SUS 316L Research Articles

Mechanistic Study of the Electrochemical Reduction of CO2 in Aprotic Ionic Liquid in Air.

The capture and electrochemical conversion of dilute CO2 in air is a promising approach to mitigate global warming. Aiming to increase the efficiency of the electrochemical reduction of CO2, we fabricated electrodes and developed a custom-designed sealed electrochemical reaction system to study the mechanism of this conversion. The performance of three metal electrodes, Ag, Cu, and SUS 316L, was compared in an aprotic ionic liquid as the electrolyte to monitor the CO2 concentration and chemical reactions using a CO2 sensor and diffuse reflectance infrared Fourier transform spectroscopy and Raman spectroscopy in CO2/N2 (400 ppm CO2 and 99.96% N2) or synthetic air (400 ppm CO2, 21% O2, and 79% N2). The CO2 concentration decreased at negative potentials and was more drastic in synthetic air than in CO2/N2. At negative potential in synthetic air, IR revealed carbon monoxide, carbonate, or peroxydicarbonate on the Ag, Cu, or SUS 316L electrodes, respectively. Reaction intermediates were identified using Raman spectroscopy. Superoxide (O2•-), produced by the reduction of O2 on each electrode, promotes the electrochemical reduction of CO2 whose reduction potential is higher on the negative side than that of O2. This research deepens our understanding of the electrochemical capture/release and conversion of dilute CO2.

Study on the forging process of artificial bone plates of SUS 316L

Abstract SUS 316L stainless steel, valued for its strength and biocompatibility, is a popular choice for artificial bone plates. Forged SUS 316L plates offer advantages like enhanced strength, improved fatigue resistance, reduced material waste, and increased production efficiency. Consequently, the forging technology and die design for SUS 316L is crucial in the manufacturing of high quality artificial bone plates. Finite element analysis simulates the forging process of SUS 316L bone plates, guiding the selection of the appropriate forging machine, die design, and process parameters based on the resulting forging load, stress distribution, and filling condition. Ultimately, this approach facilitates the creation of high-quality SUS 316L bone plates with precise dimensional accuracy.

Chloride-Induced Stress Corrosion Cracking of Friction Stir-Welded 304L Stainless Steel: Effect of Microstructure and Temperature

Dry storage canisters of used nuclear fuels are fabricated using SUS 304L stainless steel. Chloride-induced stress corrosion cracking (CISCC) is one of the major failure modes of dry storage canisters. The cracked canisters can be repaired by friction stir welding (FSW), a low-heat input ‘solid-phase’ welding process. It is important to evaluate the ClSCC resistance of the friction stir welded material. Stress corrosion cracking (SCC) studies were carried out on mill-annealed base materials and friction stir welded 304L stainless U-bend specimens in 3.5% NaCl + 5 N H2SO4 solution at room temperature and boiling MgCl2 solution at 155 °C. The engineering stress on the outer fiber of the FSW U-bend specimen was ~60% higher than that of the base metal (BM). In spite of the higher stress level of the FSW, both materials (FSW and BM) showed almost similar SCC failure times in the two different test solutions. The SCC occurred in the thermo-mechanically affected zone (TMAZ) of the FSW specimens in the 3.5% NaCl + 5 N H2SO4 solution at room temperature, while the stirred zone (SZ) was relatively crack-free. The failure occurred at the stirred zone when tested in the boiling MgCl2 solution. Hydrogen reduction was the cathodic reaction in the boiling MgCl2 solution, which promoted hydrogen-assisted cracking of the heavily deformed stirred zone. The emergence of the slip step followed by passive film rupture and dissolution of the slip step could be the SCC events in the 3.5% NaCl + 5 N H2SO4 solution at room temperature. However, the slip step height was not sufficient to cause passivity breakdown in the fine-grained SZ. Therefore, the SCC occurred in the partially recrystallized softer TMAZ. Overall, the friction-stirred 304L showed higher tolerance to ClSCC than the 304L base metal.

