The aim of topology optimization as a lightweight construction method is to reduce the mass of components while maintaining rigidity. Topology-optimized models can have highly complex geometries. Often, significant adjustments are required to ensure viability and the subsequent production of a physical component. Laser-based additive manufacturing processes are well suited to the production of such complex component geometries as they allow for a high degree of optimization. However, the production of large-volume components is energy-intensive and time-consuming, and from an environmental perspective has to be seen as an unfavorable option. One possible approach is to separate the component into additively manufactured and conventionally manufactured substructures and to combine laser-based additive manufacturing and joining processes. This paper examines the laser beam welding of dissimilar joints between additively manufactured parts made of the aluminum cast alloy AlSi12(a) and the extruded aluminum wrought alloy AlMgSi0.5. The dissimilar joints are assessed both mechanically and metallographically. The influence of heat treatment and the effects on the weld of the dissimilar joint are also considered in the homogenization of the additive joint part. The additively manufactured joining partner acts as an additional material, and the weldability of the hot-crack critical wrought aluminum alloy is significantly increased. An additional positive effect related to the hydrogen problem of aluminum-based alloys is observed when the additive joining partner is heat treated. If heat treatment is performed prior to laser beam welding, the number of gas pores in the fusion zone is significantly reduced.