Metals

22 pages, 2637 KiB

Open AccessReview

A Review of Factors Affecting the Mechanical Properties of Maraging Steel 300 Fabricated via Laser Powder Bed Fusion

by Barry Mooney and Kyriakos I. Kourousis

Metals 2020, 10(9), 1273; https://doi.org/10.3390/met10091273 - 22 Sep 2020

Cited by 58 | Viewed by 8164

Maraging steel is an engineering alloy which has been widely employed in metal additive manufacturing. This paper examines manufacturing and post-processing factors affecting the properties of maraging steel fabricated via laser powder bed fusion (L-PBF). It covers the review of published research findings [...] Read more.

Maraging steel is an engineering alloy which has been widely employed in metal additive manufacturing. This paper examines manufacturing and post-processing factors affecting the properties of maraging steel fabricated via laser powder bed fusion (L-PBF). It covers the review of published research findings on how powder quality feedstock, processing parameters, laser scan strategy, build orientation and heat treatment can influence the microstructure, density and porosity, defects and residual stresses developed on L-PBF maraging steel, with a focus on the maraging steel 300 alloy. This review offers an evaluation of the resulting mechanical properties of the as-built and heat-treated maraging steel 300, with a focus on anisotropic characteristics. Possible directions for further research are also identified. Full article

(This article belongs to the Section Additive Manufacturing)

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14 pages, 1055 KiB

Open AccessReview

A Comprehensive Review on the Corrosion Pathways of Titanium Dental Implants and Their Biological Adverse Effects

by Júlio C. M. Souza, Karin Apaza-Bedoya, Cesar A. M. Benfatti, Filipe S. Silva and Bruno Henriques

Metals 2020, 10(9), 1272; https://doi.org/10.3390/met10091272 - 22 Sep 2020

Cited by 41 | Viewed by 8471

Abstract

The main aim of this work was to perform a comprehensive review of findings reported by previous studies on the corrosion of titanium dental implants and consequent clinical detrimental effects to the patients. Most studies were performed by in vitro electrochemical tests and [...] Read more.

The main aim of this work was to perform a comprehensive review of findings reported by previous studies on the corrosion of titanium dental implants and consequent clinical detrimental effects to the patients. Most studies were performed by in vitro electrochemical tests and complemented with microscopic techniques to evaluate the corrosion behavior of the protective passive oxide film layer, namely TiO₂. Results revealed that bacterial accumulation, dietary, inflammation, infection, and therapeutic solutions decrease the pH of the oral environment leading to the corrosion of titanium. Some therapeutic products used as mouthwash negatively affect the corrosion behavior of the titanium oxide film and promote changes on the implant surface. In addition, toothpaste and bleaching agents, can amplify the chemical reactivity of titanium since fluor ions interacting with the titanium oxide film. Furthermore, the number of in vivo studies is limited although corrosion signs have been found in retrieved implants. Histological evaluation revealed titanium macro- and micro-scale particles on the peri-implant tissues. As a consequence, progressive damage of the dental implants and the evolution of inflammatory reactions depend on the size, chemical composition, and concentration of submicron- and nanoparticles in the surrounding tissues and internalized by the cells. In fact, the damage of the implant surfaces results in the loss of material that compromises the implant surfaces, implant-abutment connections, and the interaction with soft tissues. The corrosion can be an initial trigger point for the development of biological or mechanical failures in dental implants. Full article

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17 pages, 7391 KiB

Open AccessArticle

Effect of Heat Treatments on Microstructural Evolution and Tensile Properties of 15Cr12MoVWN Ferritic/Martensitic Steel

by Tingwei Ma, Xianchao Hao and Ping Wang

Metals 2020, 10(9), 1271; https://doi.org/10.3390/met10091271 - 22 Sep 2020

Cited by 6 | Viewed by 2872

Abstract

In this study, the phase transformation temperature of 15Cr12MoVWN ferritic/martensitic steel was determined by differential scanning calorimetry to provide a theoretical basis for the design of a heat treatment process. An orthogonal design experiment was performed to investigate the relationship between microstructure and [...] Read more.

In this study, the phase transformation temperature of 15Cr12MoVWN ferritic/martensitic steel was determined by differential scanning calorimetry to provide a theoretical basis for the design of a heat treatment process. An orthogonal design experiment was performed to investigate the relationship between microstructure and heat treatment parameters, i.e., normalizing temperature, cooling method and tempering temperature by evaluating the room-temperature and elevated-temperature tensile properties, and the optimum heat treatment parameters were determined. It is shown that the optimized heat treatment process was composed of normalizing at 1050 °C followed by air cooling to room temperature and tempering at 700 °C. Under the optimum heat treatment condition, the room-temperature tensile properties were 1014 MPa (UTS), 810.5 MPa (YS) and 18.8% (elongation), while the values are 577.5 MPa (UTS), 469 MPa (YS) and 39.8% (elongation) tested at 550 °C. The microstructural examination shows that the strengthening contributions from microstructural factors were the martensitic lath width, dislocations, M₂₃C₆, MX and grain boundaries of prior austenite grain (PAG) in a descending order. The main factors influencing the tensile strength of 15Cr12MoVWN steel were the martensitic lath width and dislocations. Full article

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15 pages, 6794 KiB

Open AccessArticle

Spray Cooling Heat Transfer above Leidenfrost Temperature

by Martin Chabicovsky, Petr Kotrbacek, Hana Bellerova, Jan Kominek and Miroslav Raudensky

Metals 2020, 10(9), 1270; https://doi.org/10.3390/met10091270 - 21 Sep 2020

Cited by 13 | Viewed by 3541

Abstract

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction [...] Read more.

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results. Full article

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26 pages, 6890 KiB

Open AccessFeature PaperReview

A Review of Recent Advances in Depression Techniques for Flotation Separation of Cu–Mo Sulfides in Porphyry Copper Deposits

by Ilhwan Park, Seunggwan Hong, Sanghee Jeon, Mayumi Ito and Naoki Hiroyoshi

Metals 2020, 10(9), 1269; https://doi.org/10.3390/met10091269 - 21 Sep 2020

Cited by 39 | Viewed by 5787

Abstract

Porphyry copper deposits (PCDs) are some of the most important sources of copper (Cu) and molybdenum (Mo). Typically, the separation and recovery of chalcopyrite (CuFeS₂) and molybdenite (MoS₂), the major Cu and Mo minerals, respectively, in PCDs are achieved [...] Read more.

