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Metals, Volume 11, Issue 6 (June 2021) – 150 articles

Cover Story (view full-size image): In this work, the effect of hydrogen on the local cyclic deformation behavior of austenitic stainless steels was analyzed by using cyclic indentation testing. In fatigue tests, a hydrogen-induced change in crack morphology was observed, which correlates to the transmission electron microscopy analyses performed in the vicinity of fatigue cracks, yielding differences in dislocation structure caused by hydrogen. This is in accordance with a more pronounced cyclic plastic deformation obtained in cyclic indentation tests on the hydrogen-precharged material and can be explained by hydrogen-enhanced localized plasticity (HELP). Finally, the presented results indicate a high potential of cyclic indentation testing to locally analyze the influence of hydrogen on the cyclic deformation behavior. View this paper
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Article
Research Regarding the Increase of Durability of Flexible Die Made from 50CrMo4 Used in the Typographic Industry
Metals 2021, 11(6), 996; https://doi.org/10.3390/met11060996 - 21 Jun 2021
Cited by 2 | Viewed by 768
Abstract
A large amount of packaging used mainly in the food industry is obtained by technologies that involve the use of machines such as those that cut with flexible die. The durability of the flexible die is a very important aspect and in this [...] Read more.
A large amount of packaging used mainly in the food industry is obtained by technologies that involve the use of machines such as those that cut with flexible die. The durability of the flexible die is a very important aspect and in this regard the purpose of the research was to identify technologies for its development. Thus, the research considered the analysis of the durability of the knives made of 50CrMo4 steel considering hard chrome-plating treatment, as well as laser hardening of the knives. For the analysis of the durability of the tools, two technological parameters were considered, namely the moment of the tightening force, which had values in the range of 50 Nm–110 Nm, and the die cutting speed, which was adjusted to values in the range of 50–60 m/min. For the analysis of the durability of the flexible die, the wear of the tools was taken into account, as well as the maximum length of the die cut material. Full article
(This article belongs to the Special Issue Design of Welded Steel Structures)
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Article
Effect of Double-Pulse Characteristics on Weld Bead Formation and Mechanical Properties in Metal Inert Gas Welding
Metals 2021, 11(6), 995; https://doi.org/10.3390/met11060995 - 21 Jun 2021
Cited by 2 | Viewed by 619
Abstract
Aluminum alloy has been widely used due to its excellent workability, and double-pulse metal inert gas welding (MIG) has become a popular technique in aluminum alloy welding. In this study, a cross-complementary test was performed to study the effect of double-pulse characteristics on [...] Read more.
Aluminum alloy has been widely used due to its excellent workability, and double-pulse metal inert gas welding (MIG) has become a popular technique in aluminum alloy welding. In this study, a cross-complementary test was performed to study the effect of double-pulse characteristics on weld bead formation and mechanical properties in MIG welding. The test was carried out on an AA6061 aluminum alloy using flat overlaying welding. After welding, the micro-metallographic structure and macro-mechanical performance of the weld bead were explored. The test results showed that the two methods of increasing the base current amplitude or the low-frequency of the current effectively enhanced the oscillation of the molten pool, refined the grain size of the fusion zone, and improved the mechanical properties of the weld. Additionally, by comparing the macroscopic photograph of the specimen and the corresponding welding parameters in the test, the formation characteristics of the bead’s fish-scale pattern in double-pulse MIG welding were found when appropriate welding parameters were adopted and weld bead formation was good. This test result provides a strong scientific basis for the selection of welding parameters in the actual promotion and application of double-pulse MIG welding. Full article
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Article
Medium-Frequency Electrical Resistance Sintering of Soft Magnetic Powder Metallurgy Iron Parts
Metals 2021, 11(6), 994; https://doi.org/10.3390/met11060994 - 21 Jun 2021
Viewed by 656
Abstract
The fabrication of soft magnetic Fe parts by the medium-frequency electrical resistance sintering (MF-ERS) technique is studied in this paper. This consolidation technique involves the simultaneous application to metallic powders of pressure and heat, the latter coming from the Joule effect of a [...] Read more.
The fabrication of soft magnetic Fe parts by the medium-frequency electrical resistance sintering (MF-ERS) technique is studied in this paper. This consolidation technique involves the simultaneous application to metallic powders of pressure and heat, the latter coming from the Joule effect of a low-voltage and high-intensity electric current. Commercially pure iron powder was used in the consolidation experiences. The porosity distribution, microhardness, electrical resistivity and hysteresis curves of the final compacts were determined and analysed. The results obtained were compared both with those of compacts consolidated by the conventional powder metallurgy (PM) route of cold pressing and vacuum furnace sintering, and with fully dense compacts obtained by double cycle of cold pressing and furnace sintering in hydrogen atmosphere. Full article
(This article belongs to the Special Issue Metals Powders 2021: Synthesis and Processing)
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Article
Selective Extraction of Ni from Superalloy Scraps by Molten Mg-Zn
Metals 2021, 11(6), 993; https://doi.org/10.3390/met11060993 - 21 Jun 2021
Viewed by 892
Abstract
Bearing significant concentrations of high value and critical metals, superalloy scraps require comprehensive recycling for metal reclamation. In this study, nickel-based superalloy was treated with molten Mg-Zn for the selective extraction of nickel. The influence of heating temperature, the molar ratio of Mg [...] Read more.
Bearing significant concentrations of high value and critical metals, superalloy scraps require comprehensive recycling for metal reclamation. In this study, nickel-based superalloy was treated with molten Mg-Zn for the selective extraction of nickel. The influence of heating temperature, the molar ratio of Mg to Zn in the molten metal, Mg-Zn/superalloy mass ratio, and heating time on metal extraction were investigated. Using the heating temperature of 800 °C, the Mg/Zn molar ratio of 9/1, the Mg-Zn/superalloy mass ratio of 5/1, and heating time of 240 min, the extraction rate of 97.1% was achieved for Ni, and the extraction rates of Fe, Cr and refractory metals (Nb, Mo and Ti) were all less than 1%. In the subsequent vacuum distillation process, nickel with a purity of 98.3 wt% was obtained. Therefore, the proposed method is a short, clean, and efficient process for selectively extracting nickel from the superalloy scraps. Full article
(This article belongs to the Special Issue Advanced Technologies for Extractive Metallurgy)
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Article
Influence of Process Parameters on Copper Content in Reduced Iron Silicate Slag in a Settling Furnace
Metals 2021, 11(6), 992; https://doi.org/10.3390/met11060992 - 21 Jun 2021
Cited by 4 | Viewed by 578
Abstract
During the pyrometallurgical extraction of copper, a significant fraction of this metal is lost with discard slag, which decreases profits and overall copper recovery. These copper losses can be reduced by using a settling furnace, in which suspended droplets containing copper separate from [...] Read more.
