<p>Damaged components and a lack of spare components are issues which are currently affecting military aircraft capability. Laser Cladding is an additive manufacturing technique which shows promise in repairing damaged aviation components. However, there are considerable certification requirements for critical components which stand to gain the most benefits from laser clad repair methodologies. These requirements involve establishing crack growth rate data for the laser clad material to gain confidence in the reliability of the repair's performance on in-service aircraft. This research seeks to understand the fatigue behavior of Ti-6Al-4V that has undergone a simulated laser clad repair, with unrepaired specimens also tested to allow for comparison. </p>

In order to mimic the extracellular matrix for tissue engineering, recent research approaches often involve 3D printing or electrospinning of fibres to scaffolds as cell carrier material. Within this thesis, a micron fibre printing process, called melt electrospinning writing (MEW), combining both additive manufacturing and electrospinning, has been investigated and improved. Thus, a unique device was developed for accurate process control and manufacturing of high quality constructs. Thereby, different studies could be conducted in order to understand the electrohydrodynamic printing behaviour of different medically relevant thermoplastics as well as to characterise the influence of MEW on the resulting scaffold performance. For reproducible scaffold printing, a commonly occurring processing instability was investigated and defined as pulsing, or in extreme cases as long beading. Here, processing analysis could be performed with the aim to overcome those instabilities and prevent the resulting manufacturing issues. Two different biocompatible polymers were utilised for this study: poly(ε-caprolactone) (PCL) as the only material available for MEW until then and poly(2-ethyl-2-oxazoline) for the first time. A hypothesis including the dependency of pulsing regarding involved mass flows regulated by the feeding pressure and the electrical field strength could be presented. Further, a guide via fibre diameter quantification was established to assess and accomplish high quality printing of scaffolds for subsequent research tasks. By following a combined approach including small sized spinnerets, small flow rates and high field strengths, PCL fibres with submicron-sized fibre diameters (fØ = 817 ± 165 nm) were deposited to defined scaffolds. The resulting material characteristics could be investigated regarding molecular orientation and morphological aspects. Thereby, an alignment and isotropic crystallinity was observed that can be attributed to the distinct acceleration of the solidifying jet in the electrical field and by the collector uptake. Resulting submicron fibres formed accurate but mechanically sensitive structures requiring further preparation for a suitable use in cell biology. To overcome this handling issue, a coating procedure, by using hydrophilic and cross-linkable star-shaped molecules for preparing fibre adhesive but cell repellent collector surfaces, was used. Printing PCL fibre patterns below the critical translation speed (CTS) revealed the opportunity to manufacture sinusoidal shaped fibres analogously to those observed using purely viscous fluids falling on a moving belt. No significant influence of the high voltage field during MEW processing could be observed on the buckling phenomenon. A study on the sinusoidal geometry revealed increasing peak-to-peak values and decreasing wavelengths as a function of decreasing collector speeds sc between CTS > sc ≥ 2/3 CTS independent of feeding pressures. Resulting scaffolds printed at 100 %, 90 %, 80 % and 70 % of CTS exhibited significantly different tensile properties, foremost regarding Young’s moduli (E = 42 ± 7 MPa to 173 ± 22 MPa at 1 – 3 % strain). As known from literature, a changed morphology and mechanical environment can impact cell performance substantially leading to a new opportunity of tailoring TE scaffolds. Further, poly(L-lactide-co-ε-caprolactone-co-acryloyl carbonate) as well as poly(ε-caprolactone-co-acryloyl carbonate) (PCLAC) copolymers could be used for MEW printing. Those exhibit the opportunity for UV-initiated radical cross-linking in a post-processing step leading to significantly increased mechanical characteristics. Here, single fibres of the polymer composed of 90 mol.