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Interaction mechanisms for a laser-induced metallic boiling front
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. (Manufacturing Systems Engineering)ORCID iD: 0000-0002-4569-8970
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about fundamental interaction mechanisms of laser remote fusion cutting, RFC, which is based on the formation of a quasi-stationary laser-induced boiling front that causes drop ejection, preferably downwards. Laser cutting of metals, invented in 1967, has developed from a niche to a well established high quality cutting technique in the manufacturing industry. Usually a gas jet is employed concentric to the laser beam, to eject the molten metal. One technique option, interesting though hardly applied yet because of usually low quality and speed, is remote laser cutting. Two techniques are distinguished, remote ablation cutting, grooving down through a sheet layer-by-layer, and the here addressed remote fusion cutting, by a single pass through the sheet. For the latter, the ablation pressure from laser-induced boiling at the cutting front continuously accelerates and ejects the melt downwards. Advantages of remote laser cutting, facilitated by high brilliance lasers during the last decade, are the possibility of a larger working distance along with the avoidance of cutting gas and of a gas nozzle.

The review paper of the thesis surveys different laser remote cutting techniques as well as the transition to keyhole welding, owing to similarities particularly from the boiling front and from root spatter ejection. The six papers I-VI that compose the thesis address fundamental mechanisms of laser remote fusion cutting, RFC, theoretically and experimentally. In Paper I a simplified mathematical model of the RFC cutting front enables to estimate the geometrical and energetic conditions of the process. By evidence and post-modelling from high speed imaging, HSI, the simplified smooth cutting front model is developed further to a wavy topology in Paper III, for more sophisticated absorption analysis. As a systematic support, Paper II categorizes and analyses for the first time the different wavy topologies observed at the front, from HSI. The melt dynamics induced by a pulsed laser beam was studied in Paper IV, again from HSI. Apart from other interesting transient melt phenomena it was demonstrated that the ablation pressure can push the melt to a certain position during the laser pulse while the melt retreats by surface tension during the pulse break. To engage remote fusion cutting with additive manufacturing, Paper V introduces a novel technique where the drops ejected from RFC are transferred to a substrate, about a centimetre underneath, on which a continuous track forms. This technique can even be applied as an efficient recycling approach. In Paper VI a variant of the technique is presented, to develop a boiling front along the edge of a metal sheet from which the drop transfer takes place, in a different manner. This enables to machine-off the entire sheet, which can be converted to a new product and shape.

Summarizing, the thesis provides a variety of analysis of fundamental mechanisms of a laser-induced boiling front that bear a certain simplicity and in turn controllability, of interest for established as well as for new applications, in manufacturing and in other sectors, including remote fusion cutting.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keyword [en]
Laser remote fusion cutting, Boiling front, High speed imaging, Additive manufacturing, Ejection, Absorption, Wavy surface, Deposition
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-65281ISBN: 978-91-7583-945-5 (print)ISBN: 978-91-7583-946-2 (electronic)OAI: oai:DiVA.org:ltu-65281DiVA: diva2:1136417
Public defence
2017-11-09, E632, Luleå University of Technology,S-97187, Luleå, 08:30 (English)
Opponent
Supervisors
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2017-08-28Bibliographically approved
List of papers
1. Analysis of laser remote fusion cutting based on a mathematical model
Open this publication in new window or tab >>Analysis of laser remote fusion cutting based on a mathematical model
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, no 23, 233107Article in journal (Refereed) Published
Abstract [en]

