1,706 research outputs found
Influence of single- and double-aging treatments on the mechanical and corrosion resistance of Alloy 625
No full textNickel–chromium–molybdenum Alloy 625 exhibits an excellent combination of mechanical
properties and corrosion resistance. However, the high-temperature plastic deformation process and the heat treatment represent critical aspects for the loss in mechanical strength by grain coarsening. This detrimental behavior is worsened by the absence of phase transformation temperatures. However, the chemical composition permits slow precipitation-hardening response upon single aging. Therefore, when the soft- or solution-annealed condition is associated with insufficient mechanical properties, this potentiality can be exploited to improve the mechanical strength. Since the gamma double prime precipitation can be accelerated by double-aging treatment, different time–temperature combinations of double aging at 732 °C and 621 °C are investigated. The simultaneous precipitation of intergranular carbides can dramatically affect the corrosion resistance. Such an undesired phenomenon occurs very quickly at 732 °C, but it is obtained only after very long exposure times at 621 °C. For this reason, a performance chart is developed to compare all the tested conditions. In particular, single aging at 621 °C for 72 h and 130 h are associated with an acceptable combination of mechanical and corrosion properties. Double aging permits a conspicuous acceleration of the aging response. For instance, with double aging at 732 °C 3 h and 621 °C 72 h, it is possible to obtain the same mechanical properties of single aging at 621 °C for 260 h. Such acceleration is accompanied by a more critical corrosion behavior, especially because of the primary step. However, even after its optimization, none of the tested conditions were acceptable
Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
No full textAdditive manufacturing is an innovative solution to produce components characterized by complex geometries. The use of such parts requires a deep knowledge of their behavior under different service conditions, especially from mechanical and corrosion resistance points of view. One of the most well-known and employed materials produced by selective laser melting is nickel alloy 625. It is already commonly used in its conventional form, but the additive manufacturing technology, despite its higher production costs and lower productivity, is becoming competitive because of its excellent mechanical strength. It is in fact significantly higher compared to the conventionally manufactured alloy whose properties are often limited by the difficulty in retaining a fine grain size during plastic deformation and heat treatment. Even though the as-built performance is already quite good, further strength improvement can be attained upon tailored single- and double-aging treatments that are optimized starting from the experimental results obtained in the conventional alloy and also considering the influence on corrosion resistance. In addition, considering that the stress-relieving treatment recommended for the conventional forged alloy at 870 °C is not suitable for the selective laser-melted material because of the more rapid precipitation response, this temperature is optimized to improve both the tensile deformability and the corrosion behavior
Selective laser melted 316L stainless steel: Influence of surface and inner defects on fatigue behavior
No full textIn additive manufacturing, despite its several indisputable advantages, detrimental variables to fatigue strength still remain poor surface finishing and porosities. However, because the majority of defects locate close to the surface, their mechanical removal is expected to appreciably improve fatigue strength. Considering SLMed 316L, fully-reversed rotating-bending fatigue tests in both as-built and machined conditions are performed. Fatigue failures are discussed using the Kitagawa-Takahashi diagram. In each condition, the fatigue stress is related to the equivalent micro-notch length of the killer defect. Then, this work analyses the possibility of predicting fatigue limits at 50 % probability considering the equivalent micro-notch length
Optimization of the mechanical and corrosion resistance of Alloy 625 through aging treatments
No full textIn the as-annealed condition, the nickel-based Alloy 625 has excellent mechanical and corrosion properties compared to those of common stainless steels. This peculiarity enables its exploitation in several industrial fields at cryogenic and high temperatures and in the presence of severely corrosive atmospheres. However, in this alloy, when high-temperature plastic deformation processes and heat treatments are not carefully optimized, the occurrence of excessive grain coarsening can irremediably deteriorate the mechanical strength, possibly leading to incompatibility with the standard requirements. Therefore, this research work investigated the possibility of adopting single- and double-aging treatments aimed at improving such strength loss. Their optimization involved identifying the best compromise between the hardening effect and the loss in corrosion resistance induced by the simultaneous formation of intergranular chromium-rich carbides during aging. The investigation of the aging treatments was performed using hardness, tensile and intergranular corrosion tests considering different time–temperature combinations in a range from 621 °C to 732 °C. Double aging resulted in a considerable acceleration in the hardening response compared to single aging. However, even after its optimization in terms of both temperature and time, the intergranular corrosion resistance remained a critical aspect. Among all the tested conditions, only single aging at 621 °C for 72 h was acceptable in terms of both mechanical and corrosion properties. The influence of longer exposures will be investigated in a future study
High and low cycle fatigue properties of selective laser melted AISI 316L and AlSi10Mg
