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Metals, Volume 10, Issue 5 (May 2020) – 147 articles

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Open AccessArticle
Interfacial Fracture Toughness Assessment of a New Titanium–CFRP Adhesive Joint: An Experimental Comparative Study
Metals 2020, 10(5), 699; https://doi.org/10.3390/met10050699 - 25 May 2020
Cited by 1 | Viewed by 632
Abstract
Adhesive joints between dissimilar layers of metals and composites are increasingly used by different industries, as they promise significant weight savings and, consequently, a reduction in energy consumption and pollutant emissions. In the present work, the interfacial fracture behavior of a new titanium–carbon [...] Read more.
Adhesive joints between dissimilar layers of metals and composites are increasingly used by different industries, as they promise significant weight savings and, consequently, a reduction in energy consumption and pollutant emissions. In the present work, the interfacial fracture behavior of a new titanium–carbon fiber reinforced plastic (CFRP) adhesive joint is experimentally investigated using the double cantilever beam (DCB) and end-notched flexure (ENF) test configurations. A potential application of this joint is in future large passenger aircraft wings. Four characteristic industry relevant manufacturing approaches are proposed: co-bonding with/without adhesive and secondary bonding using thermoset/thermoplastic CFRP. For all of them, the vacuum-assisted resin transfer molding (VARTM) technique is utilized. To prevent titanium yielding during testing, two aluminum backing beams are adhesively bonded onto the primary joint. A data reduction scheme recently proposed by the authors, which considers effects such as bending–extension coupling and manufacturing-induced residual thermal stresses, is utilized for determination of the fracture toughness of the joint. The load–displacement responses, fracture behaviors during testing, and fracture toughness performances of the four manufacturing options (MOs) under consideration are presented and compared. Full article
(This article belongs to the Special Issue Recent Advances in Fibre Metal Laminates)
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Open AccessArticle
Effect of Boron Content and Cooling Rate on the Microstructure and Boride Formation of β-Solidifying γ-TiAl TNM Alloy
Metals 2020, 10(5), 698; https://doi.org/10.3390/met10050698 - 25 May 2020
Viewed by 471
Abstract
Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. [...] Read more.
Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. This study seeks to correlate the chemical composition and cooling rate during solidification of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure. Several β-solidifying γ-TiAl alloys of the TNM family were cast, alloying boron to a starting Ti-44.5Al-4Nb-1Mo-0.1B (at.%) alloy. The alloys were manufactured with an induction skull melting furnace and poured into a stepped 2, 4, 8 and 16 mm thickness mold to achieve different cooling rates. On one hand, the results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides. On the other hand, boron contents above 0.5 at.% and cooling rates during solidification below 10 K/s promote the formation of a refined microstructure with blocky borides. Finally, the formation mechanisms of both ribbon and blocky borides are proposed. Full article
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Open AccessArticle
Study on Hypervelocity Impact Characteristics of Ti/Al/Mg Density-Graded Materials
Metals 2020, 10(5), 697; https://doi.org/10.3390/met10050697 - 25 May 2020
Viewed by 420
Abstract
An improved shielding structure of a bumper that constructed from Ti/Al/Mg density-graded materials was presented. Two types of Ti/Al/Mg density-graded materials with the same areal density were prepared by diffusion bonding and powder metallurgy, respectively. The characteristics of hypervelocity impact including penetration holes [...] Read more.
An improved shielding structure of a bumper that constructed from Ti/Al/Mg density-graded materials was presented. Two types of Ti/Al/Mg density-graded materials with the same areal density were prepared by diffusion bonding and powder metallurgy, respectively. The characteristics of hypervelocity impact including penetration holes in the bumper, damage patterns on the rear wall and micrographs of the crater were investigated. The results show that damage mechanism of Ti/Al/Mg density-graded materials is closely related to the interface bonding strength and matrix strength. The penetration holes of Ti/Al/Mg density-graded material obtained by diffusion bonding exhibit typical ductile characteristics. The Ti/Al/Mg density-graded material prepared by powder metallurgy shows significant mechanical synergistic response under high strain compression and appears fragile characteristic. The shielding performance of Ti/Al/Mg bumper is increased by 20.4% compared with aluminum bumper. A theoretical analysis suggests that a Ti-Al-Mg bumper can fully break the projectile and greatly increase the entropy during the impact process. Larger projectile kinetic energy is converted into the internal energy during the impact process, thereby causing an obvious increase in shielding performance. Full article
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Open AccessArticle
In Vitro Bone Cell Behavior on Porous Titanium Samples: Influence of Porosity by Loose Sintering and Space Holder Techniques
Metals 2020, 10(5), 696; https://doi.org/10.3390/met10050696 - 25 May 2020
Cited by 1 | Viewed by 442
Abstract
A great variety of powder metallurgy techniques can produce biomimetic porous titanium structures with similar mechanical properties to host bone tissue. In this work, loose sintering and space holder techniques, two frequently used metallurgical techniques, are compared to evaluate the influences of porosity [...] Read more.
A great variety of powder metallurgy techniques can produce biomimetic porous titanium structures with similar mechanical properties to host bone tissue. In this work, loose sintering and space holder techniques, two frequently used metallurgical techniques, are compared to evaluate the influences of porosity (content, size, morphology and wall roughness), mechanical properties (stiffness and yield strength) and in-vitro cellular responses (adhesion and proliferation of myoblasts and osteoblasts). These comparisons are made to achieve the best balance between biomechanical and bifunctional behavior of a partial porous implant for cortical bone replacement. Cell adhesion (filopodia presence) and spreading were promoted on both porous surfaces and fully dense substrates (non-porous control surfaces). Porous scaffold samples designed using 50 vol.% NaCl space holder technique had an improved bioactive response over those obtained with the loose sintering technique due to higher roughness and scaffold pore diameter. However, the presence of large and heterogeneous pores compromises the mechanical reliability of the implant. Considering both scenarios, the substrates obtained with 40 vol.% NH4HCO3 and pore size ranges between 100 and 200 μm provide a balanced optimization of size and strength to promote in-vitro osseointegration. Full article
(This article belongs to the Special Issue Advanced Metals and Alloys for Biomedical Applications)
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Open AccessReview
Interstitials in f.c.c. High Entropy Alloys
Metals 2020, 10(5), 695; https://doi.org/10.3390/met10050695 - 25 May 2020
Viewed by 510
Abstract
The effects of interstitials on the mechanical properties of single-phase f.c.c. high entropy alloys (HEAs) have been assessed based on a review of the literature. It is found that in nearly all studies, carbon increases the yield strength, in some cases by more [...] Read more.
