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Laser welding and laser heat treatment of high strength steels
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0002-8298-292x
2021 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Laser-svetsning och värmebehandling av höghållfast stål (Swedish)
Abstract [en]

Laser materials processing, including thermal treatment and laser welding has been undergoing continuous growth in the manufacturing industry for decades. A laser beam offers high precision and energy transfer, capable of various processing. For many cases a Gaussian beam is applied, but lately development of more complex beam shapes has been developed, where e.g. multiple beams (beam splitting) can be used for increased tolerances during welding.

This thesis presents six papers (Papers A-F) on welding of high strength steels, laser pulse shaping, thermal treatments, and microstructural investigations. Different methods for obtaining a desirable weld were investigated through tailoring of the laser beam process. This affected the resulting temperature fields and thermal histories of the specimens. Experimental analysis was supported through various in-situ observation techniques and metallurgical studies.

Papers A-C present thermal processing and chemical manipulation to obtain the desired microstructure, by introducing and applying the here introduced Snapshot method. Paper A explores tailoring a laser pulse to mimic a hybrid welding process, Paper B elaborates the simulation to a multi-cycle process, and Paper C explores dilution. The manuscripts utilize a specialized experimental setup, optical analysis methods, and standard thermal measuring techniques. Metallographic analysis showed that thermal process optimization and/or dilution rate control during welding improved weld zone characteristics.

Improvements also include joint macrostructure characteristics, which are impacted by process stability, the theme of Papers D-F. Melt pool phenomena are studied in depth in Papers D and E. Paper D explores material ejections in a single beam welding scenario. Paper E investigates six beam shapes, from a single beam to a quad-beam arrangement. Paper F studies hybrid welding, a process that was simulated in Papers A-C but focused on the stability of the process instead of thermally guiding the microstructure. 

The studies complement each other in knowledge and methods. Welding of high strength steel is joining method-dependent, which imposes a unique thermal profile that affects the microstructures. The microstructure is also influenced by the chemical composition, an important point when multiple materials are used. The studies contribute an analysis of certain aspects of thermal and chemical effects of different laser-based processes to further optimize processing of specifically high strength steels, though the aspects can be generalized to other metals

Place, publisher, year, edition, pages
Luleå University of Technology, 2021.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Laser welding, heat treatment, high strength steel, Snapshot method, microstructure
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-84181ISBN: 978-91-7790-848-7 (print)ISBN: 978-91-7790-849-4 (electronic)OAI: oai:DiVA.org:ltu-84181DiVA, id: diva2:1553166
Public defence
2021-09-09, E632, Luleå, 13:00 (English)
Opponent
Supervisors
Available from: 2021-05-11 Created: 2021-05-07 Last updated: 2022-01-17Bibliographically approved
List of papers
1. Tailored laser pulse method to manipulate filler wire melt metallurgy from thermal cycles
Open this publication in new window or tab >>Tailored laser pulse method to manipulate filler wire melt metallurgy from thermal cycles
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2019 (English)In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 31, no 2, article id 022605Article in journal (Refereed) Published
Abstract [en]

A robust method is introduced to simulate and study the filler wire metallurgy for controlled cooling conditions after melting, enabling efficient mapping with prompt analysis of trends. Proposed is a reduced, though representative, process with more controllable conditions. Short lengths of filler wires are preplaced in a cavity, drilled into a base metal sheet. Irradiation by a pulsed laser beam melts the wire to generate a sample nugget. Pulse shaping influences the cooling rate, granting the ability to tailor weldament microstructures. The method is demonstrated for S1100QL steel and undermatched filler wire, to obtain high toughness for processes like laser-arc hybrid welding, where a representative thermal cycle is needed. For high toughness, a controlled amount of acicular ferrite and, in turn, nonmetallic inclusions is desirable. This “snapshot” method has revealed a characteristic histogram of inclusion sizes, for different pulse shapes. Additional information on the thermal cycle can be acquired by employing thermocouples, a pyrometer, or advanced methods like high speed imaging or modeling. The method offers a wide spectrum of variants and applications.

