DIODE LASERS


application. If you look at copper applications, people are using 6-8kW and even 10kW lasers to join thick metals. We envisage needing a 1-2kW blue laser to process thick metals.’ Pelaprat noted that the blue laser’s speed


improvement and the higher quality welds it produces when machining thin metals diminishes for thick metals. ‘It won’t be eight times faster when welding 4mm-thick copper, as is the case for joining copper 0.5mm thick. However, it will still be faster,’ he said. ‘We don’t know how fast a blue laser can weld thicker metals because we haven’t tried it yet, but we anticipate this to be lower – it’ll still be more efficient absorption, but it will be less of a speed advantage at the same power level.’ Nuburu is focusing on copper processing at


the moment. Te first application it is targeting is for welding copper foils in lithium ion batteries. 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. ‘We have demonstrated extremely high


quality – essentially defect-free – welds of 40 foils at 2m/min,’ Pelaprat stated. ‘Tat is not possible with infrared lasers; fibre lasers cut the foil.’ Joining copper foils is carried out today


mainly by ultrasonic welding. Tis is a contact tool with a complex geometry, so only a


A 135W prototype blue diode laser developed by Coherent-Dilas


certain type of joining can be achieved. Ultrasonic welding also generates particles, which can contaminate the battery cell. ‘People are looking for a solution that will give defect-free, contactless welding,’ Pelaprat said. Using blue laser light creates a defect-free


weld with low thermal impedance, which reduces the temperature rise inside the weld. ‘Every application working with copper is


of interest,’ noted Ullmann at Laserline. ‘Tere is a lot of activity around fabricating batteries for electromobility. Te question is how to join copper with other metals, or, for example, weld 10µm copper foils, which is very difficult to do at the moment with an infrared laser. With a blue diode laser you can do heat conduction welding and get a very nice join. Also, it’s difficult to weld 300µm copper foils with infrared lasers. Tis can be done partly with a pulsed laser, but at limited speed. With the blue diode laser you can reach a few metres per minute welding speed.’ Te blue laser also minimises the


willing to start with lower power and slower speed with the understanding that power levels will increase


Customers are


Diode stacks Tere are different approaches to building high-power direct diode lasers, according to Ullmann. One is to use single emitters, which in the near infrared might emit 10-15W of laser light, and combined will give a few hundred watts of power. Te other approach is a diode bar, typically a 10mm-long chip that produces around 200-250W output power in the infrared. Te same methods can be used with blue diode lasers: blue single emitters have an output power of 3-5W, while the bar concept can produce 50W per bar, Ullmann said. Laserline bases its


technology on stacked diode bars with actively cooled heat sinks. Combining eight 50W bars, for example, will give 400W output power from a stack. Te laser light is combined by polarisation coupling, among other methods.


‘With 20 years’ experience of working with


Welding thin copper foils in batteries is one promising application for blue diode lasers


20 LASER SYSTEMS EUROPE ISSUE 38 • SPRING 2018


intermetallic when joining dissimilar metals, such as copper to aluminium, which increases the mechanical strength of the weld. Te intermetallic is a brittle alloy generated at the junction between copper and aluminium when welding with an infrared laser.


infrared diode lasers, we can transfer a lot of technology knowhow to working with blue diodes,’ Ullmann said. ‘Tere are some aspects that are new, but beam combining, stack combining and beam shaping are very similar to what we have done with infrared diode lasers.’ Coherent-Dilas is working with its T-bars


as a partner in the Blaulas project. Te firm’s core competence is building high-power diode


@lasersystemsmag | www.lasersystemseurope.com


Nuburu


Coherent-Dilas


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