Beyond the orthogonal: on the influence of build orientation on fatigue crack growth in SLM Ti-6Al-4V

Abstract

A challenge in developing an in-depth understanding of the crack growth resistance of Additively Manufactured materials is the fact that their mechanical properties have been shown to be both process and part-geometry dependent. Up to now, no studies have investigated the influence of off-axis (beyond the three orthogonal build orientations) orientations on the fatigue crack growth behaviour of selective laser melted Ti-6Al-4V. Furthermore, the widespread use of compact tension specimens for investigating the material behaviour generates data more suitable for plane-strain conditions, rather than the plane-stress state which is more applicable to many lightweight aerospace structures. To address this gap in knowledge, a comprehensive study was carried out to investigate the influence of off-axis build direction in thin SLM Ti-6Al-4V plates, with a focus on the influence of columnar grain orientation on the fatigue crack growth behaviour. It was found that although a macroscopic columnar grain structure is visible on the specimens, it had no discernible influence on the crack growth resistance when the specimen had undergone a stress relieving or HIP heat treatment.

Further Studies into Crack Growth in Additively Manufactured Materials

“…We also see that the cyclic fracture toughness associated with LENS, DMLS, and EBM processes are higher than the values associated with the SLM specimen tests discussed in [29]. However, the cyclic fracture toughness's are slightly lower than the values associated with the SLM specimen tests discussed in [30]. The fact that value of A = 58 MPa √ m used to determine the curve BC1 in Figures 2-5 is close to the value of A = 62 MPa √ m used in [17] to study the potential for the use of AM Ti-6Al-4V as replacement parts suggests that LENS, EBM, and DMLS parts, when experiencing representative flight load spectra, would present a fatigue life that is sufficiently long for use as aircraft replacement parts.…”

“…Here, it should be noted that [30] had an error in the formulae used to determine the stress intensity factor K. Therefore, the da/dN versus ∆K curves given in [30] differed markedly from other studies. The mistake is corrected in the present paper.…”

“…4We also see that the cyclic fracture toughness associated with LENS, DMLS, and EBM processes are higher than the values associated with the SLM specimen tests discussed in [29]. However, the cyclic fracture toughness's are slightly lower than the values associated with the SLM specimen tests discussed in [30]. 2) The variability in the da/dN versus ΔK curves for R = 0.1 associated with LENS and DLMS specimens is similar with that observed in conventionally manufactured materials.…”

“…c) Small crack LENS da/dN -ΔK curves presented in [25], and the small crack growth curve computed in [17] with A = 58 MPa MPa √m and ΔKthr = 0.1 MPa √m. Examining Figure 2, we see that SLM specimens have the most significant difference in the cyclic fracture toughness, with those specimens reported in [29] having the smallest value and those specimens reported in [30] having significantly larger toughness's. Indeed, the cyclic fracture toughness associated with the tests reported in [30] were similar to those seen by Bell Supplier 2, who used Powder Bed Fusion Laser (PBFL).…”

“…In this paper, these tests are labeled Coventry XY, Coventry XZ, and Coventry ZX. (iii) SLM Ti-6Al-4V [30], where the effect of stress relief (SR) and stress relief plus HIPing (HIP) was evaluated on crack growth at angles of 0, 30, 45, 60, and 90 degrees to the build direction. The tests that were performed at R = 0.1 are labeled Delft-p-q, where p takes the values 0, 30, 45, 60, or 90 depending on the orientation of the specimen to the build direction, and q is the specimen number.…”