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Pure copper is widely employed as the primary metal in thermal management and electromagnetic applications due to its exceptional electrical and thermal conductivity. Laser powder bed fusion (LPBF) is a versatile additive manufacturing technique that utilizes high laser energy to selectively melt and fuse successive layers of metal powder to create metallic components with intricate geometries. Nonetheless, LPBF of pure copper is known as a challenging manufacturing process attributed to low optical absorptivity, rapid dissipation of laser energy, and affinity to oxidation. This thesis focuses on the process development and optimization for LPBF of Cu. Firstly, the Process-structure-property relation was examined by assigning a wide range of process parameters to print Cu-LPBF coupons. The optimum process parameters were defined based on maximum relative density, which was obtained at the full laser power of the EOS M280. The results emphasized the significant impact of laser power and hatch spacing on the part quality. Second, Cu oxide exhibits higher optical absorption than pure copper, as reported in the literature. Therefore, the thin film of oxide that was created either on recycled or intentionally oxidized power particles would be a possible easy way to increase the heat energy absorbed from the laser beam. However, the current work emphasized the adverse effects of oxide presence on part quality, particularly when using a medium laser power machine. In this regard, a new method of in-situ Cu oxide reduction during LPBF was proposed to develop an easy and environment-friendly approach to recover the contaminated powder. Applying laser ablation on the powder surface and the solidified layers results in considerable improvement, where the oxygen content is reduced by 70% in the LPBF samples compared to the initial state of the oxidized powder. Finally, the power density of Cu-LPBF coils was improved by enhancing the filling factor and increasing the electrical conductivity. The dimensional limitation of Cu-LPBF fabricated parts was initially identified. The power of utilizing sample contouring was highlighted to upgrade surface quality. Adjusting beam offset associated with optimum scan track morphology upgraded the minimum feature spacing to 80 um. The electrical impedance of full-size Cu-LPBF coils, newly reported in this study, was measured and compared with solid wire. It can reflect the performance of Cu-LPBF coils (power factor) in high-frequency applications. / Thesis / Doctor of Philosophy (PhD)

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 can be used to produce complex, custom geometries, consolidating different parts into one. This reduces the required number of assemblies and allows distributed manufacturing with short lead times. Defects, such as porosity and surface roughness, associated with parts manufactured by laser powder bed fusion, can severely limit industrial application. The effect these defects have on corrosion and hence long term structural integrity must also be taken into consideration. This project aimed to characterise porosity in both solid and lattice cube samples produced by laser powder bed fusion, with the differences in porosity induced by changes in the process parameters, and subsequently, characterising the effect porosity has on corrosion. The alloy used in this investigation is AlSi10Mg, which is widely used in the aerospace and automotive industries. Samples were studied before and after corrosion using X-Ray computed tomography (CT scanning), metallographic examination and scanning electron microscopy (SEM), as well as compression testing for the lattice cubes. It was found that higher laser power leads to more porosity and lower surface roughness. CT scanning was a very effective method to study corrosion using aligned CT images of before-after states. Porosity did not have an effect on the corrosion during the early corrosion stages (168 hours). The manufacturing process parameters induced differences in porosity and surface conditions, but did not strongly affect corrosion. It is probable that crack initiation sites such as internal porosity and defects are filled with corrosion product, delaying the onset of cracking and failure, and the corrosion product that fill the voids adding to the full strength of the lattice will also slightly increase the compressive strength of the samples. / Dissertation (MEng (Metallurgical Engineering))--University of Pretoria, 2021. / Materials Science and Metallurgical Engineering / MEng (Metallurgical Engineering) / Unrestricted

