Influence of zinc evaporation-driven spattering on defect formation and mechanical properties in laser powder bed fusion of AA7075 aluminum alloy

The optimum process window for laser powder bed fusion (LPBF) of 7000-series aluminum alloys remains extremely narrow, resulting in unpredictable variations in mechanical property consistency. While previous studies attributed this challenge to hot cracking or compositional modifications, this work reveals that spattering driven by zinc evaporation is the dominant cause of defect formation in AA7075 LPBF. Comprehensive characterization demonstrates that Zn acts as the primary volatile element; it preferentially evaporates and condenses as Zn/ZnO nanostructures on the surface of spatter particles, reaching a local Zn concentration of 17.8 wt.%, which is three times higher than the nominal composition. Unlike uncoated powder, these particles enriched with Zn/ZnO degrade the flowability of the powder bed. This triggers the accumulation of defects across layers during recoater redistribution, thereby establishing a quantitative link between element volatility and porosity formation. To address this, we implemented a two-stage control strategy featuring an optimized inert gas flow rate (9.5 m/s) and a rationally designed spatial layout to minimize spatter migration between adjacent builds. This approach systematically suppressed the defect population, achieving 99.50% density alongside superior mechanical properties: 432±5 MPa yield strength, 476±12 MPa ultimate tensile strength, 8.0±0.6% elongation, and 1152±21 MPa ultimate compressive strength. By improving property consistency beyond conventional parameter optimization, the established mechanism coupling evaporation, spattering, and defects offers a new framework for processing alloys containing volatile elements, which is particularly relevant for large-scale LPBF manufacturing where spatter accumulation effects are amplified.