Dissertations / Theses on the topic 'Melt Pools'

Additive manufacturing (AM) processes are candidates for manufacturing and repair in the aerospace industry. For process stability, control of melt pool dimensions is imperative. This can be achieved via feedback and/or feedforward control approaches; however, the time needed for a change in process variables (beam power and travel velocity) to translate into changes in melt pool dimensions is a critical concern. Prior works have determined the relationship between process variables and melt pool geometry, which can be monitored in situ for feedback control. In the current work, Ti-6Al-4V is studied as it is commonly used in aerospace applications due to its desirable properties. Melt pool depth response is determined in terms of response distance rather than response time in order to develop a relationship that is not dependent on the position or path taken in the Power-Velocity (P-V) Process Map, but instead on the initial and final melt pool geometry. Research is performed through finite element simulations run in the ABAQUS™ software package and validated by experiments performed at the NASA Langley Research Center on their Electron Beam Free Form Fabrication system and at Carnegie Mellon University on their Arcam system. The work presented investigates the transient melt pool response due to changes in process variables in steady state build geometries, as well as extending relationships to transient build geometries and other AM systems yielding a comprehensive understanding of melt pool response across all direct metal AM processes. These relationships will allow for the development of effective feedback and feedforward control systems.

Laser welding is commonly used in the automotive-, steel- and aerospace industry. It is a highly non-linear and coupled process where the weld geometry is strongly affected by the flow pattern in the melt pool. Experimental observations are challenging since the melt pool and melt flow below the surface are not yet accessible during welding. Improved process control would allow maintaining, or improving, product quality with less material and contribute further to sustainability by reducing production errors. Numerical modelling with Computational Fluid Dynamics, CFD, provides complementary understanding with access to process properties that are not yet reachable with experimental observation. However, the existing numerical models lack predictability when considering the weld shape. The work presented here is the development of a model for conduction laser welding. The solver upon which the model is based is first described in detail. Then different validation cases are applied in order to test specific parts of the physics implemented. Two cases focus on thermocapillary convection in two-phase and three-phase flows with surface deformation. Finally, a third case considers the melt pool flow during conduction mode welding.It is concluded that the convection of fusion enthalpy, which was neglected in former studies, should be included in the model. The implementation of the thermo capillary force is recommended to be consistent with the other surface forces to avoid unphysical solution. Free surface oscillations, known from experimental observations, are also computed numerically. However, further investigation is needed to check that these oscillations are not disturbed b ynumerical oscillations.

Self-organization is a hot topic as it has the potential to create surface patterns on the nanoscale avoiding cost-intensive top-down approaches. Although chemists have promising results in this area, ion irradiation can create self-organized surface patterns in a more controlled manner. Different regimes of pattern formation under ion irradiation were described so far by 2D models. Here, two new regimes have been studied experimentally, which require modeling in 3D: subsurface point defect kinetics as well as ion impact-induced melt pool formation. This thesis deals with self-organized pattern formation on Ge and Si surfaces under normal incidence irradiation with heavy monatomic and polyatomic ions of energies up to several tens of keV. Irradiation has been performed using liquid metal ion sources in a focused ion beam facility with mass-separation as well as by conventional broad beam ion implantation. Irradiated samples have been analyzed mainly by scanning electron microscopy. Related to the specific irradiation conditions, investigation and discussion of pattern formation has been divided into two parts: (i) formation of Ge morphologies due to point defect kinetics and (ii) formation of Ge and Si morphologies due to melt pool dynamics. Point defect kinetics dominates pattern formation on Ge under irradiation with monatomic ions at room temperature. Irradiation of Ge with Bi and Ge ions at fluences up to 10^17 cm^(-2) has been performed. Comprehensive studies show for the first time that morphologies change from flat surfaces over hole to nanoporous, sponge-like patterns with increasing ion energy. This study is consistent with former irradiations of Ge with a few ion energies. Based on my studies, a consistent, qualitative 3D model of morphology evolution has been developed, which attributes the ion energy dependency of the surface morphology to the depth dependency of point defect creation and relaxation. This model has been proven by atomistic computer experiments, which reproduce the patterns found in real irradiation experiments. At extremely high energy densities deposited by very heavy ions another mechanism dominates pattern formation. The formation of Ge and Si dot patterns by very heavy, monatomic and polyatomic Bi ion irradiation has been studied in detail for the first time. So far, this formation of pronounced dot pattern cannot be explained by any model. Comprehensive, experimental studies have shown that pattern formation on Ge is related to extremely high energy densities deposited by each polyatomic ion locally. The simultaneous impact of several atoms leads to local energy densities sufficient to cause local melting. Heating of Ge substrates under ion irradiation increases the achievable energy density in the collision cascade substantially. This prediction has been confirmed experimentally: it has been found that the threshold for nanomelting can be lowered by substrate heating, which allows pattern formation also under heavy, monatomic ion irradiation. Extensive studies of monatomic Bi irradiation of heated Ge have shown that morphologies change from sponge-like over highly regular dot patterns to smooth surfaces with increasing substrate temperature. The change from sponge-like to dot pattern is correlated to the melting of the ion collision cascade volume, with energy densities sufficient for melt pool formation at the surface. The model of pattern formation on Ge due to extremely high deposited energy densities is not specific to a single element. Therefore, Si has been studied too. Dot patterns have been found for polyatomic Bi ion irradiation of hot Si, which creates sufficiently high energy densities to allow ion impact-induced melt pool formation. This proves that pattern formation by melt pool formation is a novel, general pattern formation mechanism. Using molecular dynamics simulations of project partners, the correlation between dot patterning and ion impact-induced melt pool formation has been proven. The driving force for dot pattern formation due to high deposited energy densities has been identified and approximated in a first continuum description.