INVESTIGATION OF THE BENDING BEHAVIOR OF INC625/SUS316L LASER-CLADDING LAYERS APPLIED TO GGG40

Laser cladding is a multi-purpose thermal coating technology whose application area has increased rapidly in recent years. It can be used for the purposes of obtaining a thick (mm to cm) layer on surfaces by the interaction of different materials in powder form (especially metallic and metal-matrix composite) with the laser beam, thus repairing, restoring, improving and protecting the surface resistance of various industrial metallic parts. It is normally used to refurbishment, create and repair metallic components, hydraulic mills, gears, shafts, turbine blades, drilling tools, and other static or dynamically loaded mechanical parts. The benefits of laser cladding over alternative technologies include better metallurgy (bonding, hardness or lower porosity) as well as reduced part deformation and stress due to lower overall heat input. The melting of substrate material ensures a good metallurgical bond between the cladding layer and the substrate material. However, the melting of the substrate material causes dilution. Therefore, the melting of the substrate material should be controlled and kept to a minimum because high substrate melting causes an increase in the dilution, which may degrade the mechanical and corrosion properties of the clad layer. The laser beam over the substrate materials creates temperature gradients in the thickness direction that can lead to mechanical deformation (such as bending) and changes in the microstructural and interface properties. Depending on the substrate type and production history, as well as the laser-clad powder properties and process parameters (power, feed rate, clad speed etc.) used in the application, the deformation properties of the cladded part and its bending behavior can change. In this study, INC625 and SUS 316L+INC625 clad layers were applied on a GGG40 substrate with optimized parameters and then subjected to bending tests. Bending-test results were examined comparatively and their microstructural properties were examined. Cladding powder feedstock material properties, substrate type and its thermo-physical properties, clad process parameters and surface preparation conditions, number and thickness of layers are the most important factors affecting the bending behaviour in laser-cladding applications and require optimization studies. The INC625 clad layer on the SUS316L bond layer has reduced the diffusional effects, and the hardness distribution has a more homogeneous profile. In this way, an increase in bending angles was observed. The highest bending angle of 32° was measured with dublex-clad layered samples.

Effect of Strain Hardening on Burr Control in Drilling of Austenitic Stainless Steel

Austenitic stainless steel has been widely used in various industries, such as aerospace, medical, and hydrogen energy, due to its high strength over a wide range of temperatures, corrosion resistance, and biocompatibility. However, stainless steel is a difficult-to-cut metal because its ductility and low thermal conductivity induce a strain hardening with significant plastic deformation at high temperatures. Burr formed at the back side of a plate is a critical issue which deteriorates the surface quality, especially in drilling. Burr removal operation, therefore, should be done in the machine shop. This study discusses the effect of strain hardening of austenitic stainless steel, SUS 316L, on burr formation. Hardness and cutting tests were conducted to compare the strain hardening effect for three types of workpieces: as-received, pre-machined, and tensile treated specimens. In the employed specimens, the tensile treated specimen is harder than the as-received specimen. Those specimens have uniform hardness in the depth direction from surfaces. Pre-machined specimen, in which the back side of the plate was finished by face milling, has a distribution of hardness in the depth direction from a surface. The highest hardness appears in the subsurface of the pre-machined specimen. The cutting forces in the steady processes, in which the entire edges remove material, were nearly the same as the tested specimens each other. However, remarkable differences were confirmed in the chip thickness and burr formation. The higher strain hardening of the tensile treated specimen is effective to suppress burr formation. The cutting characteristics are then identified to associate burr control with the shear plane model of orthogonal cutting using an energy-based force model. The shear stresses, shear angles, and friction angles of the tensile treated and as-received specimens are compared to discuss the effect of strain hardening on reduction of burr formation.

Effects of Al Addition and Annealing on Microstructure and Properties of Ni-Ti Coatings Synthesized on SUS 316L Stainless Steel Substrates Using Mechanical Alloying Method

Effects of Al Addition and Annealing on Microstructure and Properties of Ni-Ti Coatings Synthesized on SUS 316L Stainless Steel Substrates Using Mechanical Alloying Method

Microstructure Changes and Mechanical Properties of Stainless Steel Thin Metal Foils After Tensile Testing

The mechanical properties and changes of microstructure of Austenitic Stainless Steel (ASS) 316L thin metal foils (TMF) were studied through uniaxial tensile tests and SEM-EBSD investigations. The differences in the uniaxial tensile test were observed in SUS 316L with varying grain size (Gs) such as 1.5 um; 3.0 um, and 9.0 um. No. martensitic phase transformation (MPT) occurred in SUS 316 TMF fine grains and coarse grains as indicated in SEM-EBSD results. The microstructure changes were indicated by the change of grain misorientation (GMO) in both SUS 316 TMF fine grains and coarse grains. Furthermore, variations in microstructure were observed in TMF with different Gs.

