| 1. |
- Da Silva, Adrien, et al.
(författare)
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Acceleration of metal drops in a laser beam
- 2021
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Ingår i: Applied Physics A. - : Springer. - 0947-8396 .- 1432-0630. ; 127:1
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Tidskriftsartikel (refereegranskat)abstract
- Different processes require the detachment of metal drops from a solid material using a laser beam as the heat source, for instance laser drop generation or cyclam. These techniques imply that the drops enter the laser beam, which might affect their trajectory. Also, many laser processes such as laser welding or additive manufacturing generate spatters that can be accelerated by the laser beam during flight and create defects on the material. This fundamental study aims at investigating the effects of a continuous power laser beam on the acceleration of intentionally detached drops and unintentionally detached spatters. Two materials were studied: 316L steel and AlSi5 aluminium alloy. High-speed imaging was used to measure the position of the drops and calculate their acceleration to compare it to theoretical models. Accelerations up to 11.2 g could be measured. The contributions of the vapor pressure, the recoil pressure, and the radiation pressure were investigated. The recoil pressure was found to be the main driving effect but other phenomena counteract this acceleration and reduce it by an order of magnitude of one to two. In addition, two different vaporization regimes were observed, resulting respectively in a vapor plume and in a vapor halo around the drop.
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| 2. |
- Da Silva, Adrien, et al.
(författare)
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Additive Manufacturing by laser-assisted drop deposition from a metal wire
- 2021
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 209
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Tidskriftsartikel (refereegranskat)abstract
- The subject of Additive Manufacturing includes numerous techniques, some of which have reached very high levels of development and are now used industrially. Other techniques such as Micro Droplet Deposition Manufacture are under development and present different manufacturing possibilities, but are employed only for low melting temperature metals. In this paper, the possibility of using a laser-based drop deposition technique for stainless-steel wire is investigated. This technique is expected to be a more flexible alternative to Laser Metal Wire Deposition. Laser Droplet Generation experiments were carried out in an attempt to accurately detach steel drops towards a desired position. High-speed imaging was used to observe drop generation and measure the direction of detachment of the drops. Two drop detachment techniques were investigated and the physical phenomena leading to the drop detachment are explained, wherein the drop weight, the surface tension and the recoil pressure play a major role. Optimised parameters for accurate single drop detachment were identified and then used to build multi-drop tracks. Tracks with an even geometry were produced, where the microstructure was influenced by the numerous drop depositions. The tracks showed a considerably higher hardness than the base wire, exhibiting a relatively homogeneous macro-hardness with a localised softening effect at the interfaces between drops.
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| 3. |
- Da Silva, Adrien, et al.
(författare)
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The effects of laser irradiation on an aluminium powder stream in Directed Energy Deposition
- 2021
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 41
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Tidskriftsartikel (refereegranskat)abstract
- Additive Manufacturing with aluminium alloys is a subject of increasing industrial interest. Directed Energy Deposition using high power lasers and a powder feed is a useful option but the interactions between the powder stream and the laser beam are not completely understood. It is well known that the powder particles heat up in the laser beam and some theoretical models predict that they can reach their vaporisation temperature and have their flight path altered by the associated recoil pressure. In order to learn more about these phenomena, powder streams were observed with a high-speed camera at different laser powers (up to 6 kW) and with three batches of powder (AlSi10Mg) of different particle sizes. The results showed an increase of powder focussing with increased laser power. In addition, some particles were found to disintegrate in the laser beam. It is demonstrated that particle disintegration is most likely to be caused by the momentum induced by the recoil pressure.
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| 4. |
- Da Silva, Adrien, et al.
(författare)
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Vertical laser metal wire deposition of Al-Si alloys
- 2020
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Ingår i: 11th CIRP Conference on Photonic Technologies [LANE 2020]. - : Elsevier. ; , s. 341-345
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Konferensbidrag (refereegranskat)abstract
- Additive Manufacturing of aluminium alloys has become crucial for lightweight applications. However, new materials and techniques need to be developed in order to achieve more advanced properties and higher efficiency. Therefore, a new energy-efficient wire deposition strategy was developed for processing aluminium-silicon alloys with Laser Metal Wire Deposition. Three alloys with different Si-contents were studied: AlSi5, AlSi10Mg and AlSi12. Different thicknesses of partially melted zones were observed and explained. The previous layer was partly remelted only by the heat conduction in the melt pool. It was found that the thickness of the partially melted zone depends on the difference of temperature between the liquidus and solidus.
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| 5. |
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| 6. |
- Dewi, Handika Sandra, et al.
(författare)
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Impact of laser beam oscillation strategies on surface treatment of microalloyed steel
- 2020
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Ingår i: Journal of laser applications. - : American Institute of Physics (AIP). - 1042-346X .- 1938-1387. ; 32:4
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Tidskriftsartikel (refereegranskat)abstract
- The depth homogeneity of laser-treated zones is one possible factor to define the quality and efficacy of altered mechanical properties in materials. For instance, half-rounded cross-sectional shapes of laser hardened zones using Gaussian beams provide dissimilar hardened depth in the edges and center of the treated area. This means that the in-depth distribution of compressive residual stress varies between the edges and the center of the hardened area. Nonhomogeneity of compressive residual stress distributions can inhibit fatigue properties and can lead to product failure. The utilization of oscillated laser beams has been proven to improve the welding efficiency and energy input distribution to the material, which promises achieving a homogeneous depth of laser-treated zones in hardening applications. Therefore, this work examines the influence of triangular, square, and circular beam oscillation strategies on the energy input distribution during the process and the geometry of the laser-treated zones on microalloyed steel. Laser beam pathways were assembled using a vector graphic editor to visualize the energy distribution from each oscillation strategy. Cross section images of the hardened tracks were taken and related to the thermal energy input profiles. It was revealed that each oscillation strategy demonstrates characteristic temporal and spatial thermal energy input distribution, influencing the geometry of the hardened zone. The circular oscillation strategy produced a widely constant depth in contrary to the triangular and square beam oscillation due to its characteristic energy distribution that allows homogeneous heat dissemination in the material. This confirms that the laser beam oscillation strategy can tailor the energy input distribution to optimize the processing outcome.
