Corrosion Behavior of Additively Manufactured Alloy 625, A Nickel Based Superalloy

Abstract

Despite growing interest in additive manufacturing, there remains a significant gap in research concerning corrosion behavior of wire-based methods. Most of the existing studies have predominantly focused on powder-based methods, leaving wire-based techniques relatively underexplored. This disparity in research attention highlights the need for additional corrosion research involving wire-based methods to fully understand their potential benefits and applications.In this study, the corrosion behavior of alloy 625 produced by laser-wire directed energy deposition (LW-DED) and wire arc additive manufacturing (WAAM) was investigated using electrochemical techniques and compared to the wrought alloy. The differences were examined between surface conditions (polished or unpolished), heat treatment conditions, and manufacturing methods. Samples were subjected to open circuit potential, Tafel analysis, potentiodynamic, and potentiostatic testing (+1V vs. OCP) in a 3.7% hydrochloric acid electrolyte, employing a silver/silver chloride reference and platinum counter electrode. Post-test characterization included optical and scanning electron microscopy with energy dispersive spectroscopy. Additive manufacturing processes such as laser wire and wire arc directed energy deposition, build material layer by layer with localized melting and solidification of a wire feedstock. The specific microstructure is affected by many factors such as the alloy composition, temperature gradient, cooling rate, and subsequent heat-treatment. In nickel-based superalloys the as-built microstructure from additively manufacturing is generally defined by constitutional supercooling, which results in a dendritic structure oriented in the build direction. The underlying microstructure influences the characteristics of the material. For a component built using additive manufacturing to be put into service, understanding the performance in the desired application is extremely important. Some such performance standards might include tensile strength, fatigue behavior and corrosion resistance. Corrosion is of particular interest for applications such as nuclear power or aerospace, where the harsh operating environment degrades material performance. This study examines the microstructure, electrochemical behavior, and post-corrosion characterization of additively manufactured (LW-DED and WAAM) nickel-based superalloy 625. Wrought alloy 625 is also tested in the same electrochemical test sequence in order to make comparisons with the printed samples. The following has been shown: • LW-DED and WAAM both produce dendritic microstructures as a result of constitutional supercooling. • WAAM produces coarser microstructure, which includes columnar and equiaxed dendrites depending on the location relative to the weld centerline, as well as titanium-rich precipitates. • LW-DED produces a finer microstructure, including columnar dendrites along the build direction. • In corrosive conditions, the interdendritic regions, consisting of mostly niobium and molybdenum, are susceptible to accelerated attack for both LW-DED and WAAM material. • LW-DED has a higher corrosion potential and lower corrosion current/rate than WAAM. • Both LW-DED and WAAM have lower corrosion potentials than wrought alloy 625. • WAAM has a higher corrosion rate compared to the wrought alloy. • LW-DED has a similar, but slightly lower corrosion rate than wrought. • Heat treatment increases the corrosion potential and decreases the corrosion current/rate for both LW-DED and WAAM. • LW-DED produces coarse carbides during heat-treatment, which become sites for pitting at high applied potentials resulting in a higher current response/reaction rate in potentiostatic testing. • Smoother/polished surfaces lead to higher corrosion potentials and lower corrosion currents/rates.