The effects of interstitials on the mechanical properties of single-phase f.c.c. high entropy alloys (HEAs) have been assessed based on a review of the literature. It is found that in nearly all studies, carbon increases the yield strength, in some cases by more than in traditional alloys. This suggests that carbon can be an excellent way to strengthen HEAs. This strength increase is related to the lattice expansion from the carbon. The effects on other mechanical behavior is mixed. Most studies show a slight reduction in ductility due to carbon, but a few show increases in ductility accompanying the yield strength increase. Similarly, some studies show little or modest increases in work-hardening rate (WHR) due to carbon, whereas a few show a substantial increase. These latter effects are due to changes in deformation mode. For both undoped and carbon doped CoCrFeMnNi, the room temperature ductility decreases slightly with decreasing grain size until ~2–5 µm, below which the ductility appears to decrease rapidly. The room temperature WHR also appears to decrease with decreasing grain size in both undoped and carbon-doped CoCrFeMnNi and in nitrogen-doped medium entropy alloy NiCoCr, and, at least for the undoped HEA, shows a sharp decrease at grain sizes <2 µm. Interestingly, carbon has been shown to almost double the Hall–Petch strengthening in CoCrFeMnNi, suggesting the segregation of carbon to the grain boundaries. There have been few studies on the effects of other interstitials such as boron, nitrogen and hydrogen. It is clear that more research is needed on interstitials both to understand their effects on mechanical properties and to optimize their use. Full article
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Open AccessArticle
Effect of Partial Solution Treatment Temperature on Microstructure and Tensile Properties of 440C Martensitic Stainless Steel
Metals 2020, 10(5), 694; https://doi.org/10.3390/met10050694 - 25 May 2020
Viewed by 464
Abstract
The 440C martensitic stainless steel is considered to be among the hardest steels, owing to its high carbon content. Careful heat treatment of this material introduces multiple carbide particles, which can alter microstructure and mechanical properties. This study focused on the effect of [...] Read more.
The 440C martensitic stainless steel is considered to be among the hardest steels, owing to its high carbon content. Careful heat treatment of this material introduces multiple carbide particles, which can alter microstructure and mechanical properties. This study focused on the effect of austenitisation temperature on the microstructure and tensile properties of 440C steel. Austenitisation was performed on the austenite + carbide region, because 440C steel lacks a single-phase region. The steel was austenitised at two different temperatures; namely, 1160 °C and 950 °C, and subjected to oil quenching. The as-quenched samples showed a typical lath martensite structure with retained austenite phase. The treatments at 1160 °C and 950 °C promoted the formation of M7C3 and M23C6 carbides, respectively. The austenite grains in the sample treated at 1160 °C showed a higher growth rate than those in the sample treated at 950 °C. The sample treated at 1160 °C showed low-fraction and a large-size carbide phase. The Zener pinning force decreased, thereby increasing the austenite grain growth in the sample treated at 1160 °C. The hardness and 0.2% proof stress of the sample treated at 950 °C were higher than those of the sample treated at 1160 °C, owing to the higher martensite content in the former. The strength–ductility balance of the sample treated at 950 °C was higher than that of the sample treated at 1160 °C. The decreased austenitisation temperature resulted in improved mechanical properties of the steel. Therefore, the austenitisation temperature alters the microstructure and mechanical properties of 440C steel. Full article
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Open AccessArticle
High Temperature Dielectric Properties of Iron- and Zinc-Bearing Products during Carbothermic Reduction by Microwave Heating
Metals 2020, 10(5), 693; https://doi.org/10.3390/met10050693 - 25 May 2020
Viewed by 437
Abstract
In this work, the carbothermic reduction of iron- and zinc-bearing products is studied through in situ microwave heating, dielectric properties monitoring, and mass spectrometry up to high temperatures (1000 °C). The results are correlated to the information provided by conventional analysis techniques such [...] Read more.
In this work, the carbothermic reduction of iron- and zinc-bearing products is studied through in situ microwave heating, dielectric properties monitoring, and mass spectrometry up to high temperatures (1000 °C). The results are correlated to the information provided by conventional analysis techniques such as differential scanning calorimetry (DSC) and thermogravimetry (TG). This combination allows a detailed study of seven different process stages with an accurate determination of the reaction temperatures, providing new evidence about the particular conditions of this microwave-driven reduction process. The presented results suggest that molecular vibrations imposed by the microwave field are presumably the reason for reactions taking place at lower temperatures than those observed in the conventional process. This work also explores the influence of other parameters, such as the apparent density or the amount of carbonaceous material, on the resulting dielectric properties, providing useful information for the development of a potential microwave industrial application in the metallurgy field. Full article
(This article belongs to the Special Issue Researches in Microwave Assisted Metallurgy)
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Open AccessFeature PaperArticle
Compositional Optimization and Structural Properties of the Filled Skutterudite Smy(FexNi1−x)4Sb11.5Sn0.5
Metals 2020, 10(5), 692; https://doi.org/10.3390/met10050692 - 25 May 2020
Viewed by 400
Abstract
A compositional and crystallographic study was carried out on the Smy(FexNi1−x)4Sb11.5Sn0.5 filled skutterudite system (0.40 ≤ x ≤ 0.80) with the aim to determine the equilibrium Sm filling fraction (y [...] Read more.