% CL and 10 mol.% AC showed a considerable maximum tensile strength of σmax = 53 ± 16 MPa. Furthermore, sinusoidal meanders made of PCLAC yielded a specific tensile stress-strain characteristic mimicking the qualitative behaviour of tendons or ligaments. Cell viability by L929 murine fibroblasts and live/dead staining with human mesenchymal stem cells revealed a promising biomaterial behaviour pointing out MEW printed PCLAC scaffolds as promising choice for medical repair of load-bearing soft tissue. Indeed, one apparent drawback, the small throughput similar to other AM methods, may still prevent MEW’s industrial application yet. However, ongoing research focusses on enlargement of manufacturing speed with the clear perspective of relevant improvement. Thereby, the utilisation of large spinneret sizes may enable printing of high volume rates, while downsizing the resulting fibre diameter via electrical field and mechanical stretching by the collector uptake. Using this approach, limitations of FDM by small nozzle sizes could be overcome. Thinking visionary, such printing devices could be placed in hospitals for patient-specific printing-on-demand therapies one day. Taking the evolved high deposition precision combined with the unique small fibre diameter sizes into account, technical processing of high performance membranes, filters or functional surface finishes also stands to reason. / Um biomimetische extrazelluläre Matrices für das Tissue Engineering herzustellen, bedienen sich aktuelle Forschungsansätze oftmals der Produktion von Faser-Konstrukten durch additive Fertigung oder Elektrospinn-Verfahren. Das sogenannte Melt Electrospinning Writing (MEW) kombiniert Vorteile beider Techniken und weist dadurch ein hohes Applikationspotential auf. Daher bestand das Ziel der vorliegenden Arbeit in der Weiterentwicklung und Erforschung des MEW. Für diesen Zweck wurde eine neuartige Forschungsanlage konzipiert und gebaut, welche mit einzigartiger Verfahrenspräzision und Prozesskontrolle die Fertigung von hochqualitativen Konstrukten ermöglichte. Auf Basis dessen konnten die durchgeführten Studien das Verständnis des elektrohydrodynamischen Druckvorgangs und der untersuchten Prozessparameter vertiefen und letztendlich zur Ausweitung des Verfahrens auf neue medizinisch relevante Thermoplaste beitragen. Um eine reproduzierbare Herstellung von Scaffolds zu ermöglichen, wurde eine häufig auftretende Prozessinstabilität erforscht und als pulsing, oder in stark ausgeprägten Fällen als long beading, klassifiziert. Durch Prozessanalyse konnte zudem eine Methode zur Vermeidung dieser Instabilität entwickelt werden. Dafür wurden zwei unterschiedliche biokompatible Polymere verwendet: Poly(ε-Caprolacton) (PCL) als bis dahin einziger verfügbarer MEW Werkstoff, sowie erstmalig Poly(2-Ethyl-2-Oxazolin). Die aufgestellte Hypothese umfasst eine universelle Abhängigkeit der pulsing Instabilität zu involvierten Massenströmen, welche durch Anpassung des angelegten Prozessdruckes und der elektrischen Feldstärke reguliert werden kann. Um ein optimales Prozessergebnis für nachfolgende Forschungsarbeiten zu erzielen, wurde zusätzlich ein Leitfaden zur quantitativen Bewertung des Grades der Instabilität bereitgestellt. Durch Kombination kleiner Spinndüsen, kleiner Schmelze-Flussraten und hoher elektrischen Feldstärken, konnten erstmalig