This thesis aims to provide the thermal condition of melt pool convection by CFD simulation, which is important to the assessment of the invessel melt retention (IVR) strategy widely adopted in Generation III pressurized water reactors (PWRs). As a severe accident mitigation measure, the IVR strategy is realized through external cooling of the lower head of a reactor pressure vessel (RPV). To achieve the coolability and retention of the corium pool in the RPV lower head, the heat flux at the outer surface of the vessel should be less than the critical heat flux (CHF) of boiling around the lower head. Under such condition, the integrity of the RPV is guaranteed by the adequate thickness of the unmelted vessel wall. The thesis work starts from the selection and validation of a turbulence model in the CFD computational tool chosen (Fluent). Afterwards a numerical model is set up for estimation of melt pool heat transfer of a reference PWR with the power capacity of 1000 MWe, including a mesh sensitivity study. Based on the numerical model of a twolayer melt pool, four tasks are carried out to investigate the effects of Zr oxidation ratio, Fe content, and radiation emissivity on heat flux profiles, as well as the focus effect under extreme conditions. Selection and validation of the turbulence model are conducted by comparing the simulation results of different turbulence models with the DNS data on the convection of volumetrically heated fluid layer bounded by rigid isothermal horizontal walls at equal temperature. The internal Rayleigh numbers of the flow reach up to 10e6. The comparison shows a good agreement of the SST k-ω turbulence model results with the DNS data. The simulations with the Zr oxidation ratio of 0, 0.2 and 0.5, correspondingly, the oxide layer of 1.389m, 1.467m and 1.580m, and the metal layer of 0.705m, 0.646m and 0.561m in height, show that, the temperature of the oxide layer will increase with Zr oxidation ratio, while the temperature of the metal layer will decrease resulting in more heat transfer through the oxide layer sidewall and less top radiation. Nevertheless, the effect of the Zr oxidation ratio is not pronounced in the range of 00.5. The simulations with the Fe mass of 22t, 33t and 45t, and respective height of the metal layer of 0.462m, 0.568m and 0.646m, show that, the inner metal layer will significantly increase the temperatures of both the metal layer and the oxide layer. The percentage of heat transfer at the oxide layer sidewall will increase to supplement the reduction of that at the metal layer. The simulations with the radiation emissivity of 0.2, 0.35, 0.45 and 0.7 show that, the emissivity below 0.45 has an impact on heat transfer, and the temperatures and sidewall heat flux of both the oxide layer and the metal layer will increase with decreasing emissivity. The impact is negligible when the emissivity is above 0.45. The simulations under the hypothetically extreme conditions with either an adiabatic top boundary or a very thin metal layer show the focusing effect may occur, i.e., the heat flux through the metal sidewall is larger than that in the oxide layer. But the local high heat flux is flattened by the vessel wall with good heat conductivity. In summary, the simulations demonstrate that, except for the cases under extreme conditions, the heat fluxes of the melt pools in all other cases are significantly lower than the CHF of external cooling of the lower head. Therefore, the safety margin of the IVR strategy of the PWR chosen is seems sufficient. However, due to some limitations (e.g., simplification and assumptions) in the simulation cases and coupling of different influential factors, as indicated by the present study, the precise predictions of heat flux under all scenarios are still difficult. Therefore, the conclusions could not be generalized to the other conditions or other configurations of the molten pools. By discussing the model and simplifications/assumptions adopted in this work, the improvement directions of the numerical model and other perspectives are proposed at the end of the thesis.
Denna avhandling syftar till att tillhandahålla det termiska tillståndet för smältbassängskonvektion genom CFD-simulering, vilket är viktigt för bedömningen av IVR-strategin som allmänt antagits i tryckvattenreaktorer (PWR) i Generation III. Som en åtgärd för att mildra allvarliga olyckor realiseras IVR-strategin genom extern kylning av det nedre huvudet av ett reaktortryckkärl (RPV). För att uppnå kylbarhet och kvarhållning av koriumbassängen i det nedre RPV-huvudet bör värmeflöde vid den yttre ytan av kärlet vara mindre än det kritiska värmeflödet (CHF) som kokar runt det nedre huvudet. Under sådant tillstånd garanteras RPV: s integritet av den osmälta kärlväggens tillräckliga tjocklek. Examensarbetet startar från valet och valideringen av en turbulensmodell i det valda CFD-beräkningsverktyget (Fluent). Därefter sätts en numerisk modell upp för uppskattning av smältbassängens värmeöverföring av en referens PWR med en effektkapacitet på 1000 MWe, inklusive en nätkänslighetsstudie. Baserat på den numeriska modellen för en tvålagers smältbassäng utförs fyra uppgifter för att undersöka effekterna av Zr-oxidationsförhållande, Fe-innehåll och strålningsemissivitet på värmeflödesprofiler, liksom fokuseffekten under extrema förhållanden. Val och validering av turbulensmodellen utförs genom att jämföra simuleringsresultaten för olika turbulensmodeller med DNS-data för konvektionen av volymetriskt uppvärmt fluidskikt avgränsat av styva isoterma horisontella väggar vid lika temperatur. De interna Rayleigh-siffrorna i flödet når upp till 10e6. Jämförelsen visar att SST k-ω turbulensmodellresultaten överensstämmer med DNS-data. Simuleringarna med Zr-oxidationsförhållandet 0, 0,2 och 0,5, motsvarande oxidskiktet på 1,389 m, 1,467 m och 1,580 m, och metallskiktet på 0,705 m, 0,664 m och 0,561 m i höjd, visar att temperaturen av oxidskiktet kommer att öka med Zr-oxidationsförhållandet, medan metallskiktets temperatur kommer att minska vilket resulterar i mer värmeöverföring genom oxidskiktets sidovägg och mindre toppstrålning. Ändå är effekten av Zr-oxidationsförhållandet inte uttalad i intervallet 00,5. Simuleringarna med Fe-massan på 22t, 33t och 45t och respektive höjd av metallskiktet på 0,462m, 0,568m och 0,664m visar att det inre metallskiktet avsevärt kommer att öka temperaturerna för både metallskiktet och oxiden lager. Andelen värmeöverföring vid oxidskiktets sidovägg ökar för att komplettera minskningen av den vid metallskiktet. Simuleringarna med strålningsemissiviteten 0,2, 0,35, 0,45 och 0,7 visar att emissiviteten under 0,45 påverkar värmeöverföringen, och temperaturerna och sidoväggens värmeflöde för både oxidskiktet och metallskiktet kommer att öka med minskande emissivitet. Effekten är försumbar när strålningen är över 0,45. Simuleringarna under de hypotetiskt extrema förhållandena med antingen en adiabatisk övre gräns eller ett mycket tunt metallskikt visar att fokuseringseffekten kan uppstå, dvs. värmeflödet genom metallsidan är större än det i oxidskiktet. Men det lokala höga värmeflödet plattas ut av kärlväggen med god värmeledningsförmåga. Sammanfattningsvis visar simuleringarna att, förutom fall under extrema förhållanden, är värmeflödet från smältpoolerna i alla andra fall betydligt lägre än CHF för extern kylning av nedre huvudet. Därför verkar säkerhetsmarginalen för IVR-strategin för den valda PWR tillräcklig. På grund av vissa begränsningar (t.ex. förenkling och antaganden) i simuleringsfall och koppling av olika inflytelserika faktorer, vilket indikeras av den aktuella studien, är de exakta förutsägelserna av värmeflöde under alla scenarier fortfarande svåra. Därför kunde slutsatserna inte generaliseras till de andra förhållandena eller andra konfigurationer av de smälta poolerna. Genom att diskutera modellen och förenklingar / antaganden som antagits i detta arbete föreslås förbättringsriktningarna för den numeriska modellen och andra perspektiv i slutet av avhandlingen.

