Investigation into Microstructural and Electrochemical Characteristics of Laser-melted Metallic Alloys

摘要

It is generally accepted that laser surface melting (LSM) can be used for improving the corrosion resistance of metallic alloys as a result of homogenization/refinement of microstructures, dissolution/redistribution of precipitates or inclusions and phase transformations. However, some studies have shown that laser-melted surfaces do not always exhibit enhanced corrosion performance. To date, no satisfactory explanation has been given for the findings, in particular concerning the relationship between laser operating conditions, microstructure and electrochemical responses. In addition, for laser surface treatment of large components, overlapping of individual tracks is often required. This involves re-melting and re-heating of a portion of the previous track and results in microstructural changes, such as precipitate coarsening, microsegregation and phase transformation, which may have significant influence on localised corrosion within the overlapped regions.;This study investigates and compares the corrosion behaviour of laser-melted metallic alloys, including Al 2014-T6, Al 2024-T351, 304L austenitic stainless steel, and dual-phase 3CR12 steel, to gain insight into the factors influencing the pitting corrosion resistance of various alloys, with consideration of microstructural homogenization/refinement, electrochemical nature of various intermetallics with respect to the solid solution matrix, heat-affected zones produced by overlapping, and optimised cooling rate. Laser surface melting was carried out using a 3 kW CW Nd:YAG laser (1.06 mum) with a line beam profile and a 2 kW Rofin-Sinar CO2 laser (10.6 mum) with a circular beam profile. Microstructural characterization and compositional analysis were performed using optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy, electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Electrochemical studies of the laser-melted surfaces studied in this work include: open circuit potential measurement in aerated 1M NaCI solution and potentiodynamic polarization (in both 1 M and 3.5% NaC1 solution, for Al alloys and steels respectively), to determine the pitting potential, corrosion initiation and propagation; and double-loop electrochemical potentiokinetic reactivation (DL-EPR) in 0.1 M H2SO4 for detecting the chromium-depleted regions and the presence of martensite in the 3CR12 steel. (Abstract shortened by ProQuest.).