A compositional and crystallographic study was carried out on the Smy(FexNi1−x)4Sb11.5Sn0.5 filled skutterudite system (0.40 ≤ x ≤ 0.80) with the aim to determine the equilibrium Sm filling fraction (y) within the considered x range. The relevance of the material lies in its potential thermoelectric properties: in analogy with similar skutterudites systems, these features should in fact result as being improved with respect to the ones of the corresponding Sn-free system thanks to the partial substitution of Sn for Sb, which is expected to lower the phonon thermal conductivity. The results of Rietveld refinements allowed us to study the skutterudite structural properties and to discuss them, adopting a comparative approach with respect to the ones of the Sn-free system Smy(FexNi1−x)4Sb12. Relying on the refined Sm occupancy factors, the p/n crossover is shown to be located at x ~ 0.53, meaning that the introduction of Sn induces an enlargement of the p-region; moreover, at variance with the Sn-free system, the coefficient of thermal expansion does not show any significant mismatch between n- and p-compositions, which should ensure a prolonged lifetime of a device made of n- and p-legs that both derive from the studied system. Full article
(This article belongs to the Special Issue Rare-Earth Compounds for Advanced Functional Applications)
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Open AccessArticle
A Simulation and Optimization Study of the Swirling Nozzle for Eccentric Flow Fields of Round Molds
Metals 2020, 10(5), 691; https://doi.org/10.3390/met10050691 - 25 May 2020
Viewed by 390
Abstract
In continuous casting, the nozzle position may deviate from the center under actual operating conditions, which may cause periodic fluctuation of the steel-slag interface and easily lead to slag entrapment and gas absorption. Swirling nozzles can reduce these negative effects. A mathematical simulation [...] Read more.
In continuous casting, the nozzle position may deviate from the center under actual operating conditions, which may cause periodic fluctuation of the steel-slag interface and easily lead to slag entrapment and gas absorption. Swirling nozzles can reduce these negative effects. A mathematical simulation method based on a round mold of steel components with a 600 mm diameter is applied to study the flow field of molten steel in a mold. The swirling nozzle is optimized through the establishment of a fluid dynamics model. Meanwhile, a 1:2 hydraulic model is established for validation experiments. The results show that, when the submerged entry nozzle (SEN) is eccentric in the mold, it results in serious bias flow, increasing the drift index in the mold up to 0.46 at the eccentric distance of 50 mm. The impact depth of liquid steel and turbulent kinetic energy can be decreased by increasing the rotation angle of the nozzle. The nozzle with one bottom hole, which significantly decreases the bottom pressure and turbulent kinetic energy, greatly weakens the scour on nozzle and surface fluctuation. In the eccentric casting condition, using the optimized swirling nozzle that employs a 5-fractional structure, in which the rotation angle of 4 side holes is 30° and there is one bottom outlet, can effectively restrain bias flow and reduce the drift index to 0.28, a decline of more than 39%. Full article
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Open AccessArticle
Analysis of the Behavior of Dynamic Resistance, Electrical Energy and Force between the Electrodes in Resistance Spot Welding Using Additive Manufacturing
Metals 2020, 10(5), 690; https://doi.org/10.3390/met10050690 - 24 May 2020
Viewed by 504
Abstract
This work is aimed at the analysis of the dynamic resistance, electrical energy and behavior of the force between electrodes (including thermal expansion) during welding at optimized parameters, referring to the process of spot welding using additive manufacturing (AMSW). For comparative purposes, this [...] Read more.
This work is aimed at the analysis of the dynamic resistance, electrical energy and behavior of the force between electrodes (including thermal expansion) during welding at optimized parameters, referring to the process of spot welding using additive manufacturing (AMSW). For comparative purposes, this analysis also includes the conventional resistance spot welding process (RSW). The experiments were done on low carbon-zinc-coated sheets used in the automotive industry. The results regarding the welding process using additive manufacturing (AMSW), in comparison to the conventional resistance spot welding (RSW), showed that the dynamic resistance presented a different behavior due to the collapse of the deposition at the beginning of the welding, and that a smaller magnitude of electrical energy (approximately <3.35 times) is required to produce a welding spot approved in accordance with the norm. No force of thermal expansion was observed during the passage of the current, in contrast, there was a decrease in the force between the electrodes due to the collapse of the deposition at the beginning of the welding. Full article
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Open AccessFeature PaperArticle
Numerical Analysis of Stress Distribution in Offshore Wind Turbine M72 Bolted Connections
Metals 2020, 10(5), 689; https://doi.org/10.3390/met10050689 - 24 May 2020
Cited by 1 | Viewed by 746
Abstract
The use of bolted joints to connect the transition piece and monopile is nowadays widely applied in the offshore wind industry. Traditionally, grouted connections were used in the early generation of offshore wind turbines, but the experienced failures in such connections led to [...] Read more.
The use of bolted joints to connect the transition piece and monopile is nowadays widely applied in the offshore wind industry. Traditionally, grouted connections were used in the early generation of offshore wind turbines, but the experienced failures in such connections led to an increased tendency towards bolted flange connections to join the transition piece and monopile in the new generation of offshore wind turbines. The bolts used for this purpose have high strength and huge sizes, and are subjected to a preload that is applied during the tightening process. The present study is focused on the analysis of preload effects on stress distribution in M72 bolted connections by considering different friction coefficients between the bolt and nut threads. The bolt is considered to be made of grade 10.9 steel, whereas the nut is assumed to be made of grade 8.8 steel, which is a softer material. Using the finite element commercial software package Abaqus, numerical models were developed and analysed to establish trends for stress distribution and plastic strains during the bolt tightening process, and to quantify stress concentration factors in individual engaged threads. Full article
(This article belongs to the Special Issue Microstructure, Deformation, and Fatigue Behavior in Metals)
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Open AccessArticle
Analytical Model to Compare and Select Creep Constitutive Equation for Stress Relief Investigation during Heat Treatment in Ferritic Welded Structure
Metals 2020, 10(5), 688; https://doi.org/10.3390/met10050688 - 23 May 2020
Viewed by 479
Abstract
The one-dimensional analytical model was promoted to help select the creep constitutive equation and predict heat treatment temperature in a ferritic welded structure, along with neglecting the impact of structural constraint and deformation compatibility. The analytical solutions were compared with simulation results, which [...] Read more.