During the pyrometallurgical extraction of copper, a significant fraction of this metal is lost with discard slag, which decreases profits and overall copper recovery. These copper losses can be reduced by using a settling furnace, in which suspended droplets containing copper separate from slag under the influence of gravity. An industrial trial was conducted in a settling furnace to increase the knowledge of the effect of temperature and settling time on the copper content of slag, and thus enhance the settling process to increase copper recovery. Slag samples were collected from four sample points: the ingoing and outgoing slag stream, within the furnace during settling, and the granulated slag. The chemical composition of the slag samples was analyzed and compared between batches with different temperatures and settling times. The appearance of copper and its associated phases were analyzed using a scanning electron microscope with an energy-dispersive X-ray spectroscopy detector (SEM-EDS). The results indicated that the outgoing slag copper content increased with an increase in temperature, and it was also concluded to be influenced by the attachment of copper to spinels and gas bubbles. The results indicate that regulating the settling furnace temperature to a lower interval could increase copper recovery. Full article
(This article belongs to the Special Issue Metallurgy and Recycling of Nonferrous Metals)
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Article
Extraction of Rare Earth Metals by Solid-Phase Extractants from Phosphoric Acid Solution
Metals 2021, 11(6), 991; https://doi.org/10.3390/met11060991 - 21 Jun 2021
Cited by 1 | Viewed by 757
Abstract
Nowadays, solving the problem of rational, integrated use of the mined raw materials, the transition to waste-free technologies for its processing is a crucial task. The sulfuric acid technology used for the processing of apatite concentrates on a large industrial scale does not [...] Read more.
Nowadays, solving the problem of rational, integrated use of the mined raw materials, the transition to waste-free technologies for its processing is a crucial task. The sulfuric acid technology used for the processing of apatite concentrates on a large industrial scale does not provide the associated extraction of accompanying valuable components—rare earth metals (REM). During apatite concentrate processing, rare-earth metals are affected by the technology-related dispersion, being distributed between the insoluble leaching residue and phosphoric acid solution sent to the production of fertilizers. The necessity of a cost-effective method development for the extraction of rare earth metals is quite obvious already in connection with the indicated significance of the problem. Phosphoric acid solutions that simulate the composition of industrial phosphoric acid solutions of the following composition 4.5 mol/L H3PO4, 0.19 mol/L H2SO4 and 0.10–0.12% REM were selected as the object of research. The extraction of rare earth metals was carried out using polymers containing a fixed layer of an extractant—di-2-ethylhexylphosphoric acid (D2EHPA). Fixed layer was obtained by impregnation-saturation (solvent-impregnated resin (SIR)) or by the introduction of an extractant at the stage of polymer matrix synthesis (extractant-resin extraction (ERE)). The work determined the thermodynamic and technological characteristics of the solid-phase extraction of rare earth elements from phosphoric acid solutions with polymers impregnated with D2EHPA and containing a rigidly fixed extractant in a styrene-divinylbenzene resin matrix. The possibility of effective multiple use of polymeric resins containing D2EHPA, regenerated with a solution of 1 mol/L sodium citrate, was revealed. Full article
(This article belongs to the Section Extractive Metallurgy)
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Article
Corrosion Behavior of Extruded AM60-AlN Metal Matrix Nanocomposite and AM60 Alloy Exposed to Simulated Acid Rain Environment
Metals 2021, 11(6), 990; https://doi.org/10.3390/met11060990 - 21 Jun 2021
Viewed by 810
Abstract
The present work compared the initial stages of corrosion process development on the AM60-AlN metal matrix nanocomposite surface and on AM60, during their exposure for 30 days to simulated acid rain solution (SAR). The AlN nanoparticles were observed as “attached” to those of [...] Read more.
The present work compared the initial stages of corrosion process development on the AM60-AlN metal matrix nanocomposite surface and on AM60, during their exposure for 30 days to simulated acid rain solution (SAR). The AlN nanoparticles were observed as “attached” to those of Mn-rich AlMn intermetallic particles, forming clusters. The introduction of 1.0 wt.% AlN (≈ 80 nm) in the AM60 alloy carried a slight grain refinement and favored the formation of a denser and more protective corrosion layer, suggested by the electrochemical impedance spectroscopy (EIS) values of higher charge transfer resistance (R2) and capacitance, characteristic of the double layer in the presence of corrosion products, and also suggested by Rn (EN) values, compared to those of the AM60 alloy. Thus, the concentration of the released Mg-ions from the composite surface was lower. Due to the increase in time of the SAR solution pH, Al de-alloying may occur, as well as Al(OH)3 formation, as confirmed by XPS analysis. Due to the presence of Cl-ions in SAR solution, localized corrosion was observed, suggested as fractional Gaussian noise of a stationary and persistent process in time, according to the PSD of the corrosion current fluctuations (EN). Full article
(This article belongs to the Special Issue Corrosion and Protection in Aeronautical Alloys)
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Article
Effect of Boron Carbide Addition on Wear Resistance of Aluminum Matrix Composites Fabricated by Stir Casting and Hot Rolling Processes
Metals 2021, 11(6), 989; https://doi.org/10.3390/met11060989 - 21 Jun 2021
Cited by 4 | Viewed by 741
Abstract
In this study, to evaluate the effect of boron carbide (B4C) addition on the wear performance of aluminum (Al), Al6061 and 5, 10, and 20 vol.% B4C/Al6061 composites were manufactured using the stir casting and hot rolling processes. B [...] Read more.
