Some may consider laser materials processing to be relatively new. But the first gas-assisted laser cutting was demonstrated as long ago as 1967. Peter Houldcroft, deputy scientific director at The Welding Institute (TWI), showed that a combination of a focused laser beam and an oxygen assist gas could improve the precision and speed offered by thermal cutting processes.
The 300W CO2 gas laser used was initially developed for military applications and was operated at the Services Electronic Research Laboratory in Harlow, down the road from TWI in Cambridge. But industry interest quickly took hold and laser cutting is now a billion-dollar industry.
Fifty years on, significant development and innovation is still under way at TWI, working with industrial partners worldwide. By the late 1990s lasers were being used for welding complex geometries using articulated arm robots, plastics and technical textiles. Adaptive control of laser welding was introduced in 2005, and the introduction of techniques for handling thicker plate has since proved useful for nuclear decommissioning and welding heavy tanks, pressure vessels and thick pipes.
Laser welding of lightweight materials is particularly important in the automotive sector, driven by the need to boost performance and decrease the carbon footprint. Sullivan Smith, principal project leader and automotive programme manager for laser welding at TWI, points out that the first generation of weight reduction meant increasing the strength of steel. But as time has moved on, new materials are being introduced, such as aluminium in high-end vehicles, with increasing use of carbon-fibre composites on the horizon.
He says: “The automotive industry was set up for resistance spot welding and MIG/MAG arc welding of steels. The big challenge now is the move into complex mixtures of materials, where traditional welding processes aren’t applicable or the cost of application is high and production speeds are low. So we have had to find new joining solutions which can integrate steel, aluminium and potentially composites in a vehicle in a cost-effective way.”
Laser technology has been instrumental in reducing the cost of certain components. There have also been developments in resistance spot welding of aluminium, new fastening technologies using structural adhesives, friction stir welding and other specialised mechanical technologies.
TWI has been focused for many years on joining steel to aluminium using a range of technologies, including resistance welding, mechanical fastening, laser welding and friction-based welding techniques for high-volume manufacture.
Innovate UK-funded projects are under way with industrial partners, aimed at bringing composite-to-metal structures into automotive production lines using new fastening technologies. Smith says: “The initial approach to integrated composites has been focused on replacing one-for-one components, be it steel or aluminium. But in the future, components will be designed and configured for use of composites also.”
In the short term there is a focus on steel-to-aluminium joining and metal-to-composite joining. In the longer term, Smith sees more composite-to-metal processing, with modularised, smooth transitions between components.
Use of tailor-welded blanks is well established in steels and is growing in aluminium applications, in particular for small-series productions such as the Lamborghini Gallardo sports car. Laser welding is also used for stitch welding of the Audi A3 underbody. Hybrid laser-arc welding is used for improved fit-up tolerance and weld quality in the Audi A8. Laser brazing is used to join the Audi A3 roof to the side frame for a smoother joint as there is lower heat input. There are plans to use diode lasers, which are more efficient, cheaper and compact.
Two recent developments solve industry-specific problems. IPG Photonics has launched a laser stepper system that enables laser welds to be deployed in the same way as resistance spot welds for a car body application. The laser seam stepper has a novel in-built clamping and radiation shrouding system. IPG has also developed a three-spot laser brazing system for use on steel or aluminium seams to handle visible joints.
Tony Pramanik, project leader for TWI’s laser and sheet process team, highlights the reduction in the capital cost of laser equipment. “In the past, many industries considered laser welding too high a capital cost compared to arc-based processes,” he says. “As the cost has come down, industry is taking advantage of laser-based processing.”
Furthermore, laser processing offers significant advantages in terms of creating deep, penetrating welds for a given width. A 10kW laser system can produce welds up to 20mm deep, depending on what gas shielding is used.
“Most materials processing is performed using a circularly symmetric focused laser beam, delivered by a simple lens or mirror,” says Pramanik. “Laser beams in fact have a unique capability to tailor the energy in the focused beam to exactly that required for a process.”
As a result, TWI with industrial and research partners – Graham Engineering, Halitic, HOLO/OR, Impact, Nedinsco, VIP, EWF and Luleå University of Technology in Sweden – have been involved in a collaborative three-year project called TailorWeld. This is an innovative way of using tailored energy distributions for welding duplex stainless steel and other materials, using simple and robust Diffractive Optical Elements (DOEs). These have advantages for welding non-standard materials, as the applied beam energy can be shaped to a defined profile, sometimes involving multiple spots, for example.
TailorWeld technology can be used to weld printed circuit boards, using a DOE to simultaneously solder a number of components together. The technology is also used to weld nuclear waste containers made of duplex stainless steel, addressing problems with weld metal phase balance encountered using conventional welding. In this application a trailing energy distribution, positioned after the main welding beam, slows down the cooling rate after welding to control the weld microstructure. Bespoke multi-spot DOE systems are being produced by HOLO/OR.
TWI is involved in another project, LaserPipe, also funded by Innovate UK, in collaboration with OC Robotics, a Bristol-based small firm. The year-long initiative is designed to prove the feasibility of combining a small snake-arm robot with a high-powered, fibre laser welding source, for pipe manufacture and repair using multi-positional welds.
Pramanik says: “LaserPipe will help reduce process cycle time to minimise plant shutdowns, and can be used in nuclear plants for regular maintenance to replace corroded pipes, where external access is not always possible.” LaserPipe offers an autogenous laser welding process with improved tolerance to component fit-up.
In November, TWI with partners Graham Engineering, Portuguese component supplier Sodecia and CRF began the three-year ModuLase project funded by the European Commission’s Horizon 2020 Factories of the Future programme. ModuLase aims to develop a modular, adaptable process head where a single beam can be used for welding, cutting or cladding, in one flexible cell.
“There’s a gap in the market, particularly for small laser processing job shops, which could eventually realise more work by the use of such process tools,” says Pramanik. “The ModuLase system will give the operator an ‘on demand’ laser expert, by incorporating a process monitoring system that Spanish company Aimen is developing, to look at penetration depth, weld quality and other characteristics, then to adjust the beam semi-automatically.”
TWI also invented a technique called Clearweld that uses infrared absorbing dye to allow transmission laser welding of plastics, without one part being necessarily coloured with carbon black to absorb the laser beam. Ian Jones, principal project leader in laser technologies, explains: “This approach allows welding of multiple layers with the additional benefit that you are not restricted to the use of a particular colour of plastic in the joint.”
The latest version can handle micro-welds of just 500 nanometres in width. This technique is being used in medical and micro-fluidic applications, including stent rings for medical implants.
Research is also under way on plastics using Direct Laser Welding, first developed in the 1970s using a CO2 laser to weld plastic bags and thin fabrics, at very high speed. But the beam absorption on the polymer surface was so intense that the process was limited to welds of 0.1mm depth, before the material vaporised. Now, using a fibre laser with a different wavelength of 2-3microns, with much less intense absorption, laser welding can be carried out on plastic materials up to 5mm in thickness.
This process is suitable for butt welding and partial penetration stake welding to provide leak-tight joints. Direct Laser Welding is applicable to different product and lip designs for fluid containers, under-bonnet systems, medical devices and consumer items.
This is just a sample of the work at TWI, as new laser process developments benefit products from micro-scale to macro. “A revolution is under way in design capability,” says Jones.