Microstructure, properties and fracture failure mechanism of MAG welded joints of four types of stainless steels

In this study, four types of austenitic stainless steels (SUS 301L, SUS 304, SUS 316L and SUS 321) with different chemical compositions were butt welded by using MAG (metal active gas) welding. Unique information was gained by studying the solidification mode, microstructure and mechanical properties of the four welded joints under the same welding process, and their fracture mechanisms were analyzed from the perspective of crystallography. The results indicate that the microstructure of the four austenitic stainless steel base metals is uniformly sized equiaxed austenite with δ-ferrite distributed inside or at the boundary of austenite grains in 301L and 304 base metals. The welds of the four kinds of stainless steels are solidified in Austenitic-Ferrite (AF) solidification mode, and the final microstructures of the 301L, 304 and 316L weld are equiaxed grains while the 321 stainless steel weld grows into a dendritic structure due to its low temperature gradient. The microstructures of the fusion zone (FZ)/fine equiaxed zone (FQZ) and heat affected zone (HAZ) of the four types of stainless steels are closely related with the solidification mode of FZ, i.e. small FZ and HAZ regions result from Ferrite (F) solidification mode present in the 301L and 321 welded joints. However, larger FQZ regions generate in the 304 and 316L welded joints due to the Ferrite-Austenitic (FA) solidification mode. The 304 and 316L stainless steel tend to fracture at the HAZ positions due to the formation of FQZ; large angle grain boundary formed at the weld of 301L stainless steel is the reason for its fracture, and the good anisotropy of the weld and heat affected zone of 321 stainless steel also makes it easier to fracture in the base metal (BM). The section hardness distribution characteristics of the four stainless steels welded joints also verify their fracture tendency.

EFFECT OF SHIELDING GAS ON THE PROPERTIES OF STAINLESS-STEEL SUS 304L PLUG WELDED

The effect of shielding gas composition and welding current on the mechanical-physical properties of plug welding joint stainless steel SUS 304L has been investigated. Metal Inert Gas (MIG) welding was used to join SUS304L with a thickness of 3 mm. The variations of welding current were 80 A, 100 A, and 120 A, while variations of shielding gas composition were 100% Ar; 92,5% Ar-7,5% CO2; 85% Ar-15% CO2; 77,5% Ar-22,5% CO2; 100% CO2. Macro and microstructural tests were conducted to determine welded joints physical properties. Tensile-shear testing and micro hardness Vickers were done to determine welded joints physical properties. The results show that the higher level of welding current and CO2 content in the shielding gas, the more tensile-shear load bearing capacity and decreased hardness. The welding current of 120 A and shielding gas 77,5% Ar-22,5% CO2 produced welded joints with the highest tensile-shear load bearing capacity. The nugget size increased as the higher level of welding current and the CO2 content in the shielding gas due to the increase of heat input.

Investigation of Foreign Particles in Moderna COVID-19 Vaccine

Particular batches of Moderna mRNA Coronavirus Disease 2019 (COVID-19) vaccine were recalled after foreign particles were found in some vaccine vials at the vaccination site in Japan in August 2021. We investigated the foreign particles at the request of the Ministry of Health, Labour and Welfare. Energy dispersive X-ray spectroscopy analysis suggested that the foreign particles found in the vials recalled from the vaccination sites were from stainless steel SUS 316L, which was in line with the findings of the root cause investigation by the manufacturer. The sizes of the observed particles ranged from <50 μm to 548 μm in the major axis. Similar foreign particles were also detected in 2 of the 5 vaccine vials of the same lot stored by the manufacturer, indicating that the foreign particles have already been administered to some people via vaccine. Observation of the vials of the same lot by digital microscope found smaller particles those were not detected by visual inspection, suggesting that more vials were affected. Contrarily, visual inspection and subvisible particulate matter test indicated no foreign particles in the vials of normal lots. Possible root cause and strategies to prevent such a deviation were discussed from technical and regulatory aspects.

