Nano-ZrO2-enabled powder bed fusion-laser beam of an Al–Cu–Mg alloy

A hot-cracking-susceptible aluminum-copper-magnesium (Al-Cu-Mg) alloy was processed by powder bed fusion-laser beam (PBF-LB) with and without the addition of 0.5 wt.% zirconium dioxide (ZrO 2 ) nanoparticles introduced via mechanical mixing. A process window was first established for the monolithic alloy, revealing strong sensitivity of densification to volumetric energy density. Excessive energy input promoted keyhole-induced spherical porosity, whereas insufficient energy density resulted in irregular lack-of-fusion defects. An intermediate energy density of approximately 278 J/mm 3 minimized both defect types and produced the highest relative density (∼97%). The incorporation of ZrO 2 nanoparticles significantly improved processability and build quality. Under optimized processing conditions, hot cracking was suppressed and residual porosity was reduced to negligible levels, increasing the relative density to 99.9%, as determined by 2D image analysis. Concurrently, the solidification structure evolved from coarse epitaxial columnar grains spanning multiple layers to a refined mixed columnar-equiaxed morphology, with a reduction in columnar grain width of approximately 75%. This microstructural refinement resulted in a ∼62% increase in hardness, from 55 to 89 HV. The as-built ZrO 2 -modified alloy exhibited a yield strength of 222 MPa, an ultimate tensile strength of 367 MPa, and an elongation of 6.9, together with pronounced work-hardening behavior. In contrast, tensile testing of the monolithic alloy was not feasible due to extensive cracking. Fractographic analysis revealed a mixed ductile/cleavage fracture mode. The results suggest that ZrO 2 nanoparticles modify the solidification pathway by increasing nucleation frequency and disrupting epitaxial growth, thereby promoting local equiaxed grain formation, refining the microstructure, and suppressing hot-crack formation.