Effect of Laser Shock Peening on Electrochemistry and Wettability of Additively Manufactured Stainless Steel

Abstract

Laser shock peening (LSP) is investigated for its use in altering the electrochemical and wetting behavior of 316L stainless steel made with laser powder bed fusion (LPBF). The corrosion performance of LPBF stainless steel varies between studies and build parameters, thus motivating the search for postprocessing methods that enable wetted surface applications. Compressive surface stress has been demonstrated to reduce corrosion rate in additively manufactured metal, and LSP is known to impart compressive residual stress into metal targets. Wettability also affects corrosion behavior, and LSP induces hydrophobicity. LSP is, therefore, a promising tool for improving corrosion behavior of LPBF stainless steel. This paper examines the electrochemical properties of LPBF stainless steel before and after LSP with electrochemical impedance spectroscopy and potentiokinetic measurements. Contact angle, surface free energy, and surface finish are studied with dynamic contact angle measurements and profilometry. X-ray diffraction and energy-dispersive X-ray spectroscopy measure residual stress and surface chemistry. The top surface perpendicular to the build direction (XY) and the wall surface parallel with the build direction (XZ) are studied for all measurements due to the large differences in roughness and mechanical properties between these surfaces. LSP increases pitting potential for both XY and XZ surfaces and causes an increase to the surface electrochemical impedance. LSP also increases the contact angle of liquids on both surfaces. These changes to electrochemistry and wettability are attributed in part to surface morphology and surface chemistry alterations as well as the inducement of compressive residual stress.