A Study on Functional Hydrophobic Stainless Steel 316L Using Single-Step Anodization and a Self-Assembled Monolayer Coating to Improve Corrosion Resistance

Stainless steel fabricated using chromium is widely being used in various industries due to its superior corrosion resistance compared to light metals such as aluminum, titanium, and magnesium. However, despite its excellent properties, a problem of poor corrosion resistance in harsh environments remains. In this study, an economical and environmentally friendly anodizing process was applied to the surface of stainless steel (SUS 316L) to create porous nanostructures to improve its water-repellent properties. In these experiments, voltages of 30, 50, 70, and 90 V were applied to stainless steel for 3 h to form an oxide film, prior to immersion in 0.1 M phosphoric acid for 10 min to expand the oxide pores. In addition, the change of the oxide structure was observed through field-emission scanning electron microscopy (FE-SEM). In terms of the contact angle, hydrophilicity was observed at applied voltages of 70 and 90 V, in which a porous film was formed; the best water repellency was observed at a 90 V applied voltage, after the application of an FDTS (1H,1H,2H,2H-perfluorodecyltrichlorosilane) coating, a self-assembled monolayer. Finally, the corrosion behavior of a hydrophobic specimen was tested using potentiodynamic polarization (PDP) experiments. The hydrophobic SUS 316L alloy subsequently displayed improved corrosion resistance in a 3.5 wt% NaCl solution.

Synthesis of the SUS 316L Powders with a Nano-Meso Bi-Modal Structure

Pure stainless steel (SUS 316L) powders with both a nano-grain and micro-structure were prepared using a mechanical milling process. The bimodal microstructure consists of a nano-grain with a size of 50 nm in the surface region and microstructure in the core of the particle. The nano-grain with a bcc structure at the surface and the microstructure of fcc in the core of the particles were prepared by milling at 150rpm for 9h, and those compositions were 14.39% for the nano structure and 85.61% for the micro structure, respectively. Spark plasma sintering (SPS) was carried out for the compaction of the powders. The compacts forming powders with a bi-modal structure have a mesoscopic structure.

Application of Al2O3/iron-based composite abrasives on MAF process for inner surface finishing of oval-shaped tube: predicting results of MAF process using artificial neural network model

The oval-shaped tubes are the valuable product for using to transport the clean gas and high-purity solutions in the field of aerospace, semiconductor, LCD, and medical industries. However, due to their complex shape, it could significantly difficult to improve their inner surface quality by the conventional processes. In this study, the application of Al2O3/iron-based composite abrasives on magnetic abrasive finishing (MAF) process was used to finish the inner surface of oval-shaped SUS 316L tubes. Their inner surface was finished by a mixture of magnetic abrasive tools of Al2O3/iron-based composite abrasives with Fe powders, and light oil. An artificial neural network (ANN) model was used to predict the results of MAF process and then was compared with the actual test. It is found that the results predicted by the ANN model have significant agreement with the actual test results. The correlation coefficient (R2) for both training and testing of surface roughness on span and rise portion is larger than 0.92, and RMSE values for both cases are extremely small (less than 0.01). The actual test results showed that the inner surface of oval-shaped SUS 316L tube was smoothly improved from 0.24 μm to 0.05 μm and 0.04 μm on the rise and span portion, respectively. The application of Al2O3/iron-based composite abrasives on MAF process is an efficient method for inner surface finishing of the oval-shaped tubes.

Strain effect on microstructural properties of SUS 304L joint by PAW+GTAW

JIS SUS 304L stainless steel was joined by a combination welding process of plasma arc welding and gas tungsten arc welding (PAW+GTAW). Then pre-strain treatment on 304L welded joint of 9 % was carried out using uniaxial quasi-static tensile at room temperature. Effect of strain on microstructure evolution in joint was analyzed by field emission scanning electron microscope (FESEM) and on mechanical properties was also studied. The results indicated that the pre-strain rate of 304L joint showed inhomogeneity including 3 % in the weld metal and 13 % in the heat-affected zone (HAZ), which induced martensitic transformation occurring in HAZ. Tensile strength of the joint increased from 700 MPa as welded to 804 MPa after pre-strain treatment at room temperature, and it reached 1700 MPa from 1480 MPa as welded at low temperature of −196 °C. Impact energy in the HAZ was the least among the whole 304L weld, but it was still 94 J at −196 °C after pre-strain. The fracture morphology showed large numbered of cleavage steps with elongated parabolic dimples.

