FEATURE: COPPER PROCESSING IN E-MOBILITY
WELDING ACROSS THE SPECTRUM
Matthew Dale explores the range of laser solutions available for copper processing applications in e-mobility
that it suffers from almost no limitations, in terms of power.’ A high-power infrared laser can be used
‘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
22 LASER SYSTEMS EUROPE SUMMER 2019
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
“In the future, for thicknesses of copper below 4mm, green lasers will be the preferred solution”
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
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