PSI - Issue 75

Ninian Sing Kok Ho et al. / Procedia Structural Integrity 75 (2025) 35–42 Ninian Sing Kok Ho/ Structural Integrity Procedia (2025)

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has the most potential to fabricate net shaped parts. To fully take advantage of this strength of LPBF, the fatigue performance of parts with as-built surfaces must be investigated more deeply. Many studies have recognized the need to address this in the past decade, e.g. [1-4]. In general, fatigue properties are influenced by surface roughness, defects, and microstructure, ranked from most to least significant [5]. LPBF requires the use of support structures when parts are printed at very small angles from the substrate plate, since each layer is built on the previous layer. While this constraint can be mitigated at times, it is not always avoidable. For instance, parts with a larger footprint than the substrate plate must be oriented vertically to be fully within the build envelope, limiting the orientation during printing. More attention must thus be paid to the removal of support structures and its impact on fatigue life. This is investigated here using LPBF Ti6Al4V. Ti6Al4V is a widely used alloy for aerospace applications due to its low density and high yield and tensile strength [6]. Its fatigue performance must be guaranteed to match its static properties before it can be used for critical applications. Finally, these samples are compared against vertical, smaller samples with uniform as-built surface around the gauge section to show the typical scatter in S-N data that can be expected using the OEM parameter set. 2. Methodology 2.1. Sample Fabrication An SLM 280 machine was used to fabricate High cycle Fatigue (HCF) samples designed in accordance with ASTM E466 (Figure 1a, b). The laser power was 350 W, the scan speed was 1400 mm/s, the hatch spacing was 0.12 mm, and the layer thickness was 30 μm. The “Stripes” scanning strategy was used. This corresponds to the OEM recommended parameter set. OEM Ti6Al4V powder (20 – 63 μm) from SLM Solutions was used to ensure compatibility and for quality assurance. Sandblasting was performed on all samples before removal from the substrate plate. The samples were printed with the loading axis parallel to the substrate plate, resting on the thickness face. Support structures were needed for the hourglass portion which was not in contact with the substrate plate. The first two batches of samples were wirecut from the plate and the support structures were removed by manual grinding. Two levels of grinding were studied: quick, production focused grinding for the first batch (Figure 2a), and refined precision grinding for the best result with a manual grinder (Figure 2b). For the third batch, the support structure surface was milled to remove the human factor (Figure 2c) [7]. For the fourth batch, a heat treatment was applied before sample removal from the plate, and the same milling technique as in the 3rd batch was used. For the heat treatment, the parts were heated at 800 degrees C for 2 hours in a vacuum furnace while attached to the substrate plate. a) b) c)

Figure 1: a) Example of a completed print of Ti6Al4V samples, b) photo of a single sample measuring 210 mm in length, and c) fatigue test setup under stress control