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  • 1.
    Andersson, Oscar
    et al.
    KTH Royal Institute of Technology, Sweden.
    Fahlström, Karl
    RISE - Research Institutes of Sweden, Materials and Production, KIMAB. University West, Sweden.
    Melander, Arne
    RISE - Research Institutes of Sweden, Materials and Production, KIMAB. KTH Royal Institute of Technology, Sweden.
    Experiments and efficient simulations of distortions of laser beam–welded thin-sheet close beam steel structures2019In: Proceedings of the Institution of mechanical engineers. Part B, journal of engineering manufacture, ISSN 0954-4054, E-ISSN 2041-2975, Vol. 233, no 3, p. 787-796Article in journal (Refereed)
    Abstract [en]

    In this article, geometrical distortions of steel structures due to laser beam welding were analyzed. Two 700-mm-long U-beam structures were welded in overlap configurations: a double U-beam structure and a U-beam/flat structure. The structures were in different material combinations from mild steel to ultrahigh-strength steel welded with different process parameters. Different measures of distortions of the U-beam structures were evaluated after cooling. Significant factors of the welding process and the geometry of the structures were identified. Furthermore, welding distortions were modeled using two predictive finite element simulation models. The previously known shrinkage method and a newly developed time-efficient simulation method were evaluated. The new model describes the effects of expansion and shrinkage of the weld zone during welding and material plasticity at elevated temperatures. The new simulation method has reasonable computation times for industrial applications and improved agreement with experiments compared to the often used so-called shrinkage method.

  • 2.
    Fahlström, Karl
    et al.
    RISE - Research Institutes of Sweden (2017-2019), Materials and Production, KIMAB. University West, Sweden.
    Andersson, O.
    Volvo Cars, Sweden; KTH Royal Institute of Technology, Sweden.
    Melander, Arne
    RISE - Research Institutes of Sweden (2017-2019), Materials and Production, KIMAB.
    Karlsson, L.
    University West, Sweden.
    Svensson, L. -E
    University West, Sweden.
    Correlation between laser welding sequence and distortions for thin sheet structures2017In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 22, no 2, p. 150-156Article in journal (Refereed)
    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.

  • 3.
    Fahlström, Karl
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, KIMAB. RISE, Swerea, KIMAB.
    Andersson, Oscar
    Volvo Cars, Sweden.
    Todal, Urban
    Volvo Cars, Sweden.
    Melander, Arne
    RISE, Swerea, KIMAB.
    Minimization of distortions during laser welding of ultra high strength steel2015In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 27, no S2, article id S29011Article in journal (Refereed)
    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. 

  • 4.
    Fahlström, Karl
    et al.
    RISE, Swerea, KIMAB. RISE - Research Institutes of Sweden, Materials and Production, KIMAB.
    Persson, Kjell Arne
    RISE, Swerea, KIMAB.
    Larsson, Johnny
    Volvo Cars, Sweden.
    Vila Ferrer, Elisenda
    Gestamp, Spain.
    Evaluation of laser weldability of 1800 and 1900 MPa boron steels2016In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 28, no 2, article id 022426Article in journal (Refereed)
    Abstract [en]

    Ultrahigh strength steels are frequently used within the automotive industry. The driving force for use of these materials is to exchange thicker gauges to thinner and lighter structures. To get excellent strength and beneficial crash performance, the steel is microalloyed with boron which contributes to the 1500 MPa tensile strength. Increasing the carbon content will give superior tensile strength up to 2000 MPa. Welding of these components is traditionally done by resistance spot welding, but to get further productivity and increased stiffness of the structure, laser welding can be introduced. Welding of boron alloyed high strength steel is in general a stable and controlled process, but if increasing the carbon content quality issues such as cracking could possibly be a problem. In the present study, weldability of two different hardened boron steels with tensile strengths of 1800 and 1900 MPa, respectively, has been evaluated. Laser welding has been done in a lap joint configuration with 3.8-4.7 kW and varying welding speed between 3.5 and 5.5 m/min. As reference, results from more conventional 1500 MPa boron steel have been compared to 1800 and 1900 MPa boron steels to show the influence of the carbon content. Metallographic investigation, hot crack test, cold crack test, shear tensile, and cross-tension strength tests have been done. The results show that a weld quality similar to that for conventional boron steel can be achieved. Cracking and other defects can be avoided. As expected when welding martensitic steels, the failure mode in tensile testing is brittle. No weld defects have been found that influence strength. The sheet interface weld width, which together with stack-up thickness correlates with strength of the joint, could be increased by increasing the heat input and defocusing the laser beam. The effect of increased carbon content on weldability will be discussed more in detail, as well as the risk of cracking.

  • 5.
    Torkamany, Mohammad Javad
    et al.
    Tarbiat Modares University, Iran; Luleå University of Technology, Sweden.
    Kaplan, Alexander F. H.
    Luleå University of Technology, Sweden.
    Ghaini, F. Malek
    Tarbiat Modares University, Iran.
    Vänskä, Mikko
    Lappeenranta University of Technology, Finland.
    Salminen, Antti
    Lappeenranta University of Technology, Finland.
    Fahlström, Karl
    RISE - Research Institutes of Sweden, Materials and Production, KIMAB.
    Hedegård, Joakim
    RISE - Research Institutes of Sweden, Materials and Production, KIMAB.
    Wire deposition by a laser-induced boiling front2015In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 69, p. 104-112Article in journal (Refereed)
    Abstract [en]

    In laser materials processing the addition of material by wire is an option for techniques like laser welding, laser cladding or rapid prototyping. The stability of the wire deposition is strongly dependent on the wire interaction with the laser beam. For leading position wire feeding, high speed imaging was applied to study the melt transfer from the wire tip to the workpiece during keyhole welding. The observations revealed that a very stable concave processing front forms at the wire tip. A boiling front is established as an extension of the keyhole and the melt film at the front is sheared downwards by the ablation pressure of boiling. The deposition of the molten wire into the weld zone is smooth and controllable. Various wire front geometries and melt transitions are compared for different parameters. The option of laterally oscillating the laser beam is investigated and the interaction mechanism involved is discussed. Wire deposition by inducing a boiling front is explained here for the first time, which should promote future applications use of this very promising technique.

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