Porphyry copper deposits (PCDs) are some of the most important sources of copper (Cu) and molybdenum (Mo). Typically, the separation and recovery of chalcopyrite (CuFeS₂) and molybdenite (MoS₂), the major Cu and Mo minerals, respectively, in PCDs are achieved by two-step flotation involving (1) bulk flotation to separate Cu–Mo concentrates and tailings (e.g., pyrite, silicate, and aluminosilicate minerals) and (2) Cu–Mo flotation to separate chalcopyrite and molybdenite. In Cu–Mo flotation, chalcopyrite is depressed using Cu depressants, such as NaHS, Na₂S, Nokes reagent (P₂S₅ + NaOH), and NaCN, meaning that it is recovered as tailings, while molybdenite is floated and recovered as froth product. Although conventionally used depressants are effective in the separation of Cu and Mo, they have the potential to emit toxic and deadly gases such as H₂S and HCN when operating conditions are not properly controlled. To address these problems caused by the use of conventional depressants, many studies aimed to develop alternative methods of depressing either chalcopyrite or molybdenite. In this review, recent advances in chalcopyrite and molybdenite depressions for Cu–Mo flotation separation are reviewed, including alternative organic and inorganic depressants for Cu or Mo, as well as oxidation-treatment technologies, such as ozone (O₃), plasma, hydrogen peroxide (H₂O₂), and electrolysis, which create hydrophilic coatings on the mineral surface. Full article

(This article belongs to the Special Issue Advances in Selective Flotation and Leaching Process in Metallurgy)

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15 pages, 9512 KiB

Open AccessArticle

Refractory High-Entropy HfTaTiNbZr-Based Alloys by Combined Use of Ball Milling and Spark Plasma Sintering: Effect of Milling Intensity

by Natalia Shkodich, Alexey Sedegov, Kirill Kuskov, Sergey Busurin, Yury Scheck, Sergey Vadchenko and Dmitry Moskovskikh

Metals 2020, 10(9), 1268; https://doi.org/10.3390/met10091268 - 20 Sep 2020

Cited by 31 | Viewed by 3796

Abstract

For the first time, a powder of refractory body-centered cubic (bcc) HfTaTiNbZr-based high-entropy alloy (RHEA) was prepared by short-term (90 min) high-energy ball milling (HEBM) followed by spark plasma sintering (SPS) at 1300 °C for 10 min and the resultant bulk material was [...] Read more.

For the first time, a powder of refractory body-centered cubic (bcc) HfTaTiNbZr-based high-entropy alloy (RHEA) was prepared by short-term (90 min) high-energy ball milling (HEBM) followed by spark plasma sintering (SPS) at 1300 °C for 10 min and the resultant bulk material was characterized by XRD and SEM/EDX. The material showed ultra-high Vickers hardness (10.7 GPa) and a density of 9.87 ± 0.18 g/cm³ (98.7%). Our alloy was found to consist of HfZrTiTaNb-based solid solution with bcc structure as a main phase, a hexagonal closest packed (hcp) Hf/Zr-based solid solution, and Me₂Fe phases (Me = Hf, Zr) as minor admixtures. Principal elements of the HEA phase were uniformly distributed over the bulk of HfTaTiNbZr-based alloy. Similar alloys synthesized without milling or in the case of low-energy ball milling (LEBM, 10 h) consisted of a bcc HEA and a Hf/Zr-rich hcp solid solution; in this case, the Vickers hardness of such alloys was found to have a value of 6.4 GPa and 5.8 GPa, respectively. Full article

(This article belongs to the Special Issue High Energy Ball Milling and Consolidation of Nanocomposite Powders)

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13 pages, 7294 KiB

Open AccessArticle

Effect of Periodic Overloads on Short Fatigue Crack Behavior in CuNi₂Si Alloy under Rotating Bending Load

by Yahang Qin, Bing Yang, Bo Feng, Yifan Li, Shoune Xiao, Guangwu Yang and Tao Zhu

Metals 2020, 10(9), 1267; https://doi.org/10.3390/met10091267 - 18 Sep 2020

Cited by 3 | Viewed by 2489

Abstract

In this study, the short fatigue crack behavior in a precipitation-strengthened CuNi₂Si alloy was investigated using a replica technique under rotating bending loads with periodic overloads, an overload ratio of 1.5, and the stress ratio of both was R = −1. [...] Read more.

In this study, the short fatigue crack behavior in a precipitation-strengthened CuNi₂Si alloy was investigated using a replica technique under rotating bending loads with periodic overloads, an overload ratio of 1.5, and the stress ratio of both was R = −1. The results show that all the fatigue cracks originated from the surface of the specimen and displayed a trend of slow initiation and then rapid propagation. The introduction of overloads significantly reduced the fatigue crack initiation time and the fatigue life of the sample. The average life of the overloaded samples was only 31% that of the constant load samples. For overload specimens, multiple cracks grew at the same time and merged at different stages, causing the crack length to increase instantaneously after they merged, thereby considerably reducing the fatigue life. Fractographical analysis and observation of the surface-etched sample replica film showed that cracks in samples with and without overload both propagated along the grain boundaries. Full article

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12 pages, 2770 KiB

Open AccessArticle

Prediction of Tensile Strength and Deformation of Diffusion Bonding Joint for Inconel 718 Using Deep Neural Network

by Han Mei, Lihui Lang, Xiaoxing Li, Hasnain Ali Mirza and Xiaoguang Yang

Metals 2020, 10(9), 1266; https://doi.org/10.3390/met10091266 - 18 Sep 2020

Cited by 5 | Viewed by 2544

Abstract

Due to the acceptable high-temperature deformation resistance of Inconel 718, its welding parameters such as bonding temperature and pressure are inevitably higher than those of general metals. As a result of the existing punitive processing environment, it is essential to control the deformation [...] Read more.

Due to the acceptable high-temperature deformation resistance of Inconel 718, its welding parameters such as bonding temperature and pressure are inevitably higher than those of general metals. As a result of the existing punitive processing environment, it is essential to control the deformation of parts while ensuring the bonding performance. In this research, diffusion bonding experiments based on the Taguchi method (TM) are conducted, and the uniaxial tensile strength and deformation ratio of the experimental joints are measured. According to experimental data, a deep neural network (DNN) was trained to characterize the nonlinear relationship between the diffusion bonding process parameters and the diffusion bonding strength and deformation ratio, where the overall correlation coefficient came out to be 0.99913. The double-factors analysis of bonding temperature–bonding pressure based on the prediction results of the DNN shows that the temperature increment of the diffusion bonding of Inconel 718 significantly increases the deformation ratio of the diffusion bonding joints. Therefore, during the multi-objective optimization of the bonding performance and deformation of components, priority should be given to optimizing the bonding pressure and duration only. Full article

(This article belongs to the Section Metal Casting, Forming and Heat Treatment)

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14 pages, 7752 KiB

Open AccessArticle

Effect of Graphene Nanoplatelets Content on the Mechanical and Wear Properties of AZ31 Alloy

by Tianhui Lu, Mingyang Zhou, Lingbao Ren, Lingling Fan, Yangyang Guo, Xiaoni Qu, Hongtao Zhang, Xianwen Lu and Gaofeng Quan

Metals 2020, 10(9), 1265; https://doi.org/10.3390/met10091265 - 18 Sep 2020

Cited by 7 | Viewed by 2613

Abstract

Graphene, as a rising-star materials, has attracted interest in fabricating lightweight self-lubricating metal matrix composites with superior mechanical and wear properties. In this work, graphene nanoplatelets (GNPs) reinforced AZ31 alloy composites were fabricated by a powder metallurgy technique and then a hot extrusion. [...] Read more.

