<p>A simple analytical model for grain morphology selection was established, which is capable of predicting a peculiar heterogeneous grain structure in <i>in situ</i> inoculated additively manufactured Al alloys. The model derived a new formation mechanism for the heterogeneous grain structure. For conditions not subjected to extremely high cooling rates (&lt;&lt; 10⁷&#xa0;K·s⁻¹), a decay of grain nucleation density, rather than complete elimination of nucleation, governs the grain morphology selection.</p>