Laser wire vacuum additive manufacturing (LWVAM) is a highly promising metallic additive manufacturing technology. However, Ti6Al4V alloys fabricated via LWVAM have inferior corrosion resistance due to the higher fraction of acicular α′ phase and larger residual stress from rapid melting and solidification in the laser additive manufacturing process. Previous studies have shown that annealing treatment can improve corrosion resistance in Ti6Al4V parts fabricated by laser additive manufacturing, but their strength and hardness are reduced. As is known, solution-aging treatment (SAT) can not only improve the strength and hardness of titanium alloy, but also its ductility and toughness. Therefore, this study investigates the effect of solution-aging treatment on the corrosion resistance of Ti6Al4V via laser wire vacuum additive manufacturing to explain the underlying reasons for the changes in corrosion resistance based on microstructure analysis.

Ti6Al4V samples are fabricated by laser wire vacuum additive manufacturing. The solution-aging treatment parameters are shown in Table 1. To simplify the description, the as-fabricated sample and solution-aging treatment samples are labeled AM, SAT500 and SAT600, respectively. The microstructure of samples is observed by scanning electron microscope (SEM) with electron back-scatter diffraction (EBSD). The electrochemical tests, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization, are conducted in NaCl water solution with mass fraction of 3.5%. An X-Ray photoelectron spectroscopy (XPS) surface analysis system is used to analyze the elemental composition and valence state of the passive film.

The potentiodynamic polarization results show that the corrosion current density value of the AM sample is about 74.83 times and 7.39 times higher than that of the SAT500 and SAT samples, respectively, indicating that SAT is conducive to improving the corrosion resistance of the sample fabricated by LWVAM. Compared to the SAT600 sample, the SAT500 sample has better corrosion resistance. These conclusions are also borne out by EIS test results. XPS results show that the intensity of TiO₂ is much stronger than that of Ti₂O₃ and TiO, suggesting that TiO₂ is the primary component in the passive film. In addition, it is found that in solution-aging treatment samples, there is more TiO₂ than in the AM sample (Fig.5). Previous studies showed that high-valence oxides had stabler and better protection. Therefore, solution-aging treatment is helpful to obtain better corrosion resistance. The SAT500 sample has the highest TiO₂ content, indicating that the SAT500 sample has the best corrosion resistance. EBSD results show that after solution-aging treatment, the acicular α′ phase in the AM sample changes to the lamellar α phase. Compared to the SAT500 sample, the α lath is coarser for the SAT600 sample (Fig.6). It is also suggested that after solution-aging treatment, local misorientation is significantly reduced and the dislocation density and residual stress decrease significantly (Fig.9). In addition, it is found that the number fraction of low angle grain boundary (corresponding to the misorientation angle of 2°?15°) decreases from 0.343 (AM sample) to 0.203 (SAT500 sample) and 0.105 (SAT600 sample) (Fig.10). As is known, acicular α′ phase is a high energy phase with inferior corrosion resistance. After solution-aging treatment, the decrease of α′ phase is thus beneficial to improve the corrosion resistance. In addition, the dislocation density is prone to corrosion due to the high activation energy. The decrease of dislocation density is thus useful to enhance the corrosion resistance due to the solution-aging treatment. Moreover, it has been suggested that grain boundaries are prone to corrosion due to higher lattice distortion than in the grain interior. Furthermore, the high angle grain boundary energy is higher than that of the low angle grain boundary. The high angle grain boundaries are more vulnerable to corrosive solution attack. In fact, owing to the higher energy, grain boundaries are preferential sites for the nucleation of protective layers. Therefore, a higher number fraction of high grain boundary is found in the solution-aging treatment sample with better corrosion resistance. In addition, it is found that in the SAT600 sample, the αlathis coarser than in the SAT500 sample, leading to the inferior corrosion resistance of the SAT600 sample.

In the present study, the effect of solution-aging treatment on the corrosion resistance of Ti6Al4V via laser wire vacuum additive manufacturing is investigated. Results show that after solution-aging treatment, the corrosion current density of the samples decreases and the passive film formed on the solution-aging treatment sample consists of more high-valence oxide TiO₂,_{which has stabler and better protection. The reason is due to less acicular α′ phase and more high angle grain boundary in solution-aging treatment samples, showing solution-aging treatment to be conducive to improving corrosion resistance. Moreover, compared with the SAT500 sample, the αlathis coarser in the SAT600 sample, leading to inferior corrosion resistance.}