Additive manufacturing (AM) has the possibility of producing complex-shaped components which can not be produced by conventional manufacturing methods. This gives the opportunity for designers to freely think outside the design spectra which is otherwise limited by conventional manufacturing methods. AM of metal has rapidly been developed for the last three decades, and they now are reaching industrial acceptance levels, metal feedstock for use in AM is also rapidly growing. AM of metals is especially of interest for the tooling industry. The design freedom which AM offers the tooling manufacturer can design complex cooling channels within moulds, which could reduce cycle time and enhance the quality of components produced with the moulds. Maraging steels have been proven to both be able to be processed with AM but also have comparable performance to traditionally carbon-based used tool steels. Laser Powder Bed Fusion (LPBF) is one of the most promising AM systems today, by using powder as a feedstock it can produce high-resolution parts without needing to process them after they have been produced. However, there is a need to better understand processing within the LPBF system. This master thesis is aimed to process a newly developed maraging steel from Uddeholm, and conduct process parameter experiment and study their correlation to be able to produce samples with as few defects possible. It is crucial to conform to a good methodology for how to find those correlations and see how they influence the printed material. LPBF process has a multi-complex variable system, and by narrowing down the complexity by focus on the most influencing parameters as has been proven by many researchers. Even with a reduced focus, it is still a multi-variable problem. In this study a methodology of finding process parameters relations, a Design Of Experiment software was used, namely, MODDE. By screening of process parameter ranges, within the software, a statistical evaluation of operational process window can be found with fewer conducted experiment. Development of process parameter can traditionally be time-consuming and result in an unnecessary large number of experiments to find the operational window. The experiment showed that laser power and point distance had the most influencing effect on relative density, followed by exposure time and hatch distance. The experiment was firstly conducted with a layer thickness of 50 µm, the achieved relative density resulted in over 99.8 percent. However, a large lack of fusion defects was observed inside the specimens. Even though a high relative density was measured, a pore analysis has to be conducted to fully understand the size and shape of defects since they can have a severe impact on mechanical properties. It was believed that the layer thickness was too high and that the defects could be reduced by printing a set with same process parameters but with a lower layer thickness of 40 µm instead. The reduction of layer thickness did result in a significant decrease of the defects observed. However, future work after this thesis must be continued to further optimize and to increase the solidity of printed material to reach closer to its conventional produced relatives / Additiv tillverkning har möjligheten att producera komplext konstruerade komponenter som inte kan produceras med konventionella tillverkningsmetoder. Detta ger konstruktörer möjligheten att fritt tänka utanför designspektra som annars begränsas av konventionella tillverkningsmetoder. Additiv tillverkning av metall har snabbt utvecklats under de senaste tre decennierna och har nu nått industriella acceptansnivåer. Metallråvara för användning i additiv tillverkning växer snabbt. Additiv tillverkning av metaller är särskilt intressant för verktygsindustrin, designfriheten som additiv tillverkning kan erbjuda verktygstillverkaren för att kunna utforma komplexa kylkanaler inuti formar. Det kan således reducera cykeltiden och förbättra kvaliteten på komponenter som produceras med formarna. Maråldringsststål har visat sig att både kunna processas i additiv tillverkning och har jämförbara egenskaper med traditionellt kolbaserade verktygsstål. Pulverbäddsystemet är ett av de mest lovande systemen idag, genom att använda pulver som råmaterial kan systemet producera komponenter med hög noggranhet utan att behöva bearbeta dem efter att processen är klar. Det finns emellertid ett behov av att bättre förstå själva processen inom pulverbädds teknologin. Den här masteruppsatsen syftar till att additivt tillverka ett nyutvecklat maråldringsstål från Uddeholm. Samt att genomföra processparameterexperiment och studera deras korrelation för att kunna producera prover med så få defekter som möjligt. Det är avgörande att hitta en metod för hur man hittar korrelationerna och se hur de påverkar det tillverkade materialet. Pulverbäddsystemet har ett multikomplext variabelsystem. För att minska komplexiteten kan fokus läggas på de mest inflytelserika processparametrarna, vilket har bevisats av många forskare. Även med ett reducerat fokus är det fortfarande ett flervariabelsproblem. I denna studie användes en metod för att hitta relationer mellan processparametrar och en Design Of Experiment-programvara, nämligen MODDE. Genom screening av processparametrar, inom programvaran, kan en statistisk utvärdering av operativt processfönster hittas med färre genomförda experiment. Utvecklingen av processparametrar kan traditionellt vara tidskrävande och resultera i ett onödigt stort antal experiment för att hitta det operativa fönstret av processparametrar. Experimentet visade att lasereffekt och punktavstånd påverkande den relativa densiteten mest, följt av exponeringstiden och spåravståndet. Experimentet genomfördes först med en lagertjocklek av 50 mikrometer, lagertjockleken resulterade i en relativ densitet på över 99,8 procent. Emellertid observerades stora fusionsdefekter inuti proverna. Även om en hög relativ densitet mättes, måste en poranalys genomföras för att fullt ut förstå storleken och formen på defekter eftersom de kan ha en avgörande inverkan på mekaniska egenskaperna. Det misstänktes att lagertjockleken var för hög och att defekterna kunde minskas genom att tillverka en ytterligare uppsättning av samma processparametrar men med en lägre lagertjocklek på 40 mikrometer istället. Minskningen av lagertjockleken resulterade i en signifikant minskning av de observerade defekterna. Framgent efter den här avhandlingen måste dock arbetet fortsätta att ytterligare optimera och öka soliditeten i det additivt tillverkade materialet. Det för att uppnå bättre prover och komma ännu närmre det konventionellt tillverkade materialets egenskaper.