There is growing interest in using additive manufacturing for various alloy systems and industrial applications. However, existing process development and part qualification techniques, both involve extensive experimentation-based procedures which are expensive and time-consuming. Recent developments in understanding the process control show promise toward the efforts to address these challenges. The current research uses the process mapping approach to achieve control of melt pool geometry and microstructure in different alloy systems, in addition to location specific control of microstructure in an additively manufactured part. Specifically, results demonstrate three levels of microstructure control, starting with the prior beta grain size control in Ti-6Al-4V, followed by cell (solidification structure) spacing control in AlSi10Mg, and ending with texture control in Inconel 718. Additionally, a prediction framework has been presented, that can be used to enable a preliminary understanding of melt pool geometry for different materials and process conditions with minimal experimentation. Overall, the work presented in this thesis has the potential to reduce the process development and part qualification time, enabling the wider adoption and use of additive manufacturing in industry.

Additive manufacturing (AM) offers reduced material waste and energy usage, as well as an increase in precision. Direct metal AM is used not only for prototyping, but to produce final production parts in the aerospace, medical, automotive and other industries. Process mapping is an approach that represents process outcomes in terms of process input variables. Solidification microstructure process maps are developed for single bead and thin wall deposits of Ti-6Al-4V via an electron beam wire feed and electron beam powder bed AM process. Process variable combinations yielding constant beta grain size and morphology are identified. Comparison with the process maps for melt pool geometry shows that by maintaining a constant melt pool cross sectional area, a constant grain size will also be achieved. Additionally, the grain morphology boundaries are similar to curves of constant melt pool aspect ratio. Experimental results are presented to support the numerical predictions and identify a proportional size scaling between beta grain widths and melt pool widths. Results demonstrate that in situ, indirect control of solidification microstructure is possible through direct melt pool dimension control. The ability to control solidification microstructure can greatly accelerate AM process qualification potentially allow for tailored microstructure to the desired application.

