Decoding non-equilibrium solidification pathways for crack-suppressive microstructures in additively manufactured Al-Zn-Mg-Cu alloys

• A quantitative causal chain from LPBF process to molten pool attributes ( G , R c , τ ) and then to microstructure is established. • Ultra-rapid solidification ( R c ≈ 5.3 × 10 6 K/s) triggers solute trapping and yields <001>-textured fine columnar grains. • Prolonged liquid lifetime ( τ > 1000 μs) promotes Zn/Mg grain boundary segregation, driving epitaxial growth of coarse, weakly-textured grains. • Crack suppression is achieved by coupling non-directional grain boundaries (deflecting cracks) with grain boundary segregation (enhancing cohesion). Laser-driven transient molten pool physical attributes dictate non-equilibrium solidification pathways in Al-Zn-Mg-Cu alloys fabricated by laser powder bed fusion, yet their intrinsic linkage to microstructure and cracking susceptibility remains unresolved. This study elucidated the influence of processing on the physical properties of the molten pool and the non-equilibrium solidification behavior, and further investigated the correlation between microstructure and cracking susceptibility. Results showed that process-modulated thermal gradients ( G ) and cooling rates ( R c ) orchestrate solute trapping efficacy via critical molten pool residence times ( τ ). At ultra-rapid solidification ( R c ≈ 5.3 × 10 6 K/s, G ≈ 7.1 × 10 4 K/m, τ ≈ 200 μs), solute trapping suppressed Zn/Mg segregation. Homogeneous dispersion of metastable MgZn 2 precipitates (80–110 nm) with semi-coherent interfaces, including (00–2) Al ||(−10–3) MgZn2 and (020) Al ||(02–2) MgZn2 , was customized. Heterogeneous nucleation of <001> Al textured fine columnar grains was facilitated, while simultaneously exacerbating the initiation and propagation of solidification cracks. Prolonged liquid lifetimes ( τ > 1000 μs) under transitional regimes ( R c ≈ 1.2 × 10 6 K/s, G ≈ 3.6 × 10 4 K/m) exacerbated grain boundary segregation, yet enhanced cohesion via high-melting-point Al 2 Cu(Mg) phases. The epitaxial growth of coarse columnar grains with weak texture was driven, yet these microstructural features demonstrate reduced cracking susceptibility. Crack suppression in Al-Zn-Mg-Cu alloys was achieved through the concurrent application of grain boundary segregation engineering and non-directional grain boundary design. The findings of this work provide a valuable framework for the process optimization of additive manufacturing for crack-sensitive alloys.