Laser remote fusion cutting is analyzed by the aid of a semi-analytical mathematical model of the processing front. By local calculation of the energy balance between the absorbed laser beam and the heat losses, the three-dimensional vaporization front can be calculated. Based on an empirical model for the melt flow field, from a mass balance, the melt film and the melting front can be derived, however only in a simplified manner and for quasi-steady state conditions. Front waviness and multiple reflections are not modelled. The model enables to compare the similarities, differences, and limits between laser remote fusion cutting, laser remote ablation cutting, and even laser keyhole welding. In contrast to the upper part of the vaporization front, the major part only slightly varies with respect to heat flux, laser power density, absorptivity, and angle of front inclination. Statistical analysis shows that for high cutting speed, the domains of high laser power density contribute much more to the formation of the front than for low speed. The semi-analytical modelling approach offers flexibility to simplify part of the process physics while, for example, sophisticated modelling of the complex focused fibre-guided laser beam is taken into account to enable deeper analysis of the beam interaction. Mechanisms like recast layer generation, absorptivity at a wavy processing front, and melt film formation are studied too.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-4676 (URN)10.1063/1.4849895 (DOI)2a8887ff-52d9-4c15-83ac-114423a4e2cf (Local ID)2a8887ff-52d9-4c15-83ac-114423a4e2cf (Archive number)2a8887ff-52d9-4c15-83ac-114423a4e2cf (OAI)
Note
Validerad; 2014; 20140108 (ysko)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-09-14Bibliographically approved
2. Analysis of moving surface structures at a laser-induced boiling front
Open this publication in new window or tab >>Analysis of moving surface structures at a laser-induced boiling front
2014 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 317, 560-567 p.Article in journal (Refereed) Published
Abstract [en]

Recently ultra-high speed imaging enabled to observe moving wave patterns on metal melts that experience laser-induced boiling. In laser materials processing a vertical laser-induced boiling front governs processes like keyhole laser welding, laser remote fusion cutting, laser drilling or laser ablation. The observed waves originate from temperature variations that are closely related to the melt topology. For improved understanding of the essential front mechanisms and of the front topology, for the first time a deeper systematic analysis of the wave patterns was carried out. Seven geometrical shapes of bright or dark domains were distinguished and categorized, in particular bright peaks of three kinds and dark valleys, often inclined. Two categories describe special flow patterns at the top and bottom of the front. Dynamic and statistical analysis has revealed that the shapes often combine or separate from one category to another when streaming down the front. The brightness of wave peaks typically fluctuates during 20-50 μs. This variety of thermal wave observations is interpreted with respect to the accompanying surface topology of the melt and in turn for governing local mechanisms like absorption, shadowing, boiling, ablation pressure and melt acceleration. The findings can be of importance for understanding the key process mechanisms and for optimizing laser materials processing.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-11703 (URN)10.1016/j.apsusc.2014.08.190 (DOI)ab6d4dc5-7823-4b3c-8afc-bd3915f623a3 (Local ID)ab6d4dc5-7823-4b3c-8afc-bd3915f623a3 (Archive number)ab6d4dc5-7823-4b3c-8afc-bd3915f623a3 (OAI)
Note
Validerad; 2014; 20140910 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-09-14Bibliographically approved
3. Post-modelling of images from a laser-induced wavy boiling front
Open this publication in new window or tab >>Post-modelling of images from a laser-induced wavy boiling front
2015 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 357, no B, 2277-2284 p.Article in journal (Refereed) Published
Abstract [en]

Processes like laser keyhole welding, remote fusion laser cutting or laser drilling are governed by a highly dynamic wavy boiling front that was recently recorded by ultra-high speed imaging. A new approach has now been established by post-modelling of the high speed images. Based on the image greyscale and on a cavity model the three-dimensional front topology is reconstructed. As a second step the Fresnel absorptivity modulation across the wavy front is calculated, combined with the local projection of the laser beam. Frequency polygons enable additional analysis of the statistical variations of the properties across the front. Trends like shadow formation and time dependency can be studied, locally and for the whole front. Despite strong topology modulation in space and time, for lasers with 1 μm wavelength and steel the absorptivity is bounded to a narrow range of 35–43%, owing to its Fresnel characteristics.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-15906 (URN)10.1016/j.apsusc.2015.09.226 (DOI)f7a81341-7983-472b-80df-8d5a389de6a4 (Local ID)f7a81341-7983-472b-80df-8d5a389de6a4 (Archive number)f7a81341-7983-472b-80df-8d5a389de6a4 (OAI)
Note
Validerad; 2016; Nivå 2; 20151012 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-09-14Bibliographically approved
4. Transient interaction of a boiling melt with a pulsed Nd:YAG-laser
Open this publication in new window or tab >>Transient interaction of a boiling melt with a pulsed Nd:YAG-laser
2017 (English)In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 88, 28-36 p.Article in journal (Refereed) Published
Abstract [en]