In this study, to evaluate the effect of boron carbide (B4C) addition on the wear performance of aluminum (Al), Al6061 and 5, 10, and 20 vol.% B4C/Al6061 composites were manufactured using the stir casting and hot rolling processes. B4C particles were randomly dispersed during the stir casting process; then, B4C particles were arranged in the rolling direction using a hot rolling process to further improve the B4C dispersion and wear resistance of the composites. Furthermore, a continuous interfacial layer between B4C and the Al6061 matrix was generated by diffusion of titanium (Ti) and chromium (Cr) atoms contained in the Al6061 alloy. Wear depth and width of the composites decreased with increasing B4C content. Furthermore, with B4C addition, coefficient of friction (COF) improved as compared with that of Al6061. The results indicate that interface-controlled, well-aligned B4C particles in the friction direction can effectively increase the wear properties of Al alloys and improve their hardness. Full article
(This article belongs to the Special Issue New Trend in Metal-Ceramic Composite Materials)
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Article
In Situ Alloying of a Modified Inconel 625 via Laser Powder Bed Fusion: Microstructure and Mechanical Properties
Metals 2021, 11(6), 988; https://doi.org/10.3390/met11060988 - 21 Jun 2021
Cited by 1 | Viewed by 795
Abstract
This study investigates the in situ alloying of a Ni-based superalloy processed by means of laser powder bed fusion (LPBF). For this purpose, Inconel 625 powder is mixed with 1 wt.% of Ti6Al4V powder. The modified alloy is characterized by densification levels similar [...] Read more.
This study investigates the in situ alloying of a Ni-based superalloy processed by means of laser powder bed fusion (LPBF). For this purpose, Inconel 625 powder is mixed with 1 wt.% of Ti6Al4V powder. The modified alloy is characterized by densification levels similar to the base alloy, with relative density superior to 99.8%. The material exhibits Ti-rich segregations along the melt pool contours. Moreover, Ti tends to be entrapped in the interdendritic areas during solidification in the as-built state. After heat treatments, the modified Inconel 625 version presents greater hardness and tensile strengths than the base alloy in the same heat-treated conditions. For the solution annealed state, this is mainly attributed to the elimination of the segregations into the interdendritic structures, thus triggering solute strengthening. Finally, for the aged state, the further increment of mechanical properties can be attributed to a more intense formation of phases than the base alloy, due to elevated precipitation strengthening ability under heat treatments. It is interesting to note how slight chemical composition modification can directly develop new alloys by the LPBF process. Full article
(This article belongs to the Special Issue Optimization of Metal Additive Manufacturing Processes)
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Article
Polyacrylic Acid to Improve Flotation Tailings Management: Understanding the Chemical Interactions through Molecular Dynamics
Metals 2021, 11(6), 987; https://doi.org/10.3390/met11060987 - 21 Jun 2021
Cited by 3 | Viewed by 899
Abstract
Molecular dynamic simulations of polyacrylic acid polyelectrolyte (PAA) analyzed its interaction with the main minerals that make up characteristic tailings of the mining industry, in this case, quartz, kaolinite, and montmorillonite. The simulations were carried out with the package Gromacs 2020.3. The interaction [...] Read more.
Molecular dynamic simulations of polyacrylic acid polyelectrolyte (PAA) analyzed its interaction with the main minerals that make up characteristic tailings of the mining industry, in this case, quartz, kaolinite, and montmorillonite. The simulations were carried out with the package Gromacs 2020.3. The interaction potentials used were General AMBER Force Field (GAFF) for PAA and CLAYFF-MOH for mineral surfaces. The SPC/E model described water molecules and Lennard-Jones 12-6 parameters adjusted for SPC/E model were used for Na+ and Cl ions. The studied systems were carried out at pH 7, obtaining stable adsorption between the PAA and the studied surfaces. Interestingly, the strongest adsorptions were for montmorillonite at both low and high salt concentrations. The effect of salinity differs according to the system, finding that it impairs the absorption of the polymer on montmorillonite surfaces. However, a saline medium favors the interaction with quartz and kaolinite. This is explained because montmorillonite has a lower surface charge density and a greater capacity to adsorb ions. This facilitated the adsorption of PAA. It was possible to identify that the main interaction by which the polymer is adsorbed is through the hydroxyl of the mineral surface and the COONa+ complexes. Molecular dynamics allows us to advance in the understanding of interactions that define the behavior of this promising reagent as an alternative for sustainable treatment of complex tailings in highly saline environments. Full article
(This article belongs to the Special Issue Flotation and Leaching Processes in Metallurgy)
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Article
Effect of the Addition of Flocculants and KCl on Sedimentation Rate of Spodumene Tailings
Metals 2021, 11(6), 986; https://doi.org/10.3390/met11060986 - 21 Jun 2021
Viewed by 701
Abstract
Lithium has become an element of great relevance in recent times, because among its various applications is the manufacture of batteries, and it is a vital part of the growing development of new products such as electric vehicles. On the other hand, the [...] Read more.
Lithium has become an element of great relevance in recent times, because among its various applications is the manufacture of batteries, and it is a vital part of the growing development of new products such as electric vehicles. On the other hand, the geographical distribution of lithium reserves is very heterogeneous. Of the existing minerals, only some of them are important sources of exploitation, such as the salt in South America, while in other countries mineral deposits such as spodumene stand out. The process for obtaining lithium from spodumene consists of concentrating up to 3% lithium by flotation. Because other minerals associated with this mineral are mainly silicates (feldspar, clays, quartz and micas), great problems are generated in the thickening stage. This article seeks to study the effect of the addition of flocculants and KCl on the sedimentation rate, in addition to studying its effects on the turbidity of the supernatant in different types of water. This is done by Batch sedimentation tests with tailings pulps, to later characterize both the pulp and the supernatant by means of the turbidity of the clarified water. Magnafloc-338 flocculant is the most convenient to use with industrial water, since it reaches a high sedimentation rate of 34.2, 37.44, and 45.36 m/h, with doses of 5, 10, and 15 g/t respectively, and a low turbidity rate (31 Formazin Nephelometric Units (FNU)) at low flocculant dosages. Full article
(This article belongs to the Special Issue Flotation and Leaching Processes in Metallurgy)
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Review
Research Progress of First Principles Studies on Oxidation Behaviors of Ti-Al Alloys and Alloying Influence
Metals 2021, 11(6), 985; https://doi.org/10.3390/met11060985 - 20 Jun 2021
Cited by 1 | Viewed by 719
Abstract
Ti-Al alloys have good prospects in the aerospace, automobile, and other fields because of their excellent mechanical properties. However, the lack of high temperature oxidation resistance limits their practical applications. Various ways have been used to study their oxidation behaviors. Besides numerous experimental [...] Read more.
