. The influence of the SLM process parameters on the quality of the AlSi10Mg and Al6061 alloy parts is investigated. A design of experiment (DOE) is used to analyze relative density, porosity, surface roughness, dimensional accuracy, and mechanical properties according to the interaction effect between SLM process parameters. The microstructure of both materials was also characterized. A developed process map shows the range of energy densities and SLM process parameters for each material needed to achieve optimum quality of the as-built parts. This comprehensive study also strives to reduce the amount of post-processing needed. Thermal post-processing of AlSi10Mg parts is evaluated, using recycled powder, with the aim of improving the microstructure homogeneity of the as-built parts. This work is essential for the cost-effective additive manufacturing (AM) of metal optics and optomechanical systems. To achieve this goal, a full characterization of fresh and recycled powder was performed, in addition to a microstructure assessment of the as-built fabricated samples. Annealing, solution heat treatment (SHT) and T6 heat treatment (T6 HT) were applied under different processing conditions. The results demonstrated an improvement in microstructure homogeneity after thermal post-processing under specific conditions of SHT and T6 HT. A micro-hardness map was developed to help in the selection of optimal post-processing parameters for the part’s design requirements. A study is also presented, which aims to improve the surface characteristics of the as-built AlSi10Mg parts using shot peening (SP). Different SP intensities were applied to various surface textures of the as-built samples. The SP results showed a significant improvement in the as-built surface topography and a higher value of effective depth using 22.9A intensity and Gp165 glass beads. The area near the shot-peened surface showed a significant microstructure refinement up to a specific depth, due to the dynamic precipitation of nanoscale Si particles. Surface hardening and high compressive residual stresses were generated due to severe plastic deformation. Friction stir processing (FSP) was studied as a localized treatment on a large surface area of the as-built and hot isostatic pressed (HIPed) AlSi10Mg parts using multiple FSP tool passes. The influence of FSP on the microstructure, hardness, and residual stresses of parts was investigated. FSP transforms the microstructure of parts into an equiaxed grain structure. A consistent microstructure homogenization was achieved over the processed surface after applying a high ratio of tool pass overlap of ≥60%. A map of microstructure and hardness was prepared to assist in the selection of the optimal FSP parameters for attaining the required quality of the final processed parts. Micromachining to the mirror surface was performed using diamond micro fly-milling and single point diamond turning techniques, and the effect of the material properties on surface roughness after machining was investigated. The machining parameters were also tuned to meet IR mirror optical requirements. A novel mirror structure is developed using the design for additive manufacturing concept additive (DFAM). This design achieved weight reduction of 50% as compared to the typical mirror structure. Moreover, the developed design offers an improvement of the mirror cooling performance due to the embedded cooling channels directed to the mirror surface. A novel mirror structure is developed using the design for additive manufacturing concept additive (DFAM). This design achieved weight reduction of 50% as compared to the typical mirror structure. Moreover, the developed design offers an improvement of the mirror cooling performance due to the embedded cooling channels directed to the mirror surface. / Thesis / Doctor of Philosophy (PhD)

Additive manufacturing (AM) has the ability to fabricate components of high geometric complexity that are difficult or near impossible to be produced by traditional manufacturing technologies. Selective laser melting (SLM) is a commonly used AM technology for metallic fabrications. SLM offers the opportunities to customize the characteristics of the as-build part produced, by adjusting the laser settings. However, high strength aluminum (Al) alloys presents an obstacle for SLM production due to the low alloying content, which increases the alloys’ probabilities to form cracks due to thermal stress induced by the SLM build process. The current study focuses on the study of surface roughness and dimensional accuracy of SLM fabrication of Al6061 and AlSi10Mg. Using design of experiment (DOE), wide ranges SLM process parameters were experimented with, and their individual effect along with their interactive effects on the fabricated parts’ quality were evaluated. The quality characteristics studied are: microstructures, microhardness, tensile strength (ultimate tensile strength, and yield strength), density, surface roughness, and dimensional accuracy. Regression models were created for each quality characteristics, and the combination of density, surface roughness, and dimensional accuracy results was used to create processing window for SLM that ensures the production of high-quality parts. The work aims to not only be used as-is, to help with the selection of SLM process parameters for Al6061 and AlSi10Mg that will reduce the post- processing time, but also to set a foundation for future development for numerical models that could better predict and describe the relations between SLM process parameters and the part’s fundamental qualities. / Thesis / Master of Applied Science (MASc)