The one-dimensional analytical model was promoted to help select the creep constitutive equation and predict heat treatment temperature in a ferritic welded structure, along with neglecting the impact of structural constraint and deformation compatibility. The analytical solutions were compared with simulation results, which were validated with experimental measurements in a ferritic welded rotor. The as-welded and post weld heat treatment (PWHT) residual stresses on the inner and outer cylindrical surfaces were measured with the hole-drilling method (HDM) for validation. Based on the one-dimensional analytical model, different effects of Norton and Norton-Bailey creep constitutive equation on stress relief during heat treatment in a ferritic welded rotor were investigated. Full article
(This article belongs to the Special Issue Heat Treatment of Steels)
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Open AccessArticle
Kinetics Study of Solvent and Solid-Phase Extraction of Rare Earth Metals with Di-2-Ethylhexylphosphoric Acid
Metals 2020, 10(5), 687; https://doi.org/10.3390/met10050687 - 23 May 2020
Viewed by 432
Abstract
The kinetic features of solvent and solid-phase extraction of yttrium and iron (III) from simulated and industrial phosphoric acid solutions are revealed. Di-2-ethylhexylphosphoric acid (D2EHPA) was used as a liquid extractant, and D2EHPA-containing Levextrel resin—a co-polymerization product of styrene and divinylbenzene in the [...] Read more.
The kinetic features of solvent and solid-phase extraction of yttrium and iron (III) from simulated and industrial phosphoric acid solutions are revealed. Di-2-ethylhexylphosphoric acid (D2EHPA) was used as a liquid extractant, and D2EHPA-containing Levextrel resin—a co-polymerization product of styrene and divinylbenzene in the presence of D2EHPA—was used as a solid-phase extraction agent. Significant dependence of yttrium extraction rate constant on the stirring rate was revealed using the formal first-order kinetic equation. The data obtained characterizes a diffusion-limited process with an activation energy of 16.2 ± 1.3 kJ/mol. Temperature increase during the iron (III) extraction process leads to a changeover of a rate-limiting stage from kinetic to diffusion, accompanied by drop of activation energy from 40.0 ± 1.4 to 11.4 ± 1.2 kJ/mol. Effective separation of elements at the extraction stage is possible at temperatures of 283–300 K under non-equilibrium conditions of the ferric ions transport from aqueous to organic phase. This condition ensures a high yttrium–iron separation coefficient of 23.2 in 1.5–2 min. Extraction kinetics by Levextrel resin are described by Fick’s second law equation, which establishes the laws of diffusion in the solid grain of the organic phase with an activation energy of 18.5 ± 2.0 kJ/mol. Full article
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Open AccessReview
Development of AM Technologies for Metals in the Sector of Medical Implants
Metals 2020, 10(5), 686; https://doi.org/10.3390/met10050686 - 23 May 2020
Cited by 1 | Viewed by 573
Abstract
Additive manufacturing (AM) processes have undergone significant progress in recent years, having been implemented in sectors as diverse as automotive, aerospace, electrical component manufacturing, etc. In the medical sector, different devices are printed, such as implants, surgical guides, scaffolds, tissue engineering, etc. Although [...] Read more.
Additive manufacturing (AM) processes have undergone significant progress in recent years, having been implemented in sectors as diverse as automotive, aerospace, electrical component manufacturing, etc. In the medical sector, different devices are printed, such as implants, surgical guides, scaffolds, tissue engineering, etc. Although nowadays some implants are made of plastics or ceramics, metals have been traditionally employed in their manufacture. However, metallic implants obtained by traditional methods such as machining have the drawbacks that they are manufactured in standard sizes, and that it is difficult to obtain porous structures that favor fixation of the prostheses by means of osseointegration. The present paper presents an overview of the use of AM technologies to manufacture metallic implants. First, the different technologies used for metals are presented, focusing on the main advantages and drawbacks of each one of them. Considered technologies are binder jetting (BJ), selective laser melting (SLM), electron beam melting (EBM), direct energy deposition (DED), and material extrusion by fused filament fabrication (FFF) with metal filled polymers. Then, different metals used in the medical sector are listed, and their properties are summarized, with the focus on Ti and CoCr alloys. They are divided into two groups, namely ferrous and non-ferrous alloys. Finally, the state-of-art about the manufacture of metallic implants with AM technologies is summarized. The present paper will help to explain the latest progress in the application of AM processes to the manufacture of implants. Full article
(This article belongs to the Special Issue State-of-Art within 3D Printing and Advanced Machining Processes)
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Open AccessArticle
A Comparative Assessment of Six Machine Learning Models for Prediction of Bending Force in Hot Strip Rolling Process
Metals 2020, 10(5), 685; https://doi.org/10.3390/met10050685 - 22 May 2020
Viewed by 471
Abstract
In the hot strip rolling (HSR) process, accurate prediction of bending force can improve the control accuracy of the strip crown and flatness, and further improve the strip shape quality. In this paper, six machine learning models, including Artificial Neural Network (ANN), Support [...] Read more.
In the hot strip rolling (HSR) process, accurate prediction of bending force can improve the control accuracy of the strip crown and flatness, and further improve the strip shape quality. In this paper, six machine learning models, including Artificial Neural Network (ANN), Support Vector Machine (SVR), Classification and Regression Tree (CART), Bagging Regression Tree (BRT), Least Absolute Shrinkage and Selection operator (LASSO), and Gaussian Process Regression (GPR), were applied to predict the bending force in the HSR process. A comparative experiment was carried out based on a real-life dataset, and the prediction performance of the six models was analyzed from prediction accuracy, stability, and computational cost. The prediction performance of the six models was assessed using three evaluation metrics of root mean square error (RMSE), mean absolute error (MAE), and coefficient of determination (R2). The results show that the GPR model is considered as the optimal model for bending force prediction with the best prediction accuracy, better stability, and acceptable computational cost. The prediction accuracy and stability of CART and ANN are slightly lower than that of GPR. Although BRT also shows a good combination of prediction accuracy and computational cost, the stability of BRT is the worst in the six models. SVM not only has poor prediction accuracy, but also has the highest computational cost while LASSO showed the worst prediction accuracy. Full article
(This article belongs to the Special Issue Forming Processes of Modern Metallic Materials)
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Open AccessArticle
Ambivalent Role of Annealing in Tensile Properties of Step-Rolled Ti-6Al-4V with Ultrafine-Grained Structure
Metals 2020, 10(5), 684; https://doi.org/10.3390/met10050684 - 22 May 2020
Viewed by 504
Abstract
Step rolling can be used to mass-produce ultrafine-grained (UFG) Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room temperature (RT) and elevated temperature. The step-rolled alloy retained its UFG structure after subsequent annealing [...] Read more.
