PSI - Issue 54

Daniel F.O. Braga et al. / Procedia Structural Integrity 54 (2024) 631–637 Daniel F.O. Braga et al. / Structural Integrity Procedia 00 (2023) 000–000 5 Stainless steel alloys are also used extensively in additive manufacturing, but their fracture toughness is reported significantly lower than their conventionally manufactured counterparts. As example in 316L, K Ic ranges from 112 and 278 MPa √ m in non additive alloys and 63 and 87 MPa √ m when produced through SLM, Becker et al. (2021). The reasoning for this reduction might be the presence of defects, reduced ductility and the absence of transformation-induced plasticity (TRIP). Kumar et al. (2021), demonstrated this effect in SLM 304 stainless steel by studying tensile properties and fracture at room temperature and 75 º C (temperature where TRIP becomes inactive, and the deformation mechanism is dominated by dislocation glide and twinning). The authors reported that with this increase in temperature, the fracture toughness reduced by ≈ 40 % and anisotropy increased y ≈ 16%. As in fracture toughness, fatigue crack growth resistance is highly dictated by micro- and meso-structures, derived from process parameters, orientation and heat treatment. Becker et al. (2021) reviewed fatigue crack growth (FCG) characteristics of titanium, steels, Ni superalloys and Al-Si alloys made through AM. It was reported that in general, FCG in AM alloys was generally similar to cast or wrought counterparts, but fine AB microstructure inherent to AM processes lead to lower FCG thresholds. Roughness induced closure effects, related with the mesostructure, however, can lead to improvement in near-threshold FCG behaviour, at low R . The significant influence of the microstructure in the near threshold regime means that through post heat treatment, near threshold FCG resistance can be improved and anisotropy reduced. Regarding hybrid SLM-LMD AM process, given the different micro- and meso- structures are formed during each process, and remelting of the SLM alloy at the transition zone due to the LMD process, as reported by Liu et al. (2016), fracture and fatigue crack growth characteristics should be complex. Given that both fracture toughness and fatigue crack growth resistance is reported to be derived from micro-, meso- structure, and less so from residual stress, this is a hot topic that requires further study relating process parameters, and sequencing with fracture and crack resistance. This is still and incipient field, and such studies still lack in the literature. 4. Other failures in Metal AM Corrosion is a major concern in structural integrity, as environmental conditions may lead to a degra dation of key mechanical properties and result in premature failure. Given the metalography of metal AM components, corrosion resistance will differ from components made through subtractive manufacturing or casting processes. Hemmasian Ettefagh et al. (2019) studied the corrosion of Ti6Al4V manufactured through SLM. It was found that corrosion rate of as-fabricated AM parts was close to sixteen times worse than the commercial grade samples, due to the presence of non-equilibrium phases, resultant of the thermal history of the metal AM process. Post heat treatment, through annealing at 800 º C for 2h, ameliorated this accelerated corrosion rate, due to stress relief of the martensitic phase and formation of BCC phase of β Ti6Al4V, which has higher corrosion resistance. Ko et al. (2021) reviewed corrosion on metal AM stainless steels, involving various AM processes, different stainless steel types, and corrosion environments. The authors reported that in 316L manufactured through SLM, scan speed, laser power, and laser energy density, influenced significantly grain size, and the amount and size of voids, which rules corrosion resistance. Also post-treatment affected the passive film stability, the repassivation potential, and the oxide composition in components manufactured in this alloy through this AM process. It is also reported that the most important factor towards metal AM corrosion resistance in stainless steel is the post-heat treatment applied. Given the formation of complex microstructures, due to remelting of SLM processed zones, through LMD, corrosion in hybrid SLM-LMD components should be evaluated carefully. This is especially true when dealing with dissimilar material hybrid AM process, as there is the high chance of the formation of intermetallic compounds and lead to accelerated corrosion failure in these locations.

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