A novel approach for detecting undercuts within surface textures generated by Electrochemical Jet Machining (ECJM)

The present study proposes a novel method for detecting micrometric undercuts (UCs) generated by electrochemical machining for the production of surfaces with tailored functionality. Two different algorithms for the detection of UCs based on two-dimensional topographic maps are tested. The first is a traditional approach based on definition of UCs in terms of surface orientation with respect to a reference direction. The second is an innovative alternative approach designed to reduce sensitivity to numerical effects that potentially lead to overestimation of the number of detected UCs. Electrochemical Jet Machining (ECJM) is used to texture SUS 316L specimens with the aim of producing a measurable surface with a representative number of micrometric UCs. Generated surface textures, comprising craters with diameters ranging from a few microns to tens of microns, are cross-sectioned and inspected with Scanning Electron Microscopy. The extracted profiles allow the novel method for detection of UCs to be efficiently tested and compared with the traditional approach. The number of UCs is found to decrease with increasing electrolyte jet scanning speed, while remarkable differences are revealed between the two calculation approaches at scanning speeds below 2 mm s−1.

Corrosion Behavior of SUS 304L Steel in Concentrated K2CO3 Solution

IntroductionRecently, interest in hydrogen energy has been increasing as it is an environmentally friendly energy source. Water electrolysis is one method to produce hydrogen and oxygen, and can be achieved with renewable energy such as wind and solar power.[1] The alkaline polymer electrolyte membranes (AEM) type electrolysis can be used with inexpensive metal oxides and catalysts such as metal and nitrogen-doped carbon and may be expected to be able to produce hydrogen at low cost. The electrolyte used in AEM type electrolysis is an alkaline solution such as NaOH or KOH with high pH. The K2CO3 solution is also expected as a high electrical conductivity electrolyte despite lower pH than NaOH and KOH.[2] Titanium is currently used for materials of electrolysis cells. However, since titanium is expensive, steel based materials are required. In this study, SUS 304L steel was selected as a commercial steel, and electrochemical measurements were carried out in a K2CO3 solution with pH = 12. After the electrochemical measurements, the steel surface was analyzed.ExperimentalThe SUS 304L (18%Cr-10%Ni-Fe alloy) was used as a sample for electrochemical measurements, then polished with water-resistant polishing paper (# 400 to 2400) and further polished to a mirror surface using alumina powder. After the polishing, ultrasonic cleaning was performed in distilled water and acetone. An electrolyte was prepared by K2CO3 and distill water to pH = 12. A potentiostat (Hokuto Denko Co, HZ-7000) was used for the electrochemical measurements. The experimental cell was a cylindrical PTFE cell with a working electrode, counter electrode, and reference electrode attached to the lid. The SUS 304L was used for the working electrode, a platinum plate was used for the counter electrode, and Hg/HgO was used as the reference electrode. The electrolyte was controlled to 60 oC. Linear Sweep Voltammogram (LSV) measurements were performed from the open circuit potential to 1.0 V with a sweep rate of 5 mVs-1. Constant potential electrolysis was carried out at a potential of 1.0 V in the electrolyte with pH = 12 for 1, 10, and 100 hours. The surface roughness of the SUS 304L before and after the constant potential electrolysis were measured by an atomic force microscope (AFM) (Shimadzu Co: SPM-9700HT). The chemical states of Fe, Cr, and Ni on the surface of the specimens and depth profile before and after the constant potential electrolysis were analyzed by XPS (JEOL: JPS-9200) measurements.Results and DiscussionIn the LSV results, an increase of anodic current was observed from 0.76 V (vs. Hg/HgO) and attributed to oxygen evolution reaction. It was confirmed that the value of current at 1.0 V was higher than that of NaOH solution at pH = 13 and lower than that at pH = 14. In the constant potential electrolysis, gas evolution was occurred on the surface of the SUS 304L electrode during the electrolysis, without any change in the observed color of the electrolyte after the electrolysis.From the AFM measurements, the roughness before the electrolysis was 4 nm. On the other hand, the roughness of the sample after electrolysis for 1, 10 and 100 hours were 8, 21 and 58 nm, respectively. Since this result does not follow the parabolic law between the roughness value and electrolysis time, it is considered that a film formation may proceed on the substrate.In the XPS measurements of the SUS 304L surface, peaks of Cr, Fe and Ni metal and peaks of Cr and Fe oxide were detected before the electrolysis. After each electrolysis, peaks of Fe and Ni hydroxides were detected. However, after electrolysis for 100 hours, peak of Cr was hardly observed. Therefore, it is considered that Cr in the surface was dissolved to the solution during electrolysis, and Fe and Ni remained as hydroxide immediately after dissolution reaction.AcknowledgementPart of the experiments reported here are supported by the New Energy and Industrial Technology Development Organization(NEDO).[1]B. Pivovar, N. Rustagi, and S. Satyapal, Electrochem. Soc. Interface, vol. 27, 47–52, 2018.[2]H. Ito, N. Kawaguchi, S. Someya and T. Munakata, Electrochimica Acta., vol.297, 188-196, 2019.

