An experimental study of the microstructure of a hybrid part, composed of two materials fabricated through the laser powder bed fusion (LPBF) process—specifically, AlSi10Mg on an Al-Cu cast alloy substrate—revealed variations in the microstructure of the first consolidated layer. These variations were found to contribute to a robust metallurgical bonding, subsequently enhancing the mechanical properties and overall performance of the LPBF-AlSi10Mg side of the hybrid part. In this study, we conduct numerical investigations to explore the microstructures of additively manufactured dilute Al–Si on the substrate Al–Cu alloy. A thermal model has been developed to systematically address the impact of laser processing conditions on the thermal behavior of the molten pool, partial melting of the cast substrate, and the dilution of its alloying elements during the LPBF process. To simulate the microstructure evolution of the candidate alloy, we employ a multi-component, multi-order parameter phase-field model. This model allows for a comprehensive exploration of the microstructural variations across the interface of the bimetal. Notably, the presented phase-field model can self-consistently simulate both heterogeneous and homogeneous nucleation events, providing insights into the triggers for morphological transitions. Moreover, it quantitatively explains the observed microstructural variation across the bimetal interface.