PSI - Issue 66

Nur Mohamed Dhansay et al. / Procedia Structural Integrity 66 (2024) 87–101 Author name / Structural Integrity Procedia 00 (2025) 000–000

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microstructures for three build orientations. Furthermore, the aim is to identify sources of crack closure effects. In addition, to reduce the closure effects through varied R-ratio investigations and identify the effect of microstructural condition on the Δ K th behaviour of AF, SR and DA LPBF Ti-6Al-4V. Specifically, we aim to describe the role of microstructure on Δ K th in the LPBF AF, SR and DA condition and what differentiates it, if any, from CM Ti-6Al-4V. 2. Materials and method The experimental method primarily investigates the near-threshold FCGRs of LPBF Ti-6Al-4V. Specimens are produced on an ISO 13485 certified LPBF process and manufactured on standard parameter settings. The investigation is conducted on annealed specimens and compared with AF and SR results. 2.1. Specimen fabrication The investigation considers gas atomised Ti-6Al-4V ELI (extra low interstitial) powder obtained from TLS Tecknik GmbH. Via the use of inductively coupled plasma optical emissions spectroscopy, the chemical composition was found to be 6.34% Al, 3.94% V, 0.25% Fe, 0.006% N, 0.082% O and the balance Ti. The particles were atomised to produce a spherical shape and has a size distribution of d 10 = 23 µm, d 50 = 33 µm and d 90 = 46 µm. The EOSINT M280 (EOS GmbH, Krailling, Germany) LPBF machine was used to manufacture the specimens, which is housed at the Central University of Technology in South Africa. Manufacturing parameters are as follows: 30 µm layer thickness, 175 Yb-fibre laser setting, spot diameter 80 µm, hatch spacing 100 µm, scans in rows of 5 mm width with a scan rotation of 67° between each layer and no base plate heating was made use of. The building chamber was flooded with Argon with an oxygen level maintained below 0.12% during manufacture. The CT specimens were manufactured to near-net shape of individual specimens. The crack planes are described by (i) using the normal direction of the plane and, (ii) the crack propagation direction. Using the coordinate system in Fig. 1, the resulting crack plane designation are (i) ZX which is parallel to build layer, (ii) XZ which is perpendicular to build layers, and (iii) XY which is perpendicular to build layers. From here on, the specimen orientation will be referred to by their respective crack planes. Previously, the authors (Becker et al., (2020)) referred to the ZX, XZ and XY orientations as Edge, Vertical and Flat orientation, respectively, as shown in Fig. 1. After manufacture, all specimens were wire cut off the base plate. Two heat treatments were conducted using a 5kW Gallenkamp muffle furnace. One of the heat treatments was a stress-relief (SR) heat treatment, conducted at 480°C for eight hours and furnaced cooled. The purpose of the SR heat treatment was to minimise any LPBF microstructure variations and to reduce residual stress, as suggested by Ter Haar and Becker (Ter Haar and Becker, (2021)). The second heat treatment was a duplex-anneal (DA) to produce a bi-modal microstructure. Specimens were inserted into the furnace at 940°C for two hours, then lowered to 910°C and maintained for eight hours. Thereafter it was water quenched, followed by an anneal at 750°C for four hours with a furnace cool. Once the heat treatments were completed, the individual specimens were milled to a nominal dimension of W = 50mm and B = 6.5 mm, in accordance with the ASTM E647 (ASTM International, (2013)). This allowed for the removal of any alpha case hardening caused during heat treatment. The final step was to polish the CT specimen surfaces to a “mirror-like” finish to visually inspect the crack propagation. 2.2. Specimen analysis For quality purposes, the density measurements were taken using the Archimedes method, in accordance to the ASTM B311. Specimens were immersed in ethanol as to reduce air bubble formation, which could interfere with the density measurement. Specimens were measured to have a relative density of 99.7% and above. Microstructural characterisation was performed using optical microscopy (OM) on a GX51 light microscope (Olympus, Tokyo, Japan). The etchant used was Kroll’s reagent made up of: 100 ml distilled water, 6 ml HNO3 and 3 ml HF. More detailed microstructure observations and fractography were performed on a MERLIN (Zeiss, Oberkochen, Germany) and JSM 7001F (JEOL, Tokyo, Japan) scanning electron microscope (SEM). For phase transformation observations, backscatter detector (BSD) was used in conjunction with SEM.