Green light for green mobility

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Trumpf’s David Harvilla, Stefanie Bisch and Sebastian Zaske discuss why green lasers represent the perfect tool for welding e-mobility components made of copper

Better for the environment, less expensive than gas, less maintenance at a lower cost, quiet operation – these are but a few of the green attributes of the electric vehicle which make it an attractive alternative to the gasoline engine. Electric mobility is currently one of the biggest trends in the worldwide automotive industry. Fueled by a combination of technological progress, statutory requirements and evolving customer demands, electric cars are now on the verge of becoming a genuine mass-market mainstream offering.

This has prompted automakers, suppliers and machine makers to determine the best way to produce each e-mobility component in high volume and to a consistent high-quality standard. In some areas, no single manufacturing technology has emerged as an established standard. One example is the joining and welding of copper electrical components in the battery, motor and powertrain. However, the high-power green laser has proven to meet the stringent production requirements of high productivity and excellent quality welds, with reproducible results.

The green supremacy

Copper – and to a lesser degree aluminium – is an ideal choice for electrical connectors due to its high electrical and thermal conductivity. Copper conducts electricity and heat better than any other commonly used industrial metal. Unfortunately, it absorbs very little infrared light, which is exactly the wavelength regime in which most industrial lasers operate. Working at a wavelength of around 1μm, these lasers have a tough time processing copper. At room temperature, copper reflects 95 per cent of infrared light and only absorbs between 3 and 5 per cent.

As the temperature of the copper increases, so does the rate at which it absorbs infrared laser light, climbing as high as 20 per cent. Getting the welding process started requires significant energy input. The material heats up and as soon as the melting temperature is reached, the rate of absorption suddenly increases, producing a spike in energy that is difficult to control. This complicates the production process by causing a delay in the onset of welding that cannot be precisely reproduced, even when using identical parameters – so it is impossible to be sure in advance where the weld seam will start. In addition, the excess energy leads to erratic processes with spatter along the entire weld path. This is something that is completely unacceptable in a confined environment full of sensitive electronics, especially when manufacturing electric cars.

Figure 1: The surface of the copper makes no difference to green laser light. The results are the same for polished, sanded, untreated or etched surfaces – and even scratches and varying degrees of oxidation have no influence.

With green laser light, on the other hand, all these problems disappear. Scientists have long known that copper absorbs short-wavelength laser light significantly better than long-wavelength light. But only in the past few years have beam sources come onto the market that can deliver this light at the power levels required for industrial welding. Modern industrial disk lasers that operate in the green spectrum (515nm wavelength) can now deliver 2kW of power output. At room temperature, copper absorbs 40 per cent of this green laser light – a rate of absorption that is eight times higher than the near-infrared wavelengths. This significantly improves welding, leading to a much more stable process, smoother bead surfaces, low-spatter formation and more consistent penetration depths.

Green light – Cu for welding

Green light offers specific benefits when it comes to processing this challenging metal. For one thing, it enables users to carry out reproducible heat conduction welding of copper – something currently impossible with near-infrared lasers. Penetration depths of up to 500μm can be achieved in industrial settings using heat conduction welding with 2kW laser power.

Green light also makes the heat conduction welding process completely spatter-free, helping to create a smooth weld seam surface. Since there is no keyhole formation and therefore no metal vapour, the seam is free of pores. This increases the conductivity of the joint.

Deep penetration welding of copper is another area where the benefit of green light really shines. With a green laser power of 2kW users can achieve consistent weld depths of up to 1.5mm, which is unachievable with near-infrared lasers. What’s more, the higher absorption rate means welding can also be performed with defocused laser light. The green laser then produces a wedge-shaped keyhole that gets wider towards the top, creating a seam form that makes it easy for vapour to escape. This results in stable, low-spatter processes and an even weld seam surface.

Figure 2: Welding one hundred 6μm battery foils to a 2 x 0.5mm copper tab; right, a large, pore-free bonding surface and a smooth top bead with minimal spatter formation.

Another benefit to using green light is that relatively little heat is transferred to the part being welded. Absorption is high, which means processing time is relatively short, so minimal heat is lost into the material. The part distorts less and is subjected to a lower thermal load. What’s more, users do not have to resort to beam oscillation (wobbling) to achieve high seam quality, but can simply stick to linear welding. This allows for a high feed rate and thus higher productivity. It also makes the process simpler, because fewer parameters need to be defined. Not to mention that there is no need for more expensive beam delivery optics that oscillate or wobble the beam.

Numerous tests have demonstrated that surface conditions of the copper have no effect on results when welding with green light. In other words, it makes no difference whether the copper is polished, sanded, untreated or heavily oxidised. The weld depth or consistency is not affected by the degree of oxidation, superficial scratches or highly reflective, mirror-finished surfaces. In an industrial setting, this means that, in many cases, users can do without upstream surface preparation processes, such as sandblasting or tin coating, if they use a green laser.

As well as being an excellent choice for processing copper, green laser light is also suitable for other industrial metals such as aluminium, steel and gold. That makes the green laser particularly interesting for users who wish to process a range of materials with a single beam source.

