In electronics and precision devices, achieving reliable connections for composite joints of thin metal sheets such as Al/Cu is critical, as this helps reduce Cu usage costs. Conventional welding methods such as brazing and stir friction welding are limited by the sample size and tend to introduce excessive heat input into the joint. Thus, they are not suitable for joining thin metals. Suitable methods for joining thin metal sheets include ultrasonic and laser welding. Laser welding, particularly short-pulse laser welding, has precise heat input, high energy density, and good controllability, making it suitable for connecting thin sheets. Accordingly, in this study, a short-pulse laser is used to weld Cu/Al/Cu laminar joints. The effects of scanning speed on the joint microstructures and properties are investigated to provide guidance in joining laminar metal sheets.

A nanosecond-pulsed laser is used to join Cu/Al/Cu laminates. The laser scanning speed is used as a major parameter that affects the joint quality. First, the effect of scanning speed on weld formation is studied. The microstructural evolution of typical joints is then analyzed. Finally, the effects of welding speed on the mechanical properties are investigated, and the fracture mechanism of the joints is elucidated.

In terms of weld formation, the heat-affected zone at the joint edge and the penetration depth both decrease as the welding speed increases (Fig.3). The pores in the weld migrate upward as the melting depth decreases, and the joint quality improves when the welding speed is 21 mm/s (Fig.4). In terms of microstructures, the interface produces numerous bulk CuAl₂ phases at a low welding speed (15 mm/s) (Fig.6). As the welding speed increases, the number of interfacial compounds decreases. In addition, the compound Cu₉Al₄ phase appears near the copper base material, whereas the part near the aluminum is mainly subeutectic, eutectic, and perieutectic structures composed of α-Al and CuAl₂ phases. When the welding speed reaches 27 mm/s, the Cu₄Al₃ phase appears in the Cu clusters (Fig.6). In tensile tests, the maximum shear force of 95.1 N is obtained in the joint at a welding speed of 21 mm/s (Fig.9). Following fracture analysis, three fracture modes are observed. When the welding speed is low, the joint experiences the tearing failure, which is caused by deep longitudinal cracks in the outer ring of the welded joint due to considerable heat input, and brittle compounds such as CuAl₂ are produced at the weld edges (Figs.10 and 11). As the welding speed increases (21 mm/s), the cracks and porosity defects in the weld decrease, and the joint has a certain melting depth, resulting in a combined failure mode (weld shear and nugget pullout failure) in the joint (Fig.11). As the welding speed becomes excessively high, the molten nucleus has difficulty resisting the tensile shear force because of the smaller penetration depth, resulting in a button pullout fracture mode.

A nanosecond-pulsed laser is used to weld three-layer Cu/Al/Cu lamellar structures, where defects such as porosity and cracking at the joint interface can be reduced by adjusting the welding speed. The joint formation quality is found to be excellent at a welding speed of 21 mm/s. The diffusion of Al into the upper and lower Cu base materials to produce nail-shaped areas improves joint strength. The Al/Cu binary compound type is found to be related to the welding speed. The CuAl₂ phase is prevalent in the Cu welds, whereas Cu₉Al₄ and Cu₄Al₃ are present only near the Cu base material or Cu clusters. The maximum joint shear force of 95.1 N is obtained at a welding speed of 21 mm/s. Three types of fractures occur in the Cu/Al/Cu joints at different welding speeds, namely, joint tearing, mixed-mode fracture (weld shear and nugget pullout failure), and button pullout failure. The nail-shaped structure has a hindering effect on joint fracture.