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Aspects of Heat Flow in Laser Processing of Metals
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.ORCID iD: 0000-0002-9010-1555
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Since the laser was invented in 1960, its use in manufacturing industry has been growing rapidly. Laser processing of metals is based on the flow of heat that is generated by the absorbed laser beam. One outstanding aspect of laser beams is high precision along with high controllability of energy transfer, which includes creative techniques of shaping the beam and in turn the process. The thesis presents six Papers A-F on different metal processing techniques, namely welding, hardening and cutting, the latter combined with additive manufacturing. For each respective technique it was studied how desired properties can be optimized by controlled use of the laser beam and in turn of the temperature field. Addressing their different complexity of the heat transfer, various theoretical and experimental analysis methods were applied.

Laser beam welding is usually conducted with standard beam shapes, i.e. Gaussian or top-hat like, which is not always optimal for the process. Identification of an optimised weld pool shape or temperature cycle could increase the quality of welded products or even enable new applications. Papers A and B aim to increase the knowledge for non-standard beam shapes, particularly for single-pulse conduction mode welding. Paper A presents an investigation on an industrial application where a C-shaped weld joint is desired. The sensitivity to and optimization of different C-shaped beam irradiation profiles is discussed. The analysis is mainly carried out by applying Finite Element Analysis, FEA, to calculate the heat conduction contributions, showing unexpected sensitivity in certain regimes. Paper B presents a semi-analytical model for fast calculation of the temperature field from different beam profiles. Examples include multi-spots or the misalignment sensitivity of Diffractive Optical Elements.

In Paper C, for laser hardening of 11% Cr ferritic stainless steel the temperature field was studied to enable hardening. It was shown that single-track hardening without sensitisation could be achieved but overlapping tracks had a continuous network of ditched grain boundaries and is thereby at risk for sensitisation. The sensitised area is caused by a reheating cycle.

The same mechanism for the same material was studied in Paper D when applying a recently developed drop deposition technique, where additive manufacturing is fed by laser cutting. The same reheating isotherm becomes critical, but here sensitisation tests show a discontinuous network of ditched grain boundaries in the added material. The solid heat-affected zone on the other hand has a continuous network of ditched grain boundaries, which implies a sensitisation risk. The continuous network is however not in contact with the surface. The tested parameters is thus not at risk for intergranular corrosion through sensitisation.

For friction stir welding of dissimilar metals, Ti-6Al-4V with AISI 304L stainless steel, Paper E, the influence of a laser-induced preheating temperature field on the tool forces was investigated through numerical simulation. By suitable application of laser preheating, the forces acting on the tool can be substantially lowered, in a robust manner.

The temperature field from seam welding induces a residual stress field. In Paper F, for continuous wave laser keyhole welding of high strength steel butt joints, a method is presented to identify the residual stress behaviour of laser welded sheets by measurement of the fatigue crack growth rate during testing, by deriving the crack acceleration. The analysis was confirmed by hole drilling tests and by FEA.

The knowledge and methods of the above different experimental and theoretical studies complement each other. They contribute to further optimize certain aspects through laser-induced temperature fields, for different manufacturing techniques.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Mechanical Engineering Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-67586ISBN: 978-91-7790-051-1 (print)ISBN: 978-91-7790-052-8 (electronic)OAI: oai:DiVA.org:ltu-67586DiVA, id: diva2:1181658
Public defence
2018-04-11, E632, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2018-02-13 Created: 2018-02-09 Last updated: 2018-03-20Bibliographically approved
List of papers
1. Numerical optimization approaches of single-pulse conduction laser welding by beam shape tailoring
Open this publication in new window or tab >>Numerical optimization approaches of single-pulse conduction laser welding by beam shape tailoring
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2016 (English)In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 79, p. 48-54Article in journal (Refereed) Published
Abstract [en]

While circular laser beams are usually applied in laser welding, for certain applications tailoring of the laser beam shape, e.g. by diffractive optical elements, can optimize the process. A case where overlap conduction mode welding should be used to produce a C-shaped joint was studied. For the dimensions studied in this paper, the weld joint deviated significantly from the C-shape of the single-pulse laser beam. Because of the complex heat flow interactions, the process requires optimization. Three approaches for extracting quantitative indicators for understanding the essential heat flow contributions process and for optimizing the C-shape of the weld and of the laser beam were studied and compared. While integral energy properties through a control volume and temperature gradients at key locations only partially describe the heat flow behaviour, the geometrical properties of the melt pool isotherm proved to be the most reliable method for optimization. While pronouncing the C-ends was not sufficient, an additional enlargement of the laser beam produced the desired C-shaped weld joint. The approach is analysed and the potential for generalization is discussed.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-8128 (URN)10.1016/j.optlaseng.2015.12.001 (DOI)000369558900007 ()2-s2.0-84951067602 (Scopus ID)698c7324-1ed7-469f-bfdd-6d2f9ac24b38 (Local ID)698c7324-1ed7-469f-bfdd-6d2f9ac24b38 (Archive number)698c7324-1ed7-469f-bfdd-6d2f9ac24b38 (OAI)
Note
Validerad; 2016; Nivå 2; 20151228 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
2. Analytical heat conduction modelling for shaped laser beams
Open this publication in new window or tab >>Analytical heat conduction modelling for shaped laser beams
2017 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 247, p. 48-54Article in journal (Refereed) Published
Abstract [en]

