Effect of laser defocus on microstructural and mechanical properties of 316L stainless steel manufactured by laser powder bed fusion

Extensive studies have investigated the effects of process parameters such as power, scanning speed, hatch spacing, and scanning strategy on laser powder bed fusion (LPBF) part quality, however, the fundamental theoretical guidance for tailoring the LPBF part quality through laser defocus adjustment remains insufficiently explored. Accordingly, the effect of laser defocus characterized by spot size variation on the microstructural and mechanical properties of LPBF 316L stainless steel (LPBF 316LSS) was investigated in this study. Printing at the focal point (22 μm) significantly improves warping resistance across a broader processing range. However, appropriately increasing the defocus distance can not only refine the substructure but also facilitate more effective texture control, which significantly influences the mechanical properties of LPBF 316LSS. It was found that a large number of deformation twins were activated by increasing positive defocus distance to 0.68 mm (spot size of 45 μm), providing continuous and stable work hardening capability and resulting in an excellent combination of strength and ductility. Furthermore, as the positive defocus distance was increased to 0.98 mm (spot size of 65 μm), keyhole-shaped melt pools with aspect ratios exceeding 1 were observed, accompanied by a substantial weakening of the typical crystallographic texture in LPBF 316LSS. Meanwhile, spatters were ejected frequently which deteriorated the surface quality.