Study on fatigue crack growth behavior of selective laser-melted Ti6Al4V under different build directions, stress ratios, and temperatures

The experimental study of fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was carried out on an in situ fatigue testing machine. Two specimen orientations relative to the build direction (horizontal and vertical), two temperatures (room temperature, RT, and 400 degrees C), and two stress ratios (0.1 and 0.5) were considered, and the FCG curves with the threshold values were determined. The results showed that the FCG properties were affected by the stress ratio due to different degree of crack closure and temperature with grain softening at 400 degrees C in threshold and Paris region. The threshold value is highly dependent on the build direction of the alloy, which was caused by the anisotropic microstructure, while in the Paris region, the effect of material direction can be negligible and FCG rate tends to be consistent. The fracture mechanism at different stages was discussed and revealed by the fracture morphology observation using scanning electron microscopy.