Applications in e-mobility

The field of component manufacturing for electric cars offers numerous applications for powerful green laser light – from the battery and powertrain to the power electronics. Four key applications offer particular promise: Battery foil welding: battery cells consist of stacked layers of very thin copper and aluminium foils. Each foil is welded to a tab made of the same material before the cell is filled with electrolyte. Two key challenges arise during battery foil welding. First, air between the foils can potentially lead to well spatter. Second, to ensure that the foils (low thermal capacity) do not burn before enough heat has reached the tab (high thermal conductivity). Using green laser light, users can achieve a large, pore-free bonding surface and reduced spatter formation, especially with copper foils.

Welding cell connectors: the welding process used to connect battery cells often brings together different materials. A typical combination is welding copper to aluminium or vice versa. By creating a weld seam in a meandering motion using green laser light, users can achieve a large bonding surface with a high tensile strength of significantly more than 600 Newtons for connections between different materials. The high laser power enables high feed rates and high productivity, with minimal heat input to the underlying part. The high process speed also helps suppress the formation of undesirable intermetallic phases.

Figure 3: Busbar welding with copper sheets measuring 0.5 x 2mm: a consistent penetration depth in a welding time of just 0.42s.

Busbar welding: copper busbars are used in applications such as battery modules and power electronics. They are generally welded in an overlap joint. With a power output of 2kW, the green laser is the ideal choice for quickly and reliably welding copper sheets measuring 0.5 x 2mm, for example. To further increase the cross-section and thus the conductivity of the join, users can use a circular geometry, instead of a linear weld seam.

Welding contacts for DBC substrates: DBC (direct bond copper) substrates for power electronics are heat sensitive. They are generally contained in an extremely confined space, which makes access difficult. Scanning optics can be used to position green laser pulses precisely at the weld point. The rapid welding process means that very little heat enters the contact and the surrounding structure, so the thermal stress load is low. Penetration depth can be precisely controlled and reproduced – and the heat-sensitive DBC substrate remains intact while the contact is being formed. There is also significantly less spatter due to the green laser wavelength. Spatter can lead to short circuits and destruction of the entire component, especially in power electronics.

Green gives blue light the blues

In principle, similarly good results should be achievable with blue laser light (450nm wavelength), which is next to green on the wavelength spectrum. It makes no significant difference if green or blue light is used in terms of the resulting coupling behaviour in copper. But what does make a difference is the choice of beam source. Currently available beam sources for blue are based on gallium nitride diode material, which has not yet proven its durability for laser material processing in industrial settings. In contrast, disk lasers with a green wavelength use the same diode material as infrared disk lasers, which have been proven in the fi eld for many years. Green lasers are also superior to blue lasers, in terms of the power output currently achievable. However, the biggest advantage of green light from disk lasers compared to blue light from the diode is its signifi cantly better beam quality of up to 2mm*mrad with a small numerical aperture of 0.1. To obtain higher laser power output from blue diodes, manufacturers currently resort to spatial addition of diode material. However, this results in signifi cantly lower beam quality. For many industrial applications in electric vehicle construction – which require delicate weld seams and a correspondingly small focus – blue light is therefore not a viable option.

Figure 4: Connecting copper contacts to DBC substrates: green laser light offers a controlled penetration depth that keeps the DCB substrate intact. Scanning optics guide the light to the exact processing point, even when space is tight.

Yet the primary reasons for green’s supremacy over blue are practical ones: the high beam quality of green light hugely simplifi es the way machines are designed, and enables the use of focusing optics with a longer focal length. This means the optics do not have to be right above where the work is carried out. They can be guided above the workpiece at enough of a distance to allow easier access to narrow work areas, reduce contamination and extend their service life. The significantly longer Rayleigh length (larger depth of focus) paves the way for far greater tolerances, in terms of working distance. Green light requires less precision than blue light when it comes to both fixtures and part tolerances.

In addition, green lasers enable the use of scanner optics. Combined with long focal lengths, these provide a larger scan field. For example, programmable scanner optics with a focal length of 265mm cover a scan field of 140 x 100mm. The advantage is that neither part nor optics need to be moved. Moving the scanner mirrors is all it takes to process
a part – or even several parts at a time – and achieve high productivity. No additional axes are necessary.

Transmission losses for blue laser light in fibre-optic cables are higher than for green laser light by a factor of 1.4. This makes it easier to integrate green lasers in machines. It also makes it possible to use a single laser for several diff erent machines via multiple outputs, which increases productivity enormously. Green lasers also have the advantage of an extensive range of industry-standard optics, machines, process monitoring solutions and software, which have been comprehensively tested and are already available.

Based on these advantages and all the applications described, green laser light from industry-proven disk lasers is the perfect tool for all copper-processing manufacturing tasks, especially in the electric car industry.

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David Harvilla is lead instructor of laser technology at Trumpf Inc. Stefanie Bisch is product manager for high power green lasers at Trumpf Laser- und Systemtechnik. Sebastian Zaske is product manager for high power lasers at Trumpf Laser- und Systemtechnik.

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