In this study, the fatigue- and fracture properties and microstructure of a marine component of austenitic stainless steel 316L manufactured with the novel method Wire Arc Additive Manufacturing were investigated and compared with data from literature. The purpose was to find a critical flaw size in the material related to its fatigue life. It was done by studying the microstructure and interpreting fatigue- and mechanical data for the marine component in empirical models related to the fatigue- and fracture properties. Fracture properties were approximated to estimate fatigue life and critical flaw size. Fatigue limit and fatigue threshold were based on hardness test data, fracture toughness, and FADs on Charpy-V impact test data. The material manufactured with Wire Arc Additive Manufacturing had superior fatigue properties than cast and rolled equivalents and performed better in the fatigue test than recommendations for austenitic stainless steel in a seawater environment from the British Standard 7910:2019. Due to the conservative model's fatigue limit and fatigue threshold, the results are conservative. The reason for that could be the crack closure properties of the material. The results for fracture toughness are lower than the literature data. This is most likely due to conservative models based on Charpy-V impact test data. The most important properties of the fatigue life are the fatigue limit and the fatigue threshold due to their relationship with crack growth. Testing the lifetime of the component in seawater is complex and time-consuming due to the corrosion and the need for low test frequency.

No description available.

<p dir="ltr">The process of metal additive manufacturing is characterized by the layer-by-layer construction of components, where each individual layer may be subjected to distinct thermal variations, resulting in differences in cooling rates and thermal gradients. These variations can impact the microstructure and, subsequently, mechanical properties of the final product, especially as the height of the build increases. In the present investigation, an evaluation was undertaken to ascertain the impact of build height on the structural integrity of Ti-6Al-4V samples produced using the laser powder bed fusion (LPBF) technique. The study encompassed a comprehensive examination of microstructural features, the microhardness measurement, as well as an evaluation of defect characteristics including size, location, and distribution, with respect to the build height. Tensile and fatigue tests were conducted to elucidate the potential dependence of fatigue and tensile failures on the build height. Two groups of specimens were fabricated: the first, underwent continuous fabrication, while the second involved a pause at the half height, with the process resuming after a 24-hour interval. The results of this investigation unveiled a discernible influence of the height of the build on the structural integrity of components under cyclic loading. Most fatigue specimens were observed to exhibit failure in the upper portion of the gage section with respect to the build direction. Analyses of microstructure revealed a consistent grain morphology in alignment with the build direction, and a uniform distribution of hardness throughout the build height was noted. However, for the specimens in the first group, more process-induced defects were detected within the top half of the gage section in comparison to the bottom half, while there was no noticeable difference in the distribution of defects in the second group. The results suggest that in LPBF process, as the build height is increased, there is a higher likelihood of process-induced defect formation, ultimately resulting in a reduction in structural integrity at greater build heights.</p>

Additive manufacturing (AM), or 3D printing, is a collection of production processes that has received a good deal of attention in recent years from different industries. Features such as mass production of customised products, design freedom, part consolidation and cost efficient low volume production drive the development of, and the interest in, these technologies. One industry that could potentially benefit from AM with metal materials is the space industry, an industry that has become a more competitive environment with established actors being challenged by new commercial initiatives. To be competitive in these new market conditions, the need for innovation and cost awareness has increased. Efficiency in product development and manufacturing is required, and AM is promising from these perspectives. However, the maturity of the AM processes is still at a level that requires cautious implementation in direct applications. Variation in manufacturing outcome and sensitivity to part geometry impact material properties and part behaviour. Since the space industry is characterised by the use of products in harsh environments with no room for failure, strict requirements govern product development, manufacturing and use of space applications. Parts have to be shown to meet specific quality control requirements, which is done through a qualification process. The purpose of this thesis is to investigate challenges with development and qualification of AM parts for space applications, and their impact on the product development process. Specifically, the challenges with powder bed fusion (PBF) processes have been in focus in this thesis. Four studies have been carried out within this research project. The first was a literature review coupled with visits to AM actors in Sweden that set the direction for the research. The second study consisted of a series of interviews at one company in the space industry to understand the expectations for AM and its implications on product development. This was coupled with a third study consisting of a workshop series with three companies in the space industry. The fourth study was an in-depth look at one company to map the qualification of manufacturing processes in the space industry, and the challenges that are seen for AM. The results from these studies show that engineers in the space industry work under conditions that are not always under their control, and which impact how they are able to be innovative and to introduce new manufacturing technologies, such as AM. The importance of product quality also tends to lead engineers into relying on previous designs meaning incremental, rather than radical, development of products is therefore typical. Furthermore, the qualification of manufacturing processes relies on previous experience which means that introducing new processes, such as AM, is difficult due to the lack of knowledge of their behaviour. Two major challenges with the qualification of critical AM parts for space applications have been identified: (i) the requirement to show that critical parts are damage tolerant which is challenging due to the lack of understanding of AM inherent defects, and (ii) the difficulty of testing parts in representative environments. This implies that the whole product development process is impacted in the development and qualification of AM parts; early, as well as later stages. To be able to utilise the design freedom that comes with AM, the capabilities of the chosen AM process has to be considered. Therefore, Design for Manufacturing (DfM) has evolved into Design for Additive Manufacturing (DfAM). While DfAM is important for the part design, this thesis also discusses its importance in the qualification of AM parts. In addition, the role of systems engineering in the development and qualification of AM parts for space applications is highlighted.