Graphene, as a rising-star materials, has attracted interest in fabricating lightweight self-lubricating metal matrix composites with superior mechanical and wear properties. In this work, graphene nanoplatelets (GNPs) reinforced AZ31 alloy composites were fabricated by a powder metallurgy technique and then a hot extrusion. The effects of GNPs content (0.5, 1.0, and 2.0 wt.%) on the microstructures, mechanical properties, and wear performance of the extruded GNPs/AZ31 composites were studied. It was found that the addition of GNPs resulted in a weakened basal plane texture and grain refinement of the AZ31 matrix metal. Less than 1.0 wt.% GNPs in GNPs/AZ31 composites resulted in the enhancement in both Vickers hardness and tensile yield strength with acceptable elongation. The Vickers hardness and tensile yield strength of 1.0GNPs/AZ31 composite increased by 4.9% and 9.5% respectively, compared with the unreinforced AZ31. Moreover, the elongation of the composites was about the same as the AZ31 base alloy. Both the friction coefficient and the wear mass loss continuously decreased with the increasing GNPs content, which exhibited a self-lubricating effect. The relationship of the friction coefficient and wear mass loss with the GNPs content could be modeled in terms of the Holliday model and the exponential decay model, respectively. The worn surface morphology revealed that adhesive wear and abrasive wear simultaneously acted in AZ31 alloy. Nevertheless, abrasive wear became the dominant wear mechanism in the GNPs/AZ31 composites. Full article

(This article belongs to the Section Powder Metallurgy)

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22 pages, 8827 KiB

Open AccessArticle

Numerical Simulation and Multi-Objective Optimization of Partition Cooling in Hot Stamping of the Automotive B-Pillar Based on RSM and NSGA-II

by Maomao Cui, Zhao Wang, Leigang Wang and Yao Huang

Metals 2020, 10(9), 1264; https://doi.org/10.3390/met10091264 - 18 Sep 2020

Cited by 9 | Viewed by 3354

Abstract

In this study, the simulation and optimization of the partition cooling in the hot stamping process was carried out for an automotive B-pillar through minimizing the maximum thickening rate and the maximum thinning rate located in the rapid and slow cooling zones. The [...] Read more.

In this study, the simulation and optimization of the partition cooling in the hot stamping process was carried out for an automotive B-pillar through minimizing the maximum thickening rate and the maximum thinning rate located in the rapid and slow cooling zones. The optimization was implemented by investigating the process parameters such as friction coefficient, sheet austenitizing temperature, holding time, heating zone temperature, the upper binder force and the lower binder force. The optimal Latin hypercube design (OLHD), the response surface methodology (RSM) and the non-dominated sorting genetic algorithm (NSGA-II) were combined to establish the relationship between process parameters and form quality objectives. After multi-objective optimization, the maximum thickening rate and the maximum thinning rate of the slow cooling zone and rapid cooling zone were 11.1% and 12.4%, 4.7% and 7.1%, respectively. Afterwards, the simulation was performed according to the optimized parameter combinations to analyze the temperature field, microstructure, tensile strength, hardness, thickening rate and thinning rate, and forming quality. Moreover, the hot stamping test and experimental results showed that the microstructure of the ferrite and pearlite structure was uniformly distributed in the slow cooling zone, and its tensile strength reached 680 MPa, the elongation was 11.4% and the hardness was 230.56 HV, while the lath martensite structure was obtained in the rapid cooling zone, with tensile strength of up to 1390 MPa, elongation of about 7.0% and hardness reaching 478.78 HV. The results of thickness, microstructure, tensile strength and the hardness test correspond well with the simulation results. Full article

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14 pages, 7201 KiB

Open AccessArticle

Alumina Reinforcement of Inconel 625 Coatings by Cold Gas Spraying

by Sergi Dosta, Nuria Cinca, Alessio Silvello and Irene G. Cano

Metals 2020, 10(9), 1263; https://doi.org/10.3390/met10091263 - 18 Sep 2020

Cited by 6 | Viewed by 3497

Abstract

Reinforced Inconel625 coatings have been successfully deposited by means of cold gas spray (CGS). Alumina has been simultaneously sprayed achieving a homogeneous distribution along the deposit with good cohesion between particles. The aim of this study was to investigate whether ceramic reinforcement could [...] Read more.

Reinforced Inconel625 coatings have been successfully deposited by means of cold gas spray (CGS). Alumina has been simultaneously sprayed achieving a homogeneous distribution along the deposit with good cohesion between particles. The aim of this study was to investigate whether ceramic reinforcement could improve the mechanical and tribological properties of Inconel625 cold-sprayed coatings, while keeping the oxidation and corrosion resistance capacity. Furthermore, alumina particles were found to improve the spraying feasibility, by avoiding WC nozzle clogging during the process. A proper optimization of the spraying conditions was carried out in order to obtain the lowest possible porosity and best embedment of the alumina. Then, the mechanical and tribological properties as well as corrosion and oxidation behavior were characterized. Alumina acts as a reinforcement, improving the effects of abrasive and sliding wear. Regarding the oxidation and corrosion behavior, the coatings exhibit reasonably good oxidation resistance at temperatures up to 900 °C. The electrochemical corrosion performance in NaCl solution showed potentially lower noble corrosion values and corrosion current densities than bulk Inconel. Full article

(This article belongs to the Special Issue Wear, Corrosion and Fatigue Behavior of Ni-Based Superalloys Coatings Deposited via Cold Spray)

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18 pages, 12082 KiB

Open AccessArticle

Effect of Severe Shot Peening on the Very-High Cycle Notch Fatigue of an AW 7075 Alloy

by Michal Jambor, Libor Trško, Jan Klusák, Stanislava Fintová, Daniel Kajánek, František Nový and Otakar Bokůvka

Metals 2020, 10(9), 1262; https://doi.org/10.3390/met10091262 - 18 Sep 2020

Cited by 9 | Viewed by 3059

Abstract

The severe shot peening process was applied to the notched specimens from an AW 7075 alloy with the aim to improve fatigue endurance in the very-high cycle fatigue region. To reveal the stress state in the notch vicinity, finite element analysis was performed, [...] Read more.

