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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This work focused on laser powder bed fusion (LPBF) of H13 tool steel to examine microstructure and melt pool morphology. Experiments were conducted with varying laser power (P) in the range of 90-180 W and scan speed (v) in the range of 500-1000 mm/s. layer thickness (l) and hatch spacing (h) were kept constant. Volumetric energy density (γ) was calculated using the above process parameters. In order to find a relation between the recorded density and top surface roughness with changing process parameters, set of equations were derived using the non-dimensional analysis. For any chosen values of laser power, scan speed, hatch spacing and layer thickness, these equations help to predict top surface roughness and density of LPBF processed H13 tool steel. To confirm the universal relation for these equations, data of In718 and SS316L processed in LPBF was input which gave a R-square of >94% for top surface roughness and >99% for density. A closed box approach, response surface model, was also used to predict the density and surface roughness which allows only in the parametric range. Material microstructures were examined to identify the melting modes such as keyhole, transition and conduction modes. X-ray diffraction data revealed that there was a presence of retained austinite in all the H13 printed samples. Elongated and equiaxed cellular structure were observed in higher magnifications due to solidification rate and thermal gradient.

Laser powder bed fusion (LPBF), one of the most promising additive manufacturing (AM) techniques, has enabled the production of previously impossible structures. This breakthrough in AM has not only facilitated the creation of new designs, but also the redesign of existing industrial and engineering components to produce lightweight and highly efficient dies and molds, biomaterial scaffolds, aircraft brackets, heat sink and heat exchangers. In many of the mentioned applications in industries such as automotive, aerospace, heat exchanger, and electronics, aluminum (Al), Al alloys, and Al matrix composites (AMCs) are considered potential candidates. In the first phase of this study, the optimum powder particle size and size distribution of an Al alloy powder (i.e., AlSi10Mg) was determined with the aim being to achieve highest densification levels and dimensional accuracies. In the second phase, three materials with high thermal conductivities were selected, namely, pure Al, AlSi12 and AlSi10Mg alloys. Since Al/Al alloys are prone to oxidation, the LPBF process parameters were optimized not only in terms of the densification level but also oxygen content of the fabricated parts. It was found out that the rate of oxide diminishment for Al/Al alloys during the LPBF process is more than in-situ oxidation. Despite the efforts, the optimized LPBF fabricated samples showed lower thermal conductivity than their conventionally manufactured counterparts. To tackle the issue, two different potential solutions were put into test. In the third phase, the influence of preheating on thermal properties of pure Al, AlSi12, and AlSi10Mg was investigated and a huge improvement in the thermal conductivity of the optimized as-built parts was obtained. In the fourth phase, the possibility of enhancing thermal conductivity of the LPBF fabricated Al/Al alloys in as-built condition through the incorporation of a second constituent with a higher thermal conductivity (i.e., graphene) was investigated. / Thesis / Doctor of Philosophy (PhD)

SSAB has built a pilot gas atomization facility looking to expand their expertise of steel into the metal powder and additive manufacturing industry. Laser powder bed fusion is an additive manufacturing method that melts and fuse metal feedstock powder together layer by layer using a high intensity laser. The complex process requires optimization in order to be competitive. The process parameters laser power, scan speed, hatch distance and layer thickness largely govern the build rate and total production time. To increase the build rate, two iterations of test cubes with unique parameters sets were experimentally printed. Evaluation of relative density, porosity, microstructure, hardness and mechanical properties was performed. All results were compared to a reference parameter set previously studied. A candidate parameter set successfully increased the build rate by 116% while maintaining satisfactory material properties.