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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1. Introduction Investigating the structural integrity of laser powder bed fusion (LPBF) produced Ti-6Al-4V is of interest to researchers as this allows for confidence of its use in industry. These industries include aerospace, biomedical, and automotive, amongst others. This powder bed fusion additive manufacturing (AM) method produces parts in a layerwise fashion with the use of laser power to print the part. Under the standard printing process parameters, parts often contain a brittle martensitic microstructure, large residual stresses, rough surface finishes and porosity. These inherent properties are known to influence the structural integrity of the material. Fracture mechanics methodologies are commonly used to characterise the structural integrity of a component. In particular, the fatigue crack growth rate (FCGR) Paris curve/regime and fracture toughness (K ic ) are used to assess the structural integrity of a part. A less common approach used on LPBF Ti-6Al-4V found in literature is the fracture mechanics near-threshold FCGR approach. One common method of fatigue behaviour is the fatigue life or SN approach, which is a measure of the material’s ability to resist crack initiation. All the inherent LPBF properties affect the fatigue life, however, the rough surface finishes and porosity are the most severe. Whereas the fracture mechanics approaches are predominantly affected by the brittle martensitic microstructure and residual stress. It is well established that conventionally manufactured (CM) Ti-6Al-4V is microstructurally sensitive to process conditions, resulting in a variety of mechanical properties (Becker et al., (2020)). The CM Ti-6Al-4V typically has either a lamellar, bi-modal or equiaxed microstructure, which generally result in different mechanical properties to each other with each having their own benefits and drawbacks. With regards to LPBF Ti-6Al-4V, as-fabricated (AF) microstructures are typically not used for fatigue applications, thus, investigations into fatigue behaviour of heat treated LPBF Ti-6Al-4V is of significance. However, it is contested within literature as to which of these CM Ti-6Al 4V microstructures produces the best fatigue behaviour. Some investigations (Zuo et al., (2008))(Niinomi et al., (1999)) show that bi-modal microstructures has better fatigue properties than lamellar microstructures while other investigations (Nalla et al., (2002))(Hines and Lütjering, (1999)) show that lamella microstructures produce better fatigue properties than bi-modal microstructures. While this is contested, bi-modal microstructures are known to be used in the aerospace industry for applications in the low temperature stages of turbine engines (Nalla et al., (2002)). While these differences exist within literature, it is generally understood that α grain size plays an influential role in the near-threshold FCGRs (Newman, (2000); Pippan, (1991); Vasudeven et al., (1994); Wasén et al., (1988)). However, others have also noted the primary α p size, morphology, colony size, lath size, α p content (volume fraction, V α ) and α grain connectivity have an influential role (Kumar et al., (2018); Oberwinkler et al., (2010); Wu et al., (2013)). Regarding LPBF Ti-6Al-4V, various researchers (Leuders, Thöne, Riemer, et al., (2013))(Wycisk, Solbach, Siddique, Herzog, Walther, et al., (2014))(Kumar et al., (2018); Kumar and Ramamurty, (2019))(Becker et al., (2020))(Tarik Hasib et al., (2020)) have investigated the fatigue behaviour (SN and fracture mechanics approaches). However, a variety of fatigue behaviour is observed. In some cases, the as-fabricated (AF) microstructure produces fatigue behaviour comparable to CM Ti-6Al-4V, while in other cases the AF Ti-6Al-4V had inferior fatigue behaviour than CM Ti-6Al-4V. This further extends to the reasonings behind the different fatigue behaviour. In some cases, the inferior behaviour of AF Ti-6Al-4V is primarily attributed to the brittle microstructure while in other cases it is primarily attributed to the residual stress causing the inferior behaviour. While the reasonings are all valid based on the respective investigations, it highlights that a better understanding of the mechanism’s involved are required. A method which better pronounces the effects of microstructure and residual stress is required. Some of the investigations which considered the near-threshold FCGRs, also found some variation in fatigue behaviour. In some cases(Becker et al., (2020); Kumar et al., (2018); Kumar and Ramamurty, (2019); Leuders, Thöne, and Riemer, (2013); Wycisk, Solbach, Siddique, Herzog, and Walther, (2014)), an isotropic behaviour is observed while in others (Tarik Hasib et al., (2021)), anisotropy observed was negligible. Furthermore, most only considered this at a single R-ratio, R = 0.1, apart from the previous work by the authors (Becker et al., (2020)). This means that crack closure effects are still significant, and the results are not representative of the intrinsic near-threshold FCGRs for the given microstructures. Higher R-ratios would allow for a more intrinsic Δ K th result, better representing the microstructural effects on fatigue behaviour. The current study investigates the fatigue fracture mechanisms by considering the improvement in near-threshold FCGRs of LPBF Ti-6Al-4V between AF, stress relieved (SR) microstructure and duplex anneal (DA) bi-modal