As the vehicle market for advanced batteries develops, companies are realizing that new joining technology approaches are needed. Anecdotally, we have been told that the yield of good battery packs is often too low to be profitable long term.
For the past six months a team of engineers at EWI have been using a Department of Energy grant through The Ohio State University to investigate how easy or difficult it is to use laser, ultrasonic, and resistance welding to make the dissimilar materials joints required for battery pack assembly. Each of these techniques has advantages for making small tab to tab welds in packs, but each one also has limitations.
Resistance welding is used to make tab connections in packs for many industrial products. Welding copper and aluminum is difficult however because both these metals have low electrical resistance and high thermal conductivity. As a result, it is difficult to heat them enough to make a strong weld. Lasers can efficiently apply heat, but because laser welding is a fusion process, brittle, high resistance intermetallic compounds can form at the bimaterial interface creating weak, high resistance joints. Ultrasonic welding provides a solid state process to make the joint which helps avoid the formation of intermetallic compounds, but the force it requires can deform thin tab materials and create a mechanically weak connection. It is also somethimes difficult to make welds to nickel tabs and bus bars that may be included in the pack design.
Our initial results show that all three techniques can make joints that have low electrical resistance, but that low resistance and good mechanical strength are not related. While ultrasonic welding appears to be the best choice for many dissimilar material combinations, there are situations (such as the use of nickel tabs) where it has difficulty producing welds with good mechanical performance. Laser or resistance welding may be better suited to making these joints. The picture should become clearer as we continue to analyze our results.
Our investigations have also shown there is a need for a non-destructive evaluation technique that can monitor weld strength during process development if not during production. Our initial investigations of NDE options have demonstrated that we can use an energy source and thermal imaging to monitor weld strength in battery tab joints. If this technique can be applied to multiple tab joint geometries, it will provide a mechanism for reducing bad welds and improving productivity.
Stay tuned for more developments.