Laser is an efficient and widely used heat source in metal processing suchas welding and additive manufacturing. It has some great advantages compared to the other conventional heat sources like electron beam and arc namely: ability of handling complicated joint geometries and producing large components. Laser beam welding encompasses many complex physical phenomena such asheat transfer, metal melting, flow and solidification, free surface deformation, evaporation and possibly vaporization. The aim of this research work istwo-fold: gain deeper process understanding and improve the model reliability. Deeper process understanding is sought on the effect of beam shaping on themelt pool. To achieve improved model reliability, a good support consists in using qualitative experimental data representing the process. Thus, 3D validation of the melt pool geometry is performed while it was usually 2D inprevious research works. Furthermore, a new calculation procedure for laser absorption is introduced. To conduct this research work, a Computational Fluid Dynamics approach is used. A solver, capable of tracking the deformation of the melt free surface, is developed in OpenFOAM. Concerning beam shaping, it is found that not only the melt pool size as previously known but also the melt flow pattern is modified through elongating the beam shape.This last result could not be revealed by former studies as the non-transparent media hinders optical observation. New in-process quantitative measurements performed by a project partner are used to test the model. Weaknesses of the former absorptivity models are highlighted, as well as the limitations of the proposed model. Finally, the results show that the proposed absorptivity model function of local surface conditions leads to much better agreement with experimental results compared to the former constant absorptivity model. The maximum discrepancy compared to the experimental measurement, which is observed for the melt pool depth, can indeed be reduced to about 10%.
Laser är en effektiv och allmänt använd värmekälla vid svetsning och additiv tillverkning. Den har några viktiga fördelar jämfört med andra konventionella värmekällor såsom elektronstråle och elektrisk ljusbåge, nämligen: den kan ofta användas till komplicerade svetsgeometrier, och den kan producera stora komponenter. Lasersvetsning involverar olika sammansatta fysikaliska fenomen såsom värmeöverföring, metallsmältning, flöde, stelning, ytdeformation, avdunstning och i vissa fall förångning. Syftet med mitt forskningsarbete är tvåfaldigt: att få en djupare processförståelse och att förbättra modellens tillförlitlighet. Fördjupad processförståelse eftersträvades för att förstå hur formen på laserstrålen påverkar svetssmältan. För att uppnå förbättrad modellsäkerhet behövs experimentella data av hög kvalitet som representerar processen. Således utfördes 3D-validering av smältgeometrin medan det vanligtvis var 2D i tidigare forskningsarbeten. Dessutom har en ny modell för laserabsorption föreslagits. I forskningen har numerisk strömningssimulering (Computational Fluid Dynamics) använts för att simulera processen och en numerisk lösare, som kan spåra deformationen av den rörliga smälta ytan, är utveckladi programvaran OpenFOAM. Beträffande laserstrålens utbredning visar resultaten att svetssmältans storlek och även svetssmältansflöde modifieras genom att laserstråleformen förlängs. Medan den förra är känd från tidigare experimentella studier upptäcktes den senare inte före denna studie eftersomdet icke-transparenta mediet hindrar optisk observation. Nya (in-process) kvantitativa mätningar utförda av en projektpartner har använts för att testa modellerna. Svagheter i den tidigare absorptionsmodellen framhävdes, liksom begränsningarna i den föreslagna modellen. Slutligen visade resultaten att den föreslagna modellen där laserabsorptionen är en funktion av lokala ytförhållanden ledde till en bättre overensstämmelse med mätningar jämfört med den tidigare modellen med konstant laserabsorbtion. Den maximala avvikelsen jämfört med experimentell mätning, som observerades med avseende på smältbassängsdjupet, kunde reduceras till cirka 10%.

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The UK Health & Safety Executive are responsible for advising on ways to ensure the safety of employees within the workplace. One of the main areas of concern is the potential problem of unwanted fire, and it has been identified that within the area of large-scale storage in warehouses, there is an uncertainty posed by large quantities of thermoplastic. Some forms of thermoplastic exhibit melt-flow behaviour when heated, and a large vertical array exposed to a fire may melt and ignite forming a pool fire in addition to a wall fire. This project is largely experimental, and aimed at quantifying the effect of a growing pool fire fuelled by a melting wall on overall fire growth rate. The pool fire has been found to increase melting and burning rates, producing a much faster growing fire. It has also been found that - 80% of flowing and burning material will enter a potential pool fire, with only 20 - 25% of total mass loss actually burning from the original array. During the project 400+ small-scale tests and several medium-scale experiments have been undertaken at both Edinburgh University and the HSE's Fire & Explosion Laboratory, Buxton. The experiments have confirmed the main parameters governing pool fire development are molecular weight degradation rate and mechanism, which control flow viscosity. There have also been investigations into other influences, the most significant of which was found to be flooring substrate. These parameters then form the basis of a simple 1-D model. A semi-infinite heat transfer approximation is used to determine temperature profile through a thermoplastic exposed to its own flame flux, with extrapolated temperature dependant material properties. The derived profile is then inserted into a gravity driven flow model, to produce estimates of flow rate and quantity for plastics undergoing either random or end chain scission thermal degradation processes. The model identifies property data which are required to permit its use as a hazard assessment tool.

Metal Additive Manufacturing (AM) promises an era of highly flexible part production, replete with unprecedented levels of design freedom and inherently short supply chains. But as AM transitions from a technology primarily used for prototyping to a viable manufacturing method, many challenges must first be met before these dreams can become reality. In order for machine users to continue pushing the design envelope, process space must be expanded beyond the limits currently recommended by the machine manufacturers. Furthermore, as usable process space expands and demands for reduced operator burden and mission-critical parts increase, in-situ monitoring of the processes will become a greater necessity. Processing space includes both the parameters (e.g. laser beam power and travel velocity) and the feedstock used to build a part. The correlation between process parameters and process outcomes such as melt pool geometry, melt pool variability, and defects should be understood by machine users to allow for increased design freedom and ensure part quality. In this work, an investigation of the AlSi10Mg alloy in a Laser Powder Bed Fusion (L-PBF) process is used as a case study to address this challenge. Increasing the range (processing space) of available feedstocks beyond those vetted by the machine manufacturers has the potential to reduce costs and reassure industries sensitive to volatile global supply chains. In this work, four non-standard metal powders are successfully used to build parts in an L-PBF process. The build quality is compared to that of a standard powder (supplied by the machine manufacturer), and correlations are found between the mean powder particle diameters and as-built part quality. As user-custom parameters and feedstocks proliferate, an increased degree of process outcome variability can be expected, further increasing the need for non-destructive quality assurance and the implementation of closed-loop control schema. This work presents two Machine Learning-based Computer Vision algorithms capable of autonomously detecting and classifying anomalies during the powder spreading stage of L-PBF processes. While initially developed to serve as the monitoring component in a feedback control system, the final algorithm is also a powerful data analytics tool – enabling the study of build failures and the effects of fusion processing parameters on powder spreading. Importantly, many troubling defects (such as porosity) in AM parts are too small to be detected by monitoring the entire powder bed; for this reason, an autonomous method for detecting changes in melt pool morphology via a high speed camera is presented. Finally, Machine Learning techniques are applied to the in-situ melt pool morphology data to enable the study of melt pool behavior during fusion of non-bulk part geometries.