Step rolling can be used to mass-produce ultrafine-grained (UFG) Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room temperature (RT) and elevated temperature. The step-rolled alloy retained its UFG structure after subsequent annealing at 500–600 °C. The RT ductility of the step-rolled alloy increased regardless of annealing temperature, but strengthening was only attained by annealing at 500 °C. In contrast, subsequent annealing rarely improved the elevated-temperature tensile properties. The step-rolled Ti-6Al-4V alloy without the annealing showed the highest elongation to failure of 960% at 700 °C and a strain rate of 10−3 s−1. The ambivalent effect of annealing on RT and elevated-temperature tensile properties is a result of microstructural features, such as dislocation tangles, subgrains, phases, and continuous dynamic recrystallization. Full article
(This article belongs to the Special Issue Titanium Alloys and Titanium-Based Matrix Composites)
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Open AccessArticle
Investigation of Melt Pool Geometry Control in Additive Manufacturing Using Hybrid Modeling
Metals 2020, 10(5), 683; https://doi.org/10.3390/met10050683 - 22 May 2020
Viewed by 671
Abstract
Metal additive manufacturing (AM) works on the principle of consolidating feedstock material in layers towards the fabrication of complex objects through localized melting and resolidification using high-power energy sources. Powder bed fusion and directed energy deposition are two widespread metal AM processes that [...] Read more.
Metal additive manufacturing (AM) works on the principle of consolidating feedstock material in layers towards the fabrication of complex objects through localized melting and resolidification using high-power energy sources. Powder bed fusion and directed energy deposition are two widespread metal AM processes that are currently in use. During layer-by-layer fabrication, as the components continue to gain thermal energy, the melt pool geometry undergoes substantial changes if the process parameters are not appropriately adjusted on-the-fly. Although control of melt pool geometry via feedback or feedforward methods is a possibility, the time needed for changes in process parameters to translate into adjustments in melt pool geometry is of critical concern. A second option is to implement multi-physics simulation models that can provide estimates of temporal process parameter evolution. However, such models are computationally near intractable when they are coupled with an optimization framework for finding process parameters that can retain the desired melt pool geometry as a function of time. To address these challenges, a hybrid framework involving machine learning-assisted process modeling and optimization for controlling the melt pool geometry during the build process is developed and validated using experimental observations. A widely used 3D analytical model capable of predicting the thermal distribution in a moving melt pool is implemented and, thereafter, a nonparametric Bayesian, namely, Gaussian Process (GP), model is used for the prediction of time-dependent melt pool geometry (e.g., dimensions) at different values of the process parameters with excellent accuracy along with uncertainty quantification at the prediction points. Finally, a surrogate-assisted statistical learning and optimization architecture involving GP-based modeling and Bayesian Optimization (BO) is employed for predicting the optimal set of process parameters as the scan progresses to keep the melt pool dimensions at desired values. The results demonstrate that a model-based optimization can be significantly accelerated using tools of machine learning in a data-driven setting and reliable a priori estimates of process parameter evolution can be generated to obtain desired melt pool dimensions for the entire build process. Full article
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Open AccessArticle
Beta Titanium Alloys Produced from Titanium Hydride: Effect of Alloying Elements on Titanium Hydride Decomposition
Metals 2020, 10(5), 682; https://doi.org/10.3390/met10050682 - 22 May 2020
Viewed by 471
Abstract
The use of titanium hydride as a raw material has been an attractive alternative for the production of titanium components produced by powder metallurgy, due to increased densification of Ti compacts, greater control of contamination and cost reduction of the raw materials. However, [...] Read more.
The use of titanium hydride as a raw material has been an attractive alternative for the production of titanium components produced by powder metallurgy, due to increased densification of Ti compacts, greater control of contamination and cost reduction of the raw materials. However, a significant amount of hydrogen that often remains on the samples could generate degradation of the mechanical properties. Therefore, understanding decomposition mechanisms is essential to promote the components’ long life. Several studies on titanium hydride (TiH2) decomposition have been developed; nevertheless, few studies focus on the effect of the alloying elements on the dehydrogenation process. In this work, the effects of the addition of different amounts of Fe (5 and 7 wt. %) and Nb (12, 25, and 40 wt. %) as alloying elements were evaluated in detail. Results suggest that α→β transformation of Ti occurs below 800 °C; β phase can be observed at lower temperature than the expected according to the phase diagram. It was found that β phase transformation could take place during the intermediate stage of dehydrogenation. A mechanism was proposed for the effect of allying elements on the dehydrogenation process. Full article
(This article belongs to the Special Issue Powder Metallurgy of Titanium Alloys)
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Open AccessArticle
Using High-Pressure Torsion to Achieve Superplasticity in an AZ91 Magnesium Alloy
Metals 2020, 10(5), 681; https://doi.org/10.3390/met10050681 - 22 May 2020
Viewed by 496
Abstract
An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10−5−10−1 s−1 to evaluate the occurrence [...] Read more.
