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Microstructures of high strength steel welding consumables from directed thermal cycles by shaped laser pulses
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0002-8298-292x
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0003-4265-1541
Lincoln Electric Europe, Nijmegen, Netherlands.
Lincoln Electric Europe, Nijmegen, Netherlands.
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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. Vol. 109, no 9-12, p. 2653-2662
Keywords [en]
filler wire, consumable, welding, thermal cycle, microstructure, cooling rate
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-76032DOI: 10.1007/s00170-020-05749-1ISI: 000556174000002Scopus ID: 2-s2.0-85088950305OAI: oai:DiVA.org:ltu-76032DiVA, id: diva2:1353846
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
In thesis
1. Laser welding and laser heat treatment of high strength steels
Open this publication in new window or tab >>Laser welding and laser heat treatment of high strength steels
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Laser-svetsning och värmebehandling av höghållfast stål
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
Laser welding, heat treatment, high strength steel, Snapshot method, microstructure
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-84181 (URN)978-91-7790-848-7 (ISBN)978-91-7790-849-4 (ISBN)
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

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Robertson, StephanieFrostevarg, JanVolpp, JörgKaplan, Alexander F.H

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