Metals

Iron-based shape memory alloys (SMAs) have been widely studied during the last years, producing new formulations with potential applications in civil engineering. In the present paper, the microstructure and the thermomechanical behavior of the Fe-28Mn-6Si-5Cr memory alloy has been investigated. At room temperature, the presence of ε-martensite and γ-austenite was confirmed using optical and electron microscopy techniques. The martensitic transformation temperatures (A_s, A_f, M_s, and M_f) were determined by differential scanning calorimetry, together with an X-ray diffraction technique. The use of these techniques also confirmed that this transformation is not totally reversible, depending on the strain degree and the number of thermal cycles. From the kinetics study of the ε → γ transformation, the isoconversion curves (transformation degree versus time) were built, which provided the information required to optimize the thermal activation cycle. Tensile tests were performed to characterize the mechanical properties of the studied alloy. These kinds of tests were also performed to assess the shape memory effect, getting a recovery stress of 140 MPa, after a 7.6% pre-strain and a thermal activation up to 160 °C. Full article

High-entropy alloys (HEAs) have been around since 2004. The breakthroughs in this field led to several potential applications of these alloys as refractory, structural, functional, and biomedical materials. In this work, a short overview on the concept of high-entropy alloys is provided, as well as the theoretical design approach. The special focus of this review concerns one novel class of these alloys: biomedical high-entropy alloys. Here, a literature review on the potential high-entropy alloys for biomedical applications is presented. The characteristics that are required for these alloys to be used in biomedical-oriented applications, namely their mechanical and biocompatibility properties, are discussed and compared to commercially available Ti6Al4V. Different processing routes are also discussed. Full article

Within the implementation of the Industry 4.0 paradigm in the steel sector, robots can play a relevant role in improving health and safety conditions at the workplace, by overtaking cumbersome, repetitive and risky operations. However, the implementation of robotics solutions in this particular sector is hampered by harsh operating conditions and by particular features of many procedