PSI - Issue 54

Daniel F.O. Braga et al. / Procedia Structural Integrity 54 (2024) 631–637

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4 Daniel F.O. Braga et al. / Structural Integrity Procedia 00 (2023) 000–000 be somewhat mitigated through post-processing. Svensson (2009) studied fatigue strength of Ti6-Al4-V manufactured through EBM, as-fabricated and with post processing through hot isostatic pressing (HIP). It was reported an increase of fatigue strength at 10 7 cycles of 24% in the Z direction (from 407 MPa to 538 MPa) and 27% in the X-Y direction (from 441 MPa to 607 MPa) through HIP. The anisotropy difference in fatigue strength went from 8% to 11% with HIP. Feature definition and surface roughness is linked with fatigue strength, with some studies comparing different metal additive processes and demonstrating that higher resolution processes, and which result in higher surface finishes, have higher fatigue strengths and fatigue lives. Greitemeir et al. (2013) studied both fatigue strength and surface roughness of EBM and LBM of Ti6Al4V, and achieved a linear relation between both. Also LBM resulted in higher fatigue life than EBM, as EBM resulted in higher surface roughness. Rengers (2012) reported SLM Ti6Al4V as-fabricated was twice as smooth as manufactured through EBM, resulting in higher fatigue strength for the lower roughness SLM components. Given the geometric complexity of components made with metal AM technologies, this is particularly important as in some geometric features, may not be feasible to perform any post-processing. Regarding hybrid SLM-LMD AM components, there is still gap in the literature. This gap is significant given the business case for the use of these two AM processes in combination, as the mixture of complex geometrical features, high detail and surface quality of SLM with high build rate, flexibility, and large build volume of LMD enable many applications. Also, it is observed in various studies, that AM related micro porosity and surface finish, play a significant role in fatigue strength and life Frazier (2014). As such, the use of the hybrid approach to manufacturing AM parts, should be undertaken with care and further experimental data on the topic should be gathered to assist on the correct design of these components, especially regarding location and direction of transitions in metal AM processes in fatigue sensitive components. 3. Fracture and Fatigue Crack Growth in Metal AM Damage tolerant structures require accurate and deep understanding of the fracture behavior of mate rials and components. As stated throughout this review, the layer-by-layer nature and thermal history of metal AM processes result in non-homgeneous microstructures, which impact the mechanical performance of the components made with these processes. Fracture toughness, and fatigue crack growth is highly depen dant of microstructural changes, and as such, metal AM components will have different fracture behaviour when compared to other more conventional manufacturing processes. Becker et al. (2021) reviewed fracture toughness and fatigue crack growth in additive manufactured metals. Regarding fracture toughness, K Ic , it is reported that higher crack tortuosity or mixivity in the cracking mode is observed in AM alloys, due to resulting mesostructure, leads to simultaneous increase in K Ic and in crack resistance. Examples of such strengthening in AM alloys regarding their conventionally produced counterparts is reported by Suryawanshi et al. (2016) for SLM AlSi12, and by Paul et al. (2021) in SLM AlSi10Mg, by carefully selecting process parameters. Besides the changes of size of plastic zone ahead of the crack tip r p , crack tip blunting, and crack tortuosity, induced by the resulting microstructure and mesostructure of AM alloys, residual stress due to the thermal history of AM processes, also influence fracture toughness and crack resistance. As reviewed by Becker et al. (2021) metal AM processes result in defects and detrimental residual stress distributions which lower K Ic and crack resistance, while the mesostructure can significantly improve them. Apart from AM process parameters tuning, post-processing can significantly affect fracture toughness and fatigue crack strength, through diminishing defect size and normalizing residual stress. In SLM Ti6Al4V K Ic values between 75 and 106 MPa √ m were reported by Kumar and Ramamurty (2019) and Dhansay (2015) after duplex heat treatments, which is significantly higher than as-printed material. The improvement in K Ic from heat treatment in Ti6Al4V, is related to the formation of lamella α − β microstructure, which is more ductile. LMD higher build rate results in a significantly different micro and mesostructure than SLM, which as disscussed previously impacts the fracture toughness and crack resistance. Wang et al. (2023) studied fatigue crack growth in LMD TA19 and reported that crack deflection occurs at colony boundaries or columnar prior- β grain boundaries. Through this mechanism, the correct selection of scanning strategies and process parameters, and post process heat treatment, can lead to lower fatigue crack growth rate.