Ti-Al alloys have good prospects in the aerospace, automobile, and other fields because of their excellent mechanical properties. However, the lack of high temperature oxidation resistance limits their practical applications. Various ways have been used to study their oxidation behaviors. Besides numerous experimental studies focused on the oxidation resistance properties of Ti-Al alloys, theoretical research based on first principles calculations has been carried out on the oxidation mechanisms on the atomic and electronic scales. In recent years, these theoretical studies have provided strong support for understanding the oxidation mechanisms and designing anti-oxidation modification measures. This review will focus on the surface oxidation reactions, element diffusion behaviors and interface bonding properties. The influencing mechanisms of alloying elements on the oxidation properties will be discussed in detail. Full article
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Article
Determination of Critical Hydrogen Concentration and Its Effect on Mechanical Performance of 2200 MPa and 600 HBW Martensitic Ultra-High-Strength Steel
Metals 2021, 11(6), 984; https://doi.org/10.3390/met11060984 - 20 Jun 2021
Cited by 1 | Viewed by 747
Abstract
The influence of hydrogen on the mechanical performance of a hot-rolled martensitic steel was studied by means of constant extension rate test (CERT) and constant load test (CLT) followed with thermal desorption spectroscopy measurements. The steel shows a reduction in tensile strength up [...] Read more.
The influence of hydrogen on the mechanical performance of a hot-rolled martensitic steel was studied by means of constant extension rate test (CERT) and constant load test (CLT) followed with thermal desorption spectroscopy measurements. The steel shows a reduction in tensile strength up to 25% of ultimate tensile strength (UTS) at critical hydrogen concentrations determined to be about 1.1 wt.ppm and 50% of UTS at hydrogen concentrations of 2 wt.ppm. No further strength degradation was observed up to hydrogen concentrations of 4.8 wt.ppm. It was observed that the interplay between local hydrogen concentrations and local stress states, accompanied with the presence of total average hydrogen reducing the general plasticity of the specimen are responsible for the observed strength degradation of the steel at the critical concentrations of hydrogen. Under CLT, the steel does not show sensitivity to hydrogen at applied loads below 50% of UTS under continuous electrochemical hydrogen charging up to 85 h. Hydrogen enhanced creep rates during constant load increased linearly with increasing hydrogen concentration in the steel. Full article
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Article
Synthesis and Electrochemical Properties of TiNb2O7 and Ti2Nb10O29 Anodes under Various Annealing Atmospheres
Metals 2021, 11(6), 983; https://doi.org/10.3390/met11060983 - 20 Jun 2021
Cited by 3 | Viewed by 1155
Abstract
In this study, two compounds of TiNb2O7 and Ti2Nb10O29 were successfully synthesized by mechanochemical method and post-annealing as an anode material for lithium-ion batteries. The effect of annealing atmosphere on the morphology, particle size, and [...] Read more.
In this study, two compounds of TiNb2O7 and Ti2Nb10O29 were successfully synthesized by mechanochemical method and post-annealing as an anode material for lithium-ion batteries. The effect of annealing atmosphere on the morphology, particle size, and electrochemical characteristics of two compounds was investigated. For these purposes, the reactive materials were milled under an argon atmosphere with a certain mole ratio. Subsequently, each sample was subjected to annealing treatment in two different atmospheres, namely argon and oxygen. Phase and morphology identifications were carried out by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) to identify the phases and evaluate the morphology of the synthesized samples. The charging and discharging tests were conducted using a battery-analyzing device to evaluate the electrochemical properties of the fabricated anodes. Annealing in different atmospheres resulted in variable discharge capacities so that the two compounds of TiNb2O7 and Ti2Nb10O29 annealed under the argon atmosphere showed a capacity of 60 and 66 mAh/g after 179 cycles, respectively, which had a lower capacity than their counterpart under the oxygen atmosphere. The final capacity of the annealed samples in the oxygen atmosphere is 72 and 74 mAh/g, respectively. Full article
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Article
Structural Characterization, Global and Local Electrochemical Activity of Electroless Ni–P-Multiwalled Carbon Nanotube Composite Coatings on Pipeline Steel
Metals 2021, 11(6), 982; https://doi.org/10.3390/met11060982 - 20 Jun 2021
Cited by 3 | Viewed by 670
Abstract
In this work, composite Ni–P-multiwalled carbon nanotube films were produced by electroless deposition. The main goal was to investigate the influence of multiwalled carbon nanotube loading on the local electrochemical behavior of the composite films, as probed by scanning electrochemical microscopy (SECM). The [...] Read more.
In this work, composite Ni–P-multiwalled carbon nanotube films were produced by electroless deposition. The main goal was to investigate the influence of multiwalled carbon nanotube loading on the local electrochemical behavior of the composite films, as probed by scanning electrochemical microscopy (SECM). The coatings were also characterized with respect to their crystalline structure, surface, and cross-section morphologies. Adhesion strength was examined by scratch tests. The global electrochemical behavior was evaluated by potentiodynamic polarization. The local electrochemical activity was investigated by probing the Fe2+ oxidation in the surface generation/tip collection mode of the SECM. The results revealed that multiwalled carbon nanotubes increased the adhesion strength and reduced the electrochemical activity on the surface of the coated samples. Full article
(This article belongs to the Special Issue Advances in Corrosion and Protection of Materials)
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Article
Optimization of Activated Tungsten Inert Gas Welding Process Parameters Using Heat Transfer Search Algorithm: With Experimental Validation Using Case Studies
Metals 2021, 11(6), 981; https://doi.org/10.3390/met11060981 - 19 Jun 2021
Cited by 14 | Viewed by 1069
Abstract
The Activated Tungsten Inert Gas welding (A-TIG) technique is characterized by its capability to impart enhanced penetration in single pass welding. Weld bead shape achieved by A-TIG welding has a major part in deciding the final quality of the weld. Various machining variables [...] Read more.
