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Determination of mechanical state of thermal oxide growing during high temperature oxidation is a challenge. Fabrication of nanogauges for monitoring during high temperature oxidation was presently investigated up to 1000 °C. Several materials were tested in terms of mechanical behaviour and maximum working temperature for gauges used as markers during thermal loading under air. The experimental determination of the optimized gauges material for high temperature oxidation was validated. SiO2 offers particularly interesting features for gauges to determine the mechanical state of metal/oxide system. In this article, a special attention has also been paid to two nano-fabrication processes, as well as their limits. The standard electron beam lithography process is well suited to build gauges for oxidation applications, and can be improved by use of reactive ion etching process. The gauges can eventually reach several micrometers height such that the oxidation layer does not cover the gauges during thermal loading for relevant monitoring at short oxidation times. To illustrate, an application to a 17-4PH stainless steel oxidized at 480 °C is proposed and stress kinetics are deduced and related to an advanced thermomechano-chemical model, as well as identification of the numerical values of different thermomechano-chemical parameters associated to mechanisms of growth or relaxation.

Most traditional powder melting-based additive manufacturing (AM) technologies generally yield high-performance parts at the expense of additional cost and energy consumption. As an economical and efficient alternative method, material extrusion (ME) that deploys polymer-based filaments with highly filled metal particles offers the possibility of scalable, low-cost fabrication of metal components. As a critical step, sintering governs the mechanical strength and geometry of the final parts. The grain growth behavior induced during sintering should be well controlled to achieve the parts with the desired mechanical performance. Due to the nature of AM, grain growth kinetics could also involve extremely complex grain boundary migration and atomic diffusion mechanisms caused by the heterogeneous pore distribution. In this work, an analytical model was developed to predict and understand the grain growth behavior of stainless steel (SS) 316L built by ME-based sintering-assisted AM. Such a model accounts for anisotropic viscosity parameters calibrated through a three-dimensional dilatometry test, enabling the prediction of grain size evolution during sintering. Grain growth kinetic parameters were further identified by the grain size data at the heating and holding stages. To validate and generalize the model, we also conducted the grain size evolution prediction under different sintering temperature profiles for as-built SS 316L specimens. This work will provide scientific insights into the grain growth behavior of metal parts built by this AM technique and its understanding can be readily transferred to other sintering-assisted AM processes like binder jetting and ink writing for metal structure creation.

Laser powder bed fusion (L-PBF) has emerged as an additive manufacturing technique that offers unprecedented design freedom. Besides being capable of producing complex and near net shape objects, L-PBF can impact tremendously the engineering materials community due to the possibility of locally manipulating metallic microstructures. Here we exploit the latter potentiality of L-PBF, to produce site-specifically tailored stainless steel components, in terms of their crystallographic texture. The tailored materials are tested and exhibit superior energy dissipation capabilities under bending deformation compared to uniformly textured materials. This is enabled by the strong dependence of the secondary hardening mechanisms, namely the deformation twinning and/or martensite formation, of these materials on the locally tuned microstructures. With the aid of finite element simulations, it is possible to identify the stress state and hence, the crystallographic orientations that facilitate twinning or martensite formation. Then, by engineering favorable crystallographic textures, matched to the complex stress state during bending, enhanced work hardening behavior is obtained. This site-specific microstructure design enabled by L-PBF provides a new pathway for the design of “smart” components that exhibit superior mechanical response under complex stress states.

The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.

The recrystallization without previous deformation is reported in literature for a small, selected group of alloys. The present work provides evidence for the first time that the commercial stainless steel 15-5 PH also shows this recrystallization phenomenon during austenitization. A set of in-situ and ex-situ high-temperature techniques reveal that, on heating of the martensitic microstructure, recrystallization takes place after phase transformation between 900 and 1000 °C, causing a distinct reduction of the austenite grain size. This work also shows that the recrystallization correlates with the mechanisms involved in the prior martensite to austenite transformation. It is observed that increasing heating rates lead to decreasing grain sizes. This is attributed to increased defect density in the reverted austenite and increased driving pressure for the nucleation of recrystallized grains. It is proposed that, during martensite to austenite reversion, a defect arrangement of highly stable low-angle grain boundaries and, with increased heating rate, an increased density of internal, grown-in dislocations is inherited from martensite laths. This highly defect-loaded microstructure, formed without external plastic deformation, leads to a recrystallization at increased temperatures. The experimental results agree well with thermokinetic calculations based on the proposed defect arrangement, underpinning the mechanism of spontaneous recrystallization in 15-5 PH.

In Laser-based Directed Energy Deposition (L-DED) the laser-induced spatial and temporal thermal cycles strongly determine the microstructure of deposited layers. The effect of three different laser intensity profiles (beam shapes) on the shape of the melt pool and the resulting microstructure was studied. To this end, thermal gradients and growth rates, derived from measured melt pool emissions, are compared to characteristics of the microstructure in the deposited tracks. These characteristics are obtained using Electron Back Scatter Diffraction (EBSD). It was found that the shape of the laser beam strongly affects the melt pool morphology. Therefore it affects also the solidification characteristics and thus the resulting microstucture. Correlations are found between the thermal gradient - growth rate ratios and the grain shapes and amount of texture. Hence, the beam profile is a tool to steer the microstructure of deposited parts during L-DED.

Nanoparticles fabrication from RIE etching combined with a mask done by EBL technique. Oxide stress determination from nanoparticles displacement, during oxidation of 17-4PH stainless steel. [Display omitted] • Nanogauges are designed for applications to high temperature oxidation. • Nanofabrication is explored through different deposition techniques to test and optimize different gauges materials. • Design and choice of materials is performed under specific criteria. • Uncertainties for stress determination within oxide layers by use of nanogauges are calculated. • Application for 17-4PH steel is performed, with identification of thermomechano-chemical parameters for oxidation at 480°C. Determination of mechanical state of thermal oxide growing during high temperature oxidation is a challenge. Fabrication of nanogauges for monitoring during high temperature oxidation was presently investigated up to 1000 °C. Several materials were tested in terms of mechanical behaviour and maximum working temperature for gauges used as markers during thermal loading under air. The experimental determination of the optimized gauges material for high temperature oxidation was validated. SiO 2 offers particularly interesting features for gauges to determine the mechanical state of metal/oxide system. In this article, a special attention has also been paid to two nano-fabrication processes, as well as their limits. The standard electron beam lithography process is well suited to build gauges for oxidation applications, and can be improved by use of reactive ion etching process. The gauges can eventually reach several micrometers height such that the oxidation layer does not cover the gauges during thermal loading for relevant monitoring at short oxidation times. To illustrate, an application to a 17-4PH stainless steel oxidized at 480 °C is proposed and stress kinetics are deduced and related to an advanced thermomechano-chemical model, as well as identification of the numerical values of different thermomechano-chemical parameters associated to mechanisms of growth or relaxation. [ABSTRACT FROM AUTHOR]

[Display omitted] • SLM-316L exhibit much larger sensitivity to defects on the dynamic mechanical response than the quasi-static case. • Intrinsic pore defects should be responsible for the anisotropic spallation behavior of the SLM-316L. • Dislocation slip dominates the spallation mechanism, accompanying by a small amount of twinning and martensite transition. Mechanical properties of additively manufactured alloys are momentously affected by the fabrication defects, thus limiting their applications in extreme conditions. Here we report on a near fully dense 316L stainless steel via optimized laser processing parameters. The results reveal that the dynamic mechanical response exhibits much greater sensitivity to defects than the quasi-static one. The densest specimen (porosity < 0.01 %, 260W-316L) exhibits superior spall strength of 3.87 GPa and negligible damage fraction of 0.03 % at peak stress of 4.8 GPa, which are 12 % higher and 92 % smaller than those of 0.18 % porosity specimen (300W-316L). For both horizontal and vertical impacts, hardly any anisotropy of spall strength is observed in 260W-316L, demonstrating the crucial role of the pore defects on the dynamical behavior. Moreover, dislocation slip dominated spallation mechanisms have been observed in the additively manufactured 316L specimens, accompanied by a small amount of deformation twinning and martensitic transitions. This comprehensive understanding of the defect-dependent spallation behavior and deformation mechanisms provides valuable insights for optimizing the dynamic mechanical properties of additively manufactured metals and alloys. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interdigitated and spiral electrode configurations manufactured by SLM of SS316L and Al-Si10-Mg are evaluated for MECs for the first time. • The SLM SS316L anodes with controlled porosities were also pyrrole coated for the first time, targeting better cell performance. • The voltage outputs across 1 kΩ are 0.49 V and 0.61 V and full acclimation times are 26 and 12 days for Al-and Ppy-SS anode based MFCs respectively. • Pyrrole-coated stainless-steel (Ppy-SS) anodes resulted in denser biofilms compared to the spiral Al-Si10-Mg anodes. • The MEC based on the Ppy-SS spiral electrode scored the best with an overall efficiency 88.74%, compared to other cases reported so far. • The current density 322.01 ± 17.68 A/m3 and hydrogen evolution 2.89 ± 0.18 m3d-1m3 achieved are the second and third highest values respectively. In the ever-increasing quest for alternative energy sources, hydrogen emerged as a promising green option, but efficient and economical production and management have been the primary constraints. Converting wastewater into H 2 and other forms of energy attracted significant attention in terms of simultaneously and sustainably managing both the wastewater and the energy generation problems. Microbial Electrolysis Cells (MEC) evolved recently as promising options for converting wastewater into H 2 and electricity but with serious constraints on scalability. The current research aims to explore design and manufacturing solutions to build structurally strong and electrochemically effective electrodes that can also lead to scalable MEC. Two designs based on the interdigitated and spiral electrode architectures are proposed and evaluated. The added design freedom with additive manufacturing by selective laser melting of specific alloys of choice is effectively utilised in physically prototyping the interdigitated and spiral electrode forms designed with controlled porosity constraints. Microstructural, electrochemical, and cell performance characterisations led to the understanding that the spiral electrode configuration with polypyrrole-coated stainless steel 316L anode is a promising design option for both longitudinal and lateral scale-up of the MEC. [ABSTRACT FROM AUTHOR]

[Display omitted] • The angular velocity and interface temperature were simultaneously measured during the inertia friction welding. • The friction coefficient between Al alloy and steel was derived and related to the temperature, pressure, and sliding velocity. • The fiction coefficient model with linear fitting showed the best simplicity and very high accuracy and was successfully used in the simulation of temperature evolution. The precise friction coefficient at interface determines the friction behavior in inertia friction welding (IFW), its model is the basis to precisely describe the friction behavior in the numerical simulation of IFW. To measure the friction coefficient related to the temperature, pressure, and sliding velocity, the IFW trials between 2219-O Al alloy and 304 stainless steel were conducted with the simultaneous measurement of temperature and angular velocity evolutions. Two types of friction coefficient models were established by considering whether the effects of temperature and friction pressure were independent. The S-(T&P) L model, which considered the couple effect of temperature and friction pressure, was recommended as the best suitable model with its very high accuracy and best simplicity. The proposed model was successfully used in simulating the temperature evolution of IFW between Al alloy and steel tubes. The effect of the influencing variables on the friction coefficient was also discussed. [ABSTRACT FROM AUTHOR]

The goal of this work is to establish a correlation between the surface integrity and magnetic properties of stainless steels like AISI 304 (X5CrNi18-10). In metastable austenitic stainless steels, a phase transformation from the austenitic to the martensitic phase can occur even at room temperature. In this work, machine hammer peening (MHP) is used to selectively introduce the activation energy required for martensite formation on discrete areas of the surface. The phase transformation is confirmed by electron back scatter diffraction (EBSD) and micrographs. A correlation between the energy input by MHP, magnetic properties, and surface hardness is established. Using this approach, characterization of the surface integrity by measuring magnetic properties can be achieved. In addition, a novel solution to code and encode information with high information density onto metastable austenitic materials is proposed.

The present study revealed that a strong CoCrNi medium entropy alloy (MEA)/stainless steel (SS) joint could be obtained by friction welding technique with the formation of a microscale interfacial layer composed of varying degrees of the multi-principal element. A significant refinement in grain size to (∼2.8 µm) compared to the base metals (16.6 µm for MEA and 12.2 µm for SS) was observed arising from the co-occurrence of continuous and discontinuous dynamic recrystallisation where grains nucleated at triple grain boundaries, twin boundaries as well as low angle grain boundaries. A relatively strong brass texture {112} 〈110〉 appeared in the layer after recrystallisation, which was attributed to the grain growth behaviour under large deformation. The formation process of the interfacial layer from mesoscale to nanoscale was then postulated. Nanoindentation test indicated that the grains in the layer showed hardness between two base metals, but the interfacial layer demonstrated much higher yield strength (632 ± 14 MPa) and tensile strength (916 ± 4 MPa) than base metals, being higher than the relevant values reported in the literature. It was further clarified that the grain boundary strengthening mechanism rather than dislocation strengthening or solid-solution strengthening was mainly responsible for the strength enhancement.

Given the challenges in feed material supply and quality control, metal additive manufacturing has rarely been implemented in austere environments, especially underwater. This paper explores the underwater operation potential of an emerging solid-state additive technology: additive friction stir deposition, wherein material feeding and bonding are enabled by mechanical forces with minimal influences from water. It is demonstrated that additive friction stir deposition of 304 stainless steel can be successfully performed with the print head and substrate immersed in water. High temperature is reached in the deposition zone (>60% melting temperature); the material deposition behavior is similar to that in typical open-air operation. The as-deposited material is fully-dense, having fewer annealing twins and a substantially smaller grain size than the feed material (4.98 μm vs. 31.44 μm). Such microstructural changes stem from dynamic recrystallization caused by the large strain and high temperature introduced during deposition. In addition to grain refinement, small equiaxed dispersoids (∼2–3 μm or less) are formed and evenly distributed in the austenite steel matrix. Rich in Cr, Mn, and O, these particles likely result from the reaction between the elements in stainless steel and water at elevated temperatures.

The high passivation capacity of austenitic stainless steel results in their excellent corrosion resistance. There are many ways to improve the hardness of austenitic stainless steel such as cold rolling or grain refinement. Herein, we explore the possibility of improving the hardness of AISI316-Ti stainless steel by generating an amorphous-nanocrystalline microstructure. First, we have utilized our modified melt-spinning technique to fabricate AISI316-Ti stainless steel microfibers with a fully amorphous structure. Formation of a fully amorphous structure was confirmed by using X-ray diffraction (XRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM). Thermal analysis revealed a glass transition temperature of 437˚C followed by a crystallization peak of 573˚C. Nanoindentation analysis showed a fourfold increase of hardness from the initial value of ≈2.5 ± 0.1 GPa the starting AISI316-Ti stainless steel rod to the hardness of ≈8.2 ± 0.5 GPa for the amorphous AISI316-Ti structure. Further step-size heat treatment on melt-spun (amorphous) stainless steel microfibers generated a microstructure compromising adjacent nanocrystalline and amorphous grains as observed by TEM. Nanoindentation analysis of those fibers has shown an even greater increase in hardness, reaching an average value of ≈14.2 ± 1.0 GPa. We believe that both the confinement of dislocation movement in the nanocrystalline grains as well as the absence of dislocations in the amorphous grains contribute to this tremendous increase of hardness in stainless steel.

The applications of laser-powder bed fusion (LPBF), an emerging additive manufacturing (AM) technique, are rapidly growing in various industries. The superior and consistent mechanical and corrosion properties of LPBF-printed components are essential for engineering applications. The 316L stainless steel (SS) is an essential alloy with widespread applications from household items to nuclear and aerospace industries. Extensive research is conducted to understand and improve the mechanical properties of LPBF printed 316L SS. Studying the corrosion behavior of LPBF printed 316L has attracted only limited attention. Additionally, a discrepancy in the corrosion performance of LPBF printed 316L has been reported due to the complex microstructure and defects introduced during LPBF. Therefore, understanding the influence of processing parameters and feedstock on defects and microstructure becomes critical in understanding the processing-corrosion relationships and producing LPBF printed 316L components with reproducible properties. This review presents the influence of feedstock, processing parameters, and post-processing techniques on manufacturing defects, microstructure, and corrosion performance of LPBF printed 316L. Strategies and hypotheses to improve the corrosion resistance of LPBF printed 316L are also presented.

In this work, commercial 316L stainless steel (SS) powder was used to experimentally determine the impacts of severe oxidation on the powder characteristics and laser powder bed fusion (L-PBF) melting behaviors. The morphology, surface state, and laser absorptivity of both virgin and oxidized powders were systematically characterized. Their impacts on the flowability and powder bed quality were monitored by custom-designed recoating experiments and powder shear tests. The results of an in situ wetting analysis and microstructure evaluation were compared to establish a correlation between the powder characteristics and laser fusion. The powder oxidation enhanced L-PBF processability by improving the homogeneity of powder spreading and the formation of stable, consecutive laser beads. A thin ceramic layer-coated SS alloy reinforced with uniform (Si, Mn)-based oxides was synthesized by the L-PBF processing of severely oxidized powders. The mechanical strength of this alloy was found to be similar to that processed using the virgin powders, whereas the elongation was slightly decreased, likely due to the amorphous oxide feature and the mechanical oxide-Fe interface. This study provides a systematic understanding of powder reuse and new insight into the potential for economically developing high-performance parts by the positive utilization of powder oxidation and the L-PBF process.

Z3CN20–09M stainless steel, a candidate material for coolant pump shells, experiences cavitation erosion (CE) in nuclear reactor service. Z3CN20–09M steel's CE behavior is studied using scanning electron microscopy, transmission electron microscopy, electron back-scatter diffraction, and optical profilometry. The influence of CE on corrosion resistance is analyzed through potentiodynamic polarization tests and time-of-flight secondary ion mass spectrometry. During the incubation stage, damage occurs primarily in austenite. A higher Taylor factor increases the CE. Slip bands and strain-induced martensite are the main reasons for damage. After the incubation stage, brittle ferrite fracture dominates, which induces large grooves and holes. One hour of exposure improves the corrosion resistance. Slip bands, strain-induced martensite, and stacking faults provide diffusion paths for the Cr and O atoms to form a more compact passive film. Finally, approaches aiming to improve the CE resistance of Z3CN20–09M steel are suggested.

Laser-powder bed fusion (L-PBF) has gained significant research interest, not only for its profound advantage of producing near-net shape complex geometries of metallic parts, but also for the possibility of producing tailored microstructures. Here we exploit the capability of manipulating the crystallographic texture by L-PBF to tailor the deformation behavior of austenitic stainless steels. In specific, by adjusting the laser power and the laser scanning speed, tailored crystallographic textures can be obtained, along the uniaxial loading direction in 304L stainless steel samples produced by L-PBF. In situ neutron diffraction and uniaxial tension and compression tests are undertaken to investigate the extent of the transformation induced plasticity effect and to correlate it with the tailored macrostructures. The influence of the initial and the evolving crystallographic texture on the deformation behavior is demonstrated and elaborated accordingly. The observed asymmetry in the deformation behavior between tension and compression is also discussed in detail.

[Display omitted] • Low cycle fatigue (LCF) of strut-based and gyroid structures is simulated by FEM. • Cyclic elastoplastic response is reproduced by FE models with as-built density. • Strain energy density (SED) is used to predict the LCF lives of cellular materials. A finite element (FE) modelling approach is developed to reproduce the cyclic elastoplastic response and to assess the low cycle fatigue (LCF) life of two cellular materials (strut-based, gyroid) investigated in a previous experimental campaign. The cyclic response of different FE models (unit cell, one layer structure) is compared in terms of computational cost and modelling accuracy. The most satisfactory model is further updated based on the actual relative density of fabricated cellular materials. The LCF assessment exploits a volume-based strain energy density (SED) criterion, calibrated after comparing static properties of strut-based and bulk materials. The cyclic elastoplastic response is well reproduced for both cellular materials, whereas the estimated fatigue lives are in closer agreement for the gyroid structure than the strut-based one. [ABSTRACT FROM AUTHOR]

[Display omitted] • Single austenitic gradient nanostructure of 304 stainless steel is fabricated by first creating a dual-phase gradient nanostructure through USRP, followed by rapid induction heating treatment. • Fine grain, dislocation, twinning, and back-stress strengthening contribute 30%, 17.5%, 23.4%, and 29.1% respectively to the strength increase. • The excellent plasticity of the single austenite phase gradient nanostructure stems from the synergistic effects of back-stress hardening, TRIP and TWIP effect. The drawback of low strength of 304 stainless steel could be overcome by fabricating gradient nanostructures (GNS). However, deformation-induced martensite results in magnetic generation and plasticity degradation. In this work, a single austenitic GNS 304 stainless steel is fabricated by first creating a dual-phase GNS through the ultrasonic surface rolling process (USRP), followed by rapid induction heating. The yield strength of the single austenite GNS (520 MPa) is 1.68 times higher than that of the homogeneous coarse-grained structure (310 MP) without sacrificing plasticity (elongation of 65 %). Quantitative calculations indicate that fine grain, dislocation, twinning, and back-stress strengthening contribute to the strength increment by 30 %, 17.5 %, 23.4 %, and 29.1 %, respectively. Coarse-grained regions deform mainly through FCC-HCP-BCC martensitic transformation, whereas the subsurface layer forms stacking faults and twins due to increased stacking fault energy caused by the reduction in grain size. At the topmost layer, the stress required to activate dislocations is lower than that for twinning. Under high-stress conditions, martensite forms along the nanograin boundaries via a phase transition from FCC to BCC. Consequently, the excellent plasticity of the single austenite GNS stems from the synergistic effects of high back-stress hardening, TRIP and TWIP effect. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highly reliable multi-performance parts were fabricated using only a single material. • The integrated fabrication strategy shows the precise manufacturing capability from microscale to macroscale. • Wear resistance performance of 316L stainless steel could be tailored to cover a wide range from plastics to ceramics. • Ultrawide-tunable performances are attributed to ultrawide-tunable grain orientations. Additive manufacturing enables innovative design and integrated manufacturing of complex components. Metallic components with multi-performance and high reliability are always desirable for a broad range of high-end industries. Multi-material additive manufacturing is a promising way to provide multiple performances, yet there exist several persistent challenges on multi-material sustainability and recycling, interfacial defects and strength, dimensional accuracy and fineness, and manufacturing efficiency and cost, prohibiting its widespread practical applications. In this study, we introduce an additive/modification integrated manufacturing (AMIM) strategy for fabricating highly reliable multi-performance metals using only a single material with a one-step process. We demonstrate that, through a unique combination of pattern-induced orientation-consistent and layer-accumulated orientation-amplified mechanisms, the surface wear resistance performance of 316L stainless steels could be expanded to cover an exceptionally wide range from commercially available plastics to metals to ceramics. Furthermore, these strikingly different performances can be digitally programmed and precisely printed to form integrated 316L stainless steel components with excellent interfacial strength and high dimensional accuracy. This strategy paves a facile, inexpensive, and sustainable way to achieve multi-performance metals that are fully recyclable and environmentally friendly, serving as an alternative to the multi-material composite strategy. [ABSTRACT FROM AUTHOR]

[Display omitted] • DED-Arc manufacturing technique was successfully on large-scale structure. • Pressure test up to 111 bar was performer to DED-Arc structure. • Coupons extracted from the structure were subjected to quasi-static and fatigue testing. • Fatigue strength assessment considering local quality was introduced. Wire-arc direct energy deposition was used in this study to manufacture a standing pressure vessel made of stainless steel. Considering the thickness of the additively manufactured sections, the fabricated part was divided into two major components: the shell and the head. The shell with a nominal wall thickness of 5 mm was manufactured using a single-pass technique, while the relatively thicker head, with a maximum thickness of 30 mm, was fabricated using a multi pass approach. The transition from the shell to the 15–30 mm thick head was done by variating wall thickness, enabled by additive manufacturing. After additive manufacturing, the full-scale component was tested up to the maximum internal water pressure of 111 bar (defined per the vessel's industrial application). Further, to evaluate the mechanical properties of the additively manufactured steel and the effects of pressure loads on it, quasi-static tensile and fatigue tests were conducted on coupons prepared from the material in two conditions: as-built (without any preload) and preloaded (after the pressure test) extracted from various sections. Finally, metallurgical characterization was performed to establish a correlation between the microstructural features and the mechanical performance. The results showed that it is possible to manufacture high-performance and quality pressure vessels using the wire-arc direct energy deposition method. [ABSTRACT FROM AUTHOR]

[Display omitted] • The research novelty lies in evaluating the tribological and biological properties of Nickel Free Stainless Steel (NiFSS) produced through laser bed powder diffusion for metals (PBF-LB/M) additive manufacturing. • Optimized process parameters for PBF-LB/M fabrication of NiFSS resulted in samples with a relative density of 98.469% and fully ferritic microstructure in the as-built state. • Tribological experiments assessed NiFSS at constant load and sliding speed using an automated Bio-Tribometer, showing promising friction coefficients and wear rates relevant to implant applications. • NiFSS exhibited reduced Coefficient of Friction and wear rates compared to PBF-LB/M built austenitic 316L SS, attributed to its superior hardness and elastic modulus. • Biocompatibility testing with pre-osteoblastic MC3T3-E1 cells indicated better cell viability on PBF-LB/M built NiFSS compared to PBF-LB/M built 316L SS. • The present study suggests the potential of PBF-LB/M fabricated NiFSS for use in biomedical devices, particularly in joint arthroplasty applications. Due to the risk of releasing carcinogenic nickel ions from conventional 316L stainless steel under a corrosive human body environment, a new variant of steel termed nickel-free stainless steel (NiFSS) has been investigated. The present study investigates the tribological properties and biocompatibility of NiFSS manufactured via laser powder bed fusion (PBF-LB/M). The ferritic NiFSS exhibited significantly lower coefficient of friction (0.08 to 0.28) and wear rate (1.60 × 10-6 mm3/Nm to 6.60 × 10-6 mm3/Nm) compared to reported values for austenitic 316L SS, under both dry and simulated body fluid (SBF) conditions and various sliding geometries. This improvement is attributed to the superior hardness (3.394 ± 0.1340 GPa) and elastic modulus (238 ± 9.0797 GPa) of NiFSS. To assess the biocompatibility, the viability of mouse pre-osteoblastic MC3T3-E1 cells was evaluated with an Alamar Blue assay when the cells were cultured on top of PBF-LB/M built NiFSS and 316L SS samples. The results indicated that even though cell growth was most optimal on regular cell culture plastic, cell viability was better maintained on PBF-LB/M built NiFSS compared to 316L SS. Therefore, the results of the present study propose that PBF-LB/M fabricated NiFSS holds promise for application in biomedical devices for joint arthroplasty. [ABSTRACT FROM AUTHOR]

[Display omitted] • Model to identify optimization strategies and process parameters for the scaling of laser micromachining. • Experimental verification of the model for laser micromachining of pockets in silicon using a high-power ultrafast laser. • Scaling to high throughput at high average laser power is possible without exceeding the process limits for high quality. • Promising optimization strategies are low fluences, pulse bursts, increasing the beam diameter and high scanning speeds. • Removal rates exceeding 120 mm3/min achieved with an average laser power of 1.01 kW while maintaining a roughness of 1 µm. The great potential to scale the productivity of laser micromachining processes offered by the development of ultrafast lasers with average powers exceeding 1 kW comes at the cost of new physical and technical challenges. The large number of adjustable parameters, the different physical process limits, and the various possible optimization strategies complicate the design of suitable laser micromachining processes that enable high quality and high throughput. In this contribution, a comprehensive model is proposed that helps to identify the optimization strategies and process parameters required for the scaling of laser micromachining to high throughput at high average laser power without exceeding the process limits to ensure high surface quality. The model was experimentally verified on the example of laser micromachining of pockets in metals and silicon using an ultrafast laser with an average power of 1.01 kW, a pulse duration of 600 fs, different pulse energies, pulse bursts, and beam diameters. As a result, high material removal rates exceeding 120 mm3/min were achieved for silicon, stainless steel, copper, and aluminum, which exceed previously achieved removal rates by one to two orders of magnitude. The machined surfaces exhibit high quality as confirmed by the low measured roughness of about 1 µm. [ABSTRACT FROM AUTHOR]

The effect of high-temperature oxidation at 850 °C (10 h, 30 h, 100 h) and 900 °C (10 h) on porous (∼30 % porosity) ferritic stainless steel (Fe22Cr) has been investigated using synchrotron tomographic microscopy, which allowed for visualisation, separation and quantitative analysis of the metallic core, closed pores, open pores and oxide scale phase. The same regions within the samples were investigated before and after oxidation performed at different conditions. Quantitative analysis of the tomographic data provided information on changes upon oxidation of the relative volume of the different phases, the specific surface area (SSA) of the metallic core, the thickness of the oxide scale and pore size distribution. The results were discussed in the context of thermogravimetric analysis of the samples and supported by SEM imaging. It was observed that oxidation leads to an increase of the SSA of steel and the largest increase (∼50 %) was obtained for the sample processed for 100 h at 850 °C. It is demonstrated the open porosity forms a network of connected channels within the sample and it dominates in the volume. In addition, the 3D imaging revealed breakaway oxidation areas for samples, for which this phenomenon remained undetected using 2D SEM analysis.

Since 2021, cobalt (Co) is in Europe classified as carcinogen in quantities exceeding 0.1 wt-%. This affects nickel-rich stainless steels, which contain about 0.2 wt-% Co impurities. Previous findings show the bioaccessibility of Co in stainless steel to be primarily determined by the corrosion resistance. It has been unclear whether Co is distributed heterogeneously in the alloy and the outermost surface and whether a specific location would pose a risk for Co release under specific exposure conditions. This study aimed at locating Co in stainless steel 316L (0.2 wt-% Co) surfaces prior to and after exposure to different synthetic body fluids for 24 h at 37 °C. Time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICPMS) investigated the location of Co in the surface oxide and extent of release along with other metals (iron, chromium, nickel, and manganese) into synthetic biological fluids (gastric fluid, pH 1.5; lysosomal fluid, pH 4.5; phosphate buffered saline-PBS, pH 7.4). Co was homogeneously distributed along with metallic nickel beneath the surface oxide and co-released with other metals upon surface reformation and passivation. Exposure in PBS resulted in the incorporation of both Co and phosphate in the oxide.