The severe shot peening process was applied to the notched specimens from an AW 7075 alloy with the aim to improve fatigue endurance in the very-high cycle fatigue region. To reveal the stress state in the notch vicinity, finite element analysis was performed, simulating the conditions of the used 20 kHz ultrasonic fatigue loading. Modified surface characteristics by the severe shot peening process were analyzed in terms of residual stress distribution measured by X-ray diffraction methods and near-surface microstructural observations by scanning electron microscopy. The applied severe shot peening increased the fatigue limit by 11%; however, the positive effect was recorded only for the loading amplitudes corresponding to the fatigue lifetimes in the range 10⁷–10⁹ cycles. At higher loading amplitudes, the fatigue properties tended to decrease, most likely due to accelerated fatigue crack initiation on the surface damage features created by the peening process and also by rapid residual stress relaxation. Full article

(This article belongs to the Section Metal Failure Analysis)

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13 pages, 25420 KiB

Open AccessArticle

Replication Experiments and Microstructural Evolution of the Ancient Co-Fusion Steelmaking Process

by Shangxiao Qiao and Wei Qian

Metals 2020, 10(9), 1261; https://doi.org/10.3390/met10091261 - 18 Sep 2020

Cited by 1 | Viewed by 2974

Abstract

The study of co-fusion was one of the essential topics in the history of metallurgy in China. Simulation experiments were an essential concept in the study of the co-fusion steelmaking process. This paper mainly studied the simulation experiments of co-fusion from two aspects: [...] Read more.

The study of co-fusion was one of the essential topics in the history of metallurgy in China. Simulation experiments were an essential concept in the study of the co-fusion steelmaking process. This paper mainly studied the simulation experiments of co-fusion from two aspects: the replication of co-fusion swords by three different methods and the micro-analysis of the co-fusion samples. The experimental results indicated that several co-fusion swords could be made by different processes, but the carbon content and surface hardness were quite different. During repeated forge welding, the microstructure of the samples transformed from laminated to homogenized; finally, a steel with a uniform carbon content was obtained. It was challenging to determine the characteristics of co-fusion from the homogenized samples. The results prompt a rethinking of the microstructural characteristics of ancient co-fusion artifacts. Full article

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11 pages, 2422 KiB

Open AccessArticle

Fatigue Study of the Pre-Corroded 6082-T6 Aluminum Alloy in Saline Atmosphere

by Alejandro Fernández Muñoz, José Luis Mier Buenhombre, Ana Isabel García-Diez, Carolina Camba Fabal and Juan José Galán Díaz

Metals 2020, 10(9), 1260; https://doi.org/10.3390/met10091260 - 18 Sep 2020

Cited by 8 | Viewed by 3190

Abstract

This work studies the influence of the saline atmospheric corrosion on the fatigue strength of 6061-T6 aluminum alloy. For this purpose, this alloy was subjected to tests in a salt spray corrosion chamber at different exposure times (1, 2, and 3 months) according [...] Read more.

This work studies the influence of the saline atmospheric corrosion on the fatigue strength of 6061-T6 aluminum alloy. For this purpose, this alloy was subjected to tests in a salt spray corrosion chamber at different exposure times (1, 2, and 3 months) according to ASTM B117 standard. The morphological study of the pits was carried out by confocal microscopy. Subsequently, fatigue tests were performed at variable stresses whose maximum stress (S_max) was between 30% and 95% of the yield strength (S₀) in order to keep them within the zone of elastic behavior of the material. Data were analyzed using the Basquin equation and the maximum likelihood function method. The results show a similar decrease in the conventional fatigue limit (2 × 10⁶ cycles) after one month (98 MPa) and two months (91 MPa) of corrosion. After three months of corrosion, the material showed a very important reduction in the fatigue limit (68 MPa) with respect to the uncorroded material (131 MPa). The data of S_e/S₀ (fatigue limit/yield strength) versus the ratio P_m/D_m (pit average depth/pit diameter at zero depth) can be fitted to a logarithmic curve. Full article

(This article belongs to the Special Issue Metal Surface Degradation: Microstructural Analysis and Mechanical, Corrosive and Tribological Behavior)

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14 pages, 3267 KiB

Open AccessArticle

Effect of Sample Preparation Pressure on Transformation Law of Low-Valent Titanium Oxide in a Multi-Stage Reduction Process

by Shigang Fan, Zhihe Dou, Ting’an Zhang, Ji-sen Yan and Li-ping Niu

Metals 2020, 10(9), 1259; https://doi.org/10.3390/met10091259 - 18 Sep 2020

Cited by 2 | Viewed by 2243

Abstract

A novel method for preparing titanium powder by multi-stage reduction of TiO₂ was proposed. Its core is the preparation of high-quality low-valent titanium oxide. In this paper, the effect mechanism of different sample preparation pressures on the preparation of low-valent titanium oxide [...] Read more.

A novel method for preparing titanium powder by multi-stage reduction of TiO₂ was proposed. Its core is the preparation of high-quality low-valent titanium oxide. In this paper, the effect mechanism of different sample preparation pressures on the preparation of low-valent titanium oxide by the primary reduction (self-propagating high-temperature synthesis mode, SHS) of the Mg-TiO₂ system was studied. The results show that the generation of Mg thermal fluid is the key link of the self-sustaining chemical reaction of the Mg-TiO₂ system. Titanium exists inα-Ti and TiO at the end of combustion, and constitutes a non-stoichiometric low-valent titanium oxide. The sample preparation pressure determines the proportion of pores reserved for Mg diffusion in the compacts and the contact area of the reactants, thereby determining the partitioning behavior and heat transfer effect of Mg thermal fluid during the combustion process. When the sample preparation pressure is 75 MPa (relative density is 0.66 ± 0.01), the combustion effect is optimal, and the low-valent titanium oxide with oxygen content of 15.1% can be obtained. It was subjected to deep reduction to obtain a titanium powder product with an oxygen content of 0.27%. Full article

(This article belongs to the Special Issue Advances in Powder Metallurgy)

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12 pages, 5798 KiB

Open AccessArticle

The Pitting Corrosion Behavior of the Austenitic Stainless Steel 308L-316L Welded Joint

by Jinshan He, Shiguang Xu, Wenxin Ti, Yaolei Han, Jinna Mei and Xitao Wang

Metals 2020, 10(9), 1258; https://doi.org/10.3390/met10091258 - 18 Sep 2020

Cited by 6 | Viewed by 3301

Abstract

The pitting corrosion resistance of the austenitic stainless steel 308L-316L welded joint was investigated by electrochemical tests. It is found that the weld zone was the most critical for pits to initiate in the welded joint due to relatively instable passive film with [...] Read more.

