Effect of Micro-arc Oxidation on Corrosion Resistance of Ti6Al4V Electron Beam Welded Joint

Micro arc oxidation (MAO) technology is currently one of the widely used methods to improve the surface properties of titanium alloys. To improve the corrosion resistance of Ti6Al4V electron beam welded joints, MAO technology was applied in this paper. The article studied the influence of MAO treatment on the micro-structure and corrosion resistance of welded joints. The reasons for the improvement of joint corrosion resistance were analyzed. After grinding and polishing, the Ti6Al4V titanium alloy samples were sequentially cleaned by ultrasonic cleaning in acetone and alcohol, and dried for later use. An optimized welding process was adopted for titanium alloy flat plate butt welding. The welding parameters were high voltage of 65 kV, welding beam current of 10 mA, and electron beam linear velocity of 300 mm/min. After welding, the acetone and alcohol solution were used to perform ultrasonic cleaning on the welded samples. Then the base materials and the joints were treated by micro-arc oxidation simultaneously. The electrolyte was a mixed solution of 15 g/L Na2SiO3, 10 g/L Na3PO4, and 1 g/L NaOH. The power parameters included a current density of 14 A/dm2, a duty cycle of 20%, a pulse frequency of 500 Hz, and an oxidation time of 20 minutes. Optical microscope (OM), field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), electrochemical corrosion test and immersion corrosion test were taken to value the morphologies, phase structure, electrochemical corrosion performance and immersion corrosion performance of the base materials, joints and their coatings, respectively. Compared with the base material, the micro-structure of the welded joint underwent significant changes due to the high temperature and low cooling rate. The primary α phase and intergranular β phase were transformed to the acicular α' martensite, distributed in a feather-like pattern on both sides of the grain boundary. After micro-arc oxidation treatment of the base materials and the joint, the morphology of them were similar, showing unevenly distributed micro-pores of different sizes and molten protrusions resembling volcanoes. However, compared with the coatings on the base materials, the aperture and porosity of the joint coatings were relatively smaller. Compared with the base material coating, the thickness, integrity, and continuity of the joint coating were lower. The phases of the MAO coatings in the BM and WZ regions were similar, mainly showing the rutile phase TiO2 and anatase phase TiO2. And the α phase and α′ phase were also appeared in the XRD spectrum due to the small coating thickness and surface micropores, thus allowing the X-ray to penetrate the coatings. Additionally, due to the formation of an oxide film, the β phase disappeared. Compared with the base materials, the self-corrosion potential of the joint decreased by approximately 0.3 V, and the self-corrosion current increased by one order of magnitude. After the MAO treatment, polarization curves of the base materials and the joint appeared obvious passivation, and joint coating group were obviously wider. Although the corrosion resistance of coated joints was lower compared with coated base materials, their self-corrosion potential (Ecorr) was increased by 290 mV, the self-corrosion current densities (Jcorr) was reduced by an order of magnitude, and the corrosion rate was significantly reduced when immersed in 2 mol/L HCl solution compared with uncoated joints. The MAO treatment can improve the corrosion resistance of the joint in neutral and acidic corrosive medias. © 2025 Chongqing Wujiu Periodicals Press. All rights reserved.