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| 7. |
- Dewi, Handika Sandra, et al.
(författare)
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Influence of mill scale on oxygen laser cutting processes
- 2021
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Ingår i: IOP Conference Series: Materials Science and Engineering. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- Mill scale formed on the surface of hot rolled steels consists of magnetite (Fe3O4), hematite (Fe2O3) and wustite (FeO) layers, which can protect the steels from corrosion and other atmospheric effects. Existence of mill scale on the specimens' surface has shown to be able to decrease the cut edge quality. Since the mechanism behind influence of mill scale on the laser cutting process is unknown, this work performs direct observation of oxygen laser cutting processes on specimens with and without removed mill scale layers. Oxygen laser cutting processes were carried out using Ytterbium fibre laser 1070 nm along the edge of 20-mm-thick-steel specimens which were attached to a borosilicate glass. Focal point of the laser beam was positioned to be 0.7 mm below the specimens' surface. A high speed imaging system was arranged to face the glass, recording the cut front and kerf dynamics during cutting processes. It was found that cut front inclination angle increase when the mill scale was removed from the specimens' surface. This implies that mill scale on the specimens' surface seem to contribute in increasing the exothermal energy during laser cutting processes.
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| 8. |
- Dewi, Handika Sandra
(författare)
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Laser-induced phase transformations in microalloyed steels
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The application of thermal cycles below melting temperature can induce solid-to-solid phase transformation in steels, which is the transition between different crystalline structures of the same compound. There are many types of crystalline structures in steels produced, depending on the characteristics of the applied thermal cycle. For instance, rapid cooling can generate martensite structure that tends to increase the hardness of the steels, while slow cooling will more likely produce ferrite structure, which is less hard than the martensite structure. Laser heat treatment is one example where the laser becomes a thermal energy source, inducing thermal cycles below melting point and an extremely rapid cooling rate, which results in unexpected microstructures upon cooling. The mechanism of such phase transformations is still widely unknown, although the knowledge can be beneficial for many laser processes. Accordingly, studies on laser induced phase transformation are necessary.The purpose of my work is explaining underlying mechanisms of solid-to-solid phase transformation in microalloyed steels due to short thermal cycles of the laser heat treatment. My work aims to (1) find the correlation between energy input distributions from the laser beam and temperature history during the laser heat treatment process and (2) describe how changes in the thermal cycle induced by laser illumination influence the phase transformation dynamics. This work focuses on martensitic transformation and infrared laser (1070 nm).To explain martensitic transformation during laser heat treatment, this work involved ex-situ observations of the laser heat treated specimens. The study consists of varying the laser parameters, measuring the surface temperature of the specimens and simulating the in-depth temperature. Consecutively, characteristics (i.e., holding time, peak temperature, and cooling rate) of the measured and/or calculated thermal cycles were extracted, and the microstructures of the specimens were observed using microscopes. Finally, the thermal cycle characteristics and the microstructure of the specimens were related.The results show that the energy input distributions from the laser beam (e.g., laser beam profile) determine the geometry of the treated area, while processing speed and laser power influence the cooling rate and peak temperature of the thermal cycle respectively. The short thermal cycles induced by the laser beam are able to induce martensitic structure in the specimen. However, ferrite structure unexpectedly remains in the treated area. The holding time, which is the duration of temperature staying above austenisation temperature, has an inverse correlation to the appearance of ferrite structure in the treated area. This relates to the carbon diffusion occurring during the process, in which the carbon atoms have to diffuse from rich-carbon-austenite into low-carbon-austenite before cooling. Accordingly, the amount of martensite structure in the treated area depends on the holding time value of the process. There are indications that the rapid cooling induced by the laser beam can abruptly stop the diffusion process. It is clear that the laser provides an opportunity to control martensite structure.
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| 9. |
- Dewi, Handika Sandra, et al.
(författare)
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Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel
- 2020
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Ingår i: Optics and Laser Technology. - : Elsevier. - 0030-3992 .- 1879-2545. ; 127
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue property improvement for automotive components such as crankshafts can be achieved through material selection and tailored surface design. Microalloyed steels are of high interest for automotive applications due to their balanced properties, excellent hardenability and good machinability. Lasers facilitate efficient and precise surface processing and understanding the laser-material-property interrelationships is the key to process optimisation. This work examines microstructural development during laser surface treatment of 44MnSiVS6 microalloyed steel and the resulting mechanical properties. Laser beam shaping techniques are employed to evaluate the impact of beam shaping on the process. It revealed that ferrite structures remain in the treated area surrounded by martensite due to insufficient heating and dwell time of carbon diffusion.
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| 10. |
- Dewi, Handika Sandra, et al.
(författare)
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Short thermal cycle treatment with laser of vanadium microalloyed steels
- 2020
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Ingår i: Journal of Manufacturing Processes. - : Elsevier. - 1526-6125 .- 2212-4616. ; 57, s. 543-551
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Tidskriftsartikel (refereegranskat)abstract
- Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.
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