A new approach to modelling the microstructure evolution and yield strength in laser powder bed fusion components is introduced. Restoration mechanisms such as discontinuous dynamic recrystallization, continuous dynamic recrystallization, and dynamic recovery were found to be activated during laser powder bed fusion of austenitic stainless steels; these are modelled both via classical Zener-Hollomon and thermostatistical approaches. A mechanism is suggested for the formation of dislocation cells from solidification cells and dendrites, and their further transformation to low-angle grain boundaries to form subgrains. This occurs due to dynamic recovery during laser powder bed fusion. The yield strength is successfully modelled via a Hall–Petch-type relationship in terms of the subgrain size, instead of the actual grain size or the dislocation cell size. The validated Hall–Petch-type equation for austenitic stainless steels provides a guideline for the strengthening of laser powder bed fusion alloys with subgrain refinement, via increasing the low-angle grain boundary fraction (grain boundary engineering). To obtain higher strength, dynamic recovery should be promoted as the main mechanism to induce low-angle grain boundaries. The dependency of yield stress on process parameters and alloy composition is quantitatively described.

In additive manufacturing of metal matrix composites, it is difficult to ensure high strength as well as high ductility simultaneously. In this study, a FeCoNiCr high-entropy alloy was used as the strengthening phase to improve the performance of austenitic stainless steel processed via laser powder bed fusion. The microstructure of the as-printed parts was composed of high-entropy alloy nanoparticles, a solid solution matrix, and high-density dislocations. The strengthening effect caused by the microstructures resulted in a tensile strength of 627 MPa with a maximum elongation of 50%. The semi-obstructive effect of the interface on the dislocations improved the ductility, as proved through molecular dynamics analysis. The Cr, Ni, and Co elements in the high-entropy alloy melted into the stainless-steel matrix, facilitating the formation of a passive film; this improved the corrosion resistance of the composite. Thus, simultaneous improvements in strength, ductility, and corrosion resistance of the stainless-steel composites were realized. The composites inherited the performance characteristics of high-entropy alloys. This research also provides a new strategy to select the strengthening phase for composite materials. High-performance high-entropy alloy particles with lattice structures and element types similar to the stainless steel matrix have substantial potential to be used as strengthening phases.

The age hardenability of Al-alloyed lightweight stainless steels with the base chemical composition Fe–10.5Cr–3Al (wt.%) and Ni concentrations in the range 3–15 wt.% was studied. Alloys containing 3% and 6% Ni exhibited almost fully ferritic matrix microstructures and a weak age hardening response. Alloys containing 9%, 12%, and 15% Ni, on the other hand, developed primarily martensitic microstructures. Differential scanning calorimetry measurements indicated the occurrence of an exothermic reaction in the approximate temperature range 375–625 °C. Dilatometry measurements indicated that the exothermic reaction was accompanied by a net contraction. Hardness measurements after aging for 5 min indicated significant hardening of alloys already at 350 °C due to the formation of B2-(Ni,Fe)Al intermetallic precipitates. The age hardening response was significantly superior to that of conventional precipitation-hardenable martensitic stainless steels. Tensile elongation in the aged condition was negatively influenced by the presence of soft ferrite regions. Processing conditions associated with a fully martensitic microstructure prior to aging are required to render a uniform age hardening response. Guidelines for the development of a new family of lightweight precipitation-hardenable steels with lower raw material costs and a higher corrosion resistance compared to the standard 18Ni maraging steels are provided.

Laser powder bed fusion additive manufacturing is used for demanding applications in industries such as aerospace. However, machine-specific, optimized process conditions and parameters are required to assure consistent part quality. In addition, differences in supplied powder can cause variation in the mechanical properties of the final parts. In this paper, the variability in mechanical properties of 316L stainless steel produced with two different laser powder bed fusion machines from two different powder batches was studied by producing an identical set of tensile and impact toughness test specimens. The samples were subjected to stress-relieving, solution annealing and hot isostatic pressing to assess the effectiveness of standardized heat-treatments in reducing variation in the mechanical properties of the built parts. Porosity, microstructure, tensile properties, and impact toughness of the specimens were measured to study the effect of changing the material, machine, and heat treatment. The maximum differences observed between the studied machine-powder combinations were approximately 7% for tensile properties and approximately 20% for impact toughness. HIP reduced the variability in all other studied properties except elongation. All the specimens fulfil the minimum requirements set in ASTM F3184-16 for AM 316L.

[Display omitted] • A grain growth model during material extrusion-based AM was developed. • Anisotropic sintering behavior was accounted to predict the grain size. • Grain growth kinetics parameters were calibrated to apply for other sintering profiles. Most traditional powder melting-based additive manufacturing (AM) technologies generally yield high-performance parts at the expense of additional cost and energy consumption. As an economical and efficient alternative method, material extrusion (ME) that deploys polymer-based filaments with highly filled metal particles offers the possibility of scalable, low-cost fabrication of metal components. As a critical step, sintering governs the mechanical strength and geometry of the final parts. The grain growth behavior induced during sintering should be well controlled to achieve the parts with the desired mechanical performance. Due to the nature of AM, grain growth kinetics could also involve extremely complex grain boundary migration and atomic diffusion mechanisms caused by the heterogeneous pore distribution. In this work, an analytical model was developed to predict and understand the grain growth behavior of stainless steel (SS) 316L built by ME-based sintering-assisted AM. Such a model accounts for anisotropic viscosity parameters calibrated through a three-dimensional dilatometry test, enabling the prediction of grain size evolution during sintering. Grain growth kinetic parameters were further identified by the grain size data at the heating and holding stages. To validate and generalize the model, we also conducted the grain size evolution prediction under different sintering temperature profiles for as-built SS 316L specimens. This work will provide scientific insights into the grain growth behavior of metal parts built by this AM technique and its understanding can be readily transferred to other sintering-assisted AM processes like binder jetting and ink writing for metal structure creation. [ABSTRACT FROM AUTHOR]

A novel auxetic structure-based stiffener for stainless steel–carbon reinforced polymer (CFRP) T-joints with outstanding strength and toughness improvement is proposed. The stiffener was composed of a bulk web and re-entrant structure flange that was 3D printed using a selective laser melting 3D printer. The stiffener was bonded to a CFRP skin, and a pull-off test was performed to evaluate the pull-off strength, stiffness, and toughness of the joint. The results demonstrated that the proposed structure has specific strength and toughness that were 3.4 and 77 times greater than those of the baseline joint, respectively. The improvement in strength was due to the delayed damage initiation caused by the mismatch between the bending stiffness of the bulk web and the stiffener under the first re-entrant structure. However, the damage initiation in the baseline joint was located at the stiffener edge. Moreover, the negative Poisson's ratio of the re-entrant structure arrested the delamination growth after damage initiation, which enhances the toughness of the joint. By optimizing the structure of the stiffener, a joint with 2.66 and 38.2 times larger specific strengths and toughness compared to the baseline joint with the same specific stiffness was achieved. • Novel auxetic metal stiffener for metal-composite T-joints is presented. • The proposed auxetic stiffener showed 38.2 higher toughness than baseline joint. • The specific strength of the proposed joint is 266% larger than the baseline joint. • Delamination was arrested at the skin-adhesive interface for the auxetic joint. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effects of a novel functionally-graded material on the compressive performances of LPBF Corrax maraging stainless steel lattices were investigated. • The FGM-V lattice exhibited higher SEA than the FCCZ and cubic lattices due to the effective alleviation of strain localization. • The specific energy absorptions of LPBF Corrax lattices were comparable to those of LPBF Ti-based lattices in the literature. The effects of a functionally-graded material (FGM) designed with two types of unit cells, cubic and face-centered cubic with Z-axis struts (FCCZ), on the compressive performances and fracture mechanisms of powder bed fusion-laser beam\metals (PBF-LB\M) Corrax maraging stainless steel lattices were investigated. FGM lattices with gradient directions parallel and vertical to the compressive loading were respectively designated as FGM-P and FGM-V lattices. The results showed that the FCCZ lattice exhibited higher compressive properties than the cubic lattice did. The fracture modes of FCCZ and cubic lattices were respectively ∼45° shear fracture and layer-by-layer fracture. The FGM-V lattice exhibited higher specific energy absorption at 50 % strain than the FCCZ and cubic lattices did by 7.6 % and 19.4 %, respectively. This phenomenon can be attributed to the effective alleviation of strain localization in the FGM-V lattice postponing the fracture. Furthermore, the specific energy absorptions of PBF-LB\M Corrax lattices were comparable to those of PBF-LB\M Ti-based alloy lattices in the literature. Thus, the PBF-LB\M Corrax lattice appears to be a potential low-cost material for high-performance and lightweight structural applications. This lattice design extends the flexibility of additive manufactured lattices and provides a specific guideline for further improving their mechanical performances. [ABSTRACT FROM AUTHOR]

