Aluminum-copper (Al-Cu) dissimilar joints are increasingly used in e-mobility applications, particularly in battery module assembly and high-current busbar connections, where lightweight design, mechanical durability, high electrical conductivity, and cost efficiency are essential. Nevertheless, joining Al and Cu is challenged by pronounced differences in their thermophysical properties, their low mutual solubility, the formation of brittle intermetallic compounds (IMCs), and the presence of surface oxides and contaminants that hinder metallurgical bonding and reduce joint reliability.[1] Laser welding is widely employed to join Al and Cu because it enables contactless processing, precise energy input, deep penetration with a high depth-to-width ratio, short cycle times, and minimal heat-affected zones, making it attractive for industrial applications.[2] In order to avoid mixing in the liquid state, a so-called indirect laser welding is presented by [3], where joint formation occurs over indirect heating of the interface. However, the quality of the Al-Cu welds remains highly affected by the surface conditions of the substrates, which have not been thoroughly investigated.

To address this gap, experimental investigations were conducted on the influence of surface conditions on joint formation in Al-Cu laser welds, produced in an overlap configuration with copper placed on top of aluminum. The surface states examined included as-received, polished, artificially oxidized, and laser-structured conditions. Welding trials were performed using different wavelength laser beam sources, with process observations supported by high-speed synchrotron X-ray imaging during blue laser welding, providing direct insight into melt pool and keyhole dynamics at the interface. Post-process characterization included optical and scanning electron microscopy to assess weld seam geometry, interfacial morphology, and IMC distribution. Joint performance was further assessed by mechanical lap-shear testing and electrical resistance measurements to quantify joint performance under different surface conditions. Figure 1 illustrates two cross-sections of Al-Cu welds obtained by blue laser welding, where pores are visible at the interface in the as-received condition (left), while no pores are observed when the aluminum surface is artificially oxidized and the copper surface is structured with laser (right).

The investigations confirmed that surface condition plays a decisive role in weld seam quality, with surface oxidation and structuring directly influencing defect formation and joint quality. These findings underline surface engineering as an effective approach for achieving reliable Al-Cu joints in e-mobility applications.