Enhancing surface quality in laser powder bed fusion using core-shell structured powder with micrometer-sized metal core and TiO2 nanoparticle shell

Laser powder bed fusion (LPBF) is a manufacturing technique employed to produce intricate geometries, with promising applications in the medical, aerospace, and molding sectors. Nonetheless, the surface quality of components produced via LPBF, characterized by surface roughness values in the range of tens of micrometers (Ra), fails to satisfy the mold industry standard of less than 1 μm. Consequently, LPBF-fabricated components necessitate post-processing treatments, such as mechanical milling or modifications to LPBF process parameters, to mitigate surface roughness. These methods, however, are constrained by considerations of cost and time efficiency. The stability of the melt pool is influenced by thermo-fluid flow behavior, with surface roughness being affected by factors such as surface tension, Marangoni flow, and recoil pressure. In this study, a core-shell structured powder was utilized, comprising AISI 420J2 stainless steel as the core and TiO 2 nanoparticles as the shell, serving as the feedstock for LPBF. The study analyzed surface roughness and morphology through both monolayer and multilayer experiments to assess the impact of nanoparticles on surface roughness. The findings revealed that specimens incorporating nanoparticles exhibited reduced surface roughness and balling phenomena compared to those without nanoparticles. An increase in nanoparticle content correlated with a decrease in surface roughness, achieving a maximum reduction of 28.5% at a nanoparticle concentration of 0.02% and a build energy of 40 J/mm. However, beyond a nanoparticle content of 2%, agglomeration occurred, leading to diminished wettability and an increase in surface roughness. Furthermore, the influence of unmelteded TiO 2 nanoparticles on surface roughness and balling size was examined in multilayer builds ranging from 20 to 200 layers. It was observed that unmelteded TiO 2 nanoparticles affected heat flow behavior, resulting in increased viscosity within the capillary regime and enhanced Marangoni flow in the thermo-capillary context, both of which contributed to improved surface roughness. The results of this study provide valuable insights into enhancing surface quality through the incorporation of unmelteded nanoparticles in melt pools during LPBF, thereby modifying heat flow behavior.