The pitting corrosion resistance of the austenitic stainless steel 308L-316L welded joint was investigated by electrochemical tests. It is found that the weld zone was the most critical for pits to initiate in the welded joint due to relatively instable passive film with few Mo and inhomogeneous passive film induced by multiple (Mn, Al, and Si) oxides and continuous network of 13.94 vol.% δ ferrites. By statistical analysis, 53.8% pits initiated at (Mn, Al, and Si) oxides, 23.0% in austenite, and 23.2% at interface between ferrite and austenite. In addition, heat-affected zone was prone to have pitting corrosion compared with the base metal since residual strain was much higher in the region. Full article

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18 pages, 8170 KiB

Open AccessArticle

Effects of Laser Melting Distribution on Wear Resistance and Fatigue Resistance of Gray Cast Iron

by Haiyang Yang, Ti Zhou, Qingnian Wang and Hong Zhou

Metals 2020, 10(9), 1257; https://doi.org/10.3390/met10091257 - 17 Sep 2020

Cited by 9 | Viewed by 2557

Abstract

The coupling bionic surface is generally prepared by laser melting on the surface of a gray iron brake hub, which can allow the brake hub to achieve excellent wear resistance and fatigue resistance. The designs of most previous experiments have been based on [...] Read more.

The coupling bionic surface is generally prepared by laser melting on the surface of a gray iron brake hub, which can allow the brake hub to achieve excellent wear resistance and fatigue resistance. The designs of most previous experiments have been based on independent units that were uniform in their distribution patterns. Although some progress has been made in the optimization of cell features, there is still room for further improvement with respect to bionics and experimental optimization methods. Here, experiments on units with non-uniform distributions of different distances were used to rearrange and combine the bionic elements. This paper is that the original uniform distribution laser melting strengthening model was designed as a non-uniform distribution model, and the heat preservation and tempering strengthening effect of continuous multiple melting strengthening on the microstructure of the melting zone is discussed. The mechanism of crack initiation and the mode of crack propagation were analyzed. The relationship between the internal stress in the melting zone and the crack initiation resistance was also discussed. In this paper, the mechanism of different spacing distribution on the surface of gray cast iron by laser remelting is put forward innovatively and verified by experiments, which provides a solid theoretical basis for the follow-up industrial application. Full article

(This article belongs to the Special Issue Advances in Laser Materials Processing)

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11 pages, 4313 KiB

Open AccessArticle

The Evolution of Microstructure, Mechanical Properties and Fracture Behavior with Increasing Lanthanum Content in AZ91 Alloy

by Di Tie, Yi Jiang, Renguo Guan, Minfang Chen, Jufu Jiang, Fei Gao, Xiaopeng Lu and Zhanyong Zhao

Metals 2020, 10(9), 1256; https://doi.org/10.3390/met10091256 - 17 Sep 2020

Cited by 8 | Viewed by 2713

Abstract

AZ91 alloy is a widely applied commercial magnesium alloy due to its good castability, balanced mechanical properties and acceptable price, and lanthanum alloying has been proven to be one of the most effective methods to further improve its mechanical properties. Therefore, we reveal [...] Read more.

AZ91 alloy is a widely applied commercial magnesium alloy due to its good castability, balanced mechanical properties and acceptable price, and lanthanum alloying has been proven to be one of the most effective methods to further improve its mechanical properties. Therefore, we reveal the evolution of microstructure, mechanical properties and fracture behavior with increasing lanthanum content in AZ91 alloy in this study. The magnesium matrix was significantly refined by lanthanum content, and this effect became more evident with increasing addition of lanthanum. The presence of Al₃La precipitates significantly reduced the grain mobility and suppressed the formation of Mg₁₇Al₁₂ discontinuous precipitates along the grain boundaries. The rheo-cast alloys exhibited improved and balanced tensile strength and ductility after aging treatment. The fracture type of AZ91-La alloys could be classified as ductile fracture due to the presence of less quasi-cleavage planes and more dimples with a mixture of tear ridges and micropores. Due to the fully refined microstructure and the balanced mechanical properties, the AZ91–1.0La (mass%) alloy presented the greatest potential for industrial applications among the three studied AZ91-La alloys. Full article

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25 pages, 94968 KiB

Open AccessArticle

Hot Deformation Behaviour of Mn–Cr–Mo Low-Alloy Steel in Various Phase Regions

by Ivo Schindler, Petr Opěla, Petr Kawulok, Jaroslav Sojka, Kateřina Konečná, Stanislav Rusz, Rostislav Kawulok, Michal Sauer and Petra Turoňová

Metals 2020, 10(9), 1255; https://doi.org/10.3390/met10091255 - 17 Sep 2020

Cited by 4 | Viewed by 2506

Abstract

The deformation behaviour of a coarse-grained as-cast medium-carbon steel, alloyed with 1.2% Mn, 0.8% Cr and 0.2% Mo, was studied by uniaxial compression tests for the strain rates of 0.02 s⁻¹–20 s⁻¹ in the unusually wide range of temperatures (650–1280 [...] Read more.

The deformation behaviour of a coarse-grained as-cast medium-carbon steel, alloyed with 1.2% Mn, 0.8% Cr and 0.2% Mo, was studied by uniaxial compression tests for the strain rates of 0.02 s⁻¹–20 s⁻¹ in the unusually wide range of temperatures (650–1280 °C), i.e., in various phase regions including the region with predominant bainite content (up to the temperature of 757 °C). At temperatures above 820 °C, the structure was fully austenitic. The hot deformation activation energies of 648 kJ·mol⁻¹ and 364 kJ·mol⁻¹ have been calculated for the temperatures ≤770 °C and ≥770 °C, respectively. This corresponds to the significant increase of flow stress in the low-temperature bainitic region. Unique information on the hot deformation behaviour of bainite was obtained. The shape of the stress-strain curves was influenced by the dynamic recrystallization of ferrite or austenite. Dynamically recrystallized austenitic grains were strongly coarsened with decreasing strain rate and growing temperature. For the austenitic region, the relationship between the peak strain and the Zener–Hollomon parameter has been derived, and the phenomenological constitutive model describing the flow stress depending on temperature, true strain rate and true strain was developed. The model can be used to predict the forming forces in the seamless tubes production of the given steel. Full article

(This article belongs to the Section Metal Casting, Forming and Heat Treatment)

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14 pages, 3172 KiB

Open AccessArticle

Relationship and Mechanism Analysis of Soft-Melt Dropping Properties and Primary-Slag Formation Behaviors of the Mixed Burden in Increasing Lump Ore Ratio

by Yun-Fei Li, Zhi-Jun He, Wen-Long Zhan, Wei-Guo Kong, Peng Han, Jun-Hong Zhang and Qing-Hai Pang

Metals 2020, 10(9), 1254; https://doi.org/10.3390/met10091254 - 17 Sep 2020

Cited by 10 | Viewed by 3047

Abstract

At present, cost reduction and environmental protection are the mainstream of blast furnace (BF) development and the high lump ore ratio is an effective means. Therefore, it is significant to explore the relationship and mechanism of burden soft-melt dropping and its primary-slag formation [...] Read more.