PCL Fasern mit sub-mikron Durchmessern (fØ = 817 ± 165 nm) zu präzisen Scaffolds verarbeitet werden. Diese wurden anschließend durch materialwissenschaftliche Analytik charakterisiert. Dabei wurde eine molekulare Vorzugsorientierung und isotrope Kristallausrichtung entlang der Faser beobachtet, welche durch den hohen Verstreckungsgrad des erstarrenden Polymerstrahls erklärt werden konnte. Resultierende sub-mikron Fasern konnten zwar für einen akkuraten Druckvorgang verwendet werden, jedoch erwiesen sich die Strukturen als instabil und daher nicht geeignet für die Handhabung bei Zellkulturstudien. Aus diesem Grund wurde ein Beschichtungsansatz mittels hydrophilen und vernetzbaren Sternmolekülen für Substratflächen herangezogen. Während solche modifizierten Oberflächen bekanntermaßen Zelladhäsion verhindern, konnten gedruckte sub-mikron Scaffolds auf der Oberfläche haften und so für biologische Studien verwendet werden. Durch das gezielte Ablegen von Fasern unterhalb der kritischen Translationsgeschwindigkeit (CTS) des Kollektors, konnten sinusförmige Faserstrukturen erzeugt werden. Analog zu rein viskosen Fluiden, welche durch ein bewegliches Band aufgesammelt werden, schien dieser Vorgang dem sogenannten buckling zu unterliegen und daher phänomenologisch nicht oder nur geringfügig vom elektrischen Feld abhängig zu sein. Zudem konnte eine durchgeführte Studie die direkte Abhängigkeit der Fasergeometrie mit der Kollektorbewegung belegen. Unabhängig vom Prozessdruck, führte eine verminderte Kollektorgeschwindigkeit sc in den Grenzen CTS > sc ≥ 2/3 CTS zu erhöhten Amplituden bzw. Spitze-zu-Spitze Werten und verkürzten Wellenlängen. Durch das kontrollierte Ablegen der Fasern bei Geschwindigkeiten von 100 %, 90 % 80 % und 70 % CTS konnten zudem Scaffolds mit unterschiedlichen mechanischen Eigenschaften hergestellt werden. Speziell der Zugmodul wurde dadurch etwa um eine halbe Größenordnung moduliert (Es = 42 ± 7 MPa bis 173 ± 22 MPa bei 1 – 3 % Dehnung). Dies ist in Kombination mit der Strukturierung für maßgeschneiderte TE Scaffolds von großem Interesse, da zelluläre Systeme sensibel auf ihre Umgebung reagieren können. Des Weiteren wurden Poly(L-Lactid-co-ε-Caprolacton-co-Acryloylcarbonat) und Poly(ε-Caprolacton-co-Acryloylcarbonat) (PCLAC) Copolymere hinsichtlich deren MEW Verarbeitbarkeit untersucht. Solche Kunststoffe können nach dem Druckvorgang mit UV-Strahlung radikalisch vernetzt werden und dadurch deutlich erhöhte mechanische Eigenschaften ausbilden. Für Fasern aus 90 mol.% CL und 10 mol.% AC wurden beispielsweise maximale Zugfestigkeiten von σmax = 53 ± 16 MPa ermittelt. MEW gedruckte sinusförmige Faserstrukturen aus PCLAC wiesen darüber hinaus ein biomimetisches Spannungs-Dehnung-Verhalten auf, vergleichbar zu Sehnen- und Ligamentgewebe. Eine Untersuchung der Zellviabilität von L929 murinen Fibroblasten im Eluattest, sowie eine lebend/tot-Färbung von humanen mesenchymalen Stammzellen auf den Scaffolds, ergab vielversprechende Resultate und damit ein relevantes Anwendungspotential solcher Strukturen als Implantat. Neben genannten Vorteilen, weist MEW als Verfahren bislang allerdings geringe Produktionsgeschwindigkeiten auf. Diese sind daher in den Fokus aktueller Forschungsvorhaben gerückt. Einen Ansatz hierfür bieten Spinndüsen mit hohem Innendurchmesser und erhöhter Austragsrate, wobei die optimierte elektrische Feldstärke, sowie ein Verstrecken durch die Kollektorbewegung, zu den erwünschten dünnen Fasern führen können. Dadurch kann die abwärtslimitierte Düsengröße des FDM Verfahrens überwunden werden. Visionär gedacht, könnte eine solche Anlage direkt in Krankenhäusern zur Fertigung von patienten- und defektspezifischen Implantaten eingesetzt werden. Darüber hinaus ermöglicht die hohe Präzision, zusammen mit dem Drucken von Mikro-Fasern, einen technischen Einsatz zur Herstellung von Membranen, Filtern oder funktionalen Oberflächenbeschichtungen.

