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One wavelength, two approaches

Over the summer laser users learned how two types of blue diode laser are dramatically improving copper welding for fabricating e-mobility components

Among the many events that had to be cancelled or postponed due to Covid-19 was Lasys, the international trade fair for laser materials processing, due in Stuttgart, Germany, back in June.

With its postponement until 2022, Laser Systems Europe, together with Lasys organisers Messe Stuttgart, adapted the popular Lasers In Action forum – which usually takes place on the exhibition floor in tandem with the show – to an online format consisting of three webinars, ensuring laser firms could still share their latest innovations with industry users over the summer.

With e-mobility continuing to be a global megatrend, technologies for conducting laser processing in component production for electric vehicles were continually discussed throughout the webinars. Among these technologies were the two types of blue diode laser that have emerged in recent years to optimise copper welding for e-mobility applications.

Beaming in blue

Both the batteries and motors of electric vehicles comprise components made from copper, a highly reflective metal that has previously proven challenging to process using infrared fibre laser technology. This is because the material exhibits particularly low absorption in the infrared wavelength – approximately 5 per cent – which results in high laser power being required to initiate welding.

Once the copper transitions from being solid to liquid, its absorption rises significantly, leading to a sudden increase in power within the weld, that causes some of the metal to evaporate. This results in the generation of both low pressure bubbles – which stay inside the weld to create defects and voids – and high pressure bubbles, which burst and eject spatter. This leads to a sensitive process with low reproducibility that produces welds of weak mechanical and electrical performance. While wobbling techniques have been introduced to fibre laser processing to minimise this creation of bubbles and spatter, this has been known to lead to an increase in both processing time and equipment cost.

Lasers operating in the blue wavelength have emerged in recent years to address these issues. Operating at 450nm, the light of these lasers is absorbed up to 13 times more in copper than that of infrared fibre lasers – 65 per cent absorption compared to 5 per cent absorption. As a result, lower energy density is required to initiate welding when blue lasers are used. This dramatically reduces the heat impact of the laser, which minimises and even eliminates the presence of voids and spatter. These lasers can therefore be used to make fast, high-quality, defect-free welds in copper with little to no spatter. The absence of defects enables such welds to offer high mechanical and electrical performance.

Thanks to their exceptional copper processing capabilities, blue diode lasers are now finding applications in processing electrical components for e-mobility. Examples include the welding of foils within batteries, the welding of tabs to battery casings, the sealing of battering casings themselves, the welding of busbars to battery casings or other busbars, and the welding of hairpins in electric motors.

This joint, connecting 70 separate 8μm-thick foils with a 254μm-thick copper busbar, was produced in a single step with a blue laser weld. This is not possible with either ultrasonic or infrared laser welding. (Image: Nuburu)

In addition to performing welds containing a single type of metal, blue lasers are also well suited to welding dissimilar metal combinations, such as copper and stainless steel or copper and aluminium. These have proven difficult to weld in the past with fibre lasers, due to the creation of intermetallic phases, a mixture of metals that can severely impair the integrity of a joint. Using blue lasers, the creation of these intermetallic phases can be minimised, or eliminated, in highly uniform welds. This will have applications in the fabrication of electronics and lightweight structures in e-mobility.

Chip- or bar-based?

Presenters from blue diode laser makers Nuburu and Laserline took part in the webinars to share their firms’ most recent pushes in this emerging field. While they agree on the benefits that blue diode lasers can bring to materials processing, they take a different approach to delivering the blue light.

Richard Gleeson, director of European operations at Nuburu, explained to viewers how his firm takes a ‘chip-based’ approach to blue diode technology. This involves combining single gallium nitride (GaN) diodes and using individual lenses for each one, in order to correct for any pointing variability from diode to diode, and collimate them to the optimum brightness.

On the other hand, Laserline's sales director Markus Rütering described how they take a ‘bar-based’ approach to blue diode lasers. This uses a single lens to correct all the GaN diodes on a bar. Viewers were shown how each approach to delivering blue laser light carries its own set of advantages.

