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Welding across the spectrum

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Issue: 

Matthew Dale explores the range of laser solutions available for copper processing applications in e-mobility

‘No e-mobility without laser technology’. Such is the claim of the new application panel added to the programme of Laser World of Photonics in Munich this year.

The addition of the panel reflects the essential role that lasers now fulfil in the fabrication of electric vehicles – from the joining and cutting of the new materials used in their lightweight design, to the processing of key components for their motors and batteries. These components range from the hairpins used in electrical motors, to the layers of thin foils used in the creation of battery cells, and the many busbars and tabs used to connect the cells together.

Such components are widely made using copper, a material known for its excellent thermal and electrical properties. Despite these advantages, however, copper has proven to be particularly difficult to process with lasers, due to its high reflectiveness. A number of different laser solutions in the infrared, green and blue wavelengths have emerged in recent years to overcome this reflectivenesss in order to process this challenging material. 

Powering through

As might be expected from the dominant player in materials processing fibre lasers have extended their wide reach to encompass copper processing applications in e-mobility, which has led to a significant rise in demand – as described by Dr Jack Gabzdyl and Dr Ken Dzurko, of SPI Lasers, in our previous issue. 

This foothold in the e-mobility market was not secured without difficulty, however, as fibre lasers in particular – due to their use of the infrared wavelength – suffer particularly at the hands of copper’s high reflectivity when being used to process it.

‘The infrared wavelength is not ideal for welding copper, because it experiences a high level of back reflection – approximately 95 per cent – at the material’s surface at room temperature,’ explained Johannes Buehrle, Trumpf’s industry manager automotive - e-mobility. ‘The good thing about infrared, however, is that it suffers from almost no limitations, in terms of power.’

A high-power infrared laser can be used to achieve a very high brightness – in the range of megawatts per square centimetre – on a copper surface when a small spot size is used. At this level of brightness, the reflectivity of copper reduces to the point where it will couple the laser energy into the material, rather than reflect the energy away from it. A problem with this, however, is that when fusion of the material does happen, a surplus of energy flows through it, which can vaporise the material and create spatter, as well as bubble defects inside the weld joint. Such defects can increase the electrical resistivity of the copper.

This hasn’t stopped fibre lasers being increasingly used for copper processing in e-mobility, however, as Buehrle explained that there are multiple ways of overcoming these issues – one being through the use of Trumpf’s own BrightLine Weld technology. 

With BrightLine Weld, a laser’s power is coupled into a core fibre and a ring fibre simultaneously, to form a tiny spot and a larger spot respectively. The tiny spot can be used to penetrate the thickness of copper material involved in the weld, while the larger spot can be used keep the weld keyhole open at the surface. Keeping the keyhole open like this enables any gases created in the process to be released, reducing spatter projection and the development of bubbles.

Another way of reducing spatter and the creation of bubbles with a high brightness fibre laser is to first use a tightly focused spot to penetrate the surface of the copper, then use a technique known as ‘wobbling’ to move the beam laterally to the surface of the material. Moving the beam side-to-side like this ‘stirs’ the meltpool, enabling finer control of the weld parameters.

A circular wobble pattern that can be used to mitigate spatter and defects when wielding with a high-power and high-brightness CW fibre laser

Mark Thompson, director of sales at IPG Photonics UK, said: ‘Wobbling the beam is beneficial to balance laser brightness and the thermal input. High brightness overcomes the surface reflectivity, opening the weld keyhole. Maintaining a stable weld keyhole with wobbling reduces the presence pores in the weld joint, so better weld integrity is achieved.’

IPG weld heads enable wobbling to be done using a variety of weld patterns, Thompson continued, including lateral wobble, a circular wobble or a figure of eight. ‘Sophisticated wobble patterns enable improved weld integrity,’ he said. ‘The client has the opportunity to determine which wobble pattern provides the right trade-off between weld quality and weld speed for their application.’

IPG offers a welding head with built-in wobbling capability that can be used with a single-mode high-power fibre laser. ‘This combination can be optimised for the majority of copper welding applications,’ Thompson said. The single-mode fibre lasers IPG offers for copper processing in e-mobility have an average power ranging between 150w to 2kW. Thompson explained that this power range is routinely used to weld copper busbars, tabs, hairpins and foils. 

Beaming in blue

Jean-Michel Pelaprat, co-founder of diode laser manufacturer Nuburu, feels the wobbling technique is only a plaster for the issues caused by using infrared lasers, which he said works to some extent for copper thicknesses more than 1mm – below this, it doesn’t really have much effect.

‘While wobbling will minimise the issues caused by the poor absorptivity of infrared, the penalty is that you have to go at a much slower pace, due to the weld needing to be overlapped many times, so it takes much longer,’ Pelaprat said. ‘Time is money, which means the cost of the parts that take longer to produce will be more expensive, and even then you might still have defects.’

Nuburu’s diode lasers operate in the blue wavelength – around 450nm – which provides these systems with an immediate advantage over fibre lasers operating in the infrared when processing copper. This is because the metal has 65 per cent absorption in blue wavelengths versus 5 per cent absorption in the infrared – 13 times higher. This higher absorption means less power – almost an order of magnitude less, according to Pelaprat – is required to perform copper welding applications compared to a high-power infrared laser. This drop in power enables high-quality, defect-free copper welds to be achieved via heat conduction welding with a blue diode laser, which Pelaprat said leads to a dramatic increase in yield, that in turn reduces the cost of each part produced.

Blue diode lasers are absorbed extremely well by copper, enabling them to achieve defect-free welds using a low output power.

