Study on Mechanical and Corrosive Properties of CuNi2SiCr Copper Alloy Layered by Directed Energy Deposition on Nickel-aluminum Bronze (NAB) Substrate

Abstract

Marine propeller is one of the core components of ship power system. The quality of propeller manufacturing directly affects the performance and propulsion efficiency of the entire ship. Marine propellers have been immersed in a high-speed rotating environment for a long time. When rotating at high speed underwater, the propeller will be simultaneously scoured and corroded by seawater, as well as marine organisms. Therefore, various damages occurs on its surface, such as cavitation corrosion, cracks, and etc. Thus, the repair process of damaged propellers is an important research topic. Nickel-aluminum bronze (NAB) is a Cu-Al-Ni-Fe-Mn series alloy in which nickel, iron and manganese are added to the Cu-Al binary alloy. NAB is widely used to fabricate marine propellers owing to its excellent corrosion resistance in seawater. This research proposes a repair method employing the metal additive manufacturing (AM) technique, rather than conventional welding. Of the various metal AM technologies, we studied directed energy deposition (DED), an advantageous approach to repairing damaged components. The DED process uses high-power lasers to melt metal powder and then solidify it on the site to be repaired to achieve the repair of damaged parts. For repairing NAB substrate, CuNi2SiCr copper alloy powder was used. This study focused on the mechanical and corrosive properties of CuNi2SiCr deposited on the NAB substrate through DED. First of all, the optimal parameters of the DED process were established by varying the laser power, powder feed rate, scanning speed, coaxial gas flow, and powder gas flow rate. Afterward, the mechanical properties of the deposited material and substrate were studied through microhardness, tensile tests and Charpy impact test. The microstructure observation showed that the microstructure of the deposition layers was 𝛼-Cu, characterized by low strength, hardness and high toughness. The NAB substrate exhibited higher hardness and strength than the CuNi2SiCr deposited, because it was composed of a typical 𝛼 + 𝛽 solid solution and different intermetallic 𝜅 phases. Additionally, the tensile strength and elongation of the deposited specimens were lower than those of the substrate because the deposition layers have micropores. Moreover, the impact test shows that the toughness of the deposited specimen is 4.5 times that of the substrate specimen, which is attributed to the deposition layer containing more high-toughness 𝛼 phases. The result of electrochemical corrosion test shows that the corrosion current density of the deposited specimens (11.8 μA/cm2) is slightly higher than that of the substrate (13.06 μA/cm2), which indicates that the CuNi2SiCr deposited has slightly better electrochemical stability. This is because there are multiple phases in the substrate, and the corrosion potentials of different phases are different, which form a miniature galvanic cell and speed up the electrochemical reaction process. The results of the cavitation erosion test show that the corrosion rate of the deposited under the mechanical impact of bubbles is significantly higher than that of the NAB substrate, owing to its low hardness and the presence of pores. Although there were pits and cracks on the corroded surface of the substrate, it exhibits a relatively flat shape. The result of static immersion test shows that the mass loss rate of the substrate is significantly lower than that of the deposited due to a dense protective Al2O3 oxide film formed on the surface of the NAB substrate. This study indicates that it is feasible to deposit CuNi2SiCr copper alloy powder on NAB substrate by DED, which provides a new idea for repairing NAB parts through metal additive manufacturing. However, the main problem is that the performance of the repaired area is lower than that of the substrate material. In future research, the focus should be on reducing the internal porosity and enhancing its hardness and strength of the CuNi2SiCr deposited parts.