Annealing heat treatments are highly recommended to improve the properties of Ti6Al4V titanium alloy components produced by laser powder bed fusion (LPBF) and used for critical applications such as surgical implants. Therefore, this study investigates the influence of the stress ratio on the fatigue crack growth (FCG) behavior of stress-relieved and annealed Ti6Al4V-ELI. For this purpose, an as-built Ti6Al4V-ELI block was first stress-relieved at 650 °C and subsequently annealed at 950 °C. Compact tension (CT) specimens wire-cut from this block were tested at four different stress ratios of 0.01, 0.1, 0.5, and 0.8. The results demonstrated that the stress ratio influences the FCG rate and the FCG threshold of stress-relieved and annealed Ti6Al4V-ELI fabricated by LPBF. With increasing stress ratio, the FCG curve moderately changed, the FCG rate slightly increased and the FCG threshold decreased more considerably at the identical stress intensity factor ranges. Applying higher stress ratios reduced the impact of the opening load on the effective stress intensity factor range, decreasing the crack closure effect without affecting the roughness of the fracture surfaces. Therefore, the decrease in the FCG threshold and increase in the FCG rate could be simultaneously attributed to the reduced crack closure effect and increased mean stress at the higher stress ratios.
Mostahsan et al. (Wed,) studied this question.