Mechanism of melt pool structure on corrosion behavior of selective laser melted 316L stainless steels

The melt pool structure represents a critical feature in laser additive manufacturing and serves as a primary factor contributing to the disparities in microstructure and performance between the vertical ( XOY) and horizontal ( XOZ) processing directions of 316L stainless steels fabricated via selective laser melting (SLM). The mechanism of molten pool structure on the corrosion behavior of 316L stainless steels fabricated by SLM was investigated in this study. The differences in molten pool structure on the XOYand XOZplanes were explored by scanning electron microscope (SEM), electron back scattering diffraction, (EBSD), and transmission electron microscope (TEM) in various scales. The electrochemical testing and immersion experiments were conducted to study the differences in passivation and pitting behavior between the XOYand XOZplanes. The results show that the molten pool structure on the XOYplane is continuously distributed along the processing path, with equiaxed or near-equiaxed cellular substructures within the molten pool. In contrast, the molten pool structure on the XOZplane exhibits a continuous fish-scale pattern, with the cellular structure displaying clear orientation. After forming a film at a constant potential in 0.5 mol/L H ₂SO ₄solution, the XOYplane exhibits a passivation film with lower defect density of oxygen vacancy, higher electrochemical impedance, and higher relative content of Cr ₂O ₃, compared to the XOZplane, indicating the superior passivation behavior on the XOYplane. After immersion for 12 h in acidic FeCl ₃solution, the maximum pitting depth on the XOZplane is 447.4 μm, approximately twice than that of the XOYplane. Additionally, the edges of the circular pitting pits on the XOZplane show the tendency to extend into a fan-shaped molten pool boundary. It can be comprehensively known that the smaller cellular substructure on the XOYplane facilitates the passivation film growth, while the fan-shaped molten pool boundary on the XOZplane accelerates the pitting expansion.