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

Abstract

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 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 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.

Study on small crack growth behavior of laser powder bed fused Ti6Al4V alloy

“…The horizontal plane shows equiaxial β grain morphology (Figure 2A) while the vertical plane shows elongated β columnar grain morphology (Figure 2C) due to the directional nature of the SLM process, 8,23,30 and both of them are with α + β structure (Figures 2B and 2D). The light gray phases are α which is the dominant fraction and the dark gray phases are β of which the fraction is very low and the above results have been demonstrated by X‐ray diffraction (XRD) spectra in our previous study 31 . The α phase nucleates at β grain boundary called grain boundaries α ( α GB ), as shown in Figure 2D.…”

“…In this paper, small crack growth is defined as the crack growth behavior which do not penetrate specimen thickness known as the three‐dimensional (3D) crack growth stage. However, the long crack growth behavior is the crack growth behavior after penetrating the specimen thickness, known as the two‐dimensional (2D) crack growth stage and more details can be found in our previous work 31 . It can be seen that small crack can still grow below the threshold value of long crack, and at the same SIF range Δ K , the FCG rate of small crack is higher than that of long crack.…”

“…To test the small‐crack effect of SLM Ti6Al4V alloy, the small FCG data of the in situ specimen were compared with the long FCG data determined in our previous work, 31 as shown in Figure 13B,C. In this paper, small crack growth is defined as the crack growth behavior which do not penetrate specimen thickness known as the three‐dimensional (3D) crack growth stage.…”

“…The light gray phases are α which is the dominant fraction and the dark gray phases are β of which the fraction is very low and the above results have been demonstrated by X-ray diffraction (XRD) spectra in our previous study. 31 The α phase nucleates at β grain boundary called grain boundaries α (α GB ), as shown in Figure 2D. Then α plates grow and the colonies are formed from the α GB into the β grain with lamellar microstructure which consist of parallel α plates.…”

“…These observations are consistent with existing studies on AM Ti6Al4V alloy. 23,32,33 The specimens used in the in situ fatigue test were all designed in vertical direction (Figure 3A) which showed the lower FCG resistance compared with horizontal specimens in our previous study 31 and will be the first to suffer damage in the application of engineering structures and components. The specimens were electric discharge machined to the designed geometry (Figure 3B).…”