An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10−5−10−1 s−1 to evaluate the occurrence of superplasticity. Results showed that HPT processing refined the grain structure in the alloy, and grain sizes smaller than 10 µm were retained up to 623 K. Superplastic elongations were observed at low strain rates at 423 K and at all strain rates at 523 K. An examination of the experiment data showed good agreement with the theoretical prediction for grain-boundary sliding, the rate-controlling mechanism for superplasticity. Elongations in the range of 300–400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms. Full article
(This article belongs to the Special Issue Superplasticity and Superplastic Forming)
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Open AccessArticle
Integrating Flotation and Pyrometallurgy for Recovering Graphite and Valuable Metals from Battery Scrap
Metals 2020, 10(5), 680; https://doi.org/10.3390/met10050680 - 21 May 2020
Viewed by 635
Abstract
Since the current volumes of collected end-of-life lithium ion batteries (LIBs) are low, one option to increase the feasibility of their recycling is to feed them to existing metals production processes. This work presents a novel approach to integrate froth flotation as a [...] Read more.
Since the current volumes of collected end-of-life lithium ion batteries (LIBs) are low, one option to increase the feasibility of their recycling is to feed them to existing metals production processes. This work presents a novel approach to integrate froth flotation as a mechanical treatment to optimize the recovery of valuable metals from LIB scrap and minimize their loss in the nickel slag cleaning process. Additionally, the conventional reducing agent in slag cleaning, namely coke, is replaced with graphite contained in the LIB waste flotation products. Using proper conditioning procedures, froth flotation was able to recover up to 81.3% Co in active materials from a Cu-Al rich feed stream. A selected froth product was used as feed for nickel slag cleaning process, and the recovery of metals from a slag (80%)–froth fraction (20%) mixture was investigated in an inert atmosphere at 1350 °C and 1400 °C at varying reduction times. The experimental conditions in combination with the graphite allowed for a very rapid reduction. After 5 min reduction time, the valuable metals Co, Ni, and Cu were found to be distributed to the iron rich metal alloy, while the remaining fraction of Mn and Al present in the froth fraction was deported in the slag. Full article
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Open AccessArticle
Production of Oxide Dispersion Strengthened Mg-Zn-Y Alloy by Equal Channel Angular Pressing of Mechanically Alloyed Powder
Metals 2020, 10(5), 679; https://doi.org/10.3390/met10050679 - 21 May 2020
Cited by 1 | Viewed by 440
Abstract
Mg-Zn-Y alloys with long-period stacking ordered structures (LPSO) have attracted attention due to their excellent mechanical properties. In addition to the LPSO structure, Mg alloys can also be strengthened by oxide particles. In the present study, oxide dispersion strengthened Mg97Zn1 [...] Read more.
Mg-Zn-Y alloys with long-period stacking ordered structures (LPSO) have attracted attention due to their excellent mechanical properties. In addition to the LPSO structure, Mg alloys can also be strengthened by oxide particles. In the present study, oxide dispersion strengthened Mg97Zn1Y2 (at%) alloys were prepared by equal channel angular pressing (ECAP) of mechanical alloyed (MA) powder under an oxygen gas atmosphere. The 20-h-MA powder had a particle size of 28 μm and a crystallite size of 36 nm. During the MA process followed by ECAP, an Mg matrix with dispersed Y2O3 (and MgO) particles was formed. The alloy processed by ECAP exhibited a hardness of 110 HV and a compressive strength of 185 MPa. Compared to pure Mg, the increased hardness was due to the dispersion strengthening of Y2O3 and MgO particles and solution strengthening of Zn and Y. Full article
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Open AccessArticle
Numerical Simulation of the Melting Behavior of Steel Scrap in Hot Metal
Metals 2020, 10(5), 678; https://doi.org/10.3390/met10050678 - 21 May 2020
Viewed by 448
Abstract
The current study focuses on the melting behavior of a scrap bar with low carbon content in hot metal which contains high carbon concentration by applying experiments and mathematical modelings. The experiments suggest that higher temperature is favorable for the melting of the [...] Read more.
The current study focuses on the melting behavior of a scrap bar with low carbon content in hot metal which contains high carbon concentration by applying experiments and mathematical modelings. The experiments suggest that higher temperature is favorable for the melting of the bar and the melting rate of the bar is initially high while decreased to a relative stable level after 90 s in the current conditions. It can be found from the mathematical results that the bar temperature is increased near to bath temperature in about 20 s after it was immersed into the bath, and the temperature in the axis of the bar is not distributed evenly during the temperature increase stage. Moreover, the mathematical results shows that a bath circulation flow would be formed in the bath under the effects of temperature and carbon distribution during the melting process. The bath flow near the melting interface would influence the carbon concentration of the molten phase, in turn, affects the melting rate of the bar in the vertical direction. Both the experimental and mathematical results show that the melting rate in the upper part, which is in the upstream of the bath flow, is higher than that of the middle part, followed by the down part of the bar in the downstream of the flow, in which the carbon concentration is much lower than that of the bath. At this period, the main factor that dominate the bar melting is not the temperature but the carbon distribution at the melting interface after the bar temperature is increased to the bath temperature. Full article
(This article belongs to the Special Issue Numerical Modelling in Steel Metallurgy)
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Open AccessArticle
Investigation and Optimization of Load Distribution for Tandem Cold Steel Strip Rolling Process
Metals 2020, 10(5), 677; https://doi.org/10.3390/met10050677 - 21 May 2020
Viewed by 456
Abstract
In order to improve the cold rolled steel strip flatness, the load distribution of the tandem cold rolling process is subject to investigation and optimization. The strip deformation resistance model is corrected by an artificial neural network that is trained with the actual [...] Read more.
