<p>The welding of dissimilar materials, particularly copper (Cu) and aluminum (Al) alloys, has attracted significant attention in various electrical and electronic fields. However, due to the considerable differences in the physical and chemical properties of these materials, welded joints often exhibit defects and brittle intermetallic compounds (IMCs) that compromise joint performance. This study experimentally investigates the effect of laser power variation on the formation of pores and cracks in Al/Cu lap joints. The results indicate that increasing laser power reduces keyhole stability and enhances crack susceptibility. The porosity rate exhibits a nonlinear increase, rising from 1.36% at 650 W to 4.05% at 750 W. The crack length continuously increases, with rapid growth in the initial stage, where the crack propagation rate caused by pores reaches 101.3%, and the solidification crack propagation rate reaches as high as 863%. However, in subsequent stages, the growth rates decrease to 34.4% and 7.8%, respectively. This is attributed to the reduced solidification rate, which facilitates the backfilling process and diminishes the likelihood of crack initiation and secondary cracking. Tensile-shear tests reveal that the fracture surfaces exhibit typical brittle cleavage fracture characteristics, predominantly traversing the Al₂Cu phase, eutectic structure, and Al₄Cu₉ phase. By optimizing key process parameters, it is possible to effectively reduce the porosity and crack rates in the weld. This study provides a theoretical foundation and practical guidance for optimizing laser welding process parameters for dissimilar aluminum/copper metals, aiming to reduce defects and enhance welding reliability, thereby laying the groundwork for industrial applications.</p>