No description available.

No description available.

This project was initiated by Dr. Sasan Dadbaksh upon listening to the requirements I presented for my master thesis. My requirements were to do a master thesis project in the field of additive manufacturing specifically fused deposition modeling that should not only involve the research work but should also present an opportunity to develop hardware and should involve experimental testing. Then Sasan came up with the idea of developing a system capable to perform 3D printing with the extruder fixed in one position and the motion required for 3D printing will be provided by the robotic arm. The title of developing green build strategies for robot assisted plastic 3D printing came into being. The main concept behind the title of developing robot assisted plastic 3D printing platform is to develop such a system that can offer additive manufacturing services, specifically of fused deposition modeling 3D printing, as an inbound process during the manufacturing of any part through subtractive processes with the help of a robotic arm along with the repair of any kind of parts with the assistance of fused deposition modeling 3D printing. The main objectives of the master thesis include building a stationary filament extrusion module to interact with a robot hand and establishing a strategy for a robot hand to move the part to appropriate locations to complete building a part on a preform without support structures. The targets that were achieved with the completion of this thesis project includes the development of the complete hardware that consists of a mechanical structure with the option of mounting the components required to run the extrusion setup, learning the basic working of the software that are able to simulate the 3D printing process with the robotic arm (Robot Studio and Robo DK), creation of the simulation of the whole process, achieving communication between the robotic arm and the microcontroller of the extruder and finally the printing of a simple part for the demonstration. The components needed to be installed on the structure includes the motor, extruder, hot end, nozzle, filament. The structure also accumulated the required electronics that includes power supply, microcontroller, and an LCD to monitor the extrusion parameters. The developed machine runs on the state-of-the-art components that belong to the few of the best manufacturers of the technology.

The work herein is, in part, motivated by the idea of creating optimized, actuating structures using additive manufacturing processes (AM). By developing a consistent, repeatable method for designing and manufacturing multi-material compliant mechanisms, significant performance improvements can be seen in application, such as increased mechanism deflection. There are three distinct categories of research that contribute to this overall motivating idea: 1) investigation of an appropriate multi-material topology optimization process for multi-material jetting, 2) understanding the role that manufacturing constraints play in the fabrication of complex, optimized structures, and 3) investigation of an appropriate process for embedding actuating elements within material jetted parts. PolyJet material jetting is the focus of this dissertation research as it is one of the only AM processes capable of utilizing multiple material phases (e.g., stiff and flexible) within a single build, making it uniquely qualified for manufacturing complex, multi-material compliant mechanisms. However, there are two limitations with the PolyJet process within this context: 1) there is currently a dearth of understanding regarding both single and multi-material manufacturing constraints in the PolyJet process and 2) there is no robust embedding methodology for the in-situ embedding of foreign actuating elements within the PolyJet process. These two gaps (and how they relate to the field of compliant mechanism design) will be discussed in detail in this dissertation. Specific manufacturing constraints investigated include 1) "design for embedding" considerations, 2) removal of support material from printed parts, 3) self-supporting angle of surfaces, 4) post-process survivability of fine features, 5) minimum manufacturable feature size, and 6) material properties of digital materials with relation to feature size. The key manufacturing process and geometric design factors that influence each of these constraints are experimentally determined, as well as the quantitative limitations that each constraint imposes on design. / Ph. D.