Amorphization under fracture surface in hydrogen-charged and low- temperature tensile-tested austenitic stainless steel

ABSTRACT The microstructure just below the fracture surface in hydrogen-charged stable austenitic SUS 316L stainless steel, which was subjected to a low strain rate tensile test at −70°C, was studied by a combination of the focused-ion-beam method and transmission electron microscopy. An amorphous region with a chemical composition almost identical to that of the polycrystalline region was found under the lath-like structure on the fracture surface, although no deterioration of tensile properties by hydrogen appeared. In the amorphous region, band-like regions with wavy contrasts were observed, which were often accompanied by cracks at the boundaries. The presence of the amorphous region with band-like regions implies that amorphization occurred due to high-density vacancies accompanied by agglomerations of excess vacancies in the hydrogen-charged SUS 316L stainless steel that was tensile-tested at low temperatures.

Corrosion Behavior of SUS 304L Steel in pH 13 NaOH Solution

In order to investigate the corrosion behavior of SUS 304L steel in alkaline solutions, linear sweep voltammogram measurements were performed in NaOH solutions at pH 12 and 13 at 333 K. Constant potential electrolysis was carried out in an NaOH solution at 333 K at pH 13 from 5 to 20 hours. The surface analysis of the samples before and after the constant potential electrolysis was performed by atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The XPS measurements of the samples before the electrolysis showed that Fe and Cr oxide films were formed on the SUS 304L stainless steel surface, and the presence of FeOOH, CrOOH and Ni(OH)2 on the surface was confirmed after the electrolysis. It is suggested that the dissolved metal ions from the SUS 304L surface reacted with OH− and formed a precipitated film on the surface in the alkaline environment.

EXPERIMENTAL STUDY OF CHLORIDE-INDUCED STRESS CORROSION CRACKING (CISCC) FOR AUSTENITIC SUS 304L UNDER THERMAL INSULATION

Chloride-Induced Stress Corrosion Cracking (CISCC) has been frequently reported in the petrochemical industries where it usually causes failure to the austenitic stainless steel structures encased with thermal insulation in a chloride-containing environment. This study aims to analyze and evaluate the risk of chloride concentration and different types of insulation materials on CISCC of Austenitic SUS 304L. The experimental study was carried out for 14 days using U-bend specimens through the Drip Test apparatus, as per ASTM G30 and ASTM C692. U-Bend specimens were installed onto the test rig and shielded with different insulation materials. Sodium chloride (NaCl) salt solutions were periodically dripped onto the specimens, to simulate the wet and dry cycle. Insulation materials used were rockwool, calcium silicate and perlite, whereas the concentrations of NaCl solutions were set at 0.1, 1.0 and 3.5 wt.%. The specimens were inspected usingdye penetration tests (DPT), stereo microscope and optical microscope throughout the study to determine the CISCC susceptibility. Results showed that cracking was observed on the specimen with rockwool insulation at 3.5 wt% NaCl and 90°C. Rockwool has a high water absorption capacity at which saltwater will evaporate when in contact with a hot metal surface, resulting in the build-up of salt deposits of high concentration over time. SCC was not observed on other specimens with different conditions, but salt deposits, general corrosion and pitting corrosion were found. From the study, the thermal insulation of rockwool was found to have the highest tendency to cause SCC, followed by calcium silicate and perlite.Keywords: SUS304L, chloride-induced stress corrosion cracking, insulation materials, U-Bend specimen, drip test

One-step functionally graded materials fabrication using ultra-large temperature gradients obtained through finite element analysis of field-assisted sintering technique

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.