FEATURE
Issue: 

Powering to stable welds

Moving to very high power lasers for welding metals, like aluminium in car production, has some big benefits, as Greg Blackman finds out

Fibre lasers are an efficient and reliable tool for welding car powertrains. (Image: Coherent Rofin)

Laser system makers will be showing their latest offerings for cutting, welding and other types of sheet metal processing at the trade fair Euroblech, in Hanover, Germany, from 23 to 26 October. Power is one selling point for these laser systems, as a more powerful laser will be able to machine faster, although it’s been argued, mainly for sheet metal cutting, that there’s no point in having a faster machine if the factory can’t keep up with loading and unloading the parts.

In terms of welding, however, experimental work from the Institut für Strahlwerkzeuge (IFSW), at the University of Stuttgart, shows that welding aluminium at high feed rates with 16kW of continuous-wave (CW) laser power – more than double the power car manufacturers normally use – gives a very stable join. The research suggests automotive manufacturers could take advantage of higher power lasers in the future to not only increase processing speed, but also produce more stable welds.

The IFSW team experimented with welding aluminium, but the group has also applied high-power laser processing techniques to joining copper, with similar success. Working with high laser powers is ‘especially interesting for all the metals with high heat conductivity’, noted Dr Rudolf Weber, who worked on these experiments at the IFSW.

In the research, published in the Journal of Laser Applications[1], the scientists joined two 1.2mm-thick sheets of AA6016 aluminium in full penetration welding, without filler wire, using a 16kW laser at a feed rate of 30m/min. Partial penetration depths of 2mm were generated at a feed rate of 50m/min. Welds were in the overlap configuration. A large laser beam diameter of 630µm meant welds could be made at 50m/min without humping, an effect caused by moving the beam very quickly through molten material, which creates a wave that solidifies into a hump.

Pores were not found in either full or partial penetration welds, and the joints were free of centreline cracks at narrow edge distances of 3mm for partial penetration and 4mm for full penetration. The researchers noted that this is a significant reduction in the critical zone for the formation of centreline cracks at close edge positions, compared to conventional laser beam welding parameters.

Speaking to Laser Systems Europe, Weber said that it’s well known that widening the capillary – a capillary is formed by the recoil pressure of the evaporating metal – makes more stable welds. ‘The major advantage for us was to see that even in CW mode you can produce this very long capillary with quite deep welds,’ he said. ‘This makes everything very stable and this only works with high average powers.’

The lightweight properties of aluminium make it an increasingly popular metal for automobile production, for body-in-white, for doors or bonnets, but also for smaller parts like gas filters and air conditioning tubes. There is also growing demand for laser welding of aluminium cans for batteries, as electromobility becomes a higher volume prospect.

Aluminium has traditionally been more difficult to machine with a laser than steel. Dr Wolfram Rath, product line manager for high power fibre lasers at Coherent Rofin, explained that molten aluminium has a high viscosity, close to liquid water, so it can flow away easily. Interaction with air and hydrogen can form porosity in welds, and the stability of keyhole welding is sometimes a challenge. In addition, layers of aluminium oxide forming on the surface can affect the process; the melting temperature of the oxide layer is higher than the melting point of the alloy itself. Many alloys can also crack under the heat applied by the laser, which is often influenced by joint design.

‘These are the challenges of working with aluminium,’ Rath commented, ‘but processes have been developed over the last few years that make laser welding aluminium robust and reliable in production, both with and without filler wire. Adapting the laser intensity and using oscillation techniques with scanners, as well as using the right protection gas, have all contributed to stable laser welding. Today you can laser weld aluminium with high efficiency in series production.’

The efficiency of modern fibre lasers operating at 1µm wavelength has been a big step forward for machining aluminium compared to older CO2 laser technology at 10µm – aluminium absorbs better at 1µm than at 10µm. Fibre lasers also have high beam quality, which means the process can be optimised.

Coherent Rofin has developed laser tools and techniques to improve aluminium processing results. ‘There are past examples where we had cracking, but where we were able to adjust process parameters to avoid this,’ explained Rath. ‘There are also examples for keyhole welding of battery housings where the process resulted in too much spatter, which wouldn’t have been accepted by the
customer today. A new intensity distribution and process adjustments makes the process more or less spatter-free. This was important for battery applications.’

Cutting and welding can benefit a lot from dynamic beam shaping, where the beam is oscillated at high frequencies. The laser spot is not static on the workpiece, but is oscillated either in a circle or in a figure of eight or other patterns. In terms of cutting, dynamic beam shaping gives a more homogenous distribution of laser power, which gives a better quality cut, along with higher speed, according to Dr Uwe Megerle, product manager for high-power scanners at Scanlab. The technique can also be used to cut thicker sheets, which, together with the higher speed, leads to increased productivity.

Scanlab’s WelDyna scanhead can oscillate the beam at high frequencies of 4-5kHz

Oscillation at high frequencies of 4-5kHz can be achieved with galvo scanners. This is one of the developments Scanlab has contributed to in-sheet metal processing with its WelDyna scanhead, Megerle said.

Dynamic beam shaping is also beneficial for welding metals; Megerle said it can help join aluminium to copper in battery welding, for example. ‘Joining these dissimilar materials in a conventional way with a static laser beam leads to hot cracks and bubbles in the melt pool, which decreases the quality of the weld,’ he explained. This can be avoided by oscillating the beam. The melt pool is extended and the bubbles that are created from degassing and the evaporation process are released from the pool, and solidification after melting is more homogeneous. ‘This is important to ensure before these laser welding processes [in battery manufacturing] can go into series production, which is what is happening right now,’ Megerle added.