In order to improve the cold rolled steel strip flatness, the load distribution of the tandem cold rolling process is subject to investigation and optimization. The strip deformation resistance model is corrected by an artificial neural network that is trained with the actual measured data of 4500 strip coils. Based on the model, a flatness prediction model of strip steel is established in a five-stand tandem cold rolling mill, and the precision of the flatness prediction model is verified by rolling experiment data. To analyze the effect of load distribution on flatness, the flatness of stand 4 is calculated to be 7.4 IU, 10.6 IU, and 16.8 IU under three typical load distribution modes. A genetic algorithm based on the excellent flatness is proposed to optimize the load distribution further. In the genetic algorithm, the classification of flatness of stand 4 calculated by the developed flatness prediction model is taken as the fitness function, with the optimal reduction of 28.6%, 34.6%, 27.3%, and 18.6% proposed for stands 1, 2, 3, and 4, respectively. The optimal solution is applied to a 1740 mm tandem cold rolling mill, which reduce the flatness classification from 10.8 IU to 3.2 IU for a 1-mm thick steel strip. Full article
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Open AccessArticle
A Novel Microwave and Induction Heating Applicator for Metal Making: Design and Testing
Metals 2020, 10(5), 676; https://doi.org/10.3390/met10050676 - 21 May 2020
Viewed by 468
Abstract
The use of microwave heating in primary metallurgy is gaining an increasing interest due to the possibility to selectively process ores and to volumetrically heat large amounts of low-thermal conductivity minerals. In this paper the study, development and testing of a new applicator [...] Read more.
The use of microwave heating in primary metallurgy is gaining an increasing interest due to the possibility to selectively process ores and to volumetrically heat large amounts of low-thermal conductivity minerals. In this paper the study, development and testing of a new applicator combining the use of microwave and induction heating for rapid reduction of metal containing oxides is described. Numerical simulation was used in order to achieve the proper control over heat generation, considering the use of microwave solid state generators. A prototype, with a capacity up to 5 liters of standard input feed but with the predisposition for continuous processing has been designed, built and tested on reference loads like iron oxide powders and pellets. Results on the microwave heating part of the applicator indicate that it allows to efficiently and rapidly process these kinds of loads, which change from dielectric to conductors as reduction proceeds. The use of variable frequency solid state microwave generators allows to maximize energy efficiency and to controllably change the heating pattern inside the load. Full article
(This article belongs to the Special Issue Researches in Microwave Assisted Metallurgy)
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Open AccessArticle
Assessment of Chip Breakability during Turning of Stainless Steels Based on Weight Distributions of Chips
Metals 2020, 10(5), 675; https://doi.org/10.3390/met10050675 - 21 May 2020
Viewed by 462
Abstract
Currently, the available evaluation methods for determining the chip breakability in the industry are mainly based on subjective visual assessment of the chip formation by an operator during machining or on chips that were collected after the tests. However, in many cases, these [...] Read more.
Currently, the available evaluation methods for determining the chip breakability in the industry are mainly based on subjective visual assessment of the chip formation by an operator during machining or on chips that were collected after the tests. However, in many cases, these methods cannot give us accurate quantitative differences for evaluation of the chip breakability of similar steel grades and similar sets of machining parameters. Thus, more sensitive methods are required to obtain more detailed information. In this study, a new method for the objective assessment of chip breakability based on quantitative determination of the weight distribution of chips (WDC) was tested and applied during machining of stainless steels without Ca treatment (316L) and with Ca treatment (316L + Ca). The obtained results show great consistencies and the reliability of this method. By using the WDC method, significant quantitative differences were obtained by the evaluation of chips, which were collected during the machining process of these two similar grades of steel at various cutting parameters, while, visually, these chips look very similar. More specifically, it was found that the Ca treatment of steel can improve the chip breakability of 316L + Ca steel in 80% of cutting trials, since a fraction of small light chips (Type I) from this steel increased and a fraction of large heavy chips (Type III) decreased accordingly. Moreover, the WDCs that were obtained at different cutting parameters were determined and compared in this study. The obtained results can be used for the optimization of chip breakability of each steel at different cutting parameters. The positive effect of Ca treatment of stainless steel was discussed in this study based on consideration of the modification of different non-metallic inclusions and their effect on the chip breakability during machining. Full article
(This article belongs to the Special Issue Metal Machining—Recent Advances, Applications and Challenges)
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Open AccessArticle
Influence of Internal Pressure and Axial Compressive Displacement on the Formability of Small-Diameter ZM21 Magnesium Alloy Tubes in Warm Tube Hydroforming
Metals 2020, 10(5), 674; https://doi.org/10.3390/met10050674 - 21 May 2020
Viewed by 486
Abstract
In this study, the influence of internal pressure and axial compressive displacement on the formability of small-diameter ZM21 magnesium alloy tubes in warm tube hydroforming (THF) was examined experimentally and numerically. The deformation behavior of ZM21 tubes, with a 2.0 mm outer diameter [...] Read more.
In this study, the influence of internal pressure and axial compressive displacement on the formability of small-diameter ZM21 magnesium alloy tubes in warm tube hydroforming (THF) was examined experimentally and numerically. The deformation behavior of ZM21 tubes, with a 2.0 mm outer diameter and 0.2 mm wall thickness, was evaluated in taper-cavity and cylinder-cavity dies. The simulation code used was the dynamic explicit finite element (FE) method (FEM) code, LS-DYNA 3D. The experiments were conducted at 250 °C. This paper elucidated the deformation characteristics, forming defects and forming limit of ZM21 tubes. Their deformation behavior in the taper-cavity die was affected by the axial compressive direction. Additionally, the occurrence of tube buckling could be inferred by changes of the axial compression force, which were measured by the load cell during the processing. In addition, grain with twin boundaries and refined grain were observed at the bended areas of tapered tubes. The hydroformed samples could have a high strength. Moreover, wrinkles, which are caused under a lower internal pressure condition, were employed to avoid tube fractures during the axial feeding. The tube with wrinkles was expanded by a straightening process after the axial feed. It was found that the process of warm THF of the tubes in the cylinder-cavity die was successful. Full article
(This article belongs to the Special Issue Latest Hydroforming Technology of Metallic Tubes and Sheets)
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Open AccessArticle
Evaluation of Hot Deformation Behaviour of UNS S32750 Super Duplex Stainless Steel (SDSS) Alloy
Metals 2020, 10(5), 673; https://doi.org/10.3390/met10050673 - 21 May 2020
Viewed by 431
Abstract
The super-duplex stainless steel UNS S32750 consists of two main phases, austenite and ferrite, which differ not only by their morphology, physical, and mechanical properties, but also by their deformation behaviour. A heterogenous deformation can be obtained during thermomechanical processing, generating internal stresses [...] Read more.
