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Laser welding of ultra-high strength steel and a cast magnesium alloy for light-weight design
University West, Department of Engineering Science, Research Enviroment Production Technology West. (PTW)ORCID iD: 0000-0001-8933-6720
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There is a strong industrial need for developing robust and flexible manufacturing methods for future light-weight design. Better performing, environmental friendly vehicles will gain competitive strength from using light weight structures. In this study, focus has been on laser welding induced distortions for ultra-high strength steel (UHSS) where trials were performed on single hat and double hat beams simulating A-pillar and B-pillar structures. Furthermore, also laser welding induced porosity in cast magnesium alloy AM50 for interior parts were studied. For UHSS, conventional laser welding was done in a fixture designed for research. For cast magnesium, single-spot and twin-spot welding were done. Measurements of final distortions and metallographic investigations have been performed. The results show that the total weld metal volume or the total energy input were good measures for predicting the distortions within one steel grade. For comparing different steel grades, the width of the hard zone should be used. The relation between the width of the hard zone, corresponding to the martensitic area of the weld, and the distortions is almost linear. Additionally, compared with continuous welds, stitching reduced the distortions. For cast magnesium, two-pass (repeated parameters) welding with single-spot gave the lowest porosity of approximately 3%. However, two-pass welding is not considered production friendly. Twin-spot welding was done, where the first beam provided time for nucleation and some growth of pores while reheating by the second beam should provide time for pores to grow and escape. This gave a porosity of around 5%. Distortions and porosity are the main quality problems that occur while laser welding UHSS and cast magnesium, respectively. Low energy input seems to generally minimize quality issues. Laser welding shows high potential regarding weld quality and other general aspects such as productivity in light-weight design for both high strength steel and cast magnesium.

Place, publisher, year, edition, pages
Trollhättan: University West , 2019. , p. 94
Series
PhD Thesis: University West ; 29
Keywords [en]
Laser welding, ultra-high strength steel, cast magnesium alloy, light-weight design, automotive industry, distortion, porosity
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-13752ISBN: 978-91-88847-29-4 (print)ISBN: 978-91-88847-28-7 (electronic)OAI: oai:DiVA.org:hv-13752DiVA, id: diva2:1301218
Public defence
2019-04-24, F131, Trollhättan, 10:00 (English)
Opponent
Supervisors
Available from: 2019-04-02 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved
List of papers
1. Minimization of distortions during laser welding of ultra high strength steel
Open this publication in new window or tab >>Minimization of distortions during laser welding of ultra high strength steel
2015 (English)In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 27, no 2, SI, article id S29011Article in journal (Refereed) Published
Abstract [en]

Ultra high strength steels are frequently used within the automotive industry for several components. Welding of these components is traditionally done by resistance spot welding, but to get further productivity and increased strength, laser welding has been introduced in the past decades. Fusion welding is known to cause distortions due to built in stresses in the material. The distortions result in geometrical issues during assembly which become the origin of low joint quality due to gaps and misfits. U-beam structures of boron steel simulating B-pillars have been welded with laser along the flanges. Welding parameters and clamping have been varied to create different welding sequences and heat input generating a range of distortion levels. The distortions have been recorded dynamically with an optical measurement system during welding. In addition, final distortions have been measured by a digital Vernier caliper. The combined measurements give the possibility to evaluate development, occurrence, and magnitude of distortions with high accuracy. Furthermore, section cuts have been analyzed to assess joint geometry and metallurgy. The results show that final distortions appear in the range of 0-8 mm. Distortions occur mainly transversely and vertically along the profile. Variations in heat input show clear correlation with the magnitude of distortions and level of joint quality. A higher heat input in general generates a higher level of distortion with the same clamping conditions. Section cuts show that weld width and penetration are significantly affected by welding heat input. The present study identifies parameters which significantly influence the magnitude and distribution of distortions. Also, effective measures to minimize distortions and maintain or improve joint quality have been proposed. Finally, transient finite element (FE) simulations have been presented which show the behavior of the profiles during the welding and unclamping process. (C) 2015 Laser Institute of America.

Keywords
ultra high strength steel, boron steel, laser welding, distortions, finite element simulations
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-7648 (URN)10.2351/1.4906468 (DOI)000350544500020 ()2-s2.0-84943625500 (Scopus ID)
Available from: 2015-06-02 Created: 2015-06-01 Last updated: 2019-04-01Bibliographically approved
2. Correlation between laser welding sequence and distortions for thin sheet structures
Open this publication in new window or tab >>Correlation between laser welding sequence and distortions for thin sheet structures
Show others...
2017 (English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 22, no 2, p. 150-156Article in journal (Refereed) Published
Abstract [en]

Thin ultra-high strength steel shaped as 700 mm long U-beams have been laser welded in overlap configuration to study the influence of welding sequence on distortions. Three different welding directions, three different energy inputs as well as stitch welding have been evaluated, using resistance spot welding (RSW) as a reference. Transverse widening at the ends and narrowing at the centre of the beam were measured. A clear correlation was found between the weld metal volume and distortion. For continuous welds there was also a nearly linear relationship between the energy input and distortion. However, the amount of distortion was not affected by a change in welding direction. Stitching and RSW reduced distortion significantly compared to continuous laser welding.