Conduction mode laser spot welding and laser spot hardening usually employ Gaussian or top-hat-like beam modes. One main requirement of these techniques is the avoidance of overheating in the centre of the laser-material interaction zone. Process flexibility can be improved by spatially and/or temporally shaping the beam, which can enable higher process quality, robustness or speed. A desired spatial beam shape can be achieved by a suitably designed diffractive optical element. However, the prediction of a suitable beam shape for a particular process can be complex. A simplified analytical heat conduction model has been developed that can rapidly calculate the temperature field and cooling behaviour for almost any spatial and temporal beam shape. The potential and limits of the model are demonstrated and discussed by calculating and analysing temperature profiles for several cases of multi-spot welding

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-63056 (URN)10.1016/j.jmatprotec.2017.04.011 (DOI)000403133400006 ()2-s2.0-85018469004 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-05-11 (andbra)

Available from: 2017-04-18 Created: 2017-04-18 Last updated: 2018-07-10Bibliographically approved
3. Laser surface hardening of 11% Cr ferritic stainless steel and its sensitisation behaviour
Open this publication in new window or tab >>Laser surface hardening of 11% Cr ferritic stainless steel and its sensitisation behaviour
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2018 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 344, p. 673-679Article in journal (Refereed) Published
Abstract [en]

11% Cr ferritic stainless steel conforming to EN 1.4003 standard was surface hardened by a continuous-wave fibre laser beam. Both single-pass and multi-pass laser hardening was investigated. Different laser parameters were compared and their influence on hardness, microstructure, geometry of the hardened zone and sensitisation was investigated, especially for overlapping passes. The experiments showed that a surface hardness which is double that of the base material hardness was obtainable via martensitic phase transformation and high cooling rate, in spite of the low carbon and nitrogen content. This behaviour could be predicted from the chemical composition using the Kaltenhauser Ferrite Factor. Hardening at higher power levels gives more coarse-grained lath martensite but does not increase the hardness. Sensitisation was not a problem in single-pass hardening. However, the production of overlapping tracks could be detrimental to corrosion resistance in 11% Cr steel due to the formation of chromium carbides and nitrides.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Laser surface hardening, Ferritic stainless steel, Sensitisation
National Category
Mechanical Engineering Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-67581 (URN)10.1016/j.surfcoat.2018.04.002 (DOI)000437391300077 ()2-s2.0-85044924333 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-04-06 (andbra)

Available from: 2018-02-09 Created: 2018-02-09 Last updated: 2019-01-15Bibliographically approved
4. Sensitisation behaviour of drop-deposited 11% Cr ferritic stainless steel
Open this publication in new window or tab >>Sensitisation behaviour of drop-deposited 11% Cr ferritic stainless steel
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:ltu:diva-67585 (URN)
Available from: 2018-02-09 Created: 2018-02-09 Last updated: 2018-02-09
5. Numerical simulation of laser preheating of friction stir welding of dissimilar metals
Open this publication in new window or tab >>Numerical simulation of laser preheating of friction stir welding of dissimilar metals
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2018 (English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 23, no 4, p. 351-356Article in journal (Refereed) Published
Abstract [en]

Friction stir welding, FSW, of harder metal alloys is difficult to perform, like here dissimilar welding of titanium alloy to stainless steel in butt joint configuration. One major limitation is tool wear which can be reduced by preheating with a laser beam. A mathematical model to calculate the tool forces during FSW was developed further. The calculations show that the laser beam reduces forces at the pin and shoulder of the FSW-tool, accompanied by reduced heat generation through the tool. Within its operating limits, the process has low sensitivity on the lateral position of the leading laser beam. The model supports the understanding and optimisation of the complex interaction zone of forces and heat around the FSW-tool.

Place, publisher, year, edition, pages
Taylor & Francis, 2018
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-66366 (URN)10.1080/13621718.2017.1391936 (DOI)000435470000009 ()2-s2.0-85032491421 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-03-14 (andbra)

Available from: 2017-11-02 Created: 2017-11-02 Last updated: 2018-07-26Bibliographically approved
6. Identifying residual stresses in laser welds by fatigue crack growth acceleration measurement
Open this publication in new window or tab >>Identifying residual stresses in laser welds by fatigue crack growth acceleration measurement
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2015 (English)In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 27, no 4, article id 42002Article in journal (Refereed) Published
Abstract [en]

During laser welding, residual stresses are thermally induced. They can have strong impact on the fatigue behavior and fatigue life. A standardized measurement method for the fatigue crack growth rate was expanded to identify residual stress along the cracking path. The second derivative of the measured crack opening and in turn the crack acceleration corresponded well with distinct acceleration maxima and minima and accordingly with tensile and compressive stress, as was basically proven by numerical simulation. The method is simple and extendable. It provides valuable information, as was demonstrated for various situations.

National Category
Manufacturing, Surface and Joining Technology Applied Mechanics
Research subject
Manufacturing Systems Engineering; Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-10852 (URN)10.2351/1.4923472 (DOI)000362569000002 ()2-s2.0-84936882579 (Scopus ID)9bb76bdd-9c0b-4c71-8a47-fc833d81bb4e (Local ID)9bb76bdd-9c0b-4c71-8a47-fc833d81bb4e (Archive number)9bb76bdd-9c0b-4c71-8a47-fc833d81bb4e (OAI)
Note
Validerad; 2015; Nivå 2; 20151102 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved

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