The Activated Tungsten Inert Gas welding (A-TIG) technique is characterized by its capability to impart enhanced penetration in single pass welding. Weld bead shape achieved by A-TIG welding has a major part in deciding the final quality of the weld. Various machining variables influence the weld bead shape and hence an optimum combination of machining variables is of utmost importance. The current study has reported the optimization of machining variables of A-TIG welding technique by integrating Response Surface Methodology (RSM) with an innovative Heat Transfer Search (HTS) optimization algorithm, particularly for attaining full penetration in 6 mm thick carbon steels. Welding current, length of the arc and torch travel speed were selected as input process parameters, whereas penetration depth, depth-to-width ratio, heat input and width of the heat-affected zone were considered as output variables for the investigations. Using the experimental data, statistical models were generated for the response characteristics. Four different case studies, simulating the real-time fabrication problem, were considered and the optimization was carried out using HTS. Validation tests were also carried out for these case studies and 3D surface plots were generated to confirm the effectiveness of the HTS algorithm. It was found that the HTS algorithm effectively optimized the process parameters and negligible errors were observed when predicted and experimental values compared. HTS algorithm is a parameter-less optimization technique and hence it is easy to implement with higher effectiveness. Full article
(This article belongs to the Special Issue High-Productivity Welding of Metals and Alloys)
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Article
Comparison of Corrosion Resistance of the AA2524-T3 and the AA2024-T3
Metals 2021, 11(6), 980; https://doi.org/10.3390/met11060980 - 19 Jun 2021
Cited by 1 | Viewed by 709
Abstract
The 2XXX Al alloys are characterized by their superior mechanical properties resulting from alloying elements and precipitation hardening treatments. The AA2524-T3 alloy was developed to replace the AA2024-T3 alloy in the aerospace industry. However, both alloys present many intermetallic particles (IMCs) in their [...] Read more.
The 2XXX Al alloys are characterized by their superior mechanical properties resulting from alloying elements and precipitation hardening treatments. The AA2524-T3 alloy was developed to replace the AA2024-T3 alloy in the aerospace industry. However, both alloys present many intermetallic particles (IMCs) in their microstructure, and this is the main reason for their high susceptibility to localized corrosion (such as pitting and stress corrosion cracking). Despite the similarities between these alloys (e.g., chemical composition and type of intermetallics) the literature comparing their properties is scarce and focuses mainly on their mechanical properties, not their corrosion resistances. In this investigation, the corrosion resistance of the AA2524-T3 alloy was compared to the AA2024-T3 alloy. The microstructure of both alloys was analyzed by Scanning Electron Microscopy before and after immersion in the test electrolyte, and the number and area fraction of intermetallics of each alloy was determined. The corrosion resistance of both alloys was monitored as a function of exposure time by electrochemical impedance spectroscopy and the results were fitted using electrical equivalent circuits. The AA2524-T3 alloy presented not only higher impedance values but also less corroded areas than the AA2024-T3 alloy. Full article
(This article belongs to the Special Issue Advances in Corrosion and Protection of Materials)
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Article
The Interaction between the Sheet/Tool Surface Texture and the Friction/Galling Behaviour on Aluminium Deep Drawing Operations
Metals 2021, 11(6), 979; https://doi.org/10.3390/met11060979 - 19 Jun 2021
Cited by 4 | Viewed by 824
Abstract
The increasing demands for lightweight design in the transport industry have led to an extensive use of lightweight materials such as aluminium alloys. The forming of aluminium sheets however presents significant challenges due to the low formability and the increased susceptibility to galling. [...] Read more.
The increasing demands for lightweight design in the transport industry have led to an extensive use of lightweight materials such as aluminium alloys. The forming of aluminium sheets however presents significant challenges due to the low formability and the increased susceptibility to galling. The use of tailored workpieces and controlled die roughness surfaces are common strategies to improve the tribological behaviour, whilst galling is still not well understood. This work is aimed at analysing the interplay between the sheet and tool surface roughness on the friction and galling performance. Different degrees of Electro Discharge Texturing (EDT) textures were generated in AA1050 material strips, and tooling presenting different polishing degrees were prepared. Strip drawing tests were carried out to model the tribological condition and results were corroborated through cup drawing tests. A new galling severity index (GSI) is presented for a quick and quantitative determination of both galling occurrence and severity. The present study underlines the key role of die topography and the potential of die surface functionalization for galling prevention. Full article
(This article belongs to the Special Issue Forming of Aluminium Alloys)
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Review
Advances in Understanding of the Application of Unit Operations in Metallurgy of Rare Earth Elements
Metals 2021, 11(6), 978; https://doi.org/10.3390/met11060978 - 18 Jun 2021
Cited by 5 | Viewed by 1063
Abstract
Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: (1) hydrometallurgy (leaching under atmospheric and high pressure conditions, mixing of solution with gas and mechanical parts, neutralization of solution, precipitation and cementation of metals from [...] Read more.
Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: (1) hydrometallurgy (leaching under atmospheric and high pressure conditions, mixing of solution with gas and mechanical parts, neutralization of solution, precipitation and cementation of metals from solution aiming purification, and compound productions during crystallization), (2) pyrometallurgy (roasting, smelting, refining), and (3) electrometallurgy (aqueous electrolysis and molten salt electrolysis). The high demand for critical metals, such as rare earth elements (REE), indium, scandium, and gallium raises the need for an advance in understanding of the UO in NFEM. The aimed metal is first transferred from ores and concentrates to a solution using a selective dissolution (leaching or dry digestion) under an atmospheric pressure below 1 bar at 100 °C in an agitating glass reactor and under a high pressure (40–50 bar) at high temperatures (below 270 °C) in an autoclave and tubular reactor. The purification of the obtained solution was performed using neutralization agents such as sodium hydroxide and calcium carbonate or more selective precipitation agents such as sodium carbonate and oxalic acid. The separation of metals is possible using liquid (water solution)/liquid (organic phase) extraction (solvent extraction (SX) in mixer-settler) and solid-liquid filtration in chamber filter-press under pressure until 5 bar. Crystallization is the process by which a metallic compound is converted from a liquid into a crystalline state via a supersaturated solution. The final step is metal production using different methods (aqueous electrolysis for basic metals such as copper, zinc, silver, and molten salt electrolysis for REE and aluminum). Advanced processes, such as ultrasonic spray pyrolysis, microwave assisted leaching, and can be combined with reduction processes in order to produce metallic powders. Some preparation for the leaching process is performed via a roasting process in a rotary furnace, where the sulfidic ore was first oxidized in an oxidic form which is a suitable for the metal transfer to water solution. UO in extractive metallurgy of REE can be successfully used not only for the metal wining from primary materials, but also for its recovery from secondary materials. Full article
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Article
Electrochemistry, Electrodeposition, and Photoluminescence of Eu (III)/Lanthanides (III) on Terpyridine-Functionalized Ti Nanospikes
Metals 2021, 11(6), 977; https://doi.org/10.3390/met11060977 - 18 Jun 2021
Cited by 1 | Viewed by 626
Abstract
Terpyridine-functionalized Ti nanospike electrodes (TiNS-SiTpy) were developed and applied to cyclic voltammetry and amperometry of Ln (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) ions and mixed Eu (III) + Ln (III) ions in a 0.1 M [...] Read more.
Terpyridine-functionalized Ti nanospike electrodes (TiNS-SiTpy) were developed and applied to cyclic voltammetry and amperometry of Ln (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) ions and mixed Eu (III) + Ln (III) ions in a 0.1 M NaClO4 electrolyte. Electrodeposition was successfully performed over TiNS-SiTpy electrodes, which were fully examined by scanning electron microscopy, X-ray diffraction crystallography, Fourier-transform infrared spectroscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), and PL decay kinetics. The Gd and Tb ions were found to increase PL intensities with 10× longer lifetimes of 1.32 μs and 1.03 μs, respectively, compared with that of the electrodeposited Eu sample. The crystal phase and the oxidation states were fully examined for the mixed Ln (Eu + Gd and Eu + Tb) complex structures. Full article
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Article
Effects of Al-Si Coating on Static and Dynamic Strength of Spot-Welded Hot-Stamping Steel Joints
Metals 2021, 11(6), 976; https://doi.org/10.3390/met11060976 - 18 Jun 2021
Cited by 1 | Viewed by 773
Abstract
Al-Si is the most popular coating used to prevent oxidation on the surfaces of hot-stamped steel sheets during the heating process. However, like other coatings, it affects the strength of the spot welds in joining the hot-stamped steel parts. In this study, the [...] Read more.
Al-Si is the most popular coating used to prevent oxidation on the surfaces of hot-stamped steel sheets during the heating process. However, like other coatings, it affects the strength of the spot welds in joining the hot-stamped steel parts. In this study, the effects of Al-Si coating on the tensile strength of the resistance spot-welded joints in hot-stamped steel are discussed. Two types of 1.8 mm hot-stamped steel, in uncoated and Al-Si coated forms, were resistance spot-welded, and the tensile shear behavior of the welded joints was studied in both static and dynamic tests. To do this, a special fixture for impact tensile shear tests was designed and fabricated. In the case of the Al-Si coated steel, the presence of the molten Al-Si over the fusion zone, especially its aggregation in the edge of the weld nugget, caused a decrease in the maximum tensile load capacity and a failure of energy absorption in static and dynamic tests, respectively. Additionally, it increased the probability of changing its failure mode from pull out to interfacial fracture in the dynamic test. This study shows that the tensile strength behavior of the welded joints for the Al-Si coated hot-stamped steel is lower than the uncoated steel during static, and especially dynamic, force. Full article
(This article belongs to the Section Metal Casting, Forming and Heat Treatment)
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Article
Pressure Oxidation of Arsenic (III) Ions in the H3AsO3-Fe2+-Cu2+-H2SO4 System
Metals 2021, 11(6), 975; https://doi.org/10.3390/met11060975 - 17 Jun 2021
Viewed by 544
Abstract
The processing of low-grade polymetallic materials, such as copper–zinc, copper–lead–zinc, and poor arsenic-containing copper concentrates using hydrometallurgical methods is becoming increasingly important due to the depletion of rich and easily extracted mineral resources, as well as due to the need to reduce harmful [...] Read more.
The processing of low-grade polymetallic materials, such as copper–zinc, copper–lead–zinc, and poor arsenic-containing copper concentrates using hydrometallurgical methods is becoming increasingly important due to the depletion of rich and easily extracted mineral resources, as well as due to the need to reduce harmful emissions from metallurgy, especially given the high content of arsenic in ores. Ferric arsenates obtained through hydrothermal precipitation are the least soluble and most stable form of arsenic, which is essential for its disposal. This paper describes the investigation of the oxidation kinetics of As (III) ions to As (V) which is required for efficient purification of the resulting solutions and precipitation of low-solubility ferric arsenates. The effect of temperature (160–200 °C), the initial concentration of Fe (II) (3.6–89.5 mmol/dm3), Cu (II) (6.3–62.9 of mmol/dm3) and the oxygen pressure (0.2–0.5 MPa) on the oxidation efficiency of As (III) to As (V) was studied. As (III) oxidation in H3AsO-Fe2+-Cu2+-H2SO4 and H3AsO-Fe2+-H2SO4 systems was controlled by a chemical reaction with the apparent activation energy (Ea (≈84.3–86.3 kJ/mol)). The increase in the concentration of Fe (II) ions and addition of an external catalyst (Cu (II) ions) both have a positive effect on the process. When Cu (II) ions are introduced into the solution, their catalytic effect is confirmed by a decrease in the partial orders, Fe (II) ions concentration from 0.43 to 0.20, and the oxygen pressure from 0.95 to 0.69. The revealed catalytic effect is associated with a positive effect of Cu (II) ions on the oxidation of Fe (II) to Fe (III) ions, which further participate in As (III) oxidation. The semi-empirical equations describing the reaction rate under the studied conditions are written. Full article
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Review
A Review of the Boiling Curve with Reference to Steel Quenching
Metals 2021, 11(6), 974; https://doi.org/10.3390/met11060974 - 17 Jun 2021
Cited by 2 | Viewed by 593
Abstract
This review presents an analysis and discussion about heat transfer phenomena during quenching solid steel from high temperatures. It is shown a description of the boiling curve and the most used methods to characterize heat transfer when using liquid quenchants. The present work [...] Read more.