Afin de proposer un outil permettant d'étudier les phénomènes hydrodynamiques dans le bain de fusion et le capillaire de vapeur lors du soudage laser, un modèle thermohydraulique prenant en compte les trois phases en présence (vapeur métallique, bain liquide et solide) a été développé à l'aide du code de calcul Comsol Multiphysics. Pour suivre l'évolution de ces trois phases, les équations couplées de la chaleur et de Navier-Stokes sont résolues et le suivi de l'interface liquide-vapeur est traité à l'aide de la méthode level set. Les réflexions multiples du laser sont calculées avec une nouvelle méthode consistant à décrire le laser sous sa forme ondulatoire. Le modèle est d’abord appliqué à un cas de tir laser statique, cas pouvant être résolu en 2D axisymétrique permettant de réaliser les développements et une première validation. L'influence de certains paramètres, comme la puissance laser est étudiée et les mécanismes conduisant à l'apparition de porosité résiduelle sont mis en évidence. Ensuite, ces mêmes équations sont utilisées en 3D pour décrire de la façon la plus complète possible une ligne de fusion. Toute la phase de création du capillaire est analysée puis les températures et les vitesses calculées sont comparées à des mesures expérimentales. Les températures en phase solide sont obtenues grâce à des thermocouples de 25µm, les températures en surface du bain liquide par pyrométrie et enfin, les vitesses à la surface du bain son obtenues grâce à une caméra rapide. Ces comparaisons permettent de montrer la cohérence du modèle, son comportement physique à décrire les écoulements, les formes de zones fondues et la dynamique du capillaire de vapeur
To provide a tool able to study hydrodynamics phenomena in the melt pool and the vapor capillary during laser welding of tailored blanks, a heat and fluid flow model taking into account the three phases present is proposed. The metal vapor, the liquid phase and the solid base are modeled using the code Comsol Multiphysics. In order to study the evolution of these three phases, coupled equations of heat transfer and Navier-Stokes equations are solved and the liquid-vapor interface is tracked using the level set method. Multiple reflections of the laser are calculated with a new method by describing the laser in its wave form by solving Maxwell's equations. This manuscript presents the results of the model, first, in a case of a static laser shot solved in axisymmetric 2D to achieve the development and initial validation. The influence of parameters such as laser power is studied and the mechanisms leading to the appearance of residual porosities is highlighted. Then, these equations are used in three dimensions to describe the most complete as possible, a fusion line with an opened vapor capillary. All the creation phase of the capillary is analyzed. Calculated temperatures and velocities are compared to experimental measurements. Temperatures in the solid phase are obtained with thermocouples of 25µm, the surface temperature of the melt pool are obtained by pyrometry and finally velocities at the surface of the melt pool are obtained with a high speed camera. These comparisons show the consistency of the model to describe the physical flows, the molten zones shapes and the complete behavior of the vapor capillary

Additive manufacturing (AM) – and laser powder bed fusion (LPBF) specifically – constructs geometry that would not be possible using standard manufacturing techniques. This geometric versatility allows integration of multiple components into a single part. While this practice can reduce weight and part count, there are also serious drawbacks. One is that the LPBF process can only build parts with a single material. This limitation generally results in over-designing some areas of the part to compensate for the compromise in material choice. Over-designing can lead to decreased functional efficiency, increased weight, etc. in LPBF parts. Methods to control the material composition spatially throughout a build would allow designers to experience the full benefits of functionality integration. Spatial composition control has been performed successfully in other AM processes – like directed energy deposition and material jetting – however, these processes are limited compared to LPBF in terms of material properties and can have inferior spatial resolution. This capability applied to the LPBF process would extend manufacturing abilities beyond what any of these AM processes can currently produce. A novel concept for spatial composition control – currently under development at Brigham Young University – utilizes liquid or liquid-encased dopants to selectively alter the composition of the powder bed, which is then fused with the substrate to form a solid part. This work is focused on evaluating the feasibility and usefulness of this novel composition control process. To do this, the present work evaluates two deposition methods that could be used; explores and maps the laser parameter process space for zirconia-doped SS 316L; and investigates the incorporation of zirconia dopant into SS 316L melt pools. In evaluating deposition methods, inkjet printing is recommended to be implemented as it performs better than direct write material extrusion in every assessed category. For the process space, the range of input parameters over which balling occurred expanded dramatically with the addition of zirconia dopant and shifted with changes in dopant input quantities. This suggests the need for composition-dependent adjustments to processing parameters in order to obtain desired properties in fused parts. Substantial amounts of dopant material were confirmed to be incorporated into the laser-fused melt tracks. Individual inclusions of 100 $nm$ particles distributed throughout the melt pool in SEM images. Howewver, EDX data shows that the majority of the incorporated dopant material is located around the edges of the melt pools. Variations of dopant deposition, drying, and laser scanning parameters should be studied to improve the resulting dopant incorporation and dispersion in single-track line scans. Area scans and multi-layer builds should also be performed to evaluate their effect on dopant content and dispersion in the fused region.