<p>There has been a long-standing need in the marketplace for the economic production of small lots of components that have complex geometry. A potential solution is additive manufacturing (AM). AM is a manufacturing process that adds material bottom-up. It has the distinct advantages of low preparation cost and high geometric creation capability. Components fabricated via AM are now being selectively used for less-demanding applications in motor vehicles, consumer products, medical products, aerospace devices, and even some military projects.</p><p><br></p> <p>For engineering applications, high value-added components require consistency in the fatigue properties. However, components fabricated by AM have large variation in the fatigue properties compared to those by conventional manufacturing processes. To alleviate unpredictable catastrophic failures of components, it is essential to study and predict fatigue life. Previous study reported that fatigue crack initiation process accounts for a large portion of fatigue life, especially for low loading amplitude and high cycle fatigue. However, this major portion of fatigue life prediction is mostly ignored by main stream researchers working on fatigue modeling. For industrial applications, engineers often specify a lower stress condition to obtain a higher safety factor. Under these circumstances, fatigue crack initiation becomes even more important, so it is essential to further study of crack initiation.</p><p><br></p> <p>The objective of this research is to develop a fatigue crack initiation model for metal components produced by AM. To improve life prediction accuracy, the model must incorporate the effect of different microstructures, which are typically produced by AM due to a large number of repetitive cycles of re-heating and re-cooling processes. To fulfill this objective, the tasks are separated into three studies: (1) developing a temperature model to simulate temperature history, (2) modeling the component’s microstructure for the potential crack initiation zone, and (3) developing a fatigue crack initiation model for life estimation. A summary of each task is provided in the following.</p> <p>First, the role of temperature model is to understand the mechanism that leads to the variation of microstructures. The existing temperature models are computationally expensive to obtain an accurate prediction of the temperature history due to repetitive heating and cooling. The main reason is that these models considered entire boundary conditions of all the material points. In this section, we proposed and employed the concept of effective computation zone, which can save the computational time significantly for AM process. </p><p><br></p> <p>Second, it is critical to include the effect of microstructure in the fatigue life model since the microstructure variation at different locations within the real AM component is large. The grain size variation is modeled by using representative volume element, which is defined as a volume of heterogeneous material that is sufficiently large to be statistically representative of the real component’s microstructure. Regarding phase transformation, a continuous cooling transformation (CCT) diagram is a useful tool that can be used with a thermal model for microstructure design and manufacturing process control. However, traditional CCT diagrams are developed based on slow and monotonic cooling processes such as furnace cooling and air cooling, which are greatly different from the repetitive heating and cooling processes in AM. In this study, a new general methodology is presented to create CCT diagrams for materials fabricated by AM. We showed that the effect of the segmented duration within the critical temperature range, which induced precipitate formation, could be cumulative. </p><p><br></p> <p>Third, the existing fatigue crack initiation life model has poor accuracy. One of the reasons for the poor accuracy is the coefficients change due to the variation in microstructure is not accounted for. In this section, a semi-empirical fatigue crack initiation model is presented. The important coefficients include maximum persistent slipband width, energy efficiency coefficient, resolved shear stress and plastic slip rate per cycle. These coefficients are modeled and determined as a function of microstructure, which can improve the accuracy of life estimation.</p><p><br></p> <p>The contribution of this study is to provide a new engineering tool for designing the melting AM process based on scientific research. With this tool, the fundamental mechanism contributing to a large variation of the fatigue life of the metal components made by AM process can be understood, attributed, predicted and improved. The seemly ‘stochastic’ nature of fatigue life of the AM components can be changed to be more deterministic and predictable. This approach represents a major advance in fatigue research on AM materials. The model developed is considered as a tool for research, design, and control for laser-based AM process applications. </p>

<p>Extrusion deposition additive manufacturing (EDAM) has enabled upscaling the dimensions of the objects that can be additively manufactured from the desktop scale to the size of a full vehicle. The EDAM process consists of depositing beads of molten material in a layer-by-layer manner, thereby giving rise to temperature gradients during part manufacturing. To investigate the phenomena involved in EDAM, the Composites Additive Manufacturing Research Instrument (CAMRI) was developed as part of this project. CAMRI provided unparalleled flexibility for conducting controlled experiments with carbon fiber reinforced semi-crystalline polymers and served as a validation platform for the work presented in this dissertation. </p> <p>Since the EDAM process is highly non-isothermal, modeling heat transfer in EDAM is of paramount importance for predicting interlayer bonding and evolution of internal stresses during part manufacturing. Hence, local heat transfer mechanisms were characterized and implemented in a framework for EDAM process simulations. These include local convection conditions, heat losses in material compaction as well as heat of crystallization or melting. Numerical predictions of the temperature evolution during the printing process of a part were in great agreement with experimental measurements by only calibrating the radiation ambient temperature. </p> In the absence of fibers reinforcing the interface between adjacent layers, the bond developed through the polymer is the primary mechanisms governing the interlayer fracture properties in printed parts. Hence, a fusion bonding model was extended to predict the evolution of interlayer fracture properties in EDAM with semi-crystalline polymer composites. The fusion bonding model was characterized and implemented in the framework for EDAM process simulation. Experimental verification of numerical predictions obtained with the fusion bonding model for interlayer fracture properties is provided. Finally, this fusion bonding model bridges the gap between processing conditions and interlayer fracture properties which is extremely valuable for predicting regions with frail interlayer bond within a part.