No full textIn the last years, additive manufacturing has widely adopted to enable lightweight design based on the topological optimization. In fact, this technology allows generation of lattice structures with complex geometries and small thicknesses. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of selective laser melted AISI 316L and AlSi10Mg were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were studied considering the fracture mechanics principles with the Kitagawa-Takahashi diagram. The analysis of fracture surfaces revealed that crack nucleation mainly occurs close to the surface because of both poor surface quality and presence of near-surface defects. As expected, because of the face-centered cubic lattice, the final rupture of all the investigated alloys was characterized by micro-dimples confirming the presence of a ductile behavior
The spinodal decomposition of ferrite in 2507 biphasic stainless steels: embrittlement and possible toughness recovery
No full textBiphasic stainless steels provide an excellent combination of mechanical and corrosion properties. The occurrence of ferrite spinodal decomposition during processing and heat treatment can induce a dramatic drop in impact energy. In this paper, a forged rod of 480 mm diameter made of 2507 biphasic stainless steel was studied. The spinodal decomposition phenomenon upon aging at 475 degrees C (748 K) was investigated by macro- and micro-hardness tests on the constituent phases and by Charpy impact tests. Then, the influence of the microstructural constituents and their morphology on the crack path was studied by optical microscopy, noting possible correlations with the impact energy. Successively, the fracture surfaces of selected specimens were analyzed by a scanning electron microscope (SEM). Finally, reversion heat treatments at 550 degrees C (823 K) and 600 degrees C (873 K) were investigated to evaluate the possibility of recovering the detrimental effects of the alpha' embrittlement. According to the literature, this procedure works well with some biphasic steels, but these steels are characterized by a chromium content lower than that of 2507 grade steel. Regarding the 2507 grade steel studied in this work, complete reversion was obtained by solution-annealing treatment, while reversion heat treatments at both 600 degrees C (873 K) and 550 degrees C (823 K) allowed only a partial recovery
High- and low-cycle-fatigue properties of additively manufactured Inconel 625
No full textIn the last years, additive manufacturing has become a widespread technology which enables lightweight-design based on topological optimization. Therefore, generation of lattice structures with complex geometries and small thicknesses is allowed. However, a complete metallurgical and mechanical characterization of these materials is crucial for their effective adoption as alternative to conventionally manufactured alloys. Industrial applications require good corrosion resistance and mechanical strength to provide sufficient reliability and structural integrity. Particularly, fatigue behavior becomes a crucial factor since presence of poor surface finishing can decrease fatigue limits significantly. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of Inconel 625, manufactured by Selective Laser Melting, were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were deeply investigated considering fracture mechanics principles with the Kitagawa-Takahashi diagram
Correction to: When terminology hinders research: the colloquialisms of transitions of control in automated driving (Cognition, Technology & Work, (2022), 10.1007/s10111-022-00705-3)
No full textIn the original article, author affiliation published with error. The correct affiliations are: Davide Maggi—Institute for Transport Studies, Leeds, UK. Richard Romano—Institute for Transport Studies, Leeds, UK. Oliver Carsten—Institute for Transport Studies, Leeds, UK. Joost C. F. De Winter—Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Delft, The Netherlands. The original article has been corrected.Human-Robot Interactio
Endpoint of the up-down instability in precessing binary black holes
No full textBinary black holes in which both spins are aligned with the binary's orbital angular momentum do not precess. However, the up-down configuration, in which the spin of the heavier (lighter) black hole is aligned (anti-aligned) with the orbital angular momentum, is unstable to spin precession at small orbital separations [D. Gerosa et al., Phys. Rev. Lett. 115, 141102 (2015), 10.1103/PhysRevLett.115.141102]. We first cast the spin precession problem in terms of a simple harmonic oscillator and provide a cleaner derivation of the instability onset. Surprisingly, we find that following the instability, up-down binaries do not disperse in the available parameter space but evolve toward precise endpoints. We then present an analytic scheme to locate these final configurations and confirm them with numerical integrations. Namely, unstable up-down binaries approach mergers with the two spins coaligned with each other and equally misaligned with the orbital angular momentum. Merging up-down binaries relevant to LIGO/Virgo and LISA may be detected in these endpoint configurations if the instability onset occurs prior to the sensitivity threshold of the detector. As a by-product, we obtain new generic results on binary black hole spin-orbit resonances at 2nd post-Newtonian order. We finally apply these findings to a simple astrophysical population of binary black holes where a formation mechanism aligns the spins without preference for co- or counteralignment, as might be the case for stellar-mass black holes embedded in the accretion disk of a supermassive black hole...
Admiel Kosman, Siamo giunti a Dio
No full textInternational audienceSix poems from Israeli poet Admiel Kosman translated from the Hebrew into Italian. Selection of poems, presentation of the author, translation and notes by Davide Mano
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