Sparks, zap and smoke

01 Aug 2003

The next stage in the evolution process of cutting technology is laser hybrid welding, which uses a YAG laser combined with an arc system to offer high penetration welds with little distortion.
The ideal weld process for heavy sections will produce very narrow, very deep welds with minimal heat input, have very high welding speed, be tolerant of gaps, and produce a surface finish that needs no more than a coat of paint. Laser welding does this, but laser power is expensive while electric arc power is cheap, so the marriage of the two processes offers an attractive compromise.
Originally, laser welding was carried out using a CO2 laser, but this is restricted to simple straight line processes because the beam has to be directed by mirrors. The YAG laser gets around this by directing the beam using a fibre optic cable, but the strength of the laser is considerably less, thus affecting penetration.
Combining a laser beam and electric arc in an inert gas shroud has moved this closer.
David Howarth, head of materials and NDE at Lloyd?s Register, says laser hybrid welding is particularly attractive when building cruise ships where there are a high number of penetrations for pipework fitted at the block building stage. But, combined with an arc, there can be considerably improved penetration with minimal distortion.
However, because this is a specialised welding process, it comes under intense scrutiny by the classification societies. Because the depth of penetration of the weld is considerable and hidden from the surveyor, other techniques for weld approval that guarantee the dimensions of the weld have to be used.
As the process is automated, the guarantee can be achieved through close monitoring of the welding parameters, which can be shown to maintain penetration by sectioning a weld welded using those parameters.
Det Norske Veritas, the Norwegian classification society, has also been involved with laser hybrid welding. DNV has been involved in two projects: class approval of the laser hybrid welding procedures at Meyer Werft; and an EU project called Docklaser.
In the first DNV performed and/or audited the necessary tests for qualification of the welding processes at Meyer Werft.
The scope of the Docklaser project is to bring the technology one step further, by developing a hand-held device for laser hybrid welding, intended for use in dock area of a shipyard. Laser applications are to date limited to stationary systems in steel pre-manufacturing. DockLaser will develop mechanised mobile laser processing units for welding of long linear fillet welds with a tractor system, tack welding with a mechanical system and manually-guided welding or cutting. Savings of up to 50,000 man-hours are expected for a large cruise ship.
Another EU project, the Ship YAG project, has been running for about three years and is currently in its testing phase in Fincantieri?s Monfalcone yard near Venice in Italy. Dr Marco Morocutti, in charge of the project at Fincantieri, described the results as "very promising"; the system is composed of a gantry with robot installed. Fincantieri is using the equipment on flat panels (welding of the stiffeners) and Odense yard will use the same equipment on sub-assemblies.
Odense has been using CO2 laser welding equipment in its production lines for a few years ? for fillet welds for large panels on both containerships and naval vessels. The yard plans to install the demonstration YAG machine in the lab ? as close to production as possible, but not in production as it will only have the machine for three months.
In Sweden, ESAB and Permanova have brought their welding and robotised laser processes expertise to bear developing a laser-aided MIG/MAG welding systems for a wide range of applications including the shipbuilding industry. n

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