FEATURE
Issue: 

Joining giants

Building large structures like ship hulls or railcars lends itself to hybrid welding, a combination of laser and arc welding. Gemma Church explores the technique

Hybrid laser welding exploits the best of both the arc and laser welding worlds, offering a high-quality joining process for large-scale applications such as shipbuilding and railcar manufacture. This joining process allows arc welding and laser welding to be carried out simultaneously, in the same weld pool. There are many different combinations of laser source and arc welding process that can be used, but the MIG (metal inert gas) arc welding process coupled with a laser source (including CO2, Nd:YAG, fibre and disk lasers) is a common industrial choice. 

The hybrid process can produce high-quality welds in a variety of materials and thicknesses, including steels, stainless steels, nickel alloys and aluminium alloys. The depths of penetration achievable in a single pass depend on the characteristics of the laser source used, but readily approach 10mm to 15mm. 

Marc Kirchhoff, industry management automotive at Trumpf, said: ‘In this process, the laser is used for realising a deep weld penetration, and the arc welding process for bringing in additional material for closing gaps or to influence the alloy composition of the material.’ 

The presence of a filler material is particularly advantageous for large-scale applications. One example might be to weld along a 25-metre seam. For an autogenous laser process – one which doesn’t use filler material – this could require a joint gap of less than 0.3mm to be maintained, which might simply not be possible along a joint of this length. Chris Allen, principal project leader at TWI in Cambridgeshire, UK, said: ‘If you use a hybrid laser welding process, you are adding in extra material, in the form of an arc-melted filler wire into the weld pool, and that material then allows you to fill up joint gaps while welding.’ 

TWI is an independent research organisation with expertise in materials joining. ‘We have done studies in the past to demonstrate that, using appropriate parameters for the hybrid process, you can probably increase the gap tolerance by a factor of three or four, depending on the materials and equipment used, when compared with autogenous laser welding,’ Allen said.

The additional material also improves the weld profile to make it convex as opposed to a less favourable concave or undercut profile, which can otherwise result in a reduction of static and fatigue strength. Allen noted: ‘If you are adding this extra material from the arc process in hybrid welding, then you can produce a more acceptably convex weld cap profile, and this can impart improved mechanical properties.’

The extra filler material also means the chemistry, microstructure and therefore properties of the weld can be altered to improve its quality and prevent, for example, hot cracks appearing in welds in some higher strength aluminium alloys.

The thickness of the material being welded within such large-scale applications also makes laser hybrid welding an ideal process, as Kirchhoff explained: ‘Especially in shipbuilding or heavy industry, where materials with a thickness of 5mm and more are used, it is very important to realise deep penetration and to close gaps.’ 

There are also many benefits in terms of productivity to the hybrid process, as Kirchhoff added: ‘Compared to conventional arc welding, hybrid welding is much faster and the energy input per unit length is much lower; therefore the thermal distortion is much lower. Furthermore, you can reduce the number of weld beads significantly and so reduce the production time.’

Aside from improved joint gap tolerance and the elimination of hot cracks, hybrid laser welding also facilitates other improvements in weld quality. As the weld pool is larger and the freezing and cooling process immediately after welding is slower than in laser welding, gas bubbles in the weld pool can escape more readily before the weld freezes. This can reduce weld porosity content.

The production of large-scale welds requires heavy-duty machinery. Trumpf recently opened a facility in Austria where a hybrid laser welding head from Fronius International is capable of joining sheets weighing 20 tons to produce huge frames for press brakes. The welds, which are up to 8mm deep, support loads of up to 320 tons of press force, repeatedly, over the lifetime of the brake press. 

Hybrid welding is a quicker process, because only a single pass is required, compared with conventional arc welding where you would build up the weld a bead at a time. The system can join more than 20 machine frames a week; the laser can also preheat the materials and, as the process is automated, quality assurance is more efficient.

