FEATURE: COPPER PROCESSING IN E-MOBILITY
gcontrol of the weld parameters. 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
A circular wobble pattern that can be used to mitigate spatter and defects when welding with a high-power and high-brightness CW fibre laser
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. 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
40 copper foils welded together using a blue diode laser
24 LASER SYSTEMS EUROPE SUMMER 2019
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.’ 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
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Nuburu
IPG Photonics
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