Place, publisher, year, edition, pages
USA: Laser Institute of America, 2019
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-75401 (URN)10.2351/1.5096145 (DOI)000484435200093 ()2-s2.0-85065039863 (Scopus ID)
Conference
37th International Congress of Applications of Lasers & Electro-Optics (ICALEO 2018), Orlando, FL, USA, October 14-18, 2018
Projects
OptoSteel
Note

Konferensartikel i tidskrift

Available from: 2019-08-06 Created: 2019-08-06 Last updated: 2021-05-07Bibliographically approved
2. Microstructures of high strength steel welding consumables from directed thermal cycles by shaped laser pulses
Open this publication in new window or tab >>Microstructures of high strength steel welding consumables from directed thermal cycles by shaped laser pulses
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2020 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 109, no 9-12, p. 2653-2662Article in journal (Refereed) Published
Abstract [en]

Filler wire metallurgy was modified through temporally shaped laser pulses, controlling cooling cycles in a recently developed method. Trends were identified through efficient mapping while maintaining representative thermal cycles of welding processes. A primary pulse melted preplaced filler wires while a secondary, linearly ramped-down pulse elevated the nugget to re-austenization temperatures. Ramped-down pulses resulted in linear cooling rates comparable with and exceeding furnace-based methods, between 50 and 300∘C/s. The linear decay of laser output power guided the temperature through a regime to obtain desired microstructures. For three very high-strength steel filler wire chemistries, quenching resulted in smaller plates with cross-hatched microstructures, accompanied by grain boundary ferrite. Finer bainite microstructures started forming for fast linear temperature decay, about 250∘C/s. Slower decay or a weaker third cycle formed coarser microstructures with coalescent sheaves and less cross-hatching.

Place, publisher, year, edition, pages
Springer, 2020
Keywords
filler wire, consumable, welding, thermal cycle, microstructure, cooling rate
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-76032 (URN)10.1007/s00170-020-05749-1 (DOI)000556174000002 ()2-s2.0-85088950305 (Scopus ID)
Projects
OptoSteelNorFaST-HT
Funder
Interreg Nord, 304-15588-2015
Note

Validerad;2020;Nivå 2;2020-09-01 (johcin)

Available from: 2019-09-24 Created: 2019-09-24 Last updated: 2021-12-13Bibliographically approved
3. Evaluation of pre-determined dilution of high strength steels by the Snapshot method
Open this publication in new window or tab >>Evaluation of pre-determined dilution of high strength steels by the Snapshot method
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2021 (English)In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 139, article id 106512Article in journal (Refereed) Published
Abstract [en]

Dilution is an unavoidable consequence of multi-material fusion processing, i.e. welding, cladding etc. In this paper we propose a novel method for controlled dilution experiments, analyzing microstructural trends of steel filler wire diluted with steel base metal. The highlight of this method is the control of processing conditions used to melt a pre-determined dilution of two high strength steels. The materials involved are S960QL base metal machining chips and a chopped under-matched wire consumable, which is used to increase the toughness of welded joints. These materials were combined in specific mass ratios in a prepared cavity and then melted by a pulsed laser beam. A high-speed RGB camera evaluated the relative spatial temperature of the melt surface. The molten mass then solidified into a uniform nugget, confirmed by energy dispersive x-ray spectrometry (EDS) to have a homogenous chemical composition (a ‘Snapshot’ nugget). Hardness values obtained for different dilution levels were compared to a narrow gap multi-layer laser weld (NGMLW), with a decreased dilution rate yielding a decreased hardness. The Snapshot microstructures created are similar to the different regions of the NGMLW, in the weld cap and in the body of the weld. Snapshot nuggets were also evaluated for non-metallic inclusion (NMI) size distributions relating to the dilution levels (NMIs are important indicators for acicular ferrite, which has been shown to increase impact toughness).