[Display omitted] • Significant improvement in fatigue is observed after applying both aging and shot peening treatments on 17-4PH MEX. • For as-sintered 17-4PH, Al 2 O 3 dominates at high cycle, while other surface inhomogeneities dominate at low cycle regimes. • For unpeened 17-4PH, aging increases tensile strength and hardness, but does not affect fatigue properties. Material extrusion additive manufacturing (MEX) gains increasing interest due to its cost-effectiveness. However, concerns arise regarding inherent MEX characteristics potentially affecting mechanical properties. This study investigates the effects of shot peening and heat treatment on MEX 17-4PH stainless steel. After shot peening, surface nanocrystalline and compressive residual stress are induced. Aging results in higher hardness and tensile strength with lower elongation, while shot peening results in higher surface hardness with minimal change in elongation. For fatigue properties of as-sintered specimens, Al 2 O 3 particle contamination dominates at high cycle regimes, while other MEX surface inhomogeneities dominate at low cycle regimes. Furthermore, as-aged properties are dominated by all surface inhomogeneities, resulting in the non-improvement of fatigue performance compared to the as-sintered specimens. However, shot peening on the as-sintered specimens significantly enhances fatigue properties through the formation of surface nanocrystalline structures and the induction of compressive residual stress, albeit with the occurrence of wrapped corners acting as fatigue crack initiation sites. Remarkably, the combination of aging and shot peening yields the most improvement in fatigue performance, exhibiting behaviours akin to as-sintered specimens with shot peening but without the generation of wrapped corners. Instead, fatigue crack initiation sites shift to subsurface region's inhomogeneities. [ABSTRACT FROM AUTHOR]

[Display omitted] • Introducing perforated and ellipsoidal features diversifies hollow-sphere structure designs for enhanced properties. • The ellipsoid ratio has the least impact on relative density and the most positive impact on the compressive performance. • Properly sized perforations in hollow spheres result in reduced density without compromising mechanical performance. • Hollow ellipsoidal sphere, with an increased ellipsoid ratio, showcases lower density alongside improved compressive strength and energy absorption capacity. Based on the porous structure of classic simple-cubic hollow sphere (SHS) materials, two novel configurations—simple-cubic perforated hollow sphere (SPHS) and hollow ellipsoidal sphere (SHES) structures were designed by introducing perforations in sphere walls and altering the ellipsoid ratio, defined as the ratio of the major and minor axes of an ellipsoid. Using 316L stainless steel as the raw material, a series of specimens were fabricated employing additive manufacturing technology. Corresponding finite element models were established using Abaqus. Then the experimental and numerical investigations were conducted to analyze the deformation behavior of these structures subjected to quasi-static compression. Further numerical research was conducted to analyze the mechanical properties of multi-cell hollow sphere structures with varied parameters, including wall thickness, perforation diameter, and ellipsoid ratio. The results confirmed that a perforation diameter of 4 mm resulted in optimal performance for the SPHS. In comparison to the basic structure SHS with identical relative density, this configuration exhibited a 60 % higher specific yield strength. Additionally, for SHES, when the ellipsoid ratio exceeded 1 while less than 1.8, the specific yield strength demonstrated a maximum increase of 69 % ∼ 105 % and the maximum enhancement in specific energy absorption was 60 % ∼ 85 %. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new austenitic stainless steel with high strength and excellent ductility was designed and manufactured by simple method. • The elongation of samples is up to 53.5–61 % under the yield and ultimate tensile strength level of 600–707 and 977–1020 MPa. • The product of strength and plasticity of this steel is the highest value comparing with other austenitic stainless steels. • There is the highest yield strength in this steel comparing with other austenitic stainless steels at the same elongation. • The excellent mechanical property of this steel is ascribed to a new multi-element collaborative strengthening mechanism. In the pursuit of simultaneously improving the yield strength and plasticity of austenitic stainless steel, a new austenitic stainless steel was fabricated by induction smelting using a pure N 2 atmosphere, hot forging, cryogenic rolling, and annealing. The material was characterized by microstructures with 3–4 μm uniform finer grains, fine precipitates, high thermal stability austenite, and extensive high-angle grain boundaries. The elongation after fracture, yield strength, and ultimate tensile strength of the samples reached 53.5 %, 707 MPa, and 1020 MPa, respectively, as well as 61 %, 600 MPa, and 977 MPa, respectively, at the same time. Moreover, a high strain hardening rate was achieved in the new stainless steel. The appropriate uniform finer grains not only played a role in grain-refined strengthening but also provided intragranular spaces and sufficient mean free available paths for dislocation accumulation and movement. Precipitates, which were coherent or semi-coherent with the matrix, provided interfaces for dislocation accumulation and obstructions for dislocation movement. Extensive high-angle grain boundaries with appropriate finer grains served as another important factor for excellent ductility due to the inhabitation and resulting deviation of crack propagation. In addition, strain-induced mechanical twinning in the current austenitic stainless steel contributed to excellent ductility and high strength. [ABSTRACT FROM AUTHOR]

Hazard classification of metal alloys is today generally based on their bulk content, an approach that seldom reflects the extent of metal release for a given environment. Such information can instead be achieved via bioelution testing under simulated physiological conditions. The use of bioelution data instead of bulk contents would hence refine the current hazard classification of alloys and enable grouping. Bioelution data have been generated for nickel (Ni) and cobalt (Co) released from several stainless steel grades, one low-alloyed steel, and Ni and Co metals in synthetic sweat, saliva and gastric fluid, for exposure periods from 2 to 168 h. All stainless steel grades with bulk contents of 0.11–10 wt% Ni and 0.019–0.24 wt% Co released lower amounts of Ni (up to 400-fold) and Co (up to 300-fold) than did the low-alloyed steel (bulk content: 0.034% Ni, 0.015% Co). They further showed a relative bioaccessibility of Ni and Co considerably less than 1, while the opposite was the case for the low-alloyed steel. Surface oxide- and electrochemical corrosion investigations explained these findings in terms of the high passivity of the stainless steels related to the Cr(III)-rich surface oxide that readily adapted to the fluid acidity and chemistry.

The production of components using powder bed fusion presents unique possibilities for manufacturing. The process of selective laser melting (SLM) can permit fusion of powders, including powder blends and alloys comprised from elemental powders. In this context, exploring the possibility of making composites by blending stainless steel 316 L and glass powder (the latter being a waste product) was explored. Such composites were investigated on the basis of (i) significant reduction in component cost, as glass powder waste is a common industrial by-product, (ii) upgrading recycled waste, (iii) the possibility of lowering component density (the density of glass is less than three times that of stainless steel), and (iv) the possibility of unique physical properties if glass remains amorphous. Herein, laser scan strategies were optimised in order to produce solid cubes and tensile test specimens. Microstructural and phase analysis were carried out by electron microscopy and x-ray techniques. Unique CrSi oxides were observed in the manufactured microstructure. The work herein presents an exploratory approach into the development of novel engineered composites utilising additive manufacturing.