At present, cost reduction and environmental protection are the mainstream of blast furnace (BF) development and the high lump ore ratio is an effective means. Therefore, it is significant to explore the relationship and mechanism of burden soft-melt dropping and its primary-slag formation behaviors under increasing lump ore ratio. In this paper, the melt–drop test is carried out on the single ore and mixed burden, and obtained primary-slag properties are subjected to analysis. The experimental results show that the primary-slag of lump ore contains a large amount of FeO and SiO₂, so it simply produces many low melting point compounds, which cause terrible soft-melt dropping properties and primary-slag formation behaviors. Notably, mixing with sinter and pellet can effectively improve both the properties. With the increase in lump ore ratio, the CaO in the primary-slag decreases, FeO and SiO₂ increase, resulting in the melting temperature of the primary-slag sequentially decreasing and the cohesive zone moves to the low temperature zone. In addition, the maximum pressure difference increases, and the gas permeability deteriorates. Increasing the sinter ratio can overcome the defect of high lump ore ratio that can effectively improve the poor softening performance, melting performance and the position and thickness of the cohesive zone. However, because of the pulverization performance, the maximum pressure difference and gas permeability of the burden become worse. Full article

(This article belongs to the Special Issue Sustainable Steel Industry: Energy and Resource Efficiency, Low-Emissions and Carbon-Lean Production)

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3 pages, 149 KiB

Open AccessEditorial

Complex Concentrated Alloys (CCAs)—Current Understanding and Future Opportunities

by Sundeep Mukherjee

Metals 2020, 10(9), 1253; https://doi.org/10.3390/met10091253 - 17 Sep 2020

Cited by 6 | Viewed by 2889

Abstract

Complex concentrated alloys with multiple principal elements represent a new paradigm in alloy design by focusing on the central region of a multi-component phase space and show a promising range of properties unachievable in conventional alloys [...] Full article

(This article belongs to the Special Issue Complex Concentrated Alloys (CCAs) - Current Understanding and Future Opportunities)

10 pages, 1895 KiB

Open AccessArticle

Preliminary Discussion on the Highly Radiogenic Lead in Unalloyed Copper Artifacts of the Eastern Zhou Dynasty: Starting from the Huili Copper Spearheads

by Xiaoting Wang, Wugan Luo, Yingdong Yang, Dian Chen, Jing Du and Xiang Tang

Metals 2020, 10(9), 1252; https://doi.org/10.3390/met10091252 - 17 Sep 2020

Cited by 5 | Viewed by 2706

Abstract

The debate about the highly radiogenic lead in Chinese archaeology has never ceased. However, previous studies have mainly focused on high leaded bronzes and lead materials, and with little specific discussion on the unalloyed copper artifacts and the sources of copper materials in [...] Read more.

The debate about the highly radiogenic lead in Chinese archaeology has never ceased. However, previous studies have mainly focused on high leaded bronzes and lead materials, and with little specific discussion on the unalloyed copper artifacts and the sources of copper materials in China. In this work, a trace of highly radiogenic lead was found in ten copper spearheads unearthed from Huili County, Sichuan Province, southwest China, which inspired our research on this issue. The pXRF results showed that their lead content is extremely low, so the lead isotope ratios can indicate the source of copper, and the data correspond to the local copper deposits. Combined with other relevant highly radiogenic lead isotope data of unalloyed copper artifacts, the results indicate that there were multiple sources of copper ores used in the Shang Dynasty, and copper mines were continuously used in Southwest China until the Eastern Zhou Dynasty. Full article

(This article belongs to the Special Issue 10th Anniversary of Metals: Metallurgy and Metal Technology)

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12 pages, 5760 KiB

Open AccessArticle

Substrate Impact on the Structure and Electrocatalyst Properties of Molybdenum Disulfide for HER from Water

by Arūnas Jagminas, Arnas Naujokaitis, Paulius Gaigalas, Simonas Ramanavičius, Marija Kurtinaitienė and Romualdas Trusovas

Metals 2020, 10(9), 1251; https://doi.org/10.3390/met10091251 - 17 Sep 2020

Cited by 10 | Viewed by 2848

Abstract

It is expected that utilization of molybdenum disulfide (MoS₂)-based nanostructured electrocatalysts might replace the Pt-group electrodes most effectively applied for hydrogen evolution reaction from water. Therefore, in the past two decades, various approaches have been reported for fabrication of nanostructured MoS [...] Read more.

It is expected that utilization of molybdenum disulfide (MoS₂)-based nanostructured electrocatalysts might replace the Pt-group electrodes most effectively applied for hydrogen evolution reaction from water. Therefore, in the past two decades, various approaches have been reported for fabrication of nanostructured MoS₂-based catalysts, but their applications in practice are still missing due to lower activity and stability. We envisaged that the knowledge about the peculiarities of MoS₂ nanoplatelets attachment to various conductive substrates by hydrothermal processing could be helpful for fabrication of more active and stable working electrodes. Therefore, in this study, the hydrothermal syntheses at the Mo, Ti, Al, anodized Ti, and hydrothermally designed titanium suboxide substrates were performed; the electrodes obtained were characterized; and hydrogen evolution reaction (HER) activity was tested. In this way, MoS₂-based HER catalyst possessing a surprising stability and a low Tafel slope was designed via attachment of nanoplatelet-shaped MoS₂ species to the nanotube-shaped anatase-TiO₂ surface. Full article

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19 pages, 6467 KiB

Open AccessArticle

Development and Application of High-Temperature Constitutive Model of HNi55-7-4-2 Alloy

by Qiang Liang, Xin Liu, Ping Li, Ping Ding and Xianming Zhang

Metals 2020, 10(9), 1250; https://doi.org/10.3390/met10091250 - 17 Sep 2020

Cited by 14 | Viewed by 2671

Abstract

The constitutive model is still not available for theoretical and engineering analysis of HNi55-7-4-2 alloy, which is a new type of wear-resistant brass alloy widely applied to car synchronizer rings and ship condenser tubes etc. In the current investigation, a friction-corrected stress-strain curve [...] Read more.

