Taking the heat out of metal welding

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Greg Blackman speaks to two researchers who presented at the LANE conference on different aspects of laser welding

Of the areas where lasers are used in sheet metal processing, the automotive sector is one that has embraced laser technology, to a certain extent at least. Shipbuilding, on the other hand, has yet to invest in lasers to any great degree, although this is bound to change. Georg Cerwenka, a researcher at laser additive manufacturing institute Fraunhofer IAPT, believes laser processing could reduce the amount of rework time necessary in the production of a cruise liner by 30 per cent.

Cerwenka presented his latest work as part of the laser conference LANE, which was held virtually from 7 to 10 September. While being part of an additive manufacturing institute, his group is working on laser remote welding using a wide-screen scanner it has designed, and a 30kW laser installed at the institute. The scanner is built to handle 30kW of laser power, and is able to weld sheet plates at a distance of 1m over a 900 x 900mm2 area. It is a fibre-guided scanning device that can be manipulated by a robot or a gantry system.

The time and cost for rework in shipbuilding can be substantial, Cerwenka told Laser Systems Europe, with a lot of the work done manually requiring many years of experience. The group therefore worked to reduce the heat input into the weld seam, to lessen any distortion caused by heating the part – a higher quality seam needs less rework. This can be achieved by moving the beam fast over the workpiece. The group also wanted to weld over a larger area, in order to cut down on how often the laser must be stopped to change the part position and fixings. Remote welding tackles both points.

‘When joining a lot of large metal plates for ships, the best way to reduce heat input in the weld, and also weld longer seams without stopping, is to use a laser scanner,’ Cerwenka said.

He explained that distortions from putting too much energy into the part, particularly when working with thinner plate metals, is a major problem for shipbuilding, as distorted sections, when fitted together, can cause gaps of up to 5mm. ‘If you want to close this gap, you need many fixings and a large number of superimposed weld seams,’ Cerwenka said. ‘It makes sense to reduce the heat input to get a more precise part, and more precise production of sections.’ Using 30kW of laser power, the system can reach a welding speed of 10m/min.

Fraunhofer IAPT’s 30kW wide-screen scanner. (Image: Fraunhofer IAPT)

At LANE, Cerwenka presented simulations showing how to control the focus shift in the optical system. Laser optics can now withstand a lot of power, but they still heat up, which changes their refractive index, and therefore the focus position. A change in beam focus position results in poor welds or cuts, and therefore a lot of rework, or even wasted material. The metal plates used in the maritime sector, or railway engineering or civil engineering, are large and expensive, so it’s important to keep waste to a minimum, he said.

The group’s leader, Jörg Wollnack, has designed a software controller that manipulates the handling unit, the scanner and the laser. The remote scanner system is a gimbal-mounted solution with one deflecting mirror and linear drives. Almost all other scanners on the market are galvanometric scanners – they have galvo motors with two deflecting mirrors – and most are limited to 8kW of laser power.

Using a post-objective scanner design and only one scan mirror makes the behaviour of the optical system more stable, in that there is a fixed position of the laser beam on the mirror, rather than the beam hitting at various points on a second scan mirror. The focus shift can be calculated and corrected easily from this position. ‘We can precisely adjust our mirror and the laser beam on the workpiece, even with high inertia,’ Cerwenka stated.

‘[Using] the 30kW laser with the device results in a focus shift; we’ve designed software to overcome this,’ he continued. ‘We’ve modelled the entire optics setup, and with this knowledge and the laser energy distribution, we can calculate the focus shift. With the position of the sliding lens, we can adjust the laser position on the workpiece to account for focus shift in the optic. Over the whole process we always get a stable focus position.’

The focus position can be adjusted via a shifting lens over a 250mm range. It means curved contours can be welded without manipulating the part or scanner head.

The high powers that can now be produced by laser systems puts more strain on the optics. Talking about dynamic beam shaping systems used for cutting, Dr Uwe Megerle, team lead product management at laser scanning solution provider Scanlab, noted that ‘the extremely high laser powers of 10kW and beyond are a challenge for the scanning mirrors, especially since they need to be small in aperture to enable high oscillation frequencies.’ Many cutting processes can benefit from dynamic beam shaping to oscillate the laser spot on the metal sheet at kilohertz frequencies, which can increase the cutting speed and improve the cut quality.

Megerle added that advances in lightweight optical substrates and high-power coatings will push the limits of these scanning optics to match the laser powers that fixed optics can handle.

Scanlab provides dynamic beam shaping systems for laser cutting machines, plus OCT sensors for welding systems, where they can be used pre-process for seam tracking, in-process for measuring the penetration depth of the weld, and post-process for quality control.

One potential use of the Fraunhofer IAPT scanning system is to precondition parts, such as stitch welding two panels together. Stitch welding is used to hold two plate metal parts together until a second stronger weld seam is made.

Hybrid welding of sheet metal with the Fraunhofer IAPT’s 30kW laser. (Image: Fraunhofer IAPT)

It’s still early days in the uptake of lasers in shipbuilding, but two shipyards – Meyer Werft in Papenburg, Germany, and Fincantieri in Trieste, Italy – are using laser hybrid welding to make the panels that form the deck, hull or cabins. Hybrid laser joining is a combination of gas metal arc and laser welding. According to Cerwenka, the shipyards were able to reduce production time using lasers, and improve the quality of the part. He said that a manual welder can reach a maximum speed of around 0.5m/min; with the hybrid process the average speed is 2m/min.

