Battery welding | Insight E-Mobility Engineering | September/October 2023 55 resistance welding, or a high-power laser welder, according to Amada. Butt, fillet and lap welds in copper are routinely achieved up to and a little beyond a thickness of 0.02 in, says the company, which stresses the importance of using the welding system’s pulsation function to avoid porosity in the weld. Advancing processes Of these, laser and ultrasonic welding processes dominate in EV battery manufacture – with laser welding the preferred solution for mass production – and continue to be improved and refined. “We see a lot of laser welding and ultrasonic wedge bonding for the larger packs,” says Boyle. “If the packs or overall volume are smaller, then resistance welding is often used. MicroTIG comes up for specialised battery packs with low-volume production. “Recent developments in galvo scanning, beam steering and process monitoring allow for faster speeds and improved process error detection, respectively,” he adds. “We have added galvo scanning welding and process monitoring to our systems for battery pack welders. In addition, we have used lasers of more and more power.” Carr notes also that advances in laser welding have reduced the heat input into battery cells, enabling small spot sizes without sacrificing strength. He also points to the emergence of green and blue lasers, which work well on non-ferrous materials such as copper. “I can see them becoming more dominant, but the small spot of a scanning fibre laser is my preferred option for the next 5 years,” he says. When the welds required are long seams that have to go around corners, for example when sealing the lids on the cans of prismatic cells, conventional solutions such as moving the laser head or the work along a gantry, or using scanning mirrors to direct the beam, run into problems. A typical prismatic cell is about 20 mm wide by 300 mm long, and needs a continuous weld around the entire perimeter to provide a hermetic seal, even if the parts fit is less than perfect, as well as the durability to survive shock and vibration over years of service. To achieve that, the weld needs good penetration, minimal porosity and no spattering, as droplets of metal inside the cell casing can cause short-circuits, while it must also keep the resulting heat stress on the cell. Fibre lasers (FL) do all of that well, but Coherent points out that gantry systems are slow, while conventional optically scanned systems can suffer from weld variability as the spot changes from a disc to an oval shape of larger area as the beam angle shifts from the vertical towards either end of its scan travel, despite the use of lenses designed to minimise the unwanted effect. Coherent’s solution to this is its Fiber Laser – Adjustable Ring Mode (FL-ARM) technology. In a FL-ARM system, such as the company’s HighLight ARM, the beam has an additional outer ring of laser light that is concentric with the spot, and the laser power delivered to them can be varied independently using active closed-loop control to keep the energy that goes into the work much more consistent along the whole weld. As well as spot elongation, FL-ARM can also compensate for changes in scanning speed, such as when the beam is steered into and out of a corner. Weld penetration depth and seam can also be controlled independently so that there is no longer a need to maintain tight tolerances in parts fit, says the company, which reduces manufacturing costs. Other benefits include shrinking Laser-welded contacts on cylindrical cells with positive and negative poles on the cell tops. Controlling unwanted intermetallic compounds is important for weld quality (Courtesy of Hesse Mechatronics) Cell contact about to undergo an ultrasonic smart welding process, with the wedge-shaped transducer poised above the parts to be joined (Courtesy of Hesse Mechatronics)
RkJQdWJsaXNoZXIy MjI2Mzk4