On the Effect of the Stress Ratio on Fatigue Properties of Ti-6Al-4V Produced by Laser Powder Bed Fusion

Abstract

Fatigue life estimation of Ti-6Al-4V parts produced by additive manufacturing (AM) technologies has received increasing interest during the last decade. Recent studies focused mostly on the fatigue performance of Ti-6Al-4V considering a fixed stress ratio ( R ), usually 0.1 or −1. However, in order to properly design structural components subjected to variable loads, the effect of different stress ratios on the fatigue performance has to be carefully investigated. This research studies the stress ratio influence on the fatigue properties of Ti-6Al-4V specimens produced by laser powder bed fusion (L-PBF). Miniaturized Ti-6Al-4V samples were tested with the step procedure for different R values. A constant life Haigh's diagram ( 2 · 1 0 6 cycles) was generated for L-PBF Ti-6Al-4V in as-built, electro-polished, and machined surface condition. The results present for the first time the relations between alternating and mean stresses for L-PBF Ti-6Al-4V with a fine α + β microstructure when different surface posttreatments are used to enhance the coupons’ final surface quality.

Influence of Mean Stress and Building Orientation on the Fatigue Properties of Sub-Unital Thin-Strut Miniaturized Ti6al4v Specimens Additively Manufactured Via Laser-Powder Bed Fusion

“…An increase in the fatigue strength of the specimens at 10 7 cycles is recorded moving from cyclic tensile to compressive fatigue regimes. This holds for all four batches and is in line with other recently published works, where the fatigue life of AM Ti-6Al-4V specimens at different stress ratios was investigated [39,41,59]. Tensile cyclic loads are usually accelerators of the fatigue failure of the specimens, due to the opening effect that these stresses have on cracks, leading to multiple crack initiation sites on the outermost as-built surfaces of lattice struts.…”

“…A higher drop is instead registered for 45 • (0.38) and particularly 90 • specimens (0.15), showing to be less influenced by the mean stress effect. As reported by Cutolo et al [41], the Walker's coefficient α can be considered as a defect-sensitivity parameter rather than a material-dependent intrinsic property [66]. The authors indeed suggest that this coefficient can be associated to the surface defect density of the specimens also in the frame of Ti6Al4V L-PBF specimens, as previously done also for aluminum alloys [67,68].…”

“…The authors claimed a good agreement between the experimental data with the SWT method. Analogously, Cutolo et al [41], who investigated the mean stress sensitivity in a σ m > 0 regime, suggested that the exponential Walker's equation with a φ = 0.385 coefficient exhibits a good prediction of the experimental data for asbuilt parts. The latter method is an extension of the SWT model, with the adoption of a material-and defect-dependent parameter [41].…”

“…Analogously, Cutolo et al [41], who investigated the mean stress sensitivity in a σ m > 0 regime, suggested that the exponential Walker's equation with a φ = 0.385 coefficient exhibits a good prediction of the experimental data for asbuilt parts. The latter method is an extension of the SWT model, with the adoption of a material-and defect-dependent parameter [41]. Derrick et al [40] thoroughly reviewed numerous works on fatigue of AM metals and particularly as-built L-PBF Ti-6Al-4V at different stress ratios.…”

“…The authors concluded that the SWT method is a dependable predictive tool in tensile conditions, while Walker's equation yields better agreement with experimental results when the mean stress is compressive. Recently, Cutolo et al [41] investigated the influence of positive mean stress range on as-built and post-processed L-PBF Ti6Al4V coupons, confirming that the exponential Walker's equation can provide a reliable prediction of the experimental results for as-built components. However, to the authors' best knowledge, no study has focused on the role of different load ratios according to the different building directions of miniaturized specimens resembling the struts of strut-based lattices.…”