The group at Fraunhofer IAPT are working to implement its focus shift algorithm in its controller software, and validate the system for cutting or welding samples.

Aluminium welds

While shipyards are still testing the water when it comes to laser processing, the laser is now an established tool for some automotive production steps. Aluminium car doors, for example, are now often laser welded.

Dr Christian Hagenlocher, a researcher at Institut für Strahlwerkzeuge (IFSW) at the University of Stuttgart, presented work at LANE on how laser parameters influence the grain structure of welded high-strength aluminium, and how this can be used to avoid hot cracks in laser beam welding of formed parts, which contain residual stresses.

The weld seams joining aluminium car door panels together are often made close to the edge of the metal sheet. Normally, laser welding closer than 6mm to the sheet edge will result in hot cracks forming, according to Hagenlocher, because of the way molten material solidifies and because of strain within the aluminium plate.

‘Hot cracks can be avoided by changing the thermomechanical conditions, increasing the edge distance, using a partial penetration weld with beam oscillation, or using fillet welds,’ he explained. ‘However, hot cracks can also be avoided by controlling the laser welding parameters.’

The work at IFSW sought to adjust the grain structure of the solidified aluminium weld by controlling the laser parameters. ‘Hot cracks propagate between grains,’ Hagenlocher continued. ‘If you control the grain structure, you also influence how susceptible the weld is to hot cracking. It’s not an optimisation by changing the alloy or the thermomechanical conditions; it’s an optimisation by changing the grain structure through the laser welding parameters.’

Investigations were carried out on the high-strength 6016 aluminium alloy. Hagenlocher and his group formulated an equation that defines how aluminium solidifies under different laser welding parameters – the group tested spot sizes between 50μm up to 600μm, and velocities from 0.1m/min to 50m/min. Running faster at higher power gives a finer grain structure when the molten material solidifies, which is less susceptible to hot cracking.

The equation that defined grain structure was then combined with fundamental solidification equations that describe the flow of molten material between metal sheets as the metal solidifies. ‘In this way we can now describe, in one equation, the influence of the laser parameters on the critical strain rate value,’ Hagenlocher said.

If a weld is affected by a strain rate above its critical strain rate value, hot cracks will form. ‘We can now describe the critical strain rate of a laser beam weld as a function of the line energy per welded depth, so we can predict how adjusting the laser parameters will affect how resistant the weld will be to hot cracking,’ he continued. ‘It’s now not only a property of the alloy or position of the weld, but a property of the weld itself, through the laser parameters.’

The team has also added the residual stresses from forming processes into this mix. The forming process of the sheet, deep drawing for example, gives it some residual stresses in the material. Welding the sheets together will release some of the residual stresses, which in turn leads to strain and affects solidification.
The equations were validated for 6,000 different aluminium alloys, but it depends a lot on the alloy itself, as to when hot cracks will form.

Along with presenting his work at LANE, Hagenlocher was also awarded the WLT prize at the conference for this work from the Wissenschaftliche Gesellschaft Lasertechnik, a scientific society for laser technology.

The IFSW team has worked with automotive companies and with alloy manufacturers. Hagenlocher said that alloy manufacturers are now designing alloys to avoid hot cracks for almost any welding parameters, but to apply standard alloys requires the laser parameters to be optimised.

Hagenlocher has also won a scholarship for a year at the centre for additive manufacturing at RMIT University in Melbourne, Australia, to apply these equations to additive manufacturing. ‘It would be beneficial to apply 6,000 high-strength aluminium alloys in additive manufacturing, but this involves a lot of solidification, and so it becomes important to control the laser parameters to reduce hot cracks forming,’ he said.

Trumpf and Fraunhofer IPA partner to develop AI solutions for sheet metal processing

Trumpf and the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA), have formed a research alliance that over the next five years will see artificial intelligence (AI) solutions developed for sheet metal processing.

The two partners will be increasing their research activities in the field of explainable artificial intelligence, or XAI. Their goal is to make the operation of neural networks interpretable, with new findings in this area expected to improve quality, save time and cut costs in sheet-metal processing.
Trumpf and Fraunhofer IPA initially joined forces to work on smart factory topics in 2015, with the initial results of this collaboration now being close to market.

Trumpf’s Sorting Guide assistance system, which is a result of the cooperation, helps workers remove and sort laser-cut sheet metal parts. (Image: Trumpf)

These include Trumpf’s Sorting Guide assistance system, which is designed to help workers remove and sort laser-cut sheet metal parts. The AI solution detects the part removal process and automatically supplies workers with all the information they need for intralogistics. The system highlights parts in different colours to show which ones belong together, either because they are part of the same order, are destined for the same customer or are heading to the same machine for the next step in the manufacturing process. This solution replaces the documentation that would otherwise accompany each part, saving time and helping to prevent errors.

The two partners hope to build on these initial successes by continuing their strategic cooperation in the future. Set to run until 2025, the alliance will see 10 employees from Trumpf and Fraunhofer IPA working together in a joint project, which will receive approximately €2 million of funding spread over the next five years.

‘Trumpf has been working with us on connected manufacturing for years because they share our view that Industry 4.0 developments represent a major opportunity,’ said Professor Thomas Bauernhansl, director of Fraunhofer IPA. ‘Everything depends on what happens over the next few years – so these are exciting times! We expect the coronavirus pandemic to act as a kind of catalyst: those who are well prepared will be perfectly placed to exploit the huge opportunities that lie ahead. Soon we’ll see whether we have laid the right foundations for the future in our joint projects.’

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