Keen interest in additive manufacturing (AM) of refractory metals such as tungsten has been motivated by the demand for materials capable of enduring extreme temperatures in aerospace and nuclear applications. The aims of this work were to develop alloy compositions for high-temperature applications in the space propulsion and nuclear fusion sectors, and to establish processing windows for these compositions fabricated using laser powder bed fusion additive manufacturing (LPBF-AM). Tungsten (W)-based alloys are well-suited for high working temperatures because of their high melting points, excellent thermal conductivity, low corrosion resistance, and low coefficient of thermal expansion. The integrated computational materials engineering (ICME) approach was implemented to establish the connections among composition-printability-microstructure-properties-performance framework. ThermoCalc-CALPHAD software was used for Scheil-Gulliver solidification simulation (SGSS) of W-based compositions with various alloying element additions. Chromium, vanadium, and niobium were down-selected as suitable alloying elements based on SGSS results. Further, addition of carbon enhanced printability due to eutectic solidification by the formation of various carbides towards the end of solidification leading to crack-free microstructure as well as being vital for control of oxygen. This work demonstrates the successful manufacturing of multiple crack-free W-based alloy components using LPBF-AM, which had a wide range of working temperatures and enhanced mechanical properties.

Wire and Arc Additive Manufacturing is a novel Additive Manufacturing technology. As a result, the process for progressing from a solid model to manufacturing code, i.e. the Part Programming process, is undeveloped. In this report the Part Programming process, unique for Wire and Arc Additive Manufacturing, has been investigated to answer three questions; What is the Part Programming process for Wire and Arc Additive Manufacturing? What are the requirements on the Part Programming process? What software can be used for the Part Programming process? With a systematic review of publications on Wire and Arc Additive Manufacturing and related subjects, the steps of the Part Programming process and its requirements have been clarified. The Part Programming process has been used for evaluation of software solutions, resulting in multiple recommendations for implemented usage. Verification of assumptions, made by the systematic review, has been done by physical experiments to give further credibility to the results.

<p>Aeronautical products additively manufactured by Selective Laser Melting (SLM), are known to have fatigue properties which are negatively impacted by porosity defects, microstructural features and residual stresses. Little research is available studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency problem, the large focus being on the long FCG. This thesis seeks to add useful knowledge to the understanding of the mechanisms for short crack growth variability in SLM manufactured Ti-6Al-4V, with the two variables for the process conditions and build directions investigated. An in-situ FCG investigation using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually observe and quantify the short crack path evolution. Crack growth, deflections and porosity interactions were noted and discussed in relation to microstructure, build layer thickness and build layer orientation. A novel use of in-situ energy dispersive x-ray diffraction (EDD) was able to show the lattice strains evolving as a propagating crack moved through a small region of interest. The results presented show the ability to reliably obtain all six elastic strain tensor components, and interpret useful knowledge from a small region of interest. </p> <p> </p> <p>There are conflicting views in literature with respect to the damage tolerance behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et al., the more prominent studies were considered with Leuders et al. showing the highest long FCG rates for cracks parallel to the build layer and Cain et al. showing cracks propagating through successive build layers as highest [1]–[3]. Cain et al. and Vilaro et al. report significant anisotropy in long FCG for different build orientations whereas Edwards and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized build parameters without any (detectable) pores, the building direction does not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported martensitic microstructures and the presence of prior grain structures for as built SLM Ti-6Al-4V.</p> <p> </p> <p>No studies to the authors knowledge have considered the short FCG of SLM manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance behaviors reported in literature [1]. In this work small cross-sectional area (1.5 x 1.5 ) samples in two different build conditions of as built SLM Ti-6Al‑4V are studied. The short FCG rate of three different build directions was considered with cracks parallel to the build layers shown to be the most damaging. The microstructure and build layer are shown to be the likely dominant factors in the short FCG rate of as built Ti-6Al-4V. In terms of porosity, little impact to the propagating short crack was seen although there is local elastoplastic behavior around these defects which could cause toughening in the non-optimized build parameter samples tested. The fracture surfaces were examined using a Scanning Electron Microscope (SEM) with the results showing significant differences in the behavior of the two build conditions. From the microindentation hardness testing undertaken, the smooth fracture surface of the optimized sample correlated with a higher Vickers Hardness (VH) result and therefore higher strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH result and therefore strength. Furthering the knowledge of short FCG in SLM manufactured Ti-6Al-4V will have positive implications to accurately life and therefore certify additive manufactured aeronautical products.</p>

Additive manufacturing using 2 photon polymerization is of great interest as it can create nanostructures with feature sizes much below the diffraction limit. It can be called as true 3D printing as it can fabricate in 3 dimensions by moving the laser spot in any 3D pattern inside the resist. This unique property is attributed to the non-linearity of two photon absorption which makes the polymerization happen only at the focal spot of the laser beam. This method has a wide range of applications such as optics/photonics, metamaterials, metasurfaces, micromachines, microfluidics, tissue engineering and drug delivery.<br>This work focuses on utilizing 2 photon fabrication for creating a metasurface by printing diabolo antenna arrays on a glass substrate and subsequently metallizing it by coating with gold. A femtosecond laser is used along with a galvo-mirror to scan the geometry inside the photoresist to create the antenna. The structure is simulated using ANSYS HFSS to study its properties and optimize the parameters. The calculations show a reflectance dip and zero reflectance for the resonance condition of 4.04 μm. An array of antennas is fabricated using the optimized properties and coated with gold using e-beam evaporation. This array is studied using a fourier transform infrared spectrometer and polarization dependent reflectance dip to 40% is observed at 6.6 μm. The difference might be due to the small errors in fabrication. This method of 3D printing of antenna arrays and metallization by a single step of e-beam evaporation is hence proved as a viable method for creating optical metasurfaces. Areas of future research for perfecting this method include incorporating an autofocusing system, printing more complicated geometries for antennas, and achieving higher resolution using techniques like stimulated emission depletion.