The highest oscillation frequencies use lightweight mirrors, but only modulate the beam over a small field size. The beam can also be manipulated in remote welding or cutting, where there is a large standoff distance between the workpiece and the processing head. However, remote welding needs larger optics, which can be moved at up to 1kHz, but not the 4-5kHz that fixed scanners can reach.

Dynamic beam shaping is already widely used in production, in automotive production for body-in-white construction, for example, and for battery welding. There are a few industrial series processes using 4kHz oscillation, according to Megerle, although this is a newer technology compared to using a few hundred hertz.

Energy distribution of the laser beam can mean geometrically, but also temporally. Coherent Rofin’s Highlight FL-ARM (Adjustable Ring Mode) laser has an adaptive ring mode laser, where laser power is connected to the core of the fibre and different laser modules are connected to the ring of the fibre. The intensity distribution can be modulated by adjusting the power of the individual laser passes at up to 5kHz. Aluminium welding, in particular, can benefit from this technology, says Rath.

At EuroBlech, Coherent Rofin will show its CleanWeld technology, which combines all the technologies it has in its portfolio to optimise welding. These include: spatter-reduced welding, for gears but also for aluminium and copper; higher weld stability; keeping porosity low; and optimising the heat input to avoid cracking. ‘The technology portfolio we are using to achieve this is the focus geometry and focus caustic, including Adjustable Ring Mode; process head, including shield gas; and process parameter adjustments,’ Rath said.

Looking to the future

‘The laser industry has made big steps forward when it comes to aluminium machining – aluminium processing with a laser has become efficient and stable,’ Rath stated.

There are new laser technologies coming through, however, that could benefit aluminium processing and welding. High-power direct diode lasers are starting to become an industrial reality, and companies like Nuburu have released blue diode lasers that emit at 450nm wavelength, rather than the 1µm produced by a fibre laser. Aluminium is three times better at absorbing 450nm light than 1µm, and copper has 65 per cent absorption in blue wavelengths versus 5 per cent in the infrared, 13 times higher absorption. At the moment, Nuburu has a 150W blue diode laser module, with the intention to launch a 500W laser late this year and eventually go to multi-kilowatt power levels. The beam quality of these diode lasers is not as good as that produced by a fibre laser, so the blue diodes are most suited to welding applications.

Some of Nuburu’s customers using the laser for copper welding have reported a speed increase of eight to ten times versus infrared lasers, Jean-Michel Pelaprat, co-founder of Nuburu, told Laser Systems Europe earlier in the year. ‘In general, a 200-250W blue laser is equivalent to a 1.2-1.5kW infrared laser, in terms of welding speed for copper,’ he added.

However, most high-power direct diode lasers available don’t offer such a short wavelength. ‘To go from 1,070nm [fibre laser wavelength] to 940nm or even 808nm with direct diode lasers, does not affect the aluminium welding result significantly,’ commented Rath. ‘We [Coherent Rofin] have access to diode and fibre laser technology, and have done comparison studies. There might be some influence on heat conduction welding, but in keyhole welding the small difference of wavelength does not affect the efficiency or the stability of the welding process.’

The longitudinal section shows a smooth seam root and no process pores. This is due to the high feed rate of 50m/min, which stabilises the capillary during laser welding. The high laser power of 16kW allows such high feed rates

The high-power welding studies made by the IFSW at the University of Stuttgart, and others, is another future direction for manufacturers working with aluminium. Car body production usually uses lasers in the range of 6kW, sometimes a little bit less, according to Weber at IFSW. Moving to 16kW would be a big jump in terms of average power, but Weber feels that the system equipment would be similar.

‘It’s still fibre-delivered with quite long fibres; the fibre diameter is similar to what they are using, so from this point of view it’s not a big deal,’ Weber said. He noted that the scanning optics might need to be upgraded to handle higher power.

Changing production procedures in car manufacturing, however, is a lengthy process, as any new laser parameters have to be qualified to make sure they fulfil all the requirements of the joints regarding strength, oxidation, lifetime, stability against vibrations, and many others. If a process works for a certain model of vehicle, which the current laser welding procedures do, then it won’t change. ‘The automotive industry will think about changing to new lasers if they go to new [car] series – and if they see a benefit,’ Weber commented.

Rath remarked that there is still a question mark over whether welding at the high powers and high feed rates suggested by these studies is a good solution for industry. He said: ‘It’s hard for the equipment to achieve those high speeds, and the investment for the high power is relatively high. If you can provide a solution that runs at typical industrial speeds of six metres per minute, plus or minus 30 per cent, with the same stability, then you will win business. There might be certain applications, like tube welding, that run continuously where high speeds might be advantageous, but for body-in-white production I cannot see where the benefit is for such a large investment in high speeds.’

The work at IFSW did not use laser beam oscillation, because at 16kW with defocusing the seam quality was good enough to not need to oscillate the beam. ‘This is just what we’ve seen works,’ Weber said, adding that it will depend on the exact geometry of the weld as to the optimum laser parameters.

The IFSW’s work with copper showed that at average powers above 12kW the welds became stable without any additional laser beam modulation. ‘About 12kW was the limit with these parameters,’ Weber said, ‘below you get bad welds; above you get nice, stable welds.’

Weber concluded that, at the moment, most manufacturers welding aluminium have 6kW lasers, so it doesn’t make sense to develop the process for 16kW. ‘All we [IFSW] can do is show that if it comes to higher powers, [laser welding becomes] much more interesting.’

[1] Hagenlocher, C., Fetzer, F., Weber, R. and Graf, T. (2018) Benefits of very high feed rates for laser beam welding of AlMgSi aluminium alloys. Journal of Laser Applications, Volume 30, Number 1 

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