HYBRID WELDING
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
H
ybrid 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. Tis joining process allows arc welding and laser welding to be carried out simultaneously, in the same weld pool. Tere 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. Te hybrid process can produce high-quality
welds in a variety of materials and thicknesses, including steels, stainless steels, nickel alloys and aluminium alloys. Te 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.’ Te 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. Te additional material also improves the
weld profile to make it convex as opposed to a less 42 LASER SYSTEMS EUROPE ISSUE 31 • SUMMER 2016
JOINING GIANTS
Laser MIG hybrid welding can fill the gap between the two materials, making it suitable for joining large structures
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.’ Te 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. Te 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.’ Tere are also many benefits in terms of
productivity to the hybrid process, as Kirchhoff added: ‘Compared to conventional arc welding,
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TWI
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