Nuburu has initially been targeting the welding of 8 to 15µm thick copper foils in the lithium ion batteries of cars with its lasers: inside a battery cell for a car there are multiple copper foils on the cathode side, interleaved with aluminium foils on the anode side. It is crucial to join the copper foils with a high-quality weld. Currently, this application is carried out mainly by ultrasonic welding, a contact tool that is not only limited to a certain type of join due to its complex geometry, but which also generates particles that can contaminate the battery cell.

‘This particular weld cannot be done with infrared lasers … [which] will actually cut the foils,’ said Pelaprat (IPG’sThompson would not reveal how the firm’s infrared lasers could be used to weld copper foils). ‘Continuous wave (CW) blue or green lasers, on the other hand, can be used to weld these foils very easily. This is an enormous advantage, as it enables a non-contact tool to be used for this process, so there is tremendous value in using high-power blue diode lasers to weld the foils.’ Up to 70 foils can be welded together and then to a copper tab in a single process using Nuburu’s blue diode lasers, which can also be used for the welding of copper tabs and busbars at thicknesses up to 0.7mm.

Last year Pelaprat told Laser Systems Europe that the reduction in power requirement when processing with blue diode lasers compared to infrared fibre lasers is also matched by an increase in speed when welding thin amounts of copper – up to eight times faster for 0.5mm welding thicknesses.

For larger thicknesses, approximately 4mm or higher, however, Pelaprat noted that the speed and quality of welds performed with blue diode lasers will actually diminish – although they will still be faster than those done using infrared lasers.

Not only does the welding speed of larger thickness of copper diminish when using blue diode lasers, but commercially available systems arten’t currently being targeted at such welding thicknesses.

‘For the welding of thick busbars between 5 to 8mm for example, power from 12 to 16kW is needed to weld at high speeds. No blue or green laser is currently able to offer this,’ said Trumpf’s Buehrle. ‘For these welding thicknesses an infrared laser is preferable, as while a high reflection is initially experienced at the surface of the copper, once the beam is inside the copper, it can penetrate to the bottom with high power to finish the weld.’

40 copper foils weldied together using a blue diode laser

Taking into account the smaller amount of power needed by blue lasers thanks to their higher absorption, Pelaprat envisages that a blue diode laser between 1 and 2kW will be required to process larger thicknesses of copper. Currently Nuburu offers blue diode lasers at 150W and 500W – the latter having been launched at Photonics West earlier this year. Pelaprat assured, however, that a laser in the kilowatt class will also be launched in the near future. ‘Every year we’ll be increasing the power further,’ he confirmed.

Nuburu also plans to increase the brightness of its lasers in the future through a reduction in spot size, which Pelaprat says will lead to larger thicknesses of copper being able to be welded, or an increase in speed when welding smaller thicknesses. In addition, he explained that the efficiency of blue diodes made from gallium nitride – which are used in Nuburu’s lasers – is increasing each year, therefore blue diode lasers with higher wall plug efficiency can also be expected in the future.

Diode laser manufacturer Laserline announced in February that its LDMblue laser is now able to offer up to 1kW output power, however the laser is currently being targeted at copper welding up to thicknesses of 0.5mm.

For applications involving larger thicknesses of copper, Laserline said it has two strategies planned which it will show to Laser World of Photonics attendees in June. In addition to a blue diode laser with even higher output power, the firm plans to reveal a hybrid laser able to use a combination of blue and infrared wavelengths to weld larger thicknesses of copper. Further details of both could not be released at the time of writing. 

Going green

In instances where material thicknesses are low and the need for control of heat input is high, rather than using high-power infrared lasers – which according to Buehrle are limited in their suitability to perform controlled welds depths of thin thicknesses of copper – Trumpf subsidiary SPI Lasers has developed a nanosecond welding process using a 100W pulsed fibre laser that can achieve excellent welds in 0.3mm-thick copper tabs.

As described by Gabzdyl and Dzurko in our previous issue, this technique enables multiple spots to be made to give appropriate bonding to the focus area using a spiralled spot. An issue with this process, however, is that while control of heat input and penetration is extremely high, the resulting welding time can be quite slow, due to the low average power used. For welding such copper tabs at depths from 0.2 to 0.5mm, Trumpf has developed its own solution that uses a CW green laser to perform either heat conduction or deep penetration welding. Similar to a blue laser, a green laser’s wavelength of around 515nm is highly absorbed by copper – around 40 per cent of its power is absorbed at room temperature. The laser can also be used for welding the layers of copper foils in battery cells.

‘Our green laser is based on our infrared disk laser technology. However, when the light exits the resonator, it is converted from infrared to a green wavelength,’ explained Buehrle. ‘The good thing about a disk laser is that its robust design prevents any back-reflected radiation from damaging the optics of the laser, which is a potential issue with fibre lasers.’

In addition, Beuhrle continued, Green lasers are offered in a compact cabinet, similar to a fibre laser but with integrated cooling.

Green lasers are suited to copper processing applications due to the material exhibiting high absorption in their wavelength

They can also offer several outputs from a single laser source, rather than a fibre laser, which needs external components to offer this capability.

Trumpf’s CW green laser – the TruDisk 1020 – provides both 1kW of power and a beam parameter product of 2mm.mrad, which according to Buehrle means a 50µm fibre can be connected to the laser, making it suitable for scanning applications. He added that in the lab Trumpf is already working with more kilowatts, so a green laser with more power will be available commercially in the near future. 

Buehrle believes that while infrared fibre and disk laser technologies are suited to a range of applications in e-mobility involving higher thicknesses of copper, for tab and foil welding, green lasers have the advantage and are the better solution.

‘I think in the future for thicknesses of copper below 4mm, green lasers will be the preferred solution,’ he said. 

‘The future will require green lasers of higher laser power in order to weld thicknesses exceeding 1mm, which is the current limit with a 1kW green laser. Green lasers up to 3kW in power would be suitable for welding copper thicknesses exceeding 1mm at a comfortable rate.’

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