Effect of stress relief annealing on microstructure and corrosion behavior ofselective laser melted TC4 titanium alloy

To clarify the influence of stress-relief annealing temperature on the microstructure, mechanical properties, and corrosion behavior in artificial seawater of selective laser melted (SLM) TC4 titanium alloy (SLMed TC4), specimens were vacuum-annealed at 600, 700 and 800 ℃ for 1 h. X-ray diffractometer, electron backscatter diffraction, scanning electron microscope, nanoindentation, Vickers hardness testing, and electrochemical measurements were employed to characterize the microstructure, mechanical response, and passivation behavior. The results show that with increasing annealing temperature, the acicular α′ martensite gradually coarsens and decomposes, and an α+β two-phase structure forms after annealing at 800 ℃. The elastic modulus increases from 102.40 to 133.98 GPa, and the plastic work increases from 1 862.29 to 2 371.29 pJ, whereas the Vickers hardness decreases, indicating that the release of residual stress and microstructural evolution jointly alter the mechanical response. The corrosion current density decreases from 1.43×10^-7 to 9.83×10^-8 A·cm^-2, and the polarization resistance increases from 7.33×10⁵ Ω·cm² to approximately 7.58×10⁵ Ω·cm². The specimen annealed at 800 ℃ exhibits the lowest steady-state current density and the largest slope of the Mott-Schottky plot, suggesting a reduced carrier density and enhanced stability of the passive film. Overall, elevating the stress-relief annealing temperature enables synergistic regulation of the phase constitution, microstructural scale, and internal stress state in SLMed TC4, promoting the formation of a more stable and protective passive film, thereby improving its corrosion resistance in artificial seawater.