Abstract This study investigates the effect of Ultrasonic Nanocrystal Surface Modification (UNSM) on the tribological properties of Inconel 625 alloy produced by selective Laser Melting (SLM). In this research, Inconel 625 samples with simple geometries were produced using the SLM process, followed by surface modification through UNSM. The focus of this study is to analyze the impact of UNSM on microstructure, hardness, surface roughness, friction coefficient, wear resistance, and corrosion behavior of the alloy. Various analytical tools and methods, including optical microscopy, hardness testing, scanning electron microscopy (SEM), roughness measurement, wear testing, X-ray diffraction (XRD) analysis, and corrosion testing using electrochemical impedance and polarization methods, were employed to eva‎luate these properties. The results indicate that the microstructure formed by SLM, characterized by relatively large melt pools, was modified by UNSM to a depth of approximately 15 microns, resulting in a new microstructure comprising deformed grains without altering the chemical composition. This microstructural change led to a more than 63% increase in surface hardness following UNSM treatment. Additionally, surface roughness, which is a notable limitation of parts produced by the SLM method, was reduced by over 90% due to surface peening, which resulted in a tenfold reduction in the friction coefficient. Wear test results demonstrate that after UNSM treatment, the wear rate under test forces of 10 N and 20 N decreased by 37% and 30%, respectively. Wear path analysis indicates that, while the abrasive wear mechanism of the alloy remains unchanged, UNSM-modified samples exhibit increased debris generation and delamination due to reduced work hardenability. Improvements in surface properties, including reduced crystallite size, increased lattice strain, reduced grain size, enhanced hardness, and decreased surface area, have been identified as critical factors contributing to increased hardness and wear resistance, as well as reduced surface roughness of the alloy following UNSM treatment. However, the results obtained from the electrochemical impedance and polarization tests indicate that if the UNSM process is performed at high power, it reduces charge transfer resistance, decreases corrosion potential, and increases corrosion current. Conversely, when the UNSM process is limited to 100 W, the polarization resistance increases from 217 ohms to 360 ohms, and the Tafel curve is obtained without a significant decrease in corrosion properties. The maintenance of a uniform surface passive layer during corrosion following UNSM treatment at 100 W, as confirmed by microstructural examination, can be attributed to a reduction in grain size and an increase in low-angle grain boundaries. However, the microstructural analysis of the corroded surfaces indicated that increasing the UNSM treatment power beyond 100 W led to incomplete formation of a uniform passive layer and an exacerbation of pitting corrosion.

مرتضي هادي

محمد خدائي