The initiative of a sustainable material system needs to lower the environmental and economic impact of production processes and adopt new ways of synthesizing and re-using materials. Even though the current conventional manufacturing processes, such as powder metallurgy, casting, forging, and rolling, have already shown their excellent ability to manufacture a large variety of parts and efficiently yield high volume products. Nevertheless, there are still many obstacles in manufacturing metallic components, such as complicated process procedures, high-energy consumption, large material waste, and high machinery cost for reasons that the excess materials need to be removed and extra post-processing time needs to be taken to acquire desired shapes during the machining stage. Thus, finding innovative solutions for producing complex structures is becoming increasingly desirable in the industry. Innovative additive manufacturing (AM, also known as 3D printing) techniques have proved their capacity to manufacture metallic materials with designed complex shapes and tailored properties. The selective laser melting (SLM) is one of the most popular AM techniques, which has the ability to manufacture a wide range of metallic powders in a layer-wise manner and fabricate complex shapes without compromising dimensional accuracy. The toxicity, biocompatibility, corrosion resistance, and stress shielding effect are the key challenges for developing titanium biomaterials for orthopedic applications. Adding nontoxic alloying elements into titanium can solve the issues of toxicity and biocompatibility. One of the best solutions for minimizing the stress-shielding effect and prolonging implant lifetime is to tailor the modulus of implant materials closer to that of bones. Nb is a nontoxic alloying element and an excellent β phase stabilizer, which plays a significant role in reducing the elastic modulus and in improving the corrosion resistance of Ti-based alloys. Accordingly, obtaining a highperformance simple alloy by reducing the alloying elements and substituting toxic elements can facilitate the improvement of sustainability. Thus, the β-metastable Ti-Nb alloys with relatively low elastic modulus have been studied for orthopedic implants due to their high strength to weight ratio, excellent corrosion resistance, and high biocompatibility in the human body. In addition, the high reactivity of titanium with hydrogen and oxygen as well as the high melting points of titanium alloys make conventional manufacturing difficult and cost intensive. As such, the SLM provides an innovative solution to manufacture shape-complicated products in a building chamber under the flow of high purity argon gas to minimize oxidation. However, the availability, printability, and high cost of high-quality raw metallic alloy powder are the limits for the SLM process. The individual elemental powder is relatively cheap and easy to manufacture. Thus, the use of elemental powder mixture results in greater alloy choices as well as lower cost and wider commercial availability. The issues of resultant microstructural and chemical inhomogeneity of the produced parts using the powder mixture have been the major concerns and challenges in the field. Since the mechanical behaviors and chemical properties directly depend on the microstructural homogeneity and phase composition, an in-depth understanding of the effect of inhomogeneity is required. It is necessary to have further advances in manufacturing optimization to extend the benefit of low production costs. In particular, in-situ alloying prospects make SLM a potential route to use a powder mixture with near infinite chemical compositions to synthesize desired titanium alloys for broad applications. As such, synthesizing the proper titanium alloys using the SLM technique, minimizing defect formation, controlling phase composition, evaluating their properties, and investigating the performances of SLM-processed products could significantly advance the applications in various industries and academia. The aim is to apply the SLM technique to process titanium alloys for biomedical and industrial applications. The results help to improve the scientific understandings of the interrelation among alloy compositions, processes, microstructures, defects, properties, and deformation behaviors of 3D-printed parts. Chapter 1 introduces additive manufacturing (AM) has huge potential to realize new alloys with flexible design and easy manufacturing. Especially for the customized healthcare products and services, such as biomedical implants, prosthetics, and hip replacement. Titanium alloys have desirable properties for various applications. Combining additive manufacturing with affordable and biocompatible titanium alloys can further advance and benefit the healthcare industry. Accordingly, the objectives are to fabricate titanium alloys by SLM and to investigate the microstructure, mechanical performance, and corrosion properties. Chapter 2 overviews the type, utilization, and advantage of AM techniques, biomaterials, and titanium alloys. The SLM process can manufacture parts with high precision and superb asbuilt surface quality but relatively high residual stress due to the rapid cooling rate. The raw powder properties and processing parameters play important roles in the densification and mechanical property of built products. The physical factors in the melting process and simulation are shown to understand the melt pool characteristics and stability, which is the critical factor to a successful and desired part. The microstructure, mechanical properties, and corrosion performance of different titanium alloys are also reviewed in order to design the powder, understand the mechanism, and improve the properties. Chapter 3 shows insight into the manufacturing of a Ti-35Nb composite using SLM and post heat treatment. The results emphasize the capability of SLM to fabricate alloys from elemental powder mixtures, even suitable for those with a significant difference in melting point. It provides a significant advance in the understanding of the effect of microstructural inhomogeneity on the resultant mechanical and chemical properties. Heat treatment can further enhance the corrosion resistance of SLM-produced Ti-35Nb samples because the improved chemical homogeneity can facilitate the homogeneous formation of titanium oxides and niobium oxides. It presents a different method of synthesizing novel β-type composites at a relatively lower cost and in easy manufacture. Chapter 4 shows the microstructure, phase response, and mechanical properties of the SLM-fabricated Ti-35Nb using an elemental powder mixture with reduced Nb particle size and its heat-treated counterpart. The results provide significant advances in the understanding of the role of undissolved Nb particles, Nb-rich interfaces, and Ti-Nb-based β phases on the mechanical performance. The nanoindentation mappings provide direct evidence of the contribution of the different phase responses to overall mechanical properties. The Nb particle segregation zones have lower hardness and higher deformation compared to the Ti-Nb matrix. The as-SLMed Ti- 35Nb exhibits relatively high tensile yield strength (648 ± 13 MPa) due to the formation of dendritic β grains. However, the ductility is relatively low (3.9 ± 1.1%) as a result of the weak bonding of undissolved Nb particles within the matrix. The heat-treated counterpart shows a slightly lower yield strength (602 ± 14 MPa) but a nearly 43% increase in ductility (5.6 ± 1.9 %) due to the improved homogeneous Ti-Nb β phase. Chapter 5 shows the microstructure, phase composition, melt pool morphology, and mechanical properties of a prealloyed Ti-35Nb alloy manufactured using SLM and compares it to one produced using an elemental powder mixture. The SLM-processed Ti-35Nb from both feedstocks retained a high volume fraction of β phase due to adequate β stabilization by the Nb and the fast cooling of the SLM process; however, other phase compositions were quite different. The chemical heterogeneity and inhomogeneous microstructure of the SLM-produced sample from powder mixture are results of the fast cooling rate of the melt pool and the high difference of melting temperature and density between elemental powders. However, a uniform microstructure and chemical composition can be achieved in the SLMed prealloyed Ti-35Nb. The variances of powder morphology, density, and melting point between mixed powder and prealloyed powder induce different melt pool status, where the stability of the melt pool plays a critical role in the homogeneity and microstructure. The SLMed Ti-35Nb prealloyed powder samples present a slightly lower yield strength (485 ± 28 MPa) but higher plastic strain (23.5 ± 2.2 %). The excellent ductility has been attributed to the high homogeneity, strong interface bonding, and the existence of a large amount of β phase. Chapter 6 shows the understanding of the homogeneity effect on the coexistence of the acicular α″, β grains, and melt pool boundary for a homogeneous microstructure. It provides some new insight into the phase response and the effect of homogeneity on the SLMed Ti-35Nb alloy using prealloyed powder. The reduced elastic modulus of β phase (89.6 ± 2.1 GPa) is close to that of α″ phase (86.3 ± 2.0 GPa) from the indentation measurement, which is in favor of orthopedic implants application. It also reveals that the nanoindentation test can provide a fast mapping and considerable potential to evaluate the homogeneity, microstructural features, individual phase strength, and deformation behavior in a fine microstructure of SLM-fabricated metallic alloys. Chapter 7 shows the preliminary design in porous structures and compressive behavior of different prealloyed Ti-35Nb sandwich composite porous structures manufactured using SLM. The simulation results were in good agreement with the compression tests. The compression tests show that the sandwich composites with different layers have different deformation behavior and mechanical properties. The rhombic dodecahedron porous structure with added layers could achieve balanced compressive strength and ductility. The preliminary sandwich design with the verified finite element method (FEM) models can be employed in other metallic porous structures to improve the strength and ductility without affecting the porosity. Chapter 8 concludes the present findings in this thesis and suggests the future challenges and development using SLM to tailor titanium alloys for specific applications. As such, the SLM technique is a promising route to develop titanium alloys from powder mixture with wider alloy choices at a cheaper cost and in easier availability. Even though a uniform microstructure and chemical composition can be achieved in the SLM-produced Ti-35Nb using prealloyed powder, there are still challenges on how to achieve full melting of elemental powder particles and obtain a homogeneous β phase microstructure. With the investigation of β- type Ti-Nb alloys, this thesis aims to further understand the effect of the unmelted Nb particles in the synthesized Ti-Nb alloys and melt pool stability as well as improve the Nb melting, microstructure, and mechanical properties for industrial and biomedical applications. Understanding the effect of powder feedstock type and phase features of the SLM-produced Ti- 35Nb using prealloyed powder further provides insights into the homogeneity, microstructure, and resultant properties. The novel design in Ti-35Nb sandwich composite cellular structures can benefit biomedical and industrial applications. By taking advantage of the commercial availability and lower cost of elemental powder, finding solutions to achieve full melting and homogeneous microstructure for nontoxic and biocompatible β-type Ti-Nb alloys with promising mechanical and corrosion properties is significant in future research and development.