The main aim of this thesis is to investigate the manufacturability of different gyroid designs using Selective laser melting (SLM) process . This study paves the way for a better understanding of design aspects, process optimization, and characterization of titanium alloy (Ti6Al4V) gyroid lattice structures for bone implant applications. First, A MATLAB® code was developed to create various gyroid designs and understand the relationship between the implicit equation parameters and the measurable outputs of gyroid unit cells. A novel gyroid lattice structure is proposed, where the porosity is graded in a radial direction. Second, gyroid designs were investigated by developing a permissible design map to help choose the right gyroid parameters for bone implants. Third, response surface methodology was used to study the process-structure-property relationship and understand the effect of SLM process parameters on the manufacturability of Ti6Al4V gyroid lattice structures. Laser power was found to be the most significant factor affecting the errors in relative density and strut size of gyroid structures. A volumetric energy density between 85 and 103 J/mm3 induces the least errors in the gyroid’s relative density. Fourth, the quasi-static properties of the novel designs were compared to uniform gyroids. The proposed novel gyroids had the highest compressive strength reaching 160 MPa. Numerical simulations were studied to give insight into how manufacturing irregularities can affect the mechanical properties of gyroids. Last, an in-depth defect analysis was conducted to understand how SLM defects may influence the fatigue properties of different Ti6Al4V gyroids. Thin struts have less internal defects than thick ones; thus, they show less crack propagation rate and higher normalized fatigue life. These favorable findings contributed to scientific knowledge of manufacturability of Ti6Al4V porosity graded gyroids and determined the influence of SLM defects on the mechanical properties of gyroid designs for bone implants. / Thesis / Doctor of Philosophy (PhD) / This thesis studies the integration of design aspects, SLM manufacturability, and mechanical characterization of Ti6Al4V gyroid lattice structures used for bone implants. A MATLAB® code was developed to design novel porosity graded gyroids, and develop permissible design map to aid the choice of different gyroid designs for bone implants.. Process maps were also developed to investigate the relationship among laser power, scan speed, and the errors in the relative density of lattice structures. Moreover, the normalized fatigue strength of thin struts gyoid was found to be higher than that of thicker struts.Analytical models and finite element analysis (FEA) models were compared to experimental results. The variation of the results gives a better understanding of the effect of manufacturing defects. An improved insight of gyroids manufacturability has been obtained by integrating the permissible design space with the process-structure-property relationship, and the defect analysis of porosity graded gyroids.

The goal of this research is developing a computational framework to study mechanical fatigue and fracture at different length scales for a broad range of materials. The developed multiscale framework is utilized to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. The highly-localized stress in a relatively small contact area at the wheel-rail interface promotes micro-crack initiation and propagation near the surface of the rail. 2D and 3D microstructural-based computational frameworks are developed for studying the rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions. The results obtained from studying RCF behavior in different conditions will help better maintenance of the railways and increase the safety of trains. The developed framework is employed to study the fracture and fatigue behavior in 3D printed metallic alloys fabricated by selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing the manufacturing sector and is being utilized across a diverse array of industries, including biomedical, automotive, aerospace, energy, consumer goods, and many others. Since experiments on 3D printed alloys are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. In this research, a computational framework is developed to study fracture and fatigue in different scales in 3D printed alloys fabricated by the SLM method. Our method for studying the fatigue at the microstructural level of 3D printed alloys is pioneering with no similar work being available in the literature. Our studies can be used as a first step toward establishing comprehensive numerical frameworks to investigate fracture and fatigue behavior of 3D metallic devices with complex geometries, fabricated by 3D printing. Composite materials are fabricated by combining the attractive mechanical properties of materials into one system. A combination of materials with different mechanical properties, size, geometry, and order of different phases can lead to fabricating a new material with a wide range of properties. A fundamental problem in engineering is how to find the design that exhibits the best combination of these properties. Biological composites like bone, nacre, and teeth attracted much attention among the researchers. These materials are constructed from simple building blocks and show an uncommon combination of high strength and toughness. By inspiring from simple building blocks in bio-inspired materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired composites compared to their building blocks. Furthermore, an optimization methodology is implemented into the designing the bio-inspired composites for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure. This study can be used as an effective reference for creating damage-tolerant structures with improved mechanical behavior. / Doctor of Philosophy / The goal of this research is developing a multiscale framework to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. Different computational models are developed for studying rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions and the results will help better maintenance of the railways and increase the safety of trains. The developed model is employed to study the fracture and fatigue behavior in 3D printed metals created by the selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing industries including biomedical, automotive, aerospace, energy, and many others. Since experiments on 3D printed metals are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. Our method for studying the fatigue at the microstructural level of 3D printed alloys can help to create more fatigue and fracture resistant materials. In the last section, we have studied fracture behavior in bio-inspired materials. A fundamental problem in engineering is how to find the design that exhibits the best combination of mechanical properties. Biological materials like bone, nacre, and teeth are constructed from simple building blocks and show a surprising combination of high strength and toughness. By inspiring from these materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired structure compared to its building blocks. Furthermore, an optimization method is implemented into the designing the bio-inspired structures for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure.