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Close-up on heavy steel welding technologies

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onsdag 31. oktober 2012

In several industries the welding paradigm is about to shift from conventional electric arc technologies to advanced beam technologies and this might also apply to thick-walled steel components for wind turbine foundations, substructures and towers. OceanWise takes a look on how three welding technologies work and how they would work for welding of the abovementioned wind turbine bodies: traditional submerged arc welding, the emerging technology - laser beam welding, and electron beam welding, which industry and academia use as a reference point for laser welding.

Submerged Arc Welding – the present
A common welding technology for heavy steel plates is submerged arc welding (SAW), which is by far the most widely used technology in the wind energy sector. In order to melt and fuse steel objects it uses an electric arc, produced by a low voltage (tens of volts) and a high current (ranging from hundreds and up to thousands of amperes). Often wind turbine components are welded using direct current, but alternating current may be used as well.

Submerged Arc Welding. Graphic: LORC

The arc is started at an electrode, meaning the tip of a consumable wire which is fed into the weld. To protect the melt pool against the chemically reactive influence of the ambient air, flux materials are added evenly as granulate, consisting mainly of calcium oxide, manganese oxide and silicon oxide. The flux melts and covers the entire welding area. It thermally insulates the area while also conducting the electric current, thereby enhancing the process. The melted flux material minimizes spattering, noise, fumes and suppresses the intense, harmful ultraviolet radiation from the electric arc.

Laser Beam Welding – the imminent
Laser welding is a technology that uses a laser beam to provide the weld pool joining the materials that are to be joined. The power density of a focused laser beam can be as high as 1 MW/mm2 depending on the power of the laser. Such concentrated power levels cause evaporation of the surface material it interacts with, but also allows generation of a weld pool at substantial depth beneath the sample surface.

Laser Beam Welding. Graphic: LORC

The weld pool is typically merely tens of a millimetre wide. This is an advantage as well as a drawback. Among the advantages is the fact that laser welding is fast, thus steel plates tens of millimetres thick may be welded together using only little filler wire, if any, whereas one drawback is the extreme cooling-rate (1000’s of K/s), which can cause cracks and brittleness if proper precautions are not taken. Another drawback is the need for high precision fixtures to meet the narrow gap requirements of less than a milimeter.

Research shows that the use of a vacuum chamber can further increase the laser penetration depth. (Source: Katayama Seiji et al. “Deep Penetration Welding with High-Power Laser under Vacuum”).

Of special interest for the heavy industries is hybrid laser welding in which the laser beam is combined with an electric arc and a fed-in wire. Such a combination has proven to combine the best of the traditional and advanced technologies. Laser sources can be of different types like solid state, gas or fiber lasers.

Electron Beam Welding – the experimental
Laser welding is often compared with electron beam welding. Both technologies are capable of delivering energy very focused on the work piece. Electron beam welding works by bombarding the work piece with an intense ray of electrons, accelerated by a strong electric field and focused with a magnetic lens. In principle, it works just like the now outdated television tube, but on a larger scale. The power density can be more than 1 MW/mm2.

Electron Beam Welding. Graphic: LORC

The kinetic energy of the electrons is converted to heat in collisions with the material to be welded resulting in temperatures beyond the evaporation point, even faster than laser welding. As in the old TV set, the electron beam is manipulated via a magnetic field. This must be performed in vacuum, which is one drawback of the electron beam compared with the laser. Other drawbacks are the extremely fast cooling-rate and the need for shielding of X-rays generated as the electron beam interacts with the work piece.

The need for the work piece to be located in a vacuum chamber together with the electron gun constitutes a practical and economical limit to the size of the welded objects.

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