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

Technical Summary: Nano-ZrO2-enabled Powder Bed Fusion-Laser Beam of an Al-Cu-Mg Alloy

Problem Statement
High-strength wrought aluminum alloys, specifically the Al-Cu-Mg (2xxx series) system, are critical for aerospace and structural applications but remain notoriously difficult to process via Powder Bed Fusion-Laser Beam of Metals (PBF-LB/M). Unlike cast-type Al-Si alloys, which possess a narrow solidification range and high fluidity, Al-Cu-Mg alloys exhibit a wide solidification interval and strong susceptibility to hot cracking. This susceptibility arises from the formation of continuous epitaxial columnar grain structures aligned with the build direction. During solidification, these grains create continuous boundaries where low-melting eutectic films persist, unable to accommodate the tensile stresses generated by thermal contraction and solidification shrinkage. Consequently, conventional PBF-LB/M processing of these alloys typically results in extensive cracking, rendering the fabrication of dense, mechanically sound components unfeasible. While process parameter optimization can mitigate porosity, it has proven insufficient to eliminate the underlying microstructural drivers of cracking.

Methodology
To address these challenges, the study employed a two-stage experimental approach using a 300 W continuous-wave fiber laser system:

1. Process Window Establishment: A monolithic Al-Cu-Mg alloy (pre-alloyed gas-atomized powder) was processed to map the relationship between volumetric energy density (VED) and defect formation. Nine parameter combinations were tested by varying hatch spacing (40, 60, 80 µm) and scan speed (440, 520, 600 mm/s) while maintaining constant laser power (260 W) and layer thickness (30 µm).
2. Nanoparticle Modification: A composite powder was created by mechanically mixing the Al-Cu-Mg powder with 0.5 wt.% ZrO2 nanoparticles (<50 nm) via low-energy ball milling. This composite was processed under the optimal parameters identified in the first stage (60 µm hatch spacing, 520 mm/s scan speed, ~278 J/mm³ VED).
3. Characterization: The resulting specimens were analyzed for relative density, defect morphology (cracks, porosity), and microstructural evolution (grain size, morphology, phase identification via XRD and SEM). Mechanical properties were evaluated through Vickers microhardness and uniaxial tensile testing. Notably, tensile testing was only performed on the ZrO2-modified alloy, as the monolithic specimens were too cracked to be machined into test coupons.

Key Results

• Process Window and Defects: The monolithic alloy exhibited a strong sensitivity to energy density. High energy densities (>328 J/mm³) induced keyhole-mode porosity (spherical pores), while low energy densities (<246 J/mm³) resulted in lack-of-fusion (LoF) defects. An intermediate energy density of ~278 J/mm³ minimized both defect types, achieving a maximum relative density of ~97.1% for the monolithic alloy. However, hot cracking remained pervasive across all conditions, with crack density decreasing only slightly as energy density increased.
• Effect of ZrO2 Addition: The incorporation of 0.5 wt.% ZrO2 nanoparticles completely suppressed hot cracking and reduced residual porosity to negligible levels, increasing the relative density to 99.9%.
• Microstructural Evolution: The addition of ZrO2 fundamentally altered the solidification structure. The monolithic alloy displayed coarse, epitaxial columnar grains spanning multiple layers (average width ~120 µm). In contrast, the ZrO2-modified alloy exhibited a refined mixed columnar-equiaxed morphology. The columnar grain width was reduced by approximately 75% (to ~30 µm), and the continuous grain boundary network was interrupted by fine equiaxed regions. XRD analysis confirmed the presence of only the α-Al phase, with no detectable ZrO2 peaks or reaction products, suggesting the nanoparticles acted as nucleation sites or influenced solidification dynamics without forming distinct secondary phases detectable by XRD.
• Mechanical Performance: The microstructural refinement led to a ~62% increase in hardness, rising from 55 ± 2 HV (monolithic) to 89 ± 2 HV (modified). The ZrO2-modified alloy demonstrated a yield strength of 222 ± 12 MPa, an ultimate tensile strength of 367 ± 10 MPa, and an elongation of 6.9 ± 0.7%. Fractographic analysis revealed a mixed ductile-cleavage-intergranular fracture mode. In contrast, the monolithic alloy could not be tested due to catastrophic cracking.

Significance and Claims
The paper claims that the addition of trace amounts of ZrO2 nanoparticles offers a cost-effective and practical strategy to enable the PBF-LB/M processing of high-strength, crack-susceptible Al-Cu-Mg alloys. The study demonstrates that oxide nanoparticles can modify the solidification pathway by increasing nucleation frequency and disrupting epitaxial grain growth. This promotes a columnar-to-equiaxed transition (CET), which refines the microstructure and suppresses the formation of continuous crack paths.

The authors emphasize that while process parameter optimization alone is insufficient to eliminate hot cracking in these alloys, grain structure engineering via nanoparticle inoculation successfully achieves near-fully dense, crack-free parts with significantly improved mechanical properties. The work suggests that ZrO2 nanoparticles act through mechanisms such as heterogeneous nucleation (potentially via in-situ formation of Al3Zr or other Zr-bearing phases) and Zener pinning, although the exact reaction pathways require further investigation as no secondary phases were directly detected. Ultimately, this approach extends the applicability of PBF-LB/M beyond eutectic Al-Si alloys to high-strength precipitation-hardenable systems.