| Abstract Scope |
Introduction:
Arc-based welding processes, such as gas metal arc welding (GMAW) or submerged arc welding (SAW), are widely used in heavy industries for joining thick-walled steels. However, the multi-layer technique used in these processes can result in productivity losses due to high welding times. Welding thick steels using multi-layer techniques presents process-specific challenges, including high heat input and resulting thermal-induced distortion. Therefore, alternative welding processes that offer higher productivity and efficiency are actively sought. Beam-based welding processes such as laser beam or laser hybrid welding provide a suitable alternative to arc-based welding processes. Laser-hybrid welding processes, in particular, are characterized by deep penetration welds and lower heat input, making them ideal for joining thick-walled steels.
Experimental setup:
The welding experiments were conducted in a flat position (1G) using a 20-kW-Yb fiber laser YLR-20000 with a 1064 nm wavelength and an 11 mm x mrad beam parameter product. The laser beam was transmitted using a 200 µm optical fiber, with a focus diameter of 0.56 mm and a focal length of 350 mm. The Qineo Pulse 600A welding machine was used in pulse mode at 180 Hz, with an arc leading orientation and a torch angle of 25°. The laser beam's focal position was set to -6 mm. To prevent root drops, an electromagnetic backing solution developed at the BAM research center was used. This solution involves the use of oscillating electromagnetic fields to generate Lorentz forces in the weld pool. The AC magnet was positioned 2 mm below the workpiece, with an oscillating frequency of 1.2 kHz and a magnetic flux density of 80-100 mT.
For the tests steel of grade S355 were used to investigate laser hybrid weldability up to 30 mm in a single-pass, with various seam preparations (sawed, plasma cut, laser cut and flame cut edges).
Results:
Sheets up to a thickness of 25 mm can be welded with a square groove butt joint. For thicker sheets (30 mm), a single-V seam preparation with an overall seam opening angle of 30° - 45° and a root face of 22 mm - 25 mm is preferred. These results show that thicker sheets can be joined with lower laser powers in a single layer. In addition, the experiments have shown that laser hybrid welding is less sensitive to gap and edge misalignment and that the quality of the plasma cut is sufficient to weld the joining partners without internal or external defects. High gap bridgeability of up to 1 mm and 2 mm was achieved with laser-cut and plasma-cut samples, which is a threefold increase in gap bridgeability compared to the state of the art. Even with 25 mm thick sheets, a gap of up to 1 mm could be safely bridged.
Additionally, the results indicate that the electromagnetic backing can lead to a more homogeneous distribution of filler metal throughout the weld depth and allow for the extension of process parameters to achieve desired cooling times. This approach also has a positive effect on mechanical properties, particularly notched impact strength. Using laser hybrid welding in heavy industries like wind tower manufacturing can provide economic benefits by increasing productivity, reducing the number of layers, and decreasing filler material and energy consumption. For example, when welding a typical 120 m high, 5 m diameter wind tower, SAW requires five to six layers of welding while laser hybrid welding only needs one pass. This results in over 80% reduction in welding time and up to 90% savings in filler material, flux, and energy costs, based on real industry data and material and energy cost in Germany.
Despite the advantages of laser hybrid welding, its industrial use is still limited to a few applications. However, it offers not only economic benefits but also ecological advantages. Studies have shown that laser hybrid welding has a better ecological balance sheet than arc-based welding processes. This is because laser hybrid welding requires less filler material, produces less welding spatter, and emits lower levels of hazardous substances. As a result, it contributes to a more sustainable production process, making it an attractive option for industries looking to reduce their environmental impact.
Conclusion:
Laser hybrid welding presents itself as a promising alternative to Submerged Arc Welding (SAW) for thick plate welding in multilayer technology due to its numerous benefits. One major advantage is the potential for reducing wind turbine tower welding time by over 85% or increasing productivity by a factor of 8. Moreover, laser hybrid welding allows for a substantial reduction in filler metal (wire and powder) and electricity costs by around 90%. Beyond its economic benefits, laser hybrid welding is also more environmentally friendly than SAW. However, the use of this process in industry is subject to process-specific challenges. To address this limitation, this study describes a novel non-contact electromagnetic backing that expands the potential of laser hybrid welding in the thick plate range of over 15 mm. The advantages of this electromagnetic bath support include the ability to work without contact, eliminating the need for mechanical attachment and removal, and allowing for the joining of sheets with a wall thickness of up to 30 mm in a single layer. The electromagnetic bath support also eliminates the need for time-consuming seam preparation, with a plasma or flame cut being sufficient. It can safely bridge gaps and edge offsets of up to 2 mm, achieve a more homogeneous distribution of filler metal over the entire weld depth, and extend process parameters to meet desired cooling times, positively impacting mechanical properties. |