Gleeson, of Nuburu, explained that their chip-based approach enables higher brightnesses to be achieved compared to bar-based technologies. He gave an example using the firm's latest 1.5kW model, the AI-1500, which has a BPP (beam parameter product) of 11mm-mrad from a 100μm delivery fibre. He pointed out that comparatively, the best bar-based 1.5kW blue laser (that he was aware of) has a BPP of 60mm-mrad from a 600μm delivery fibre. This translates into a power density difference of 36 in favour of the chip-based approach. Gleeson also highlighted that the low BPP of the new AI-1500 makes it compatible with scanning technology, enabling it to be used for a wider range of applications.

How does higher power density affect weld performance? According to Gleeson, it enables both faster and deeper welds to be made in copper. He demonstrated this using a graph (figure 1) that compared two 500W lasers, one chip-based with a BPP of 30mm-mrad creating a 200μm spot, the other bar-based with a BPP of 60mm-mrad creating a 400μm spot. Using the same welding optics, the power density of the chip-based laser was four times higher than the bar-based one. The graph shows that at a similar welding speed, the chip-based laser's penetration depth was 1.6 times that of the bar-based laser, and that at the same penetration depth the chip-based laser was three to four times faster than the bar-based laser.

Figure 1: Higher brightness leads directly to increased energy density at the workpiece. For welding, higher brightness translates into faster weld speeds, increased weld penetration depth, or a combination of both. (Image: Nuburu)

Rütering, of Laserline, explained that the advantages of the bar-based approach is that it is both simpler and more cost effective than chip-based blue diode lasers. From his firms’ perspective, using individual lenses to correct each diode is too expensive. Instead, using fewer optics the firm is able to correct all of its emitters at once, rather than individually. With this approach, Laserline is able to couple 1.5kW of power into a 400μm fibre, which according to Rütering, can be used to address more or less all of the applications the firm wishes to target with its blue laser technology. He also highlighted that due to the size of spot required, having a higher BPP is not necessary when performing gap bridging, or when creating welds with large cross sections to ensure low electrical resistance in components.

Since the launch of its 1kW LDMblue laser in 2018, Laserline launched a 1.5kW system last year and a 2kW system this year – the LDMblue2000-60. Rütering hinted that attendees to the Laser World of Photonics next year (should the event go ahead as planned) can expect to see even higher power models, such as those that exceed the 3kW mark. He explained that this continually increasing power of blue diode lasers is enabling the technology to address a wider range of applications.

Rütering also pointed out to viewers that despite now offering blue diode lasers, Laserline has not dismissed infrared laser technology when it comes to copper processing. The firm has instead developed a solution that brings both blue and infrared wavelengths together, enabling the high-quality, spatter-free aspects of blue diode lasers to be combined with the large penetration depths enabled through using multi-kW infrared lasers. The ‘Hybrid Welding’ technique uses a large, blue spot to form and stabilise the melt pool, and then a centred infrared beam to create and maintain the keyhole. This enables welding depths larger than those achievable solely with Laserline’s blue diode lasers to be achieved. Rütering showed examples of copper welds up to 3mm in depth using a 1kW blue diode laser combined with a 1, 2 and 5kW infrared beam. Such welds could be made while minimising voids and spatter.

Laserline has developed a solution that combines the benefits of both blue and infrared laser technology. (Image: Laserline)

Future thoughts

Both presenters agreed that at the moment blue diode lasers are more expensive than infrared lasers, however the prices are coming down. While the maturity of infrared diode bar technology prevents it from getting any more efficient, according to Gleeson, GaN blue laser diodes do still have a long way to go in terms of wall plug efficiency, and the price of blue diode lasers will resultantly come down as this efficiency increases.

With regards to applications, both presenters hinted at additive manufacturing being another big future area for blue diode laser technology, with Gleeson highlighting that a smaller spot size could be particularly advantageous in this area.

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