Laser defocus regulates melt pool stability and performance in laser powder bed fused 316 L stainless steels

Laser defocus is an often-overlooked but highly influential processing parameter in laser powder bed fusion, as it directly alters energy delivery, melt-pool stability, and solidification conditions. In this study, 316 L stainless steel was fabricated over a wide defocus range (+10 to –40 mm) to elucidate how beam offset modulates melt-pool morphology, microstructure, corrosion behavior, and mechanical performance. Large negative defocus conditions produced stable conduction-mode melt pools, yielding coarse columnar grains, stronger texture, uniform passive-film behavior, and high tensile ductility. In contrast, the intermediate negative defocus condition (D = –10 mm) generated a narrow, deep, keyhole-prone melt pool, leading to local microstructural irregularities and heterogeneous defect landscapes. These features resulted in a unique combination of performance responses, including high pitting potential and polarization resistance but reduced repassivation ability, elevated metastable pitting activity, and the lowest tensile elongation among all conditions. These findings establish laser defocus as a critical, tunable parameter for process optimization and provide guidance for mitigating defect-driven degradation pathways in additively manufactured stainless steels.