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
Dilution, Narrow gap multi-layer welding, Laser welding, Snapshot method, Energy dispersive x-ray spectroscopy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-82060 (URN)10.1016/j.optlaseng.2020.106512 (DOI)000614093000044 ()2-s2.0-85097904003 (Scopus ID)
Funder
Interreg Nord, 2014-2020European Regional Development Fund (ERDF), 304-15588-2015Vinnova, 2019-00781
Note

Validerad;2021;Nivå 2;2021-01-01 (johcin);

Finansiär: EC Research Fund for Coal and Steel (709954)

Available from: 2020-12-18 Created: 2020-12-18 Last updated: 2021-05-07Bibliographically approved
4. Multi-keyhole separation during multi-spot laser welding of duplex steel
Open this publication in new window or tab >>Multi-keyhole separation during multi-spot laser welding of duplex steel
2021 (English)In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 143, article id 107382Article in journal (Refereed) Published
Abstract [en]

Separated keyholes generated during a multi-beam laser welding process were observed, analyzed, and quantified. Two high-speed cameras were utilized to capture process phenomena of bead on plate experiments, with a near vertical and a horizontal view. The films were analyzed for: the ability to form a unique keyhole for each beam, the time and location of observed fully penetrated keyhole, and other associated trends. The number of beams was correlated to fully penetrated, separate keyholes. Keyhole separations were observed to be stable while full penetration was less regular. The location of a fully penetrating keyhole correlated with the position of the stronger beam. The dynamic melt wall width separating the keyholes was measured with beam orientation and power ratios between beams having a significant impact. Beam orientation showed that the lagging inter-keyhole wall of a quad-beam process was impacted by the presence of a fully molten front in the in-line beam arrangement. For the cross-beam orientation, the lagging wall did not seem impacted by the melt front. Large power differences between leading beams for the quad-beam, in-line orientation formed a melt column in the location of the weaker beam and spattering from the column.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
Multi-focal, Multi-spot, Multi-beam, Beam shaping, Keyhole dynamics, Laser welding
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-86334 (URN)10.1016/j.optlastec.2021.107382 (DOI)000687055900007 ()2-s2.0-85110145313 (Scopus ID)
Funder
Vinnova, 2019-00781
Note

Validerad;2021;Nivå 2;2021-07-12 (beamah);

Forskningsfinansiär: EC Research Fund for Coal and Steel (800726)

Available from: 2021-07-12 Created: 2021-07-12 Last updated: 2021-09-06Bibliographically approved
5. Material ejection attempts during laser keyhole welding
Open this publication in new window or tab >>Material ejection attempts during laser keyhole welding
2021 (English)In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 67, p. 91-100Article in journal (Refereed) Published
Abstract [en]

Material loss during keyhole mode laser welding often leads to surface imperfections that can negatively impact component performance. High-speed imaging identified four types of material ejection attempts: classical spatter, re-captured spatter, protuberances, and scalloping. The momentum attributed to the melt body, which is influenced by the keyhole properties, dictates whether the ejection attempt is successful or if the material is re-captured. The relationship between the dynamics of the keyhole and melt pool was elaborated in an extended systematic description of melt ejection attempts, which were then classified. Ejection attempts were often observed to be proceeded by a co-current swelling of the melt, adjacent to the keyhole, followed by a melt depression. The melt swell would elongate into a melt column with a concentration of momentum, where excessive momentum causes the melt to neck and separate into spatter. Trajectory determines if the spatter becomes a permanent fixture or re-incorporates into the melt body, with the latter having the possibility to cause further melt body disturbances leading to more ejection attempts. If the melt column fails to neck and separate, or an additional force acts upon the column, a protuberance or a scallop could then form. Keyhole and melt pool fluctuations were sometimes observed to be accommodated, avoiding material ejection. In these cases, a stable weld could be obtained with large variations in the dimensions of the melt pool and the keyhole.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
spatter, protuberance, material ejection, keyhole stability, melt pool dynamics, laser welding
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-83449 (URN)10.1016/j.jmapro.2021.04.052 (DOI)000658516300001 ()2-s2.0-85104925644 (Scopus ID)
Funder
Vinnova, 2019-00781
Note

Validerad;2021;Nivå 2;2021-05-06 (alebob)

Available from: 2021-03-30 Created: 2021-03-30 Last updated: 2021-06-17Bibliographically approved
6. Effects of surface and cut oxides on laser arc hybrid welding stability of high strength steels
Open this publication in new window or tab >>Effects of surface and cut oxides on laser arc hybrid welding stability of high strength steels
2021 (English)In: Article in journal (Other academic) Submitted
Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2021
Keywords
Laser beam welding, high strength steels, heat treatment, microstructure, Snapshot method
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-83451 (URN)
Available from: 2021-05-07 Created: 2021-05-07 Last updated: 2021-05-07

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