This review presents an analysis and discussion about heat transfer phenomena during quenching solid steel from high temperatures. It is shown a description of the boiling curve and the most used methods to characterize heat transfer when using liquid quenchants. The present work points out and criticizes important aspects that are frequently poorly attended in the technical literature about determination and use of the boiling curve and/or the respective heat transfer coefficient for modeling solid phase transformations in metals. Points to review include: effect of initial workpiece temperature on the boiling curve, fluid velocity specification to correlate with heat flux, and the importance of coupling between heat conduction in the workpiece and convection boiling to determine the wall heat flux. Finally, research opportunities in this field are suggested to improve current knowledge and extend quenching modeling accuracy to complex workpieces. Full article
(This article belongs to the Special Issue Heat Treatment of Steels)
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Article
A Ternary Map of Ni–Mn–Ga Heusler Alloys from Ab Initio Calculations
Metals 2021, 11(6), 973; https://doi.org/10.3390/met11060973 - 17 Jun 2021
Cited by 1 | Viewed by 578
Abstract
In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of [...] Read more.
In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of first-principles approaches struggle with the concept of a non-stoichiometric system. In this work, we attempt a systematic computational study of magnetic and structural properties of Ni–Mn–Ga, mapped onto ternary composition diagrams. Compositional stability was examined using the convex hull analysis. We show that the cubic austenite has its stability region close to the stoichiometric Ni2MnGa, in agreement with experimental data, while the tetragonal martensite spreads its stability over a wider range of Mn and Ni contents. The unstable compositions in both austenite and martensite states are located in the Ga-rich corner of the ternary diagram. We note that simultaneous stability of the austenite and martensite should be considered for potentially stable compounds suitable for synthesis. The majority of compounds are predicted to be ferrimagnetically ordered in both austenitic and martensitic states. The methodology used in this work is computationally tractable, yet it delivers some predictive power. For experimentalists who plan to synthesize stable Ni–Mn–Ga compounds with ferromagnetic order, we narrow the target compositional range substantially. Full article
(This article belongs to the Special Issue Recent Development in Magnetic Shape Memory Alloys)
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Article
Optimization of the Continuous Casting Process of Hypoeutectoid Steel Grades Using Multiple Linear Regression and Genetic Programming—An Industrial Study
Metals 2021, 11(6), 972; https://doi.org/10.3390/met11060972 - 17 Jun 2021
Cited by 1 | Viewed by 584
Abstract
Štore Steel Ltd. is one of the major flat spring steel producers in Europe. Until 2016 the company used a three-strand continuous casting machine with 6 m radius, when it was replaced by a completely new two-strand continuous caster with 9 m radius. [...] Read more.
Štore Steel Ltd. is one of the major flat spring steel producers in Europe. Until 2016 the company used a three-strand continuous casting machine with 6 m radius, when it was replaced by a completely new two-strand continuous caster with 9 m radius. For the comparison of the tensile strength of 41 hypoeutectoid steel grades, we conducted 1847 tensile strength tests during the first period of testing using the old continuous caster, and 713 tensile strength tests during the second period of testing using the new continuous caster. It was found that for 11 steel grades the tensile strength of the rolled material was statistically significantly lower (t-test method) in the period of using the new continuous caster, whereas all other steel grades remained the same. To improve the new continuous casting process, we decided to study the process in more detail using the Multiple Linear Regression method and the Genetic Programming approach based on 713 items of empirical data obtained on the new continuous casting machine. Based on the obtained models of the new continuous casting process, we determined the most influential parameters on the tensile strength of a product. According to the model’s analysis, the secondary cooling at the new continuous caster was improved with the installation of a self-cleaning filter in 2019. After implementing this modification, we performed an additional 794 tensile tests during the third period of testing. It was found out that, after installation of the self-cleaning filter, in 6 steel grades out of 19, the tensile strength in rolled condition improved statistically significantly, whereas all the other steel grades remained the same. Full article
(This article belongs to the Special Issue Applied Artificial Intelligence in Steelmaking)
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Article
Theoretical Study on Thermoelectric Properties and Doping Regulation of Mg3X2 (X = As, Sb, Bi)
Metals 2021, 11(6), 971; https://doi.org/10.3390/met11060971 - 17 Jun 2021
Cited by 2 | Viewed by 710
Abstract
For searching both high-performances and better fits for near-room temperature thermoelectric materials, we here carried out a theoretical study on thermoelectric properties and doping regulation of Mg3X2 (X = As, Sb, Bi) by the combined method of first principle calculations [...] Read more.
For searching both high-performances and better fits for near-room temperature thermoelectric materials, we here carried out a theoretical study on thermoelectric properties and doping regulation of Mg3X2 (X = As, Sb, Bi) by the combined method of first principle calculations and semi-classical Boltzmann theory. The thermoelectric properties of n-type Mg3As2, Mg3Sb2, and Mg3Bi2 were studied, and it was found that the dimensionless figures of merit, zT, are 2.58, 1.38, 0.34, and the p-type ones are 1.39, 0.64, 0.32, respectively. Furthermore, we calculated the lattice thermal conductivity of doped structures and screened out the structures with a relatively low formation energy to study the phonon dispersion and thermal conductivity in Mg3X2 (X = As, Sb, Bi). Finally, high thermoelectric zT and ultralow thermal conductivity of these doped structures was discussed. Full article
(This article belongs to the Special Issue Advances in First-Principles Calculations on Metallic Materials)
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Article
Variability Study of Bond Work Index and Grindability Index on Various Critical Metal Ores
Metals 2021, 11(6), 970; https://doi.org/10.3390/met11060970 - 17 Jun 2021
Cited by 7 | Viewed by 705
Abstract
It is a well-known fact that the value of the Bond work index (wi) for a given ore varies along with the grinding size. In this study, a variability bysis is carried out with the Bond standard grindability tests on [...] Read more.