This research describes the process of manufacturing and machining of wire and arc additive manufactured (WAAM) thin wall structures on integrated and non¬integrated WAAM systems. The overall aim of this thesis is to obtain a better understanding of deposition and machining of WAAM wall parts through an integrated system. This research includes the study of the comparison of deposition of WAAM wall structures on different WAAM platforms, namely an Integrated SAM Edgetek grinding machine, an ABB robot and a Friction Stir Welding (FSW) machine. The result shows that WAAM is a robustly transferable technique that can be implemented across a variety of different platforms typically available in industry. For WAAM deposition, a rise in output repeatedly involves high welding travel speed that usually leads to an undesired humping effect. As part of the objectives of this thesis was to study the travel speed limit for humping. The findings from this research show that the travel speed limit falls within a certain region at which humping starts to occur. One of the objectives of this thesis was to study the effect of lubricants during sequential and non-sequential machining/deposition of the WAAM parts. Conventional fluid lubricants and solid lubricants were used. In addition, the effect of cleaning of deposited wall samples with acetone was also studied. A systematic study shows that a significant amount of solid lubricant contamination can be found in the deposited material. Furthermore, the results indicate that even cleaning of the wire and arc additive manufactured surfaces with acetone prior to the weld deposition can affect the microstructure of the deposited material.

The medical device regulatory system, as well as the medical device market in the European Union (EU), is now facing challenges posed by the newest regulation, Medical device regulations (MDR). Researches have shown concerns and possible consequences related to this new regulation system from both the regulatory approval procedure and market development perspectives. This study aims to elaborate on a practical and objective situation of this latest shift and picture out a predictable scenario for the implementation of future technology like Additive Manufacturing (AM) in healthcare. These two objectives are addressed from the perspective of the core role in this system, Notified Bodies (NBs). Specifically, it answers the following questions: What is the impact of the MDR on the NBs’ operations? What is the impact of the MDR on the device building on AM from NBs’ perspective? A literature review is conducted on existing researches in the relevant fields mentioned in the research questions of this study. Then a self-completion questionnaire is generated and sent to NBs who offer the CE marking granting service for the medical devices around the EU. The eight responses for the survey indicate that the MDR influences NBs and the device building on AM from several perspectives. For the NBs, the number of NBs will decrease while the workload and new recruitment will increase. Also, the independence and competences of NBs will be improved by MDR. In the case of AM-relevant medical devices, MDR will pose specific issues on them while the market will be developed by ensuring the product quality and raising public awareness. These findings are valuable practical evidence to examine the application of MDR and the implementation of technology like AM in healthcare under MDR. Overall, it found that the MDR will cause a tough situation in the short term. At the same time, the far-reaching influence for the regulatory system, as well as the medical device market, is affirmative and expectable worthy.

<p>An existing thermal model for laser melting and additive manufacturing (AM) was expanded to include phase transformation and hardness predictions for an alloy steel and coupled to experimental results. The study was performed on AISI 8620, a popular case-hardening, steel to understand microstructural and property effects for potential repair applications. The experimental samples were polished, etched with nital and picral for comparison, imaged, and Vicker’s microhardness was taken at 0.5 and 0.2 kg loads. The etched images revealed a transformation zone slightly larger than the melt zone in all cases including a gradient in transformation along the outer edges of the transformation zones. The microhardness measurements revealed that the lower energy cases provided a higher hardness in the melted region even after tempering due to multiple passes. But the overall hardness was higher than what is to be expected of a fully martensitic structure in AISI 8620. The phase transformation model qualitatively shows a similar microstructure where molten regions turn completely to martensite. The model also predicts a transformation zone larger than the melt pool size, as well as the transformation of pearlite but not ferrite near but not in melt pool. This observation is experimentally verified showing a heat affected zone where pearlite is clearly transformed but not ferrite outside the transformation zone comprised of complete martensite. The hardness model predicts a lower hardness than the experiments but is similar to what is expected based on published Jominy End Quench tests. The cases in the regime dominated by conductive heat transfer show good agreement with the predictions of melt pool shape and hardness by the thermal model. However, at higher powers and lower speeds, the fluid flow influenced the shape of the melt pool and the hat transfer in its vicinity, and the model was less accurate.</p>

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<p>Explosives and rocket fuel are just two prime examples of energetic materials, compounds that contain a combustible fuel and oxidizer within the same substance. Recent advances have enabled the construction of energetic materials through multiple variations of additive manufacturing, principally inkjet, direct-write, fused filament fabrication, electrospray deposition, and stereolithography. Many of the methods used for creating multiple layered objects (three-dimensional) from energetic materials involve the use of highly viscid materials.</p> <p>The focus of this work was to design a process capable of additively manufacturing three-dimensional objects from melt-castable energetic materials, which are known for their low viscosity. An in-depth printer design and fabrication procedure details the process requirements discovered through previous works, and the adaptations available and used to construct an additive manufacturing device capable of printing both energetic and non-energetic (also referred to as inert) melt-castable materials. Initial characterization of three proposed inert materials confirmed their relative similarity in rheological properties to melt-castable energetic materials and were used to test the printer’s performance.</p> <p>Preliminary tests show the constructed device is capable of additively manufacturing melt-castable materials reproducibly in individual layers, with some initial successful prints in three-dimensions, up to three layers. An initial characterization of the printer’s deposition characteristics additionally matches literature predictions. With the ability to print three-dimensional objects from melt-castable materials confirmed, future work will focus on the reproducibility of multi-layered objects and the refined formulation of melt-castable energetic materials.</p>

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