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Laser Beam Shaping Increases Welding Speed of EV Battery Coolers

May 08, 2023

When it comes to temperature extremes, electric vehicle (EV) batteries are a lot like people. EV batteries perform best in the same sort of temperature ranges as humans do. EV thermal management systems maximize battery performance and extend its life. Cooling plates in an EV thermal management system allow liquid coolant to remove heat from the battery.

One cooling plate design circulates coolant between two thin aluminum (Al) plates. The coolant flows through stamped channels in the base plate, which is joined to a top plate. To prevent coolant leakage, the base and top plates must be joined to create a tight, hermetically sealed joint (Figure 1). The welded joints must also be free of cracks that can lead to mechanical failure in the field.

Manufacturers started joining battery cooling plates using vacuum brazing technology. These earlier plates (Figure 2) were much smaller than the cooling plates required for today's EV battery systems, which rely on cooling plates measuring up to 2.1 × 1.3 m.

As demand for larger cooling plates increases, vacuum brazing inefficiencies become apparent. Brazing is slow and consumes a lot of energy (>4 MW), which leads to high operating cost. A single production line can take up 800 square meters of production floor space. The increasing size of the cooling plates also requires significant capital investment in larger vacuum furnaces, which can cost more than 5 million euro for a single furnace.

Brazing also requires the use of Al 3003, a special aluminum alloy that can be brazed. Manufacturers want to switch to more economical alloys such as Al 5754, which can be brazed but requires a postprocessing treatment, and Al 6xxx series alloys, which have the advantage of being recyclable but can't be brazed at all. They are searching for faster, more efficient joining methods that will help them keep up with increased demand and speed adoption of new metal alloys.

Adoption of laser materials processing technology accelerates with increased reliability, robustness, and availability of multikilowatt lasers. Compared to the traditional welding processes, laser welding reduces production costs, and increases manufacturing flexibility and selectivity.

Laser welding technology also requires less heat input, which minimizes distortion potential while maximizing speed. All welding methods involve melt-pool formation and subsequent rapid solidification. However, the high energy of laser welding not only melts material but also evaporates it. 1

Evaporation of material during the welding process creates a keyhole, which gives laser welding the advantage of a very ratio of penetration depth to weld seam width (Figure 3). Consequently, many manufacturers have switched from traditional brazing and welding to laser materials processing, which can join a variety of materials, reduce power consumption, and improve process yields.

Large in scale and complex in geometry, battery cooling plates must meet stringent requirements to achieve robust seams that can provide a long leak-free service life. To avoid mechanical failure, the joints cannot have any cracks, humping, undercut, or porosity defects in the interface (Figure 4).

While laser welding's high aspect ratio translates into lower part distortion potential when compared to thermal welding, it can also pose challenges, since keyhole stability is crucial for achieving high weld quality.

The laser keyhole generally remains stable during the welding of high-absorption materials such as steel and nickel. Unfortunately, when welding copper, aluminum, and high-alloy materials such as those required in cooling plate production, the keyhole can be inherently unstable, making the process susceptible to irregularities that compromise weld quality. One common method to overcome these defects is wobbling the beam and beam shaping, which varies laser beam spot shape and size. 2

Three broad categories of beam-shaping include static, variable, and dynamic. Static and variable methods rely on diffractive optical elements (DOEs), which provide cost-effective beam shaping via a thin pattern on a robust window that diffracts and modulates the phase of the light passing through it.3 For static beam shaping, a variety of DOEs can tailor the shape of the laser beam output at the workpiece. Limited flexibility of static solutions makes them suitable for applications with very well-defined process parameters.

Using adjustable ring shapers that split the beam into a central spike, or core beam, and a surrounding ring beam, DOEs can provide variable beam-shaping options that increase laser flexibility. This option requires a single-axis shift or rotation to change the ratio intensity between the core and ring beams. Another approach uses variable superimposed intensity distribution with a two-in-one (dual-core) fiber.

While such beam-shaping solutions can improve the flexibility of a given process – enabling a single machine to carry out specialized tasks in serial production, for instance – a static beam cannot adequately stir the melt pool to accomplish the frequently changing operations that constitute the daily business of industry. 4

Known for overcoming welding defects, dynamic beam shaping methods currently include four options: galvanometer scanners, piezo-driven actuators, micro-electrical-mechanical (MEM) scanners, and optical phased arrays (OPAs).

Galvanometric scanners can be used to oscillate single-mode fiber lasers during the welding process in the pattern of, for example, a circle or a figure eight. However, such solutions have power and frequency limitations. Inherent mechanical and kinetic trade-offs related to moving parts limit the maximum achievable frequency due to the mass of the scanner's oscillating mirrors. Smaller, lightweight mirrors limit laser power.

In contrast, OPA technology, a type of coherent beam combining (CBC), merges many single-mode laser beams into one larger beam (Figure 5). Each laser emits its own light, which overlaps with other beams in the far field to create a diffraction pattern, providing the flexibility to easily manipulate the beam shape in real time, without any moving parts, creating a dynamic beam laser (DBL).

To overcome cooling-plate welding challenges, tailored beam shaped were needed and designed (Figure 6). These beam shapes use high shape frequency together with a sequence of beam shapes, this enables fast switching between beam shapes, adding more flexibility. For example, if one shape stabilizes the keyhole and prevents spatter while a different shape prevents cracking, then a well-designed sequence of these two shapes can achieve all three goals.

Processes for welding cooling plate configurations have recently been developed, including designs with channel and dimple geometries made in Al 3003 and Al 5754 alloys. Simulations created by Professor Andreas Otto at the Institute for Production Engineering and Photonic Technologies at the Vienna University of Technology (TU Wien), Wein helped optimize the many process parameters (Figure 7).

Simulations reveal that humping is a periodic phenomenon. When the melt pool is long and the speed is fast, cooling starts from the sides, narrowing the molten channel. As the molten channel narrows, molten material flows up and creates the hump. 5

Changing the beam shape to concentrate energy input on the sides of the melt pool maintains channel width in the trailing edge, ensuring channel stays open and reducing the flow velocity of the melt behind the keyhole, which decreases the risk of humping. Combining this with the introduction of a different period into the process interrupts the periodicity of the humping, avoiding the defect altogether. Switching the beam shapes in sequence every few microseconds eliminates humping and enables welding at higher speeds without defects (Figure 8).

For large-scale part production, SLTL (Sahajanand Laser Technology Limited), a leading manufacturer of laser welding machines in India has incorporated DBL technology in a 3D cutting and welding machine based on CBC. The project, funded by the Israeli Innovation Authority and the Global Innovation & Technology Alliance, has resulted in defect-free production of full-scale cooling plates.

This article was written by Ami Shapira, Marketing Manager, Civan Lasers. For more information visit here .

This article first appeared in the March, 2023 issue of Battery & Electrification Technology Magazine.

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