The constitutive model is still not available for theoretical and engineering analysis of HNi55-7-4-2 alloy, which is a new type of wear-resistant brass alloy widely applied to car synchronizer rings and ship condenser tubes etc. In the current investigation, a friction-corrected stress-strain curve was obtained through a hot-compression test to develop the high-temperature constitutive model of HNi55-7-4-2 alloy based on the Hansel–Spittel model. By comparing predicted flow stress and a simulated force-stroke curve with experimental results, the proposed constitutive model was verified. The developed constitutive model was applied to numerically simulate the hot precision forging of a synchronizer ring. The simulation results based on two process plans on material flow and forging defects were validated by process experiment. The Hansel–Spittel high-temperature constitutive model proposed in this work enables the theoretical and engineering analysis of HNi55-7-4-2 alloy. Full article

(This article belongs to the Section Metal Casting, Forming and Heat Treatment)

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16 pages, 5766 KiB

Open AccessArticle

Prediction of Longitudinal Curvature Radius of 3D Surface Based on Quadratic Relationship between Strain and Coordinates in Continuous Roll Forming

by Jiaxin Gao, Dongye He, Lirong Sun, Xi Zhang and Zhongyi Cai

Metals 2020, 10(9), 1249; https://doi.org/10.3390/met10091249 - 17 Sep 2020

Viewed by 2626

Abstract

Continuous roll forming (CRF) is a new method for the rapid forming of three-dimensional (3D) surfaces developed in recent years, and the significant advantage of CRF compared with traditional die forming is that the longitudinal dimension of the sheet metal is not limited. [...] Read more.

Continuous roll forming (CRF) is a new method for the rapid forming of three-dimensional (3D) surfaces developed in recent years, and the significant advantage of CRF compared with traditional die forming is that the longitudinal dimension of the sheet metal is not limited. By controlling the curvature radius and gap shape of upper and lower bending rolls, three-dimensional parts with different shapes and sizes can be precisely formed. When the elastic deformation is ignored during the forming process, the transversal curvature radius of the three-dimensional surface is consistent with the radius of the roll gap centerline. Therefore, the calculation of longitudinal curvature radius is the key to improve the accuracy of the 3D surface in CRF. In this paper, the basic principle of CRF is described. The modified formulas for calculating the longitudinal curvature radius of convex and saddle surfaces based on the quadratic relationship between the strain and coordinates are deduced in detail, and the corresponding design method of the roll gap is derived. Furthermore, the mathematical equations of convex and saddle surfaces are given. Through numerical simulation and theoretical analysis, it is found that the relative errors of the longitudinal centerline radius are reduced from 13.67% before modification to 4.35% after modification for a convex surface and 6.81% to 0.41% for a saddle surface when the transversal curvature radius is 800 mm and the compression ratio is 5%. The experimental and measured results indicate that the shapes of formed parts are more consistent with the target parts after modification, which further proves the applicability of the modified formulas. Full article

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15 pages, 5647 KiB

Open AccessCommunication

Gauge-Strain-Controlled Air and PWR Fatigue Life Data for 304 Stainless Steel—Some Effects of Surface Finish and Hold Time

by Marc Vankeerberghen, Michel De Smet and Christian Malekian

Metals 2020, 10(9), 1248; https://doi.org/10.3390/met10091248 - 16 Sep 2020

Cited by 2 | Viewed by 2957

Abstract

We performed environmental fatigue testing in simulated primary water reactor (PWR) primary water and reference fatigue testing in air in the framework of an international, collaborative project (INCEFA-PLUS), where the effects of mean strain and stress, hold time, strain amplitude and surface finish [...] Read more.

We performed environmental fatigue testing in simulated primary water reactor (PWR) primary water and reference fatigue testing in air in the framework of an international, collaborative project (INCEFA-PLUS), where the effects of mean strain and stress, hold time, strain amplitude and surface finish on fatigue life of austenitic stainless steels in light water reactor environments are being studied. Our fatigue lives obtained on machined specimens in air at 300 °C lie close to the NUREG/CR6909 mean air fatigue curve and are in line with INCEFA-PLUS air fatigue lives. Our environmental fatigue lives obtained in simulated PWR primary water at 300 °C lie relatively close to the NUREG/CR6909 mean fatigue curve; derived from the NUREG/CR6909 mean air fatigue curve and the applicable environmental correction factor (Fen). The PWR results show that (1) a polished surface finish has a slightly higher and a ground surface finish a slightly lower fatigue life than the NUREG/CR6909 prediction; (2) the ratio of polished to ground specimen life is ~1.37 at 300 °C and ~1.47 at 230 °C; (3) holds—at zero strain after a positive strain-rate—have a slightly detrimental effect on fatigue life. These results are in line with the INCEFA-PLUS PWR fatigue lives. A novel gauge-strain extensometer was deployed in order to perform a true gauge-strain-controlled fatigue test in simulated PWR primary water. Full article

(This article belongs to the Special Issue Environmental Fatigue Assessment of Metallic Materials and Components)

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16 pages, 10115 KiB

Open AccessFeature PaperArticle

Magnetic and Magnetocaloric Effect of Laves Phase Compounds Er(Fe_0.8−xMn_0.2−yCo_x_+y)₂ with x, y = 0.0 or 0.1

by Safa Othmani, Ichrak Chaaba, Sonia Haj-Khlifa, Patricia de Rango and Daniel Fruchart

Metals 2020, 10(9), 1247; https://doi.org/10.3390/met10091247 - 16 Sep 2020

Cited by 4 | Viewed by 2744

Abstract

Magnetic and magnetocaloric effect (MCE) of the Er(Fe_0.8−xMn_0.2−yCo_x_+y)₂ Laves phase-type compounds have been investigated. X-ray diffraction (XRD) analysis has revealed that these compounds crystallize with the C15 type Laves phase structure (Space [...] Read more.

