Microstructure-electrochemical behavior relationship in post processed AISI316L stainless steel parts fabricated by laser powder bed fusion

Abstract

AISI316L stainless steel components produced via additive manufacturing techniques have quickly found new applications across several industrial sectors. However, parts manufactured through this method generally exhibit poor surface quality and performance in the as-built state. This work addresses the influence of laser polishing and water jet assisted recirculating shot peening on the surface quality, microstructure and electrochemical properties of AISI316L samples produced by the laser powder bed fusion method. To do so, surface roughness analysis, residual stress measurement, scanning electron microscope and electron backscatter diffraction analysis were employed along with electrochemical tests, including cyclic potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. Laser polished samples exhibited a smooth surface with high tensile residual stress on the surface, which led to the reduction of pitting potential and formation of passive layers, including more crystal defects. Microscopical analysis evidenced that the higher density of lattice defects and local microstrain on the surface of the shot peened samples promoted surface hardness and induced compressive residual stress. Therfore, the shot peened samples exhibited a wider passive range in cyclic potentiodynamic polarization measurements, higher polarization resistance in electrochemical impedance spectroscopy measurements, and less defective passive film in Mott-Schottky analysis. Moreover, it was calculated that the passive film thickness of the shot peened sample was slightly larger than the other samples. Low surface roughness, high crystal defect density, and compressive residual stresses enhanced the passive layer's resistance to defect transport, lowered the point defect concentration in the passive film, and improved the pitting resistance of the samples.