In ship building, large steel plates, which are approximately 15mm thick and up to 30 metres long, are welded together. Hybrid laser welding is used as the gap between plates is simply too large to bridge using a conventional laser beam alone. The laser delivers the power densities needed to achieve high welding speeds and deep welds. As mentioned, this subsequently reduces the heat input and distortion, while the MIG torch bridges the gap using filler wire.

Mitsubishi Heavy Industries recently used laser-arc hybrid welding to reduce the thermal deformation and improve the cosmetic appearance of a passenger ship. The introduction of an 8kW IPG fibre laser at the Nagasaki Shipyard Koyagi factory raised the finishing precision of the hull block and decreased the need for additional working processes, such as on-site cutting to adjust and correct deformations in the weld, which improved productivity at the site.

Light on the tracks

Welded aluminium fabrications are also popular in industries needing to reduce the weight of certain components, for example in the automotive, aerospace and rail industries. 

Railcar structures can be made from extruded aluminium alloy sections, which offer lightweight performance and also have a specific stiffness and strength. The intense heat inputs of arc welding processes can lead to strength losses in these alloys, which are usually counteracted by local joint thickening. However, this adds weight to the railcar.

TWI developed a hybrid laser-MIG welding process capable of high-speed, low heat input welding of these alloys, with improved fit-up gap tolerance when compared with conventional laser welding. Ytterbium fibre laser sources were used in the development of this process, owing to their high wall-plug efficiency, small footprint and the processing flexibility from their optical fibre beam delivery. Allen added: ‘Another driver for developing hybrid welding for this application was to reduce the distortion that would otherwise result if arc welds were made over the very long lengths of joint made in railcar seam welds.’ 

A series of welding trials demonstrated that the process is capable of producing low porosity content, high-quality butt welds, at speeds of up to five metres per minute in 3mm thick material. Joint fit-up gaps of up to 1mm width with constant width gaps, or up to 1.5mm width with tapering gaps were also bridged successfully. 

Aluminium alloys are not the only materials that can be used to reduce weight: TWI is also investigating using hybrid welding with alternative grades of higher strength carbon and stainless steels.

Future tech 

There is still much research being done into both laser and hybrid welding. One hot topic is the development of real-time sensor systems to monitor and perfect the application of these joining processes. When laser or hybrid welds are being carried out, a change in a range of different signals generated by the process (for example, sounds, electrical charges or photo-emissions), can indicate that unacceptable defects are being produced in the welds, incorrect welding parameters are being used or joints are being welded that are out of tolerance. 

TWI is working with a consortium of research and industrial partners in the Radicle project (www.radiclelaser.com) – coordinated by the Manufacturing Technology Centre in the UK and standing for ‘Real-time dynamic control system for laser welding’ – to develop new combinations of laser welding sensors to detect these changes at frequencies approaching tens of thousands of times a second. The ultimate goal is to make such sensors fast and smart enough to adapt intelligently to control these processes in real time and improve the quality of the resulting welds.

The technology, although being targeted at laser welding, could then cross over to the hybrid laser process, as Allen remarked: ‘As that monitoring and control capability improves, there is no reason why we couldn’t then apply it to the hybrid process, to have real-time quality assurance and control of that process as well.’

Industry also continues to drive hybrid laser welding research and development in other directions. For example, the use of hybrid laser welding to clad surface layers onto substrates and, potentially, then additively manufacture structures, are possibilities.

A number of industries, among them the nuclear waste containment industry, are also pushing for more productive processes to weld thick-walled storage vessels for nuclear waste. Narrow gap multi-pass laser welding is one candidate for making such thick-walled welds, where a U-shaped joint profile can be filled with a number of overlapping passes. Allen said: ‘Existing narrow gap arc processes can be quite slow, and one way to increase the rate at which these joints can be made will be to hybridise the arc welding processes used with a beam from a suitable laser source.’ 

A great deal of research and development is being carried out to bring improvements to the weld quality and productivity in these large-scale industrial applications. Hybrid welding clearly has a bright and interesting future ahead of it, in terms of the processes, materials and areas to which it could be applied. 

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