Narrow gap multi-layer hybrid welding technology is an efficient way to join thick plates of metal or alloys. However, the coarse microstructure and solute segregation were caused by rapid cooling. In this paper, a magnetic field was utilized to achieve better microstructural control during welding. Using analytical electron microscopy combining with EDS and EBSD, microstructure evolution including grain growth and solute segregation during electromagnetic stirring (EMS) was systematically studied. EMS promoted the refinement and suppressed the overgrowth of austenite (γ) grains. Besides, γ exhibited equiaxed grains as seen from both grain size and texture evolution. For ferrite (δ) dendrites, EMS promoted the morphological transition from the coarse and long vermicular morphology to the fine lathy forms. EMS also induced deviation and discontinuity in δ dendrite growth. From chemical compositions point-of-view, a homogeneous distribution was obtained at both macro- and micro-scales by EMS. These results were attributed to δ “dendrite fragmentation” behavior brought by electromagnetic force at solid/liquid interface. Such δ fragments grew and acted as a substrate for epitaxial γ grains nucleation, with specific γ/δ orientation relationships, suppressing continuous epitaxial growth of γ grains via increased nucleation. Moreover, δ fragments result in reduced fraction of δ phase and dispersed distribution.

Gaussian laser intensity profiles are standard in laser-based metal additive manufacturing, although recent work in single-layer melt tracks showed that beam shaping could offer a feasible route towards microstructural control. Since thermal cycling and grain orientation templating in multilayer builds can alter microstructures, we compare three-dimensional 316 L stainless steel cubes built using Gaussian and elliptical laser intensity profiles. Microstructural characterization confirms that elliptical beams result in a modified and improved microstructure compared to Gaussian beams. This assessment favoring the elliptical beam is based on: (1) the observed refinement of the columnar and equiaxed grains; (2) more importantly, the volume fraction occupied by equiaxed grains increases dramatically such that the average grain area is reduced by nearly 50%; (3) reduced texture in cubes built using an elliptical beam. The random orientation of small equiaxed grains in samples built using an elliptical beam also suggests a higher nucleation frequency. High-fidelity finite element simulations that deliver accurate thermal profiles by incorporating laser ray tracing and fluid dynamics were performed. Using a time-dependent solidification map based on local thermal gradients (G) and growth rates (R), our simulation results confirm the experimentally observed trend that an elliptical beam results in a favorable thermal profile.

One of the biggest challenges of nuclear industries is to fabricate safe, stable and efficient materials for the storage and transportation of spent nuclear fuels (SNFs), these materials are known as neutron shielding and absorbing materials. Recently, Gd has received much attention as a neutron absorbing material in SNF applications owing to its high neutron absorption capability. Herein, we propose novel FeGd alloys as binary alloys and master alloys for potential materials in nuclear applications. FeGd alloys with 20, 55 and 80 wt% Gd were selected based on the FeGd phase diagram. All fabricated FeGd alloys had higher hardnesses than pure Fe due to the formed FeGd phases, and the Fe-rich phase had a higher hardness than the Gd-rich phase. On the other hand, corrosion resistance of the FeGd alloys decreased as the Gd content increased. Satisfactory Gd-based intermetallic dispersion and modifying effects were obtained by casting Fe and stainless-steel alloy with Fe80Gd. The average intermetallic size in the stainless-steel alloy remarkably decreased with a decrease in the average distance between Gd-based intermetallics. FeGd alloys possess requisite hardness, thermal conductivity, and corrosion resistance as well as dispersion ability of intermetallics; hence, they show potential as promising candidates for SNF applications.

Wire and arc additive manufacturing (WAAM) is a method of 3D printing that enables large elements to be built, with reasonable printing times and costs. There are, however, uncertainties relating to the structural performance of WAAM material, including the basic mechanical properties, the degree of anisotropy, the influence of the as-built geometry and the variability in response. Towards addressing this knowledge gap, a comprehensive series of tensile tests on WAAM stainless steel was conducted; the results are presented herein. As-built and machined coupons were tested to investigate the influence of the geometrical irregularity on the stress-strain characteristics, while material anisotropy was explored by testing coupons produced at different angles to the printing orientation. Non-contact measurement techniques were employed to determine the geometric properties and deformation fields of the specimens, while sophisticated analysis methods were used for post processing the test data. The material response revealed a significant degree of anisotropy, explained by the existence of a strong crystallographic texture, uncovered by means of electron backscatter diffraction. Finally, the effective mechanical properties of the as-built material were shown to be strongly dependent on the geometric variability; simple geometric measures were therefore developed to characterise the key aspects of the observed behaviour.

Functionally graded materials (FGMs) exhibit good performance owing to their gradual and directional property and compositional changes occurring in the material. Field-assisted sintering is one method to fabricate FGMs by utilizing a heating geometry that provides a temperature gradient within the sample. Here, a heating geometry that provides ultra-large temperature gradients is proposed. Using finite element analyses, the geometrical parameters of the proposed design were optimized through a systematic parametric investigation. The resulting temperature gradients were evaluated for electrically conductive stainless steel (SUS) 304L and insulating 8 mol% yttria-stabilized zirconia (8YSZ). The temperature gradients within the samples were 80 and 122 °C/mm for SUS 304L and 8YSZ, respectively. The heating geometry was used to fabricate functionally graded versions of the two materials, which showed gradual changes in the porosities, grain sizes, and hardness. Lastly, the temperature gradients were then quantitatively validated through temperature measurements and single-temperature sintering experiments.

[Display omitted] • The stress-free d 0 was determined utilizing the RS measurement itself. • The maximum compressive RS was obtained as the phases composed by α′-bcc and γ-fcc with the same fraction of 50%. • The compressive RS was attributed to the type II intergranular RS. • The compressive RS arose from the α′-bcc formed directly from the γ-fcc matrix. The strain-induced martensitic transformation (α′-martensite) and residual stress (RS) introduction in the gradient nanostructured (GNS) austenitic stainless steel have been extensively reported in the last three decades, however, their correlation is still unclear. Here, the XRD, EBSD, ECCI and TEM were used to study the RS distribution and martensite behaviors in a GNS SS304 fabricated by ultrasonic surface nanocrystallization modification. We found the non-linear relationship between the magnitude of RS and the fraction of α′-martensite due to the role of α′-martensite in the development of type II intergranular RS. The maximum compressive RS of about 1,244 MPa was obtained as the γ-austenite and α′-martensite phases were each at approximately 50 %. Otherwise, the RS would decrease with the second phase fraction. The developed maximum compressive RS arises from the α′-martensite that formed directly from the γ-austenite. By contrast, the RS from other deformation structures, i.e., α′-martensite produced by γ → ε → α′, ε-martensite, nanotwin and dislocations, is relatively low. The current study classified the correlation of RS and the role of martensite for the first time, which may inspire new approaches for improving the fatigue properties of materials by controlling the phase fraction. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new novel FeNiCoCr high-entropy alloy has been additively manufactured via in situ alloying. • This new HEA has broken the ductility-strength trade-off due to a synergy of deformation mechanisms. • High-density hybrid crystal-amorphous precipitates are unexpectedly found, which has never been reported before. • A new in situ strategy is provided to additively manufacture HEA with superior mechanical properties. Equiatomic high-entropy alloys (HEAs) fabricated by additive manufacturing (AM) have gained increasing attention since they present superior mechanical properties over their counterparts made by traditional synthesis routes. However, few studies have investigated AM of non-equiatomic HEAs, although this type of HEAs showing excellent mechanical properties. In this paper, we additively manufactured a novel non-equiatomic FeCoCrNi HEA via in-situ alloying 316L stainless steel and CrCoNi medium-entropy alloy. Besides the fine grain size (9 μm) and high dislocation density (3.8 × 1014 m−2), we observed unexpected high-intensity of hybrid crystal-amorphous precipitates, which have never been reported in NiCoCr-based alloys made by AM. These unique microstructures led to exceptional combination of strength (561 MPa) and elongation (41 %) exceeding those of additively manufactured equiatomic FeCoCrNi HEAs. Our observation from EBSD, TEM and atomistic simulations reveals a group of deformation mechanisms of stacking faults, deformation twins, TB-dislocation and dislocation-precipitates interactions. Our findings not only provide new insights into AM of non-equiatomic HEAs via in situ alloying, also shed lights on understanding the microstructure-properties relationship of non-equiatomic HEAs fabricated by AM. [ABSTRACT FROM AUTHOR]

