Synergistic enhancement of strength and ductility in Arc-DED Al-Cu alloys via in-situ liquid nitrogen cooling-induced grain structure heterogeneity and porosity suppression

Achieving a favorable strength-ductility balance in heat-treated Al-Cu alloys fabricated by arc-directed energy deposition (Arc-DED) remains challenging due to the difficulty in effective microstructure control and porosity suppression. To address these issues, a synchronized liquid nitrogen cooling (LNC) strategy is introduced during Arc-DED to enhance the strength-ductility synergy of T6-treated Al-Cu components. Compared to the deposits without LNC, LNC-processed samples exhibit a 42 % increase in uniform elongation and a 7.5 % rise in ultimate tensile strength, achieving 482.1 MPa with 10.9 % elongation, outperforming the existing Arc-DED Al-Cu alloys. This improvement results from the coupled effect of 61 % porosity suppression and enhanced grain heterogeneity. Porosity reduction is attributed to a higher cooling rate that promotes hydrogen supersaturation and thereby suppresses hydrogen bubble nucleation during molten pool solidification. Grain heterogeneity arises from reduced peak temperature and the shorter melting duration at the molten-pool bottom, promoting Al₃Ti particle retention, increasing nucleation sites and refining equiaxed grains. Further analysis reveals that 69 % of the ductility improvement derives from the hetero-deformation-induced (HDI) strain-hardening, while the remaining 31 % stems from the porosity suppression. Moreover, HDI stress elevates the saturation stress, contributing to the enhanced tensile strength.