Numerous manufacturers are working to improve electric vehicle (EV) performance and economics. A major part of this effort is focused on improving battery technology; in particular, developing methods and materials to simultaneously bring down fabrication costs (especially reducing production cycle time) and also extend reliability. Can-to-cap welding of prismatic battery cells is a standout example of this work, and one in which recent developments in laser welding are addressed with great success.
Can-to-cap welding specifically refers to the process of sealing the lid on the casing (can), which contains all the electrode structures for the battery. This sealing is done after these internal parts have been assembled into the can. Since this occurs near the end of the production cycle—after most of the value has been built into the assembly—scrapping parts at this stage is particularly costly.
The sealing operation requires making a fairly long, continuous weld. A typical prismatic battery is around 20 mm wide by up to 300 mm long, and the weld goes around the entire battery perimeter. There are several key requirements for manufacturers to perform this process:
- The weld seam must provide a hermetic seal with no gaps along the entire weld—even if the original part fit-up wasn’t perfect or isn’t consistent (especially at the corners).
Welding must achieve adequate penetration depth and low weld porosity. This is necessary to create a seal strong enough to last the lifetime of the battery without cracking open, even when subjected to vibration and mechanical shock.
- The welding process cannot create any spattered metal. Most importantly, spatter cannot occur inside the battery where it might create electrical short circuits. Spatter is especially a problem when welding aluminum, which is commonly used for battery cans. This is due to several factors, but primarily because of aluminum’s high reflectivity and low melting temperature. These may lead to keyhole instability, which causes weld spatter.
- The heat input into the battery must be limited to avoid damaging internal parts.
Fiber lasers can deliver on all these requirements. As a result, they have already established themselves as useful production tools for prismatic battery can-to-cap welding. In the most common implementation, the beam focusing optics are gantry-mounted, and then moved around to follow the shape of the desired weld seam.
This gantry approach delivers highly precise mechanical alignment and great weld consistency. This is because the laser beam always hits the workpiece at the exact right place and the same angle of incidence. However, moving the optics (or alternately, the battery) makes a gantry system slow. This is an issue because slower speed translates directly into higher production costs.
Faster way to weld large areas
As manufacturers strive to dramatically increase their production capacity and also go to larger-sized prismatic batteries, the speed limitations of gantry-type welding systems become a significant issue (see Fig. 1). It’s well known that scanning systems can deliver higher weld speeds. This is because it is much easier to move the weightless laser beam using scanner mirrors than it is to physically translate the entire focusing optics assembly.