Keywords
Automotive, Distortion, High strength steel, Laser welding, Welding sequence
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9678 (URN)10.1080/13621718.2016.1207046 (DOI)2-s2.0-84978705979 (Scopus ID)
Funder
VINNOVA
Available from: 2016-12-16 Created: 2016-08-10 Last updated: 2019-04-01Bibliographically approved
3. Metallurgical effects and distortions in laser welding of thin sheet steels with variations in strength
Open this publication in new window or tab >>Metallurgical effects and distortions in laser welding of thin sheet steels with variations in strength
2017 (English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 22, no 7, p. 573-579Article in journal (Refereed) Published
Abstract [en]

Geometrical distortions occur while welding, but the understanding of how and why they occur and how to control them is limited. The relation between the weld width, weld metal volume, total energy input, width of hard zone and distortions when laser welding three different thin sheet steels with varying strength has therefore been studied. Weld metal volume and total energy input show a good correlation with distortion for one steel at a time. The best correlation with the when including all three steel grades was the width of the hard zone composed of weld metal and the martensitic area in the heat affected zone. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.

Keywords
Distortion (waves); Hardness; Heat affected zone; High strength steel; Laser beam welding; Martensitic steel; Metallurgy; Metals; Sheet metal; Welding; Welds, automotive; Geometrical distortion; Good correlations; Steel grades; Thin sheet; Total energy; Weld metal; Weld widths, Steel metallurgy
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-10936 (URN)10.1080/13621718.2016.1275483 (DOI)000406522500004 ()2-s2.0-85009275116 (Scopus ID)
Funder
Vinnova, 2012-03656
Note

Published online: 12 Jan 2017

Available from: 2017-08-25 Created: 2017-08-25 Last updated: 2019-05-23Bibliographically approved
4. Effect of Laser Welding Parameters on Porosity of Weldsin Cast Magnesium Alloy AM50
Open this publication in new window or tab >>Effect of Laser Welding Parameters on Porosity of Weldsin Cast Magnesium Alloy AM50
2018 (English)In: Modern Approaches on Material Science, ISSN 2641-6921, Vol. 1, no 2, p. 25-32Article in journal (Refereed) Published
Abstract [en]

Pores in the weld metal lower the mechanical properties of the weld. It is therefore important to understand the pore formation mechanisms and find procedures that could reduce porosity. This study focused on laser welding of 3 mm thick magnesium alloy AM50, investigating how different parameters affect porosity formation. Low levels of porosity content were achieved by either increasing the welding speed or using a two-pass welding approach. It was found that higher welding speeds did not allow pores,which were pre-existing from the die-casting process, to have sufficient time to coalesce and expand. In the two-pass welding technique, pores were removed as a result of a degassing process which occurred through the second pass.

Keywords
Laser welding; Magnesium, Cast; Metallurgy; Porosity; Automotive; AM50
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-13754 (URN)10.32474/MAMS.2018.01.000106 (DOI)
Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved
5. Low Porosity in Cast Magnesium Welds by Advanced Laser Twin-Spot Welding
Open this publication in new window or tab >>Low Porosity in Cast Magnesium Welds by Advanced Laser Twin-Spot Welding
2019 (English)In: Materials Sciences and Applications, ISSN 2153-117X, E-ISSN 2153-1188, Vol. 10, no 1, p. 53-64Article in journal (Refereed) Published
Abstract [en]

Porosity is reported to be a major issue when welding cast magnesium. Therefore, it is important to understand the pore formation mechanisms and find procedures that could be used to reduce porosity. This study investigated the possibility of using twin-spot optics for reducing the porosity in laser welded cast magnesium. Two twin-spot welding setups were compared using either a beam splitter or twin-spot welding with primary and secondary (placed in front of the primary optic) optics. The results showed that welding with a dual optic setup with a defocused secondary beam reduced the volumetric porosity in the weld to 5%. The highest levels of volumetric porosity were 30%, and were a result of using the dual optic setup, but with a defocused primary beam. No clear relation between the level of porosity and power or welding speed was found. It was found that the amount of porosity depended on the balance of the energy input (controlled by defocusing) between the two beams. Porosity formation can be reduced if the energy from the first beam results in the nucleation and initial growth of pores. Reheating by the second beam then allows the pores to grow and escape from the molten material without melting additional base material. Furthermore, twin-spot welding is shown to be a promising combination of a production friendly solution and high quality welding.

Keywords
Laser Welding, Cast Magnesium, Twin-Spot, Metallurgy, Porosity, Automotive, AM50 Alloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-13753 (URN)10.4236/msa.2019.101006 (DOI)
Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved

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