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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.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24849号 / 工博第5166号 / 新制||工||1987(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 泉井, 一浩, 教授 松原, 厚, 教授 平山, 朋子 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Laser powder bed fusion (LPBF) is a popular additive manufacturing (AM) technique that has demonstrated the capability to produce sophisticated engineering components. This work reports the crack-free fabrication of an SS316L/IN718 bimetallic structure via LPBF, along with compositional redistribution, phase transformations and microstructural development, and nanohardness variations. Constituent intermixing after LPBF was quantitatively estimated using thermo-kinetic coefficients of mass transport and compared with the diffusivity of Ni in the austenitic Fe-Ni system. The intermixing of primary solvents (Ni and Fe) in SS316L/IN718 bimetallic structures was observed for an intermixing zone of approximately 800 µm, and their intermixing coefficient was estimated to be in the order of 10−5 m2/s based on time of 10 ms. In addition, to understand the high temperature behavior, SS316L/IN718 bimetallic structures were annealed at 850, 950, and 1050 °C, for 120, 48, and 24h respectively, followed by water quenching (WQ). Furthermore, to better understand the intermixing of individual components (Ni and Fe) and to predict the varying (maximum) temperatures in LPBF of SS316L/IN718 bimetallic structures, solid-to-solid SS316L vs IN718 diffusion couples were examined at 850, 950, and 1050 °C, for 120, 48, and 24h respectively, followed by WQ. The investigation of SS316L vs IN718 diffusion couples yielded a maximum temperature of approximately 3400 K in the LPBF of SS316L/IN718 bimetallic structures. Finally, compositional redistribution, phase transformations and microstructural development, and nanohardness variations after LPBF of SS316L/IN625 bimetallic structure were also investigated to provide a better understanding of the LPBF process via bimetallic fabrication.

Heat exchangers are integral parts for thermal management and find countless applications in automotive, aerospace, energy, nuclear power plants, HVAC, etc. Due to intensive research & development and technological advancements in manufacturing technologies, there is an increasing rise in demand for high-performance heat exchangers. In the automotive and aerospace industries, heat exchangers are expected to deliver better thermal efficiency and improve the system’s overall functionality in which they are installed by saving space and being lightweight. Additive Manufacturing (AM) is a ground-breaking and promising technology that offers avenues of opportunities to manufacture parts that were almost impossible to be produced with conventional manufacturing and can improve part performance with lightweight and compact designs. Laser-Based Powder Bed Fusion (LPBF), one of the well-known AM techniques, provides freedom to design complex geometries and fabricate them in a layer-by-layer fashion by exposing a high-density laser on a vertically moving powder bed. The study focuses on the application of AM in re-designing heat exchangers under given design requirements using LPBF. It includes exploring Triply Periodic Minimal Surfaces (TPMS) based structures such as gyroid and realizing them as heat exchanger core. Computational gyroid-based heat exchanger core models were designed and analyzed for thermal and fluid dynamics characteristics. A parametric study and analysis based on gyroid TPMS network type, periodic length, thickness, aspect ratio, and functional grading were carried out to optimize heat exchanger performance as per design conditions and validate their manufacturability using LPBF. Successful printable designs were further used to develop and manufacture prototypes. The study concludes with a comparison between additively manufactured gyroid-based design and conventional shell-and-tube design based on the thermal performance from CFD analysis and the weight of prototypes. It was found that the thermal performance from CFD analysis showed an 18.96% improvement, whereas weight was reduced by 14.8% for the gyroid-based design as compared to the conventional shell-and-tube design. / Thesis / Master of Applied Science (MASc)

Laser Powder Bed Fusion (L-PBF) is a metal additive manufacturing technique that uses a laser beam as a heat source to melt metal powder selectively. Because of the process small layer thicknesses, laser beam diameter, and powder particle size, L-PBF allows the fabrication of novel geometries and complex internal structures with enhanced properties. However, the main disadvantages of the L-PBF process are high costs and a lengthy production time. As a result, shortening the manufacturing process while maintaining comparable properties is exceptionally beneficial. Inconel 625 (IN625) is a nickel-based superalloy becoming increasingly popular in marine, petroleum, nuclear, and aerospace applications. However, the properties of IN625 parts produced by casting or forging are challenging to control due to their low thermal conductivity, high strength and work hardening rate, and high chemical complexity. Furthermore, IN625 alloy is regarded as a difficult-to-machine material. As a result, it is worthwhile to seek new technologies to manufacture complex-shaped IN625 parts with high dimensional accuracy. IN625 alloy is known for its excellent weldability and high resistance to hot cracking; thus, IN625 alloy appears to be a promising candidate for additive manufacturing. This thesis presents an experimentally focused study on optimizing L-PBF processing parameters in IN625 superalloy to increase process productivity while maintaining high material density and hardness. This study had four stages: preliminary, exploratory, modelling, and optimization. The first stage was devoted to conducting a literature review and determining the initial processing parameters. The second stage concentrated on determining the process window, for which single tracks were printed with two high levels of laser power (300, 400 W), five levels of scan speed (500, 700, 900, 1100, 1300 mm/s), and five levels of powder layer thickness (30, 60, 90, 120, 150 µm). Then, the process window was defined after investigating the top views and cross-sections of the tracks. Stage 3 involved printing 48 cubes (10 × 10 × 10 mm^3) with a laser power of 400 W, scan speeds of (700, 900, 1100, 1300 m/s), layer thicknesses of (60, 90, 120, 150 µm), and overlap percentages of (10, 30, 50%). As a result, the density of cubes was measured, and a statistical multiple regression analysis was used to predict it. Stage 4 involved estimating four sets of ideal processing parameters (based on statistical modelling of relative density) and printing 24 cubes (10 × 10 × 10 mm^3), six samples for each set. Finally, the relative density, hardness, and productivity of the samples were assessed, and a trade-off was determined. Even with the thickest powder layer of 150 µm (highest process productivity), samples with a mean relative density greater than 99% (i.e., 99.31% by Archimedes principle and 99.82% by image analysis) were printed. These findings are consistent with previously published results for L-PBF IN625 samples manufactured with smaller layer thicknesses ranging from 20 to 40 µm while maintaining comparable material hardness. The findings of this study are noteworthy because IN625 parts can be printed with higher powder layer thicknesses (less production time) while retaining similar material properties to those published with typical layer thicknesses ranging from 20 to 40 µm. Reduced production time due to optimized processing parameters can lead to significant energy and cost savings. / Thesis / Master of Applied Science (MASc)