52 September/October 2023 | E-Mobility Engineering Challenges facing engineers can be very different, depending on their individual background experience and specialisation, according to Holtkamper. “However, what they have in common is the need to keep an eye on multiple welding technologies,” he says. “Laser, ultrasonic smart welding, wire bonding and others can be applied to most typical products, but only a deep evaluation of the alternatives helps to uncover big advantages for particular products or product variants.” The alternatives differ in their process requirements, and affect the choice of connector materials. For ultrasonic smart welding, copper alloys of a particular hardness are favourable for certain processes, Holtkamper says, while the clearance for top access for the welding tool must be provided and cleanliness ensured. For laser welding, however, surface requirements and maintaining zero gap between components to be joined are much more important. And as a fusion process, material combinations and safety issues have to be considered in design and production. Engineers are also increasingly faced with tight timelines to the start of production (SOP), Holtkamper adds. “The timelines to SOPs are shorter than ever, and collide with the long lead times of high-performance equipment,” he says. “In these situations, cooperative development and reliable relationships are of high value.” While there are many kinds of welding, in EV battery applications the most common are resistance welding and laser welding, along with ultrasonic welding and wire bonding, and benefit from standardisation for mass production. However, these techniques differ in terms of speed, creation of unwanted intermetallic compounds and process controllability, according to Holtkamper. Outliers include microTIG welding, ultrasonic-assisted laser welding and electron beam welding. Resistance is useful Resistance welding passes an electric current between a pair of electrodes and through the materials to be joined, relying on the heat generated by ohmic resistance to melt and fuse them. It is a proven and relatively cheap process, but has limitations in the applications and geometries to which it is suited. As a process that relies on physical contact, it requires the battery design to allow good access. It is not suited to seam welds, it is relatively slow – as explained above – and electrodes need to be maintained and/or replaced, and cannot easily handle conductive materials and increasing material thicknesses. The problem with highly conductive materials stems from the process’ reliance on resistance to generate heat. Contacts need to be highly conductive in service, so a balance has to be struck between conductivity and weldability of the joint. Light fantastic Laser welding has become the industry’s preferred option for speed and flexibility, notes Carr. Much of that flexibility comes from the ability to reposition the beam rapidly, using technology such as ‘galvo’ scanning heads, which use tiny electromagnetic actuators to angle beam-steering mirrors, Boyle says. Quantifying welding times for laser techniques, Holtkamper says they can be as short as a few milliseconds per cell. Modern systems also come with standardised clamp-and-weld functionality to hold the work as the joint is formed, improving repeatability, he adds, and can shape the beam to enable both spot and seam welding. Connections formed by lasers exhibit low electrical resistance, and the area around the weld affected by heat is small. While the high capital cost of laser welding is coming down, the process still has a number of other drawbacks. In addition to the extra complexity brought by the need for tooling to hold the work in place, it can be difficult to control the formation of the intermetallic compounds, such as aluminium-copper and aluminium-iron. However, this is true for all fusion welding processes, not just laser welding, Boyle points out. Further, capital costs grow with laser power, which also mandates more laser safety measures, and there is a need as well to control residues and spatter, and to minimise the risk of penetrating the cell, he adds. Insight | Battery welding Laser welding offers the flexibility to join thick and thin cell tabs made from materials including copper, aluminium, steel and nickel, as well as dissimilar metal combinations (Courtesy of Amada Weld Tech)
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