How Fast is Too Fast: Increasing the Weld Speed for Bipolar Plates
This blog was written in conjunction with Woo-Sik Chung from Fraunhofer ILT, a leading development and contract research institute for laser technology
With dire predictions about climate change pouring in, it’s no wonder that many of the major OEMs have big plans to shift production fully (or mostly) to electric vehicles by 2030-2035. While this is good news for consumers who want to make the eco-conscious decision to drive an electric car, the efficient production of electric vehicles does not come without challenges.
In contrast to a standard combustion engine found in traditional vehicles, electric cars require a fuel cell stack built of approximately 200 bipolar plates. These plates are stacked together, to form the fuel cell, along with membrane-electrode assembly (MEA), and must be tightly and precisely welded with no defects to function properly and maximize the space allocated to them.
Currently, it takes ten times as long to weld the plates as it does for the rest of the manufacturing process. As welding is the critical path to production, cutting down this time will have a significant effect on production time and the environmental impact of the production process.
The Challenge: How to Make a Bipolar Plate
There are three steps in the process of producing the bipolar plates needed to make fuel cell stacks:
Step One: Cut & Shape - this process is generally referred to as punching, and involves cutting the sheet metal plate out of a coil in the right shape, as well as cutting a hole in the plates if necessary.
Step Two: Mold with the Flow Field - a flow field must be formed before the joining process to allow for the cooling water to flow between the welded bipolar plates.
Step Three: Welding - the final step is when the two sheets are welded together.
The first two steps are very fast, taking about 100-300 millisecond each. It’s the third step - which is the crux of the production process - where the challenges arise. Welding the plates together properly requires welding up to 2m of metal. Currently, the top possible speed is around 0.5m per second, which means 4 seconds per weld. Because each car needs 200 bipolar plates, this part of the process takes 800 seconds, or 13 minutes per car, slowing down the entire production process considerably.
Not So Fast: Increasing Speed Creates Defects
The first two steps in the production process are already as fast as possible, so the question becomes how can the final step take less time? While the obvious answer would seem to be simply to increase the speed of the weld, doing this causes significant defects.
While existing technology has the capacity to go faster, bipolar plates require deep and narrow seams on 0.1mm sheets. When the feed rate increases to above 0.5m/s, the resulting melt stream leads to the well-known humping-effect.
For the bipolar plate to be welded properly, it has to pass a hydrogen leakage test to make sure that the weld is tight and there are no leaks. It could pass this test, but still have humping. In which case, the plates still cannot be used because the humping changes their shape and the plates will not fit into a single stack together.
Because increasing the speed is not a viable option at this time, companies incorporate alternative solutions such as using 10 lasers at once on each sheet of metal to speed the process, but this is not cost effective or eco-friendly.
The Solution: Dynamic Beam Lasers
More humping occurs at high speeds with high power lasers as control of the melt pool flow and vapor capillary decrease. When we can control the shape of the laser beam to a degree that influences the melt dynamic, changing the shape could change the outcome and prevent this humping.
The shape of standard laser beams cannot be easily changed, especially mid-process. With Dynamic Beam Lasers, the shape can be changed - for example to an oval - which will offer less resistance and should reduce the flow velocity of the melt behind the keyhole and decrease the risk of humping. It will then be possible to use one high intensity laser at a higher speed without causing defects.
Another possibility is to use a higher intensity beam combined with additional laser points at lower intensity in order to pre and post heat the material. These processes that are now achievable with Dynamic Beam Lasers will enable an increased feed rate and greatly decrease production time.
Civan is currently working in partnership with Fraunhofer ILT and Smart Move on the “Eureka Project” whose aim is to find a solution for welding bipolar plates at high speed. The potential solutions described above, which rely on Civan’s technology, will be tested to create a viable solution for producing bipolar plates at scale, efficiently.