It is a well-known fact that the value of the Bond work index (wi) for a given ore varies along with the grinding size. In this study, a variability bysis is carried out with the Bond standard grindability tests on different critical metal ores (W, Ta), ranging from coarse grinding (rod mills) to fine grinding (ball mills). The relationship between wi and grinding size did not show a clear correlation, while the grindability index (gpr) and the grinding size showed a robust correlation, fitting in all cases to a quadratic curve with a very high regression coefficient. This result suggests that, when performing correlation studies among ore grindability and rock mechanics parameters, it is advised to use the grindability index instead of the Bond work index. Full article
(This article belongs to the Special Issue Grinding and Concentration Technology of Critical Metals)
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Article
MWCNT-Reinforced AA7075 Composites: Effect of Reinforcement Percentage on Mechanical Properties
Metals 2021, 11(6), 969; https://doi.org/10.3390/met11060969 - 17 Jun 2021
Cited by 1 | Viewed by 700
Abstract
Metal–matrix composites (MMC) of aluminium alloy 7075 (AA7075) containing 1 wt.% and 0.5 wt.% multiwall carbon nanotubes (MWCNTs) were developed by powder metallurgy, using a high energy ball milling (HEBM) process for dispersion of the MWCNTs. The powder of the AA7075-MWCNT obtained was [...] Read more.
Metal–matrix composites (MMC) of aluminium alloy 7075 (AA7075) containing 1 wt.% and 0.5 wt.% multiwall carbon nanotubes (MWCNTs) were developed by powder metallurgy, using a high energy ball milling (HEBM) process for dispersion of the MWCNTs. The powder of the AA7075-MWCNT obtained was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructural changes produced during the milling process, such as the modification of the crystallite size, as well as the micro-deformation of the matrix crystal lattice, were determined using the Scherrer formula. After consolidation into a strip shape using the hot powder extrusion (HPE) process at 500 °C, no porosity was detected and a fine homogeneous dispersion of the reinforcement into the matrix was obtained. After performing a 0.2 HV test and tensile tests in the extruded profiles of both composites, a better combination of properties was found in samples of AA7075-0.5 wt.% MWCNT, with the increase in measured ductility being especially remarkable. Full article
(This article belongs to the Section Powder Metallurgy)
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Article
High-Temperature Elastic Properties of Yttrium-Doped Barium Zirconate
Metals 2021, 11(6), 968; https://doi.org/10.3390/met11060968 - 16 Jun 2021
Cited by 2 | Viewed by 742
Abstract
The elastic properties of 0, 10, 15, and 20 mol% yttrium-doped barium zirconate (BZY0, BZY10, BZY15, and BZY20) at the operating temperatures of protonic ceramic fuel cells were evaluated. The proposed measurement method for low sinterability materials could accurately determine the sonic velocities [...] Read more.
The elastic properties of 0, 10, 15, and 20 mol% yttrium-doped barium zirconate (BZY0, BZY10, BZY15, and BZY20) at the operating temperatures of protonic ceramic fuel cells were evaluated. The proposed measurement method for low sinterability materials could accurately determine the sonic velocities of small-pellet-type samples, and the elastic properties were determined based on these velocities. The Young’s modulus of BZY10, BZY15, and BZY20 was 224, 218, and 209 GPa at 20 °C, respectively, and the values decreased as the yttrium concentration increased. At high temperatures (>20 °C), as the temperature increased, the Young’s and shear moduli decreased, whereas the bulk modulus and Poisson’s ratio increased. The Young’s and shear moduli varied nonlinearly with the temperature: The values decreased rapidly from 100 to 300 °C and gradually at temperatures beyond 400 °C. The Young’s modulus of BZY10, BZY15, and BZY20 was 137, 159, and 122 GPa at 500 °C, respectively, 30–40% smaller than the values at 20 °C. The influence of the temperature was larger than that of the change in the yttrium concentration. Full article
(This article belongs to the Special Issue Strength and Fracture of Metal Parts in Batteries)
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Article
Influence of Austempering Temperatures on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron
Metals 2021, 11(6), 967; https://doi.org/10.3390/met11060967 - 16 Jun 2021
Cited by 4 | Viewed by 745
Abstract
The influence of the austempering temperatures on the microstructure and mechanical properties of austempered ductile cast iron (ADI) was investigated. ADI is nodular graphite cast iron, which owing to higher strength and elongation, exceeds mechanical properties of conventional spheroidal graphite cast iron. Such [...] Read more.
The influence of the austempering temperatures on the microstructure and mechanical properties of austempered ductile cast iron (ADI) was investigated. ADI is nodular graphite cast iron, which owing to higher strength and elongation, exceeds mechanical properties of conventional spheroidal graphite cast iron. Such a combination of properties is achieved by the heat treatment through austenitization, followed by austempering at different temperatures. The austenitization conditions were the same for all the samples: temperature 890 °C, duration 30 min, and quenching in a salt bath. The main focus of this research was on the influence of the austempering temperatures (270 °C, 300 °C, and 330 °C) on the microstructure evolution, elongation, toughness, and fatigue resistance of ADI modified by certain amounts of Ni, Cu, and Mo. The Vickers and Rockwell hardness decreased from 535.7 to 405.3 HV/1 (55.7 to 44.5 HRC) as the austempering temperature increased. Optical images showed the formation of graphite nodules and a matrix composed of ausferrite; the presence of these phases was confirmed by an XRD diffraction pattern. A fracture surface analysis revealed several types of the mechanisms: cleavage ductile, transgranular, and ductile dimple fracture. The stress-controlled mechanical fatigue experiments revealed that a 330 °C austempering temperature ensures the highest fatigue life of ADI. Full article
(This article belongs to the Special Issue Fatigue Behavior of Structural Steel)
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