Magnetic and magnetocaloric effect (MCE) of the Er(Fe_0.8−xMn_0.2−yCo_x_+y)₂ Laves phase-type compounds have been investigated. X-ray diffraction (XRD) analysis has revealed that these compounds crystallize with the C15 type Laves phase structure (Space Group Fd-3m). The magnetization curves indicate a ferri-magnetic-ordering resulting of the antiparallel coupling between the moments of the heavy rare earth Er and the transition metal (TM). The partial substitution of Fe/Mn by Co increases the Curie temperature from 355 K for Er(Fe_0.8Mn_0.2)₂ to 475, 550, and 555 K for Er(Fe_0.7Mn_0.2Co_0.1)₂, Er(Fe_0.8Mn_0.1Co_0.1)₂, and Er(Fe_0.7Mn_0.1Co_0.2)₂, respectively. According to the nature of the TM elements, arguments were presented forwards either Molecular Field or Spin Fluctuation Theory, even Stoner type pictures should be considered for. MCE was calculated according to the Maxwell relation based on isotherm magnetization measurements. The magnetic entropy change (−∆S_M) observed on a 300–400 K temperature range can be understood in terms of a Spin Fluctuation Theory picture supported by both the different magnetic polarization levels that were shared by the TM elements and the related interatomic exchange forces. Full article

(This article belongs to the Special Issue Advanced Magnetic Materials)

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14 pages, 5985 KiB

Open AccessArticle

Effect of Grain Size on the Friction-Induced Martensitic Transformation and Tribological Properties of 304 Austenite Stainless Steel

by Bo Mao, Shuangjie Chu and Shuyang Wang

Metals 2020, 10(9), 1246; https://doi.org/10.3390/met10091246 - 16 Sep 2020

Cited by 20 | Viewed by 4093

Abstract

Friction and wear performance of austenite stainless steels have been extensively studied and show a close relationship with the friction-induced martensitic transformation. However, how the grain size and associated friction-induced martensitic transformation behavior affect the tribological properties of austenite steels have not been [...] Read more.

Friction and wear performance of austenite stainless steels have been extensively studied and show a close relationship with the friction-induced martensitic transformation. However, how the grain size and associated friction-induced martensitic transformation behavior affect the tribological properties of austenite steels have not been systematically studied. In this work, dry sliding tests were performed on an AISI 304 stainless steel with a grain size ranging from 25 to 92 μm. The friction-induced surface morphology and microstructure evolution were characterized. Friction-induced martensitic transformation behavior, including martensite nucleation, martensite growth and martensite variant selection and its effect on the friction and wear behavior of the 304 stainless steel were analyzed. The results showed that both the surface coefficient of friction (COF) and the wear rate increase with the grain size. The COF was reduced three times and wear rate was reduced by 30% as the grain size decreased from 92 to 25 μm. A possible mechanism is proposed to account for the effect of grain size on the tribological behavior. It is discussed that austenite steel with refined grain size tends to suppress the amount of friction-induced martensitic transformed and significantly alleviates both the plowing and adhesive effect during dry sliding. Full article

(This article belongs to the Special Issue Friction and Wear of Metals)

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19 pages, 16757 KiB

Open AccessArticle

Scale Formation on HSLA Steel during Continuous Casting Part II: The Effect of Surface Conditions

by Rosa Maria Pineda Huitron, Pavel Ernesto Ramírez López, Esa Vuorinen, Pooria Nazen Jalali, Leonardo Pelcastre and Maija Kärkkäinen

Metals 2020, 10(9), 1245; https://doi.org/10.3390/met10091245 - 16 Sep 2020

Cited by 5 | Viewed by 3970

Abstract

The present research addresses the effect of surface condition on oxide scale formation at high temperatures such as those experienced during secondary cooling in Continuous Casting. Tests were carried out in clean, as-cast and surfaces covered with casting powder to replicate the oxidation/re-oxidation [...] Read more.

The present research addresses the effect of surface condition on oxide scale formation at high temperatures such as those experienced during secondary cooling in Continuous Casting. Tests were carried out in clean, as-cast and surfaces covered with casting powder to replicate the oxidation/re-oxidation after the mould. Specimens oxidized at 1000, 1100 and 1200 °C under dry air and water-vapour conditions revealed that the oxide scale formation is strongly influenced by temperature, environmental and surface conditions. The oxide scale thickness increases with temperature alterations in the surface (e.g., as-cast and covered with powder) where oxides and carbonates from the casting powder accelerate oxidation kinetics leading to thick and unstable scales. A high amount of carbon is present on surfaces covered with casting powder where it diffuses through the oxide scale forming CO and CO₂ which lead to stress accumulation that makes scales prone to defects such as pores, voids and micro-cracks. Ultimately, if wüstite remains attached to the steel surface or inside oscillation marks, it may disturb heat transfer during secondary cooling which has deep industrial implications for crack formation and overall casting yield. Therefore, accurate insights on scale type and growth mechanisms could lead to accurate control of its formation during casting. Full article

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19 pages, 9149 KiB

Open AccessArticle

Assessing Microstructure-Local Mechanical Properties in Friction Stir Welded 6082-T6 Aluminum Alloy

by Hossein Monajati, Mariem Zoghlami, Amevi Tongne and Mohammad Jahazi

Metals 2020, 10(9), 1244; https://doi.org/10.3390/met10091244 - 16 Sep 2020

Cited by 13 | Viewed by 3874

Abstract

The severe deformation and temperature paths in the stir zone, also called weld nugget, of friction stir welded joints result, at very local levels, in significant microstructural variations, such as major differences in grain size or precipitation. One of the most common features [...] Read more.

The severe deformation and temperature paths in the stir zone, also called weld nugget, of friction stir welded joints result, at very local levels, in significant microstructural variations, such as major differences in grain size or precipitation. One of the most common features of friction stir welds is the presence of successive material layers, known as onion rings; however, little data is available on the mechanical properties of the different regions of the weld nugget, and particularly within the onion ring bands. Such information becomes very important for the integrity of large size friction stir welded structures. In the present study, a comprehensive characterization of onion rings produced during friction stir welding of a 6082-aluminum alloy was carried out. Advanced techniques such as in-situ SEM nanoindentation, EBSD, and high-resolution EDS were used to validate and compare the characteristics of the different bands in the onion rings. The analyses consisted of quantifying variations in grain size, precipitate composition and distribution, crystallographic orientations, and mechanical properties in each band. Furthermore, the tensile strengths of different regions of the weld nugget were evaluated using shear punch testing and correlated with those for the onion ring region in order to determine the impact of the presence of onion rings on weld nugget mechanical properties. The main difference between the alternate bands in the onion ring was found to be due to the difference in their grain size, misorientation, and precipitate content. It was also observed that the bands originate from the base metal and stir zone successively due to the nature of the stirring process, which pulls BM into SZ. Comparison of the shear punch testing results in different regions of the nugget revealed that, in spite of having local differences in the hardness of alternate bands in the onion ring, the presence of onion rings has no significant impact on the deterioration of the mechanical properties of the weld nugget. Full article

(This article belongs to the Special Issue Friction Stir Welding Prospective on Light-Alloys Joints)

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