The super-duplex stainless steel UNS S32750 consists of two main phases, austenite and ferrite, which differ not only by their morphology, physical, and mechanical properties, but also by their deformation behaviour. A heterogenous deformation can be obtained during thermomechanical processing, generating internal stresses and sometimes fissures or cracks on sample lateral surfaces, due to ferrite’s phase lower potential of plastic deformation accommodation in comparison with austenite phase. The research objective is to determine the optimum range of the applied deformation degree, during hot deformation processing by upsetting of the super-duplex steel (SDSS) UNS S32750. In the experimental program several samples were hot deformed by upsetting, by applying a deformation degree between 5–50%, at 1050 °C and 1300 °C. The most representative hot-deformed samples were selected and analysed by scanning electron microscope-Electron Backscatter Diffraction (SEM-EBSD), to determine the main microstructural characteristics obtained during thermomechanical processing. When considering the experimental results, the influence of the applied deformation degree on the microstructure has been evaluated. Microstructural features, such as nature, distribution, morphology and relative proportion of constituent phases, Grain Reference Orientation Deviation (GROD), and recrystallization (RX), were analysed, in correlation with the applied deformation degree. Finally, it was concluded that the UNS S32750 alloy can be safely hot deformed, by upsetting, at 1050 °C and 1300 °C, with a maximum applied deformation degree of 20% at 1050 °C and, respectively, by 50% at 1300 °C. Full article
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Open AccessArticle
Wear Resistance of High C High Si Steel with Low Retained Austenite Content and Kinetically Activated Bainite
Metals 2020, 10(5), 672; https://doi.org/10.3390/met10050672 - 21 May 2020
Viewed by 423
Abstract
A novel high C high Si carbide free bainitic steel was developed for the production of cold work tools, knives, and rolls, requiring high hardness, toughness, as well as abrasive/adhesive wear resistance and resistance to galling at low costs. The steel was tribologically [...] Read more.
A novel high C high Si carbide free bainitic steel was developed for the production of cold work tools, knives, and rolls, requiring high hardness, toughness, as well as abrasive/adhesive wear resistance and resistance to galling at low costs. The steel was tribologically tested in dry sliding conditions under abrasive and adhesive wear mode, facilitated by using alumina and bearing steel ball as a counter-material, respectively. It was determined that carbide dissolution occurs under high contact pressures, thereby enriching the surrounding matrix with carbon and locally increasing the retained austenite content. The high retained austenite at the sliding interface increases the steels work hardening capacity and promotes superior wear resistance when compared to much more alloyed cold work tool steel, such as AISI D2. The steel has a high resistance to galling as determined by sliding against a soft steel bar due to its chemical composition. Full article
(This article belongs to the Special Issue Friction and Wear of Metals)
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Open AccessFeature PaperArticle
Particle-Stimulated Nucleation (PSN) in the Co–28Cr–5Mo–0.3C Alloy
Metals 2020, 10(5), 671; https://doi.org/10.3390/met10050671 - 21 May 2020
Viewed by 436
Abstract
The present work is aimed at refining the grain size in the Co–28Cr–5Mo–0.3C (wt%) cast alloy using particle-stimulated nucleation (PSN) of recrystallization. It is pointed out that PSN resulted in considerable grain refinement (≈80%) of the as-cast structure, leading to an increased yield [...] Read more.
The present work is aimed at refining the grain size in the Co–28Cr–5Mo–0.3C (wt%) cast alloy using particle-stimulated nucleation (PSN) of recrystallization. It is pointed out that PSN resulted in considerable grain refinement (≈80%) of the as-cast structure, leading to an increased yield and tensile strength (around 30%). Partial solutionizing is associated with the formation of γfcc and athermal martensite. During PSN, the intensity of the hexagonal close-packed (hcp) phase increases due to the formation of isothermal martensite. It appears that new dynamic recrystallized (DRX) grains are formed around coarse undissolved particles (≈10 μm in size), especially where these particles are present in large clusters. The high-resolution TEM image shows the formation of heavily faulted regions and subgrains, with maximum misorientation near the carbides providing the driving force for the nucleation of new grains. Full article
(This article belongs to the Special Issue Heat Treatment of Non-ferrous Alloys)
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Open AccessArticle
Numerical Analysis on Temperature Field of Grinding Ti-6Al-4V Titanium Alloy by Oscillating Heat Pipe Grinding Wheel
Metals 2020, 10(5), 670; https://doi.org/10.3390/met10050670 - 21 May 2020
Cited by 1 | Viewed by 583
Abstract
When grinding hard-to-machining materials such as titanium alloys, a massive grinding heat is generated and gathers in the grinding zone due to the low thermal conduction of the materials. The accumulated grinding heat easily leads to severe thermal damages to both the workpiece [...] Read more.
When grinding hard-to-machining materials such as titanium alloys, a massive grinding heat is generated and gathers in the grinding zone due to the low thermal conduction of the materials. The accumulated grinding heat easily leads to severe thermal damages to both the workpiece and the grinding wheel. A novel oscillating heat pipe (OHP) grinding wheel is one of the solutions to this phenomenon. The oscillating heat pipe grinding wheel can transfer the grinding heat directly from the grinding zone to avoid heat accumulation and a high temperature rise. In this paper, the temperature field of the grinding Ti-6Al-4V alloy is investigated, via the oscillating heat pipe grinding wheel, by numerical analysis. The three-dimensional thermal conduction model is built accordingly, containing the grinding wheel, grinding zone and Ti-6Al-4V workpiece. Due to the enhanced heat transport capacity of the oscillating heat pipe grinding wheel, the highest temperature in the grinding zone and the temperature on the ground surface of the workpiece decrease dramatically. For example, under a grinding heat flux of 1 × 107 W/m2, when using the grinding wheel without OHP and with OHPs, the highest temperature in the grinding zone drops from 917 °C to 285 °C by 68.7%, and the ground surface temperature decreases from 823 °C to 244 °C by 71.2%. Moreover, the temperature distribution on the grinding wheel is more uniform with an increase of the number of oscillating heat pipes. Full article
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