Influence of subtle particle size distribution differences between two AA6061 powders on melt pool behavior and defect formation in laser powder bed fusion

Aluminum alloys exhibit high laser reflectivity and thermal conductivity, making defect formation in laser powder bed fusion (LPBF) highly sensitive to powder characteristics and processing conditions. Although particle size distribution (PSD) affects powder spreading, packing density, and laser absorption, the influence of subtle PSD differences between nominally similar powders remains insufficiently investigated. In this study, two commercially sourced AA6061 powders with similar compositions, nominal size ranges, and flowability, but slightly different PSDs, were compared to determine their effects on melt pool behavior and defect formation. The powder with a broader PSD and higher fraction of fine particles (mean size 29.5 μm, span 1.36) produced deeper melt pools, smoother surfaces, and lower lack-of-fusion porosity when the 50 μm layer thickness was comparable to powder D90. In contrast, the powder with a narrower PSD and larger mean particle size (36.4 μm, span 0.86) exhibited shallower melt pools and higher porosity, indicating reduced laser absorption associated with lower packing density. Increasing laser power for the narrower-PSD powder reduced porosity and deepened melt pools, supporting insufficient energy absorption as a primary contributor. Doubling the layer thickness to 100 μm reduced the sensitivity of packing density, melt pool behavior, and defect formation to PSD, producing comparable defect levels. Scheil-Gulliver simulations indicated that minor compositional differences may contribute to cracking behavior but are secondary to PSD-driven packing effects in governing melt pool size and porosity. These findings demonstrate that subtle PSD variations can significantly influence LPBF printability and require parameter re-optimization to achieve consistent defect control.