Objective Ti-6Al-4V alloy is a typical alpha+ beta-type titanium alloy combining the advantages of alpha and beta titanium alloys. It has excellent mechanical properties and is the most widely used titanium alloy in marine engineering. Generally, corrosion occurs on the surfaces of titanium alloys due to the severe ocean environments. Thus, the corrosion resistance of repaired surfaces is essential for the service safety of the structural parts composed of titanium alloys. Recently, the in-air directed energy deposition (in-air DED) technique has been rapidly developed to repair damaged structures with high performances. Thus, we propose a novel technique, namely, underwater directed energy deposition (UDED), which introduces in-air DED into underwater environments. UDED has many advantages such as concentrated heat input and small heat-affected zones. Previous studies show that most of the current research has focused on the corrosion resistance of titanium alloys fabricated using in-air additive manufacturing methods. A few research studies have reported the electrochemical corrosion behavior of Ti-6Al-4V repaired using the powder-blown UDED technique. Therefore, studying the corrosion behavior of the UDED-repaired samples is essential. The results of this study are useful for the performance assessment of components repaired using UDED and optimizing the UDED process. Furthermore, this work provides an experimental basis and theoretical foundation for advancing the on-site repair of titanium alloys using UDED. Methods Herein, the Ti-6Al-4V alloy was employed as the testing material. First, a trapezoidal groove was machined on the as-received Ti-6Al-4V plates using wire-cut electrical discharge machining before UDED. Then, the preprepared Ti-6Al-4V was repaired in a water tank using UDED. For comparison, the preprepared Ti-6Al-4V was also repaired using the in-air DED in a homemade protective bag filled with high-purity argon. The as-deposited samples were subjected to standard metallographic preparation for OM, SEM, TEM, and EBSD observations. Then, we conducted microhardness measurements on the repaired Ti-6Al-4V zone and determined the average microhardness. Next, we conducted the electrochemical tests on the samples in 3.5% NaCl solution at room temperature using a PARSTAT 2273 electrochemical workstation with a conventional three-electrode cell. Before the electrochemical tests, the test specimens were immersed in the 3.5% NaCl solution for 24 h. Finally, the open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) measurements were performed on the samples using the electrochemical workstation. Results and Discussions Several key findings are reported in this study. 1) The microstructure of Ti-6Al-4V fabricated using UDED is significantly finer than that using in-air DED (Fig. 3). This is attributed to the water quenching effect caused by the underwater environment, which increasing the cooling rates of the underwater melt pool and reducing the heat accumulation of the as-deposited material. This contributes to the rapid solidification of the melt pool and fast solid-state phase transformation, leading to the formation of an acicular martensite microstructure. 2) The results of EBSD show that the microstructures of Ti-6Al-4V repaired by UDED and in-air DED have multiple orientations (Fig. 4). The volume fraction of the beta-phase in the sample repaired using in-air DED was slightly higher than that in the sample repaired by UDED. 3) The TEM results show that the dislocation density in the sample repaired using UDED is high due to the steep temperature gradient and rapid cooling rates in the UDED process (Fig. 5). However, the dislocation density in the sample repaired using in-air DED is low due to the significantly intrinsic heat treatment during the in-air DED process. 4) The microhardness of the sample repaired using UDED is relatively high (Fig. 6). The microstructure is strengthened by three factors, namely, the supersaturation of Al and V elements within the Ti matrix, small size of the acicular martensite, and high dislocation density within the lath martensite. 5) The Tafel polarization curves show that the UDED-repaired samples are the least susceptible to corrosion. However, when corrosion starts, their corrosion rate is relatively fast (Fig. 7). Additionally, pitting corrosion occurs on the surface of UDED-repaired samples. 6) The EIS results show that the Ti-6Al-4V repaired using UDED has the largest arc radius (Fig. 8), indicating that it has better corrosion resistance. 7) The influencing factors of microstructural features on the corrosion behavior of the Ti-6Al-4V are the grain size of the as-deposited microstructure, distribution of alloy elements, and surface activity state. Conclusions Herein, we repaired the preprepared trapezoidal grooves on the Ti-6Al-4V substrate using UDED and in-air DED. The mechanisms of different deposition processes influencing the microstructural formation/evolution, microhardness, and electrochemical corrosion behavior were revealed. The main conclusions are as follows. 1) The water quenching effect increases the cooling rates of the melt pool and decreases the thermal accumulation of the as-deposited metal. This results in the formation of acicular martensite with a high dislocation density. 2) The supersaturation of Al and V elements in the Ti matrix, fine acicular martensite, and high dislocation density increase the microhardness of UDED-repaired samples. 3) The corrosion resistance of UDED-repaired samples is better than that of in-air DED-repaired samples. The corrosion resistance of the former is mainly determined by three factors: the grain size, distribution of alloy elements, and surface state of the microstructure. The satisfactory corrosion resistance of titanium alloy repaired using UDED is important for their service safety. Additionally, the results of this work provide an experimental and theoretical basis for improving the corrosion resistance of titanium alloy structural parts repaired using UDED.