[Display omitted] • Study focuses on impact of wall thickness (<1 mm) on properties of 316L after L-PBF. • Melt pool size and microstructure vary, affecting grain size and orientation. • Thicker samples had finer cells and slender columnar grains, whose orientation shifted rom 〈100〉 to 〈101〉 with increasing thickness. • Tensile tests revealed thickness-dependent properties: thinner samples had lower strength, thicker ones exhibited higher strength and ductility. • Findings highlight the significance of wall thickness in L-PBF for material properties. Laser powder bed fusion (L-PBF) allows for the fabrication of samples with complex geometries based on thin struts or walls. However, only few studies have focused on the effect of these geometries on the properties of the material fabricated using this technology. In this work, we studied the impact of wall thicknesses below 1 mm on microstructure formation and mechanical properties in 316L parts fabricated by L-PBF. The size and geometry of melt pools varied significantly between different wall thicknesses due to powder denudation and local preheating, resulting in non-symmetrical melt pools for thicker samples. Furthermore, in the sub-grain microstructure, the thinnest samples consisted of solidification cells oriented almost parallel to the building direction. In the thicker walls, side branching and slender columnar grains were observed in the center lines of the melt pools. On the grain size scale, the thinnest samples consisted of finer grains with a more pronounced texture 〈100〉, while large grains growing parallel to the build direction and texture 〈101〉 were found for the thicker samples. Mechanical tests showed that the strength and ductility were higher in thicker samples, which was attributed to finer solidification cells. [ABSTRACT FROM AUTHOR]

[Display omitted] • Roughness metrics inadequately assess corrosion of 3D-printed 316L stainless steel. • True active surface area (A TA) proves to be effective for quick corrosion assessment. • Electrochemical testing results align with a stochastic pitting model when using A TA. • Advocates paradigm shift: A TA is essential over roughness for corrosion characterization. Laser powder bed fusion (LPBF) additively manufactured (AM) 316L stainless steels (SS) possess much more complex surfaces than their wrought counterparts which affects the corrosion behavior. Surface roughness, a typical metric for assessing corrosion of conventionally manufactured metals, is often ineffective as an independent parameter in characterizing corrosion of the AM metals for their higher surface roughness ranging from 5 to 50 µm. This study experimentally shows that the true active surface area (A TA) is a proper parameter for quick assessment of localized corrosion response of AM 316L SS. Through the potentiodynamic polarization testing on surrogates under full immersion in 0.6 M NaCl solution, the pitting corrosion susceptibility was evaluated. While no consistent correlation to surface roughness was displayed, the pitting breakdown potential (E p) showed a clear statistical trend at 1 / A T A 0.5 . In addition, normalization of the polarization resistance with the A TA reveals the corresponding surface roughness did not significantly affect the change in open-circuit corrosion phenomenon. This correlation fits well with a previously reported stochastic pitting model on metal surfaces. The results suggest that the importance of A TA as a predictor for predisposition to corrosion in AM 316L SS extends far beyond what has been established for wrought materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Identification of favorable and evolving texture during V-bending by finite elements. • Production of site-specific tailored microstructures with L-PBF. • Tailored material outperfoms non-tailored material up to 30%. • Highlighting L-PBF as a tool for designing superior microstructures. • The methodology can be extended to lattice and open cell structures. Laser powder bed fusion (L-PBF) has emerged as an additive manufacturing technique that offers unprecedented design freedom. Besides being capable of producing complex and near net shape objects, L-PBF can impact tremendously the engineering materials community due to the possibility of locally manipulating metallic microstructures. Here we exploit the latter potentiality of L-PBF, to produce site-specifically tailored stainless steel components, in terms of their crystallographic texture. The tailored materials are tested and exhibit superior energy dissipation capabilities under bending deformation compared to uniformly textured materials. This is enabled by the strong dependence of the secondary hardening mechanisms, namely the deformation twinning and/or martensite formation, of these materials on the locally tuned microstructures. With the aid of finite element simulations, it is possible to identify the stress state and hence, the crystallographic orientations that facilitate twinning or martensite formation. Then, by engineering favorable crystallographic textures, matched to the complex stress state during bending, enhanced work hardening behavior is obtained. This site-specific microstructure design enabled by L-PBF provides a new pathway for the design of "smart" components that exhibit superior mechanical response under complex stress states. [ABSTRACT FROM AUTHOR]

[Display omitted] • An simulation-driven-design tool is developed to design metal lattices with a target compressive stress–strain curve. • The automated design algorithm is informed by Design for Manufacture rules based on powder bed fusion process capability. • The design tool is applied to five distinct curves and validated via manufacturing and testing of optimised designs. • The unit cell aspect ratio and strut taper design variables improve strength and energy absorption in BCC lattices. • Strong repeatability and ductility suitable for energy absoption applications is observed in as-built 17-4PH lattices. Additive manufacturing (AM) allows for the creation of novel complex structures previously impossible using traditional subtractive methods; however, there is a need for new design approaches to fully exploit this potential. Metal lattice structures have attracted attention for their broad customisable range of configurations and potential properties, and to take advantage of these possibilities there is a need for intelligent design tools to optimise the lattice according to the desired application and properties. This study presents and demonstrates a method to automatically design lattice structures which achieve a desired compressive stress–strain curve and satisfy user-defined manufacturing limits. A genetic algorithm iterates selected design variables (unit cell aspect ratio, strut taper, and thickness gradient) and minimises the error between the target stress–strain curve and predicted curve which is determined via automated finite element (FE) modelling of each lattice design. The optimised design is validated through manufacture and compression testing of 17-4PH stainless steel lattice structures, and micro-CT imaging to assess build quality. The tool's versatility is demonstrated by successfully designing lattices to fit a range of target curves. This work demonstrates the potential of this smart inverse design tool to exploit AM and optimise lattice design based on desired performance. The custom written design tool presented in this study is available open source at https://github.com/I-Form/LiDOT. [ABSTRACT FROM AUTHOR]

[Display omitted] • The brittle sigma and chi phases were inhibited while the Laves phase was promoted by Al modification. • The pinning of Laves phase and the solute drag of Al hindered the movement of grain boundaries. • Al modification reduces the risk of material embrittlement and promotes improved mechanical properties. • Al-modified ultra-super ferritic stainless steels have been successfully developed and require further research. An isothermal ageing experiment was carried out to investigate the influence of Al modification on the brittle phase precipitation behaviour, grain coarsening and mechanical properties of newly designed ultra-super ferritic stainless steel. Experimental results demonstrated that the modification of Al significantly inhibited the formation rate of the sigma phase, retarded the induction period of the chi phase, and promoted the growth process of the Laves phase. Meanwhile, the calculated grain growth kinetics of the surface layer and core layer were compared with the measured values. It was found that the Al modification enhanced the effect of Laves phase precipitation pinning and solid solution drag, which slowed the grain coarsening rate. With the modification of Al, the values of ultimate tensile strength, yield strength (0.2% proof stress) and total elongation increased compared to those of the Al-free alloys. The plasticity of the Al-modified alloys was improved with the retardation of sigma and chi phases at the grain boundary. This study provided valuable insights for the development of Al-modified ultra-super ferritic stainless steels with low precipitation brittleness tendency. [ABSTRACT FROM AUTHOR]