<p>Heat treatment is crucial for the optimization of additively manufactured components. Laser-powder bed-fused (L-PBFed) magnesium-rare earth (Mg-RE) alloys are highly suitable for lightweight engineering applications because of their high specific strengths and top-down fabrication. However, L-PBFed Mg-RE alloys have unique non-equilibrium features that cause difficulty in the context of conventional heat treatments based on equilibrium phase diagrams or quasi-equilibrium solidification; these challenges severely limit their performance. This study systematically reveals the unique governing mechanism of the solution-treatment temperature over the precipitation evolution and age-hardening responses of L-PBFed WE43 alloy. The results indicate grain coarsening and increased heterogeneity following solution treatment. Under a low solution heat-treatment temperature (400&#xa0;°C), the in-situ phases partially dissolve but coarse Mg₄₁Nd₂Y phases form; under a high solution heat-treatment temperature (500&#xa0;°C), the precipitates dissolve but the nucleation site density decreases through excessive defect annihilation. Under optimized solution treatment (450&#xa0;°C), a defect network is retained via lattice distortions, which enables nucleation of shearable β′/β₁ phases and stacking faults (SFs) during aging. These shearable β′/β₁ phases and SFs impede the dislocations, as an anti-phase boundary energy barrier is established, causing dislocation accumulation and strong strain gradients at the interfaces. Consequently, optimized solution treatment coupled with aging heat treatment yields stronger precipitation strengthening and heterodeformation-induced strengthening owing to the higher density of shearable microstructures and growing grain-size heterogeneity. This work elucidates solute-defect coupling in L-PBFed WE43 solution aging and provides new insights for the heat treatment of L-PBFed Mg-RE alloys.</p> Graphical abstract <p></p>