This research aims to contribute to the safe methodology for additive manufacturing (AM) of energetic materials. Coating formulation processes were investigated to find a suitable method that may enable selective laser sintering (SLS) as the safe method for fabrication of high explosive (HE) compositions. For safety and convenience reasons, the concept demonstration was conducted using inert explosive simulants with properties quasi-similar to the real HE. Coating processes for simulant RDX-based microparticles by means of PCL and 3,4,5- trimethoxybenzaldehyde (as TNT simulant) are reported. These processes were evaluated for uniformity of coating the HE inert simulant particles with binder materials to facilitate the SLS as the adequate binding and fabrication method. The critical constraints being the coating effectiveness required, spherical particle morphology, micron size range (>20 μm) and a good powder deposition and flow, and performance under SLS to make the method applicable for HEs. Of the coating processes investigated, suspension system and single emulsion methods gave required particle near spherical morphology, size and uniform coating. The suspension process appears to be suitable for the SLS of HE mocks and potential formulation methods for active HE composites. The density was estimated to be comparable with the current HE compositions and plastic bonded explosives (PBXs) such as C4 and PE4, produced from traditional methods. The formulation method developed and the understanding of the science behind the processes paves the way toward safe SLS of the active HE compositions and may open avenues for further research and development of munitions of the future. / Dissertation (MSc (Applied Science:Chemical Technology))--University of Pretoria, 2019. / Chemical Engineering / MSc (Applied Science:Chemical Technology) / Unrestricted

There is an upsurge of research interest on Ni-based superalloys additively manufactured (AM) in aerospace sectors. However, achieving the accuracy and quality of the AM part is a challenging task because it is a process of adding material layer by layer with different process parameters. Hence, defects can be observed, and these defects have a detrimental effect on the mechanical properties of the material. Also, AM materials commonly portray a columnar grain structure which also makes it difficult to determine the average grain size because while using the commonly used intercept method, the grain boundaries do not intercept to the test line appropriately. It is important to measure the defects and grain size before performing mechanical testing on the material. Defect measurement and grain size measurements are usually measured manually which results in longer lead time. This work is addressed towards testing recipes in the automated image analysis software to optimize the lead time with good accuracy. Haynes 282, a γ' strengthened superalloy is used in this work. It was assumed that 1,5mm of material from the surface will be machined away so defects had to be measured in this region of interest. The image analysis tools used to test its potentials are MIPAR and ImageJ. Initially, five images in MIPAR and Image J were tested keeping the manual measurements as a benchmark. From this part, it was concluded that metallography and image quality play an important role in the automated measurement. Also, basic Image J software cannot give the measurements of lack of fusion in terms of caliper diameter (longest measurable diameter). Hence, MIPAR was chosen for the application because it was more promising. In the next part, 15 samples were used with manual measurements from a stitched sample and batch processing with MIPAR. The total caliper diameter results were plotted to compare manual measurements and MIPAR. It was observed that scratches were measured as lack of fusion defects at few instances by MIPAR which were further refined using a post-processing function. The defect density results were plotted and compared as well. Due to the difference in calculation of region of interest, the difference in results was observed.To perform the grain size measurement, Haynes 282 was used in HIP and heat treated condition, achieving equiaxed grains. The etchant should be appropriate to reveal the grains. Hence four different etchants were used in this study hydrogen peroxide+HCl, Kallings (electro etch), Kallings (swab) and diluted oxalic acid. This measurement was performed on the material which was cut along the build direction as well as 90º to the growth direction. Since there is no standard for additively manufactured material yet, the results were tested with hall-petch equation to be convinced of the results obtained. It was observed that MIPAR recipe portrayed good results. The results of manual measurements and MIPAR measurements were plotted and compared. It was observed that Hydrogen peroxide and Kallings (swab) showed the grains evidently but twin boundaries were revealed as well. MIPAR calculated the twin boundaries as grains so it over calculated than manual measurements. Kallings (electro etch) and diluted oxalic acid did not reveal the grains so it was difficult for MIPAR to identify the grains.

