FEATURE
Issue: 

Ship Shape

Welding ship hulls is a relatively new application area for laser processing, but one where the laser can add value, as Rachel Berkowitz discovers

In the fourth millennium BC, sewing wooden planks together into a ship hull was the latest and greatest joining technology for Egyptian shipbuilders. Jumping forward a couple of millennia, the Vikings honed ‘clinker built’ boat building, overlapping the edges of hull planks and riveting them together to construct a seaworthy vessel. But it wasn’t until the twentieth century that the maritime sector took up laser joining technologies.

In 1992, the European shipbuilding industry published a feasibility study about the use of laser welding in shipbuilding[1]. Meyer Werft shipyard in Germany was, at the time, the only yard with industrial experience in laser welding techniques. Even then, CO2 laser welding was only used for pre-fabricating non-structural decks and internal walls for its enormous cruise liners. Strict regulatory requirements hindered its further adoption until research led to unified guidelines for the approval of CO2 laser welding in shipbuilding[2].

While a review from 2005[3] observed that ‘very few ship construction yards use laser welding, so it is clear that in shipbuilding the technology is still in its infancy’, the technology continues to gain popularity in shipyards. Now, research and development is bringing the advantages of laser welding systems to shipyards around the world. 

Laser-arc hybrid welding

Long before laser welding, arc welding offered the modern solution to joining in ship construction. By applying intense heat, metal at the joint between two parts is melted, and allowed to fuse together as it cools and solidifies. In arc welding, the heat to melt the metal is produced by an electric arc formed between the metal surface and an electrode. Still, arc welding is an energy-intense process and can lead to significant stress and distortion at the joints.

Using a laser beam as the heat source offers the advantages of lower distortion and higher welding speed. Hybrid laser-arc welding combines a laser beam with an arc welding process in a single weld pool. It provides the deep penetration of a thick metal offered by laser welds, along with better tolerance to joint fit and improved weld cap profile offered by arc welding.

Laser arc system adapted to weld shipbuilding steel. Credit: Turichin et al 2017

‘Hybrid laser-arc welding is a joining process that is beneficial for ship building applications, as it would reduce the costs associated with distortion, correction, and rework,’ said Paola De Bono, team manager of laser and sheet processes at TWI in Cambridge, UK. Over the last decade, numerous new laser concepts have been developed for materials processing. Fibre and disk laser sources are currently among the most promising in the materials laser welding market, thanks to their high beam quality, excellent scalability and high electrical-to-optical conversion efficiency, compared to CO2 and Nd:YAG lasers.

‘Hybrid laser-arc welding increases the geometrical precision of the ship body sections,’ said Mikhail Kuznetsov, research assistant at the Institute of Laser Welding Technologies at Peter the Great Saint Petersburg Polytechnic University in Saint Petersburg, Russia.

Along with Professor Gleb Turichin at the Saint Petersburg State Marine Technical University, Kuznetsov develops laser welding systems for a wide range of applications. Most recently, his group developed a laser arc technological system intended for agglomeration of panels during ship building. Their experimental studies test different joint geometries in high strength steels, in a portal system comprising a complete solution for welding panels. They demonstrated that the weld quality meets shipbuilding industry standards for panels of thicknesses 7mm to 45mm[4].

Their system uses a ytterbium fibre laser with maximum power of 16kW. The laser head and arc torch are fixed to a carriage that moves along the overhead crossrail of the portal at up to 10 metres per minute. All controls are automatic, and the system is capable of welding seams on panels 6 x 6m, but it ‘can be scaled to size’, added Kruznetsov.

For some configurations, the welding system yielded interior defects in the weld seam, but the researchers found that by minimising the distance between laser spot and arc, and using metal powder welding wire, they could achieve the mechanical properties required by the Russian Maritime Register of Shipping.

‘Using hybrid laser-arc welding in shipyards is new. It has not yet made its way into the world practice of the shipyards industry,’ said Kuznetsov. 

Different materials

It’s not just welding thick sheets of steel that’s important in the maritime industry. A push toward reducing fuel consumption has led to the incorporation of weight-reducing materials such as aluminium in shipbuilding, particularly for deck structures on top of a steel hull.

So far, different metals have been joined to one another using a separate adapter component. This is done via an expensive and complicated joining process called explosive cladding. Now, scientists and engineers are working with the shipbuilding industry to develop new methods for joining different metals.

Cross-section of T-joint weld. Credit: Turichin et al 2017

In 2015, ten partners of the Laser Welding of Steel to Aluminium for Applications in Shipbuilding (LaSAAS) research project began a collaboration with ship manufactures including Meyer Werft and Lürssen Werft to develop a laser process that would let them directly weld dissimilar joints of steel and aluminium.

The main benefit of laser welding for joining steel with aluminium is the ability to control the mixing ratio of the two melted metals. To obtain a relatively high quality weld seam in terms of imperfections and joint strength, the mixing ratio must consist of more melted steel than aluminium.

Led by Laser Zentrum Hannover (LZH), the LaSAAS industrial partners developed a laser processing head with which to control how deep the weld penetrates. That’s key to the process: with the steel sheet positioned on top of the aluminium, the operator can weld all the way through the steel and only a predetermined amount into the aluminium. The optimised penetration depth into the aluminium depends on the sheet thickness and laser spot diameter. They used a TruDisk 16002 disk laser with maximum beam power of 16kW.

‘The adapter welded with optimised welding parameters and a certain number of weld seams is able to exceed the yield strength of the aluminium by more than 152 per cent,’ said Rabi Lahdo, engineer at LZH.

The laser processing head controls the penetration depth using two different measurement principles based on: firstly, a non-contact optical sensing technology called short coherence interferometry and secondly, spectral analysis of emissions from the weld plume during welding. The operator can establish a clear relation between the characteristics of these spectral emissions and the penetration depth, as the intensity is indicative of the weld depth. During the welding process, the operator compares the spectral intensity with the intensity that has been determined to indicate optimal depth. The laser power is adapted to ensure a constant penetration depth.

‘Using the depth-control laser processing head, a consistent and comparatively high weld seam quality can be achieved independently of the weld speed, different material batches, [or other variations],’ added Lahdo.

Laser welded adapter of steel and aluminium for shipbuilding applications

The laser welding process has only been developed under laboratory conditions, however. ‘Our project partners are intent on transferring the process to the field of ship manufacturers,’ Lahdo explained. Key challenges will be to weld very long seams of dissimilar materials.

Laser welding dissimilar joints of steel and aluminium for shipbuilding applications in this thickness range is very new. ‘It’s hard to know what will work and what won’t long term,’ said Lahdo. The goal is to achieve the standards for dissimilar joints set by DNV GL, a Norway-based company that sets standards for ships and offshore structures.

Shipbuilding has its roots in human beings’ earliest transportation, exploration, and commercial efforts. It will be interesting for mariners and scientists alike to see how laser technologies shape the industry’s future.  

References

[1] S. T. Riches, J. Klaestrup Kristensen et al, Laser welding in ship construction phase 1 – feasibility study, TWI-UK/Force Institute-Denmark, June 1992.

[2] Guidelines for the approval of CO2 laser welding, Lloyd’s Register, March 1997.

[3] Christoph H.J. Gerritsen (TWI), David J. Howarth (Lloyd’s Register), A Review of the development and application of laser and laser-arc hybrid welding in European shipbuilding; paper presented at the 11th CF/DRDC International Meeting on Naval Applications of Materials Technology, Halifax, Nova Scotia, Canada, 7-9 June 2005.

[4] Turichin et al., Physics Procedia 89, 156-163 (2017).

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