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Minimization of distortions during laser welding of ultra high strength steel
University West, Department of Engineering Science, Research Enviroment Production Technology West. Joining Technology, Swerea KIMAB, Kista 164 40, Sweden . (PTW)ORCID iD: 0000-0001-8933-6720
Volvo Cars, Torslanda 418 78, Sweden and XPRES, KTH Royal Institute of Technology, Stockholm 100 44, Sweden.
Volvo Cars, Torslanda 418 78, Sweden.
Joining Technology, Swerea KIMAB, Kista 164 40, Sweden and XPRES, KTH Royal Institute of Technology, Stockholm 100 44, Sweden.
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.

Place, publisher, year, edition, pages
2015. Vol. 27, no 2, SI, article id S29011
Keywords [en]
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: urn:nbn:se:hv:diva-7648DOI: 10.2351/1.4906468ISI: 000350544500020Scopus ID: 2-s2.0-84943625500OAI: oai:DiVA.org:hv-7648DiVA, id: diva2:815829
Available from: 2015-06-02 Created: 2015-06-01 Last updated: 2019-04-01Bibliographically approved
In thesis
1. Laser welding of ultra-high strength steel and a cast magnesium alloy for light-weight design
Open this publication in new window or tab >>Laser welding of ultra-high strength steel and a cast magnesium alloy for light-weight design
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
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:nbn:se:hv:diva-13752 (URN)978-91-88847-29-4 (ISBN)978-91-88847-28-7 (ISBN)
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

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