Indiana University-Purdue University Indianapolis (IUPUI) / Material jetting (MJ) additive manufacturing (AM) has experienced an increased adoption in several industry areas and as well as research applications. One of MJ’s distinct benefits is the ability to print tunable composites, digital materials (DM) by carefully adjusting the ratio of droplets of heterogeneous base-polymeric inks. However, the lack of material information usable in computer simulations has hampered its acceptance in some end-use applications. For these materials to be used in Finite Element Analysis (FEA) simulations the mechanical properties of the DMs need to be characterized into usable material models. DMs printable with an MJ printer has a wide variety of materials properties, ranging from flexible silicone rubber to rigid Acrylonitrile Butadiene Styrene (ABS). Therefore, to cohesively express the mechanical behavior of the DMs it is necessary to utilize non-linear material models. The objective this research is to conduct physical testing to characterize the mechanical behavior of DMs printable with an MJ. Subsequently, to validate the effectiveness of the material models for multi-DM prints. Utilizing the newly characterized material models two use cases were investigated, with the goal of improving the performance of printed parts through simulation. In this study, an MJ printer was used to fabricate the test specimens as well as the components used in the use case studies. The study was focused on the family of six DMs printable from the mixture of the base polymers Tango Black+ (TB+) and Vero White+ (VW+). To characterize the mechanical properties of the materials a tensile test was conducted utilizing the KS-M6518 standard as a basis. The mechanical properties of the DMs were then fitted into four non-linear models and the results compared. The fitted models were, the Neo Hookean model, a two-parameter, three-parameter, and a five-parameter Mooney Rivlin model. To confidently use the material models for multi-DM prints FEA simulations need to validate the accuracy to which they can predict the deformation of the samples under load. To compare the results of the computer simulations and the physical test, strain maps for both results were analyzed. Four different test specimens were printed and tested. A baseline single material samples were compared to three multi-material samples with different embedded structures. The results confirmed the validity of the material models even when used for multi-DM prints. The recently characterized models are utilized in two use case studies which showcase the potential of DMs. The first use case was focused on printing multi-DM substrates for the use of stretchable electronics. The second use case investigated the benefits of utilizing multiple materials to create 3D conductive traces utilizing a new method, the “swollen-off” method. Both case studies showed the benefits of utilizing DMs as well as the applicability of the material models in predictive simulations.

Indiana University-Purdue University Indianapolis (IUPUI) / Ceramics have been extensively used in aerospace, automotive, medical, and energy industries due to their unique combination of mechanical, thermal, and chemical properties. The objective of this thesis is to develop an extrusion based ceramic 3D printing process to digitally produce a casting mold. To achieve the objective, an in-house designed ceramic 3D printer was developed by converting a filament based plastic 3D printer. For mold making applications, zircon was selected because it is an ultra-high temperature ceramic with high toughness and good refractory properties. Additionally, alumina, bioglass, and zirconia slurries were formulated and used as the feedstock material for the ceramic 3D printer. The developed 3D printing system was used to demonstrate successful printing of special feature parts such as thin-walled high aspect ratio structures and biomimetically inspired complex structures. Also, proof of concept with regard to the application of 3D printing for producing zircon molds and casting of metal parts was also successfully demonstrated. To characterize the printed parts, microhardness test, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analyses were conducted. The zircon samples showed an increase in hardness value with an initial increase in heat treatment temperature followed by a drop due to the development of porosity in the microstructure, caused by the decomposition of the binder. The peak hardness value for zircon was observed to be 101±10 HV0.2. Similarly, the microhardness values of the other 3D printed ceramic specimens were observed to increase from 37±3 to 112±5 HV0.2 for alumina, 23±5 to 35±1 HV0.2 for bioglass, and 22±5 to 31±3 HV0.2 for zirconia, before and after the heat-treatment process, respectively. Finally, a system model for the ceramic 3D printing system was developed through the application of the model-based systems engineering (MBSE) approach using the MagicGrid framework. Through the system engineering effort, a logical level solution architecture was modeled, which captured the different system requirements, the system behaviors, and the system functionalities. Also, a traceability matrix for the system from a very abstract logical level to the definition of physical requirements for the subsystems was demonstrated.

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