Laser welding has become a critical process in the assembly of high-current busbars for battery packs, particularly in electric vehicles and energy storage systems. The technique offers precision, speed, and repeatability, which are essential for maintaining electrical performance and mechanical integrity. However, welding multi-layer busbars introduces challenges related to joint quality, electrical resistance, and thermal effects. The choice of busbar material—pure copper or aluminum—further complicates the process, requiring tailored strategies in beam shaping, shielding gas selection, and process parameter optimization.

Multi-layer busbar welding presents unique difficulties due to the need for consistent penetration across all layers while minimizing defects such as porosity, cracks, or incomplete fusion. The high thermal conductivity of copper and aluminum exacerbates heat dissipation, making it harder to achieve stable keyhole formation. Inconsistent penetration can lead to increased electrical resistance at the joint, which is detrimental in high-current applications where even minor resistive losses generate significant heat. To address this, laser power modulation and beam oscillation techniques are employed. Pulsed laser welding, for instance, allows better control over heat input, reducing the risk of excessive melting or burn-through. Beam oscillation, whether circular or linear, improves weld pool dynamics, ensuring uniform energy distribution across the joint.

Electrical resistance optimization is a key consideration in busbar welding. The resistance at the welded joint must be as low as possible to minimize power losses and heat generation during operation. For copper busbars, the formation of intermetallic compounds or oxides at the weld interface can increase resistance. Laser welding in an inert gas environment, such as argon or nitrogen, helps mitigate oxidation. However, aluminum busbars introduce additional complexity due to their natural oxide layer, which must be broken or removed before welding. Pre-weld cleaning methods, such as mechanical brushing or laser ablation, are often necessary. Additionally, the use of filler materials with compatible electrical properties can help bridge gaps and improve joint conductivity.

Thermal management during welding is critical to avoid distortion or material degradation. The high energy density of laser welding can lead to rapid heating and cooling cycles, inducing residual stresses that may affect mechanical performance. For copper, which has a higher thermal conductivity than aluminum, heat input must be carefully balanced to prevent excessive spreading, which could weaken the joint. Aluminum, while less conductive than copper, is more susceptible to hot cracking due to its solidification characteristics. Controlling the cooling rate through post-weld heat treatment or optimized shielding gas flow can alleviate these issues. Helium, for example, offers higher thermal conductivity than argon, promoting faster cooling in aluminum welds.

The choice between copper and aluminum busbars dictates specific welding strategies. Copper’s high reflectivity at common laser wavelengths (e.g., 1 µm) necessitates higher power densities or alternative wavelengths, such as green or blue lasers, which are more readily absorbed. Beam shaping techniques, such as top-hat or ring-mode profiles, can also improve energy coupling. Aluminum, on the other hand, absorbs laser energy more efficiently but requires careful management of its oxide layer. Shielding gas selection plays a pivotal role here; argon is commonly used for aluminum, while nitrogen may be preferred for copper due to its ability to reduce oxidation without forming nitrides.

Shielding gas selection further influences weld quality and process stability. For copper, nitrogen is often chosen over argon because it reduces oxidation without introducing significant nitrides that could increase electrical resistance. In aluminum welding, argon remains the standard, though mixtures with helium can enhance heat dissipation. Gas flow rate and nozzle design are equally important; insufficient flow may fail to protect the weld pool, while excessive flow can cause turbulence, leading to porosity.

Process monitoring and control are essential for ensuring consistent weld quality. Real-time monitoring systems, such as pyrometers or high-speed cameras, can detect deviations in weld penetration or surface quality. Closed-loop control systems adjust laser parameters dynamically, compensating for variations in material thickness or fit-up tolerances. This is particularly important in high-volume production, where manual inspection is impractical.

In summary, laser welding of high-current busbars demands a multifaceted approach to address multi-layer welding challenges, electrical resistance optimization, and thermal management. Copper and aluminum busbars require distinct strategies, from beam shaping and shielding gas selection to process parameter tuning. Advances in laser technology, such as wavelength diversification and beam modulation, continue to push the boundaries of what is achievable, enabling more reliable and efficient busbar joints for next-generation battery systems. The ongoing development of real-time monitoring and adaptive control systems further enhances process robustness, ensuring consistent performance in demanding applications.