A bright future for blue
Jean-Michel Pelaprat, co-founder of Nuburu, discusses the benefits of materials processing using emerging blue diode lasers with increased brightness
For decades lasers have offered unmatched flexibility for materials processing applications, but they have been capable of producing rapid high-quality welds in only a few metals. Industrially important metals such as gold and copper poorly absorb the infrared wavelengths of traditional materials processing lasers. Now, high-power, high-brightness blue lasers have demonstrated the ability to produce copper welds of unprecedented quality at unmatched speed.
In materials processing applications, efficiency and quality are determined by a combination of wavelength, power, and brightness. The wavelength comes into the picture because, as seen in Figure 1, every material demonstrates different absorption for light of different wavelengths. Aluminum, stainless steel, gold, and especially copper all absorb better in the blue than in any other visible wavelength, and more than ten times better than they absorb in the traditional industrial laser neighborhoods – around both 1µm and 10.6µm.
The power and brightness together determine the energy density that can be delivered to a target material. The key here is to match the energy density to an application’s needs. The beam-parameter-product of an optical system can’t be reduced, but it can be increased, so an ideal system will produce a higher energy density than the application requires. On the other hand, making the basic system too bright doesn’t help – it’s like putting a diesel locomotive engine in a lawnmower: you’re going to need so many modifications to reduce the output that you’ll spend more than necessary and still end up with an inefficient system.
Figure 1: Metals absorb differently at different wavelengths. For example, copper absorbs blue light 13 times more efficiently than it absorbs 1µm infrared radiation.
The fundamental mechanism of welding is the transfer of thermal energy to a workpiece, which melts the target material or materials. The energy transfer locus moves on, and the melted materials mix to create a joint. If too much energy is deposited into the material, it will produce miniature explosions that eject material outside of the joint and leave holes behind. Those defects are called, respectively, spatter and voids, and they degrade the mechanical strength and increase the electrical resistance of the joint.
Spatter and voids are unavoidable consequences of welding with traditional infrared lasers because the energy required to initiate a weld is much higher than that required to sustain the weld, so the infrared laser always delivers excess energy density. The same thing is true of any laser system that’s too bright. And in welding, it’s all about making defect-free joints.
In addition to making defect-free joints, the blue laser has another advantage: it can produce very compact joints of almost arbitrary geometries. Whereas alternative welding methods require a weld head of some finite dimension to be in contact with the material — and often require a unique head for each geometry — the blue laser just needs a few simple process adjustments to accommodate any geometry.
Two years ago, Nuburu introduced the first, 150W version of its blue diode laser, which was adopted almost immediately by battery fabricators due to its ability to create compact, defect free foil joints at a speed unmatched by any alternative approach. Two years later, we have introduced a second system that outputs 500W at 450nm through a 400µm core optical fibre. With a beam-parameter-product of better than 30mm.mrad, the system offers exceptional brightness. The firm also introduced a high-efficiency non-contact welding head this year, as well as a dual-lens assembly that efficiently combines the outputs of two 500W systems for a power-to-the-target of 1kW.
Nuburu’s blue laser design is based on combining 2D arrays of GaN (gallium nitride) blue diodes. The high power is achieved by individually collimating the beamlets and combining them with a combination of spatial interleaving and polarisation optics. Although the design is efficient, improvements are possible in all three aspects, meaning the next generation of refinements are already under way.
Figure 2: 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.
In addition, GaN diode technology is relatively immature. Current chips convert about 38 per cent of their input electrical energy into output laser energy. When the technology is mature, it’s expected to approach the 70 per cent efficiency of the GaAs (gallium arsenide) diodes used in fibre lasers.
The ongoing system design improvements and the improvement in output power from the chip-based packages are both leading to higher brightness systems. This is important because higher brightness leads directly to faster weld speeds and/or increased penetration depth, as shown in Figure 2. It also opens up the application space further: for example, our initial system is ideal for joining foils in rechargeable batteries, but the currently available higher brightness system can not only weld the heavier gauge leads that connect the foils within a battery, but also the busbars that connect those assemblies, as shown in Figure 3. In fact, the current high-brightness blue laser system can also weld windings within solenoids or drive motors and lighting assemblies – essentially every copper joint within a motor vehicle is now a candidate for high-quality, high-efficiency blue laser welding.
Figure 3: 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
Copper is not the only target material standing to benefit from the blue laser’s advantages. Aluminum, gold, stainless steel – even the tough problem of joining dissimilar materials benefits from the blue laser advantage. High brightness also opens up other materials processing applications, such as etching and cutting, so it appears as if the blue laser is poised to bring increased flexibility and efficiency to a wide range of industrial applications.
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