This thesis is about the investigation of the spectral interaction of electromagnetic radiation with metal powders. For this purpose, spectral data of powders for laser powder bed fusion processes are investigated in three papers using different techniques. In paper A the spectral radiation behavior of the laser interaction zone is considered, in paper B and C the absorbance behavior of different metal powders depending on their state and measurement method.   Paper A investigates the spectral signal of the process light generated by laser material interaction in laser powder bed fusion. The detection is performed by a coaxially guided measuring beam and a quasi-coaxial measuring beam simultaneously guided by another scanning optics. The signal characteristics depend on the angle of incidence of the measuring beam to the laser material interaction zone. Using high-speed recordings and optical simulations, a model for describing the signal behavior could be determined. The measured spectral intensity distribution representing the degree for energy coupling can be corrected with a correction factor over the whole field for solid materials. This correction includes a function describing the numerical aperture of the measuring channel and the laser intensity on the working field. For the investigated powder, the measurement signal fluctuated strongly and no transferable model could be formed. The reason for this was the different absorbance behavior of the powders investigated. Paper B therefore deals in detail with the spectral absorbance behavior of metal powders for additive manufacturing. Using a high-precision spectrometer, 39 powders were measured reflectively over a wide spectral range and the absorbance determined. By varying the degree of use, aging, grain size and impurities, various influence parameters are determined experimentally and discussed theoretically. Based on 20 derived laser wavelengths, technically usable wavelengths with better process efficiency and stability are proposed. From the obtained absorbance, the efficiency of energy coupling can be estimated and form a broad data base for the optimization of laser parameters. In order to perform the absorbance determinations also in situ in a laser powder bed fusion system paper C describes a possibility of an inline absorbance determination by high resolution coaxial imaging. A method is discussed for geometrically correct and gapless imaging of the processing plane, recorded through the laser optics. By imaging at six different wavelengths, metal powders can be distinguished by their absorbance spectrum and impurities can be detected. In an experimental implementation the functionality of the method is proven. The results are validated by optical simulations, ray tracing and comparative measurements with a high-precision spectrometer.

It is known that altering different processing parameters will yield completely different results in additive manufacturing. Some of the most common ones to alter to increase material quality or to increase productivity are laser power, hatch distance, layer thickness and scan speed. These parameters directly affect the material quality and are well understood. This study investigates how the process for additive manufacturing is being affected by the more unexplored minor process parameters stripe width, stripe overlap, and gas flow, with a goal to increase to knowledge of the process stability in additive manufacturing. Density measurements and investigations in optical tomography were made to determine the minor process parameters effect on the material density and process stability. The density is measured using white light interferometry and the results from the density analysis showed that the minor process parameters does not affect the density of the produced material within the interval used in this study. The minor process parameters effect on the process stability were investigated using the measured gray value obtained from optical tomography. A higher gray value means that the process is kept at a higher temperature for a longer period of time. A decrease in stripe width increased the measured gray value, while an increase of the stripe overlap increased the measured gray value. To understand what the measured gray value means for the process, the spatter area was measured using ImageJ, and a strong correlation between measured spatter area and measured gray value was found, showing that a larger spatter area will be visible as higher measured gray value. The effect of the scan angle was investigated in optical tomography, comparing the mean gray value to the scan angle. Results showed that the mean gray value increases when the angle is close to perpendicular to the gas flow at higher stripe widths and higher stripe overlaps. / Det är känt att olika processparametrar erhåller olika materialkvalitéter när de ändras. Några av de vanligaste att variera för att öka materialkvalitén eller öka produktiviteten är lasereffekten, hatch-avståndet, lagertjocklek and skannhastighet. Dessa parametrar påverkar materialkvalitén och är väl undersökta. Denna studie undersöker hur processen påverkas av de mer okända parametrarna skannbredd, överlapp och gasflödet, med målet att utöka kunskapen kring processtabiliteten i additiv tillverkning. Densitetsmätningar och undersökningar i optisk tomografi gjordes för att bestämma påverkan av de sekundära processparametrarnas påverkan på materialdensiteten och processtabiliteten. Densiteten mäts med en vit-ljus interferometer och resultatet från densitetsanalysen visade att de sekundära parametrarna inte påverkade densiteten av de producerade materialet inom intervallen som användes i denna studie. Påverkan av de sekundära parametrarna på processtabiliteten undersöktes med det uppmätta gråvärdet från optisk tomografi. Ett högra gråvärde innebär att temperaturen är högre under en längre period. En ökning av skannbredden sänkte det uppmätta gråvärdet och en ökning av överlappet ökade de uppmätta gråvärdet. För att förstå vad gråvärdet innebär för processen så mättes arean av stänk i ImageJ. En stark korrelation mellan uppmätt area av stänk och uppmätt gråvärde upptäcktes, vilket visades i att det uppmätta gråvärdet var högre när arean av stänk var större. Påverkan av skannvinkeln undersöktes också i optisk tomografi där jämföranden mellan gråvärdet och skannvinkeln gjordes. Resultatet visade att gråvärdet ökar när skannvinkeln är nära vinkelrät mot gasflödets vid höga värden på skannbredden och överlappet.

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