Les applications utilisant les procédés de fabrication directe par laser et projection de poudre sont en pleine expansion, en particulier, dans l'aéronautique. Néanmoins, cette technologie prometteuse fait état de quelques points durs et est confrontée aux problèmes d'instabilités du procédé. Lorsque ces phénomènes ne sont pas maîtrisés, cela conduit à des défauts (résistances mécaniques insuffisantes, porosités trop importantes, mauvais états de surface,….etc), qui, selon leur répartition et leur taille, risquent d'engendrer des non conformités, de détériorer les caractéristiques mécaniques des pièces et qui peuvent représenter un coût de post-traitement non négligeable. Par conséquent, il est primordial de maîtriser le procédé d'élaboration, afin de rendre le procédé de fabrication robuste et préserver l'intégrité structurelle de la pièce. Cela requiert la mise en place de système d'instrumentation du procède de projection laser, et par l'intermédiaire du contrôle procédé, d'avoir un système de commande temps réel permettant d'adapter les paramètres procédés en cours d'élaboration, afin de de maintenir une haute qualité de la pièce fabriquée. Dans cette perspective, nous avons développé une solution technologique (matérielle et algorithmique) à base de caméras (vision) permettant de suivre des paramètres clefs lors de la fabrication. L'application de ce système de vision a permis la mise en œuvre de méthodes innovantes, utilisant des outils de l'automatique moderne, pour surveiller l'état de pièces projetées, voire même corriger leurs défauts lors de la fabrication, en ayant un suivi et un contrôle du procédé en temps réel. De plus ce système de vision a permis à partir de mesures effectuées sur les entrées et les sorties du procédé, d'identifié un modèle dynamique qui ont conduit à la réalisation du système de contrôle procédé
Applications using the direct metal deposition laser process have been expanded rapidly, particularly in aeronautics. However, this promising technology reported some difficult points and faced several problems, mainly the process instability. When these phenomena are not controlled, several defects was obtained (lack of mechanical strength, excessive porosity, poor surface, ... etc.). According to their distribution and size, non-conformity, deteriorate the mechanical characteristics of the parts was recorded and result in a significant cost of post-processing. Therefore, it is important to control the process, to make the process both robust and preserve the structural integrity of the piece. This requires the development of instrumentation through the control process, in order to have a real-time system able to adjust the process parameters to keep a high quality of the manufactured part. In this perspective, the studied thesis developed a technological solution (hardware and algorithms) based on cameras (vision) to monitor key parameters during manufacture. The application of this vision system has been allowed for the implementation of innovative methods by using modern automatic tools to monitor the status of the built part or even correct their defects during the manufacture parts, having a monitoring and process control in real time. Furthermore this vision system performed measurements for the inputs and outputs of the process, matched to a dynamic model that lead to the realization of the process control system

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.

Self-organization is a hot topic as it has the potential to create surface patterns on the nanoscale avoiding cost-intensive top-down approaches. Although chemists have promising results in this area, ion irradiation can create self-organized surface patterns in a more controlled manner. Different regimes of pattern formation under ion irradiation were described so far by 2D models. Here, two new regimes have been studied experimentally, which require modeling in 3D: subsurface point defect kinetics as well as ion impact-induced melt pool formation. This thesis deals with self-organized pattern formation on Ge and Si surfaces under normal incidence irradiation with heavy monatomic and polyatomic ions of energies up to several tens of keV. Irradiation has been performed using liquid metal ion sources in a focused ion beam facility with mass-separation as well as by conventional broad beam ion implantation. Irradiated samples have been analyzed mainly by scanning electron microscopy. Related to the specific irradiation conditions, investigation and discussion of pattern formation has been divided into two parts: (i) formation of Ge morphologies due to point defect kinetics and (ii) formation of Ge and Si morphologies due to melt pool dynamics. Point defect kinetics dominates pattern formation on Ge under irradiation with monatomic ions at room temperature. Irradiation of Ge with Bi and Ge ions at fluences up to 10^17 cm^(-2) has been performed. Comprehensive studies show for the first time that morphologies change from flat surfaces over hole to nanoporous, sponge-like patterns with increasing ion energy. This study is consistent with former irradiations of Ge with a few ion energies. Based on my studies, a consistent, qualitative 3D model of morphology evolution has been developed, which attributes the ion energy dependency of the surface morphology to the depth dependency of point defect creation and relaxation. This model has been proven by atomistic computer experiments, which reproduce the patterns found in real irradiation experiments. At extremely high energy densities deposited by very heavy ions another mechanism dominates pattern formation. The formation of Ge and Si dot patterns by very heavy, monatomic and polyatomic Bi ion irradiation has been studied in detail for the first time. So far, this formation of pronounced dot pattern cannot be explained by any model. Comprehensive, experimental studies have shown that pattern formation on Ge is related to extremely high energy densities deposited by each polyatomic ion locally. The simultaneous impact of several atoms leads to local energy densities sufficient to cause local melting. Heating of Ge substrates under ion irradiation increases the achievable energy density in the collision cascade substantially. This prediction has been confirmed experimentally: it has been found that the threshold for nanomelting can be lowered by substrate heating, which allows pattern formation also under heavy, monatomic ion irradiation. Extensive studies of monatomic Bi irradiation of heated Ge have shown that morphologies change from sponge-like over highly regular dot patterns to smooth surfaces with increasing substrate temperature. The change from sponge-like to dot pattern is correlated to the melting of the ion collision cascade volume, with energy densities sufficient for melt pool formation at the surface. The model of pattern formation on Ge due to extremely high deposited energy densities is not specific to a single element. Therefore, Si has been studied too. Dot patterns have been found for polyatomic Bi ion irradiation of hot Si, which creates sufficiently high energy densities to allow ion impact-induced melt pool formation. This proves that pattern formation by melt pool formation is a novel, general pattern formation mechanism. Using molecular dynamics simulations of project partners, the correlation between dot patterning and ion impact-induced melt pool formation has been proven. The driving force for dot pattern formation due to high deposited energy densities has been identified and approximated in a first continuum description.

碩士
國立中山大學
機械與機電工程學系研究所
99
The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.

博士
國立成功大學
材料科學及工程學系
106
A three dimensional selective laser melting process simulation is developed to investigate (1) the influence of normalized process parameters on surface morphology and (2) the melt-pool behavior of a randomly-distributed powder bed with keyhole formation by Nd-YAG laser. Results showed that when the scanning speed was increased, the surface morphology initially became flatter, but then roughness developed again at high speed case. Further, as the laser power was increased, the surface morphology gradually roughened. To better describe the surface morphology phenomenon according to different laser parameters, the melt pool volume and melt pool lifetime were also investigated. With these two factors constrained, a fine surface could be obtained with a low melt pool volume and proper lifetime (approximately 100 µs to 130 µs). Also, to show the importance of evaporation during laser melting, the melt-pool temperature, melt-pool dimensions and the surface morphology are used as metrics for comparison. Through simulation, the transition from keyhole formation to the final convex surface at a local area was discovered. The simulation results are all validated via good agreement with the experiment.

碩士
國立成功大學
機械工程學系
107
This research built up a relatively low cost lateral off-line high-speed imaging system to observe the melt pool and the spatter behavior in selective laser melting with stainless steel 316L powder. The (1) melt pool geometry including melt pool length and width (2) spatter size distribution (3) total spatter volume comparing to the evaporation volume over 3200K with the simulation, which is the evaporation point of SS316L material. These three objectives are studied. The imaging system is constructed on the side of the melt pool and therefore the image taken from the camera exists an angle of view. Thus, the perspective transformation is applied to transform the image from the side view to the top view mathematically and the spatial resolution was calculated. The melt pool was recognized using the liquid to the solid transition point of the material and used the second derivative method to identify the transition point. However, the transition point in the image was not clear enough to be identified because of the noises. As a result, the automatically detect algorithm to identify the transition point to measure the melt pool length and width was developed. The monitored length showed the maximum average error 15% while the error of the measured width was 24%. A large number of spatters eject from the melt pool with very high speed during the process. Therefore, it is necessary to perform spatter tracking so as to get the spatter size distribution in a certain period. The tracking algorithm based on Kalman filter was developed. The result shows that the higher energy density, the more spatters generate. The volume of the spatter was also compared to the evaporation zone in the simulation. The result showed that the spatter volume was smaller than the evaporation volume in the simulation. This result should be further studied because of the contrary to physics.