The boiling front induced by a pulsed Nd:YAG-laser at very slow translation speed was studied. The purpose is to understand fundamental melt movement mechanisms. The melt was observed by high speed imaging, with and without illumination. When switching on the laser beam a hole is drilled through a bulk of melt. The hole expands and the boiling pressure gradually opens the melt bridge, instead developing an interaction front similar to cutting. These conditions remain in quasi-steady state during the pulse. The ablation pressure from boiling shears waves down the front and keeps the melt downwards in a stable position. When switching off, the waves smoothen and in absence of boiling the surface tension drags the melt back upwards, to semi-torus-like Catenoid shape. Evidence on the large melt pool and its shape was achieved by three-dimensional reconstruction from cross section macrographs. The basic findings how melt can move with and without ablation pressure can enable controlled melt dynamics for various laser processing techniques, like remote cutting, ablation, keyhole welding or drilling.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-2565 (URN)10.1016/j.optlaseng.2016.07.008 (DOI)000385319500005 ()02fc10a6-5cd3-4344-b21e-01891485dc6e (Local ID)02fc10a6-5cd3-4344-b21e-01891485dc6e (Archive number)02fc10a6-5cd3-4344-b21e-01891485dc6e (OAI)
Note

Validerad; 2016; Nivå 2; 20160816 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-09-14Bibliographically approved
5. Using laser cutting as a source of molten droplets for additive manufacturing: A new recycling technique
Open this publication in new window or tab >>Using laser cutting as a source of molten droplets for additive manufacturing: A new recycling technique
2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 125, 76-84 p.Article in journal (Refereed) Published
Abstract [en]

A new variant of additive manufacturing is proposed which involves transferring molten droplets via a laser beam to a substrate. The droplets are generated by laser remote fusion cutting of a supply sheet that could be a waste material, for recycling purposes. The laser-induced ablation pressure at the cutting front continuously drives melt downwards below the supply sheet in the form of a liquid column. Droplets separate from the column and solidify as a track on a substrate below. The droplets, surrounded by vapour, had in this case an average diameter of 500 μm and a speed of 2 m/s, with deviations up to 50%. Sound clad tracks were generated on steel and aluminium substrates. In the case of a copper substrate discontinuous clad tracks were produced as a result of poor wetting. The droplet jet had a small divergence of about 5°, which is suitable for controlled deposition. The transmitted part of the laser beam interacted with the clad track but did not affect the process result. High speed imaging was found to be a suitable tool for qualitative and quantitative analysis of the technique.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-62854 (URN)10.1016/j.matdes.2017.03.080 (DOI)000402490400009 ()2-s2.0-85016979838 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-04-07 (rokbeg)

Available from: 2017-04-03 Created: 2017-04-03 Last updated: 2017-09-14Bibliographically approved
6. Laser-induced boiling and melt transfer from a metal edge
Open this publication in new window or tab >>Laser-induced boiling and melt transfer from a metal edge
2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197Article in journal (Refereed) Submitted
Abstract [en]

A new technique for additive manufacturing was recently presented, by depositing droplets as a continuous track on a substrate, where the droplets were ejected from laser remote fusion cutting of a metal sheet. For the here presented approach, the droplets are instead ejected from the sheet edge, termed the machining mode, which is compared to cutting. Here the transmitted part of the laser beam does not hit and interact with the deposited track because of lateral dislocation. High speed imaging has shown that laser-induced boiling, which drives the melt downwards, causes asymmetric conditions in the machining mode by lateral pushing of the generated drop jet under the sheet, where the melt even can attach. Compared to machining, the cutting mode keeps less deviation of the drop trajectories, higher precision and a smoother surface finish. It was demonstrated that the edge conditions after machining are sufficient to repeat the process. This enables additional technique opportunities, including recycling of a whole sheet of waste metal. By the aid of high speed imaging from two different perspectives, the melt flow behaviour, the drop jet precision as well as process trends with respect to parameters, drop ejection and deposition were studied.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
additive manufacturing, melt drop ejection, laser-induced ablation, high speed imaging, metal recycling
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-64442 (URN)
Note

In the thesis

Interaction mechanisms for a laser-induced metallic boilling front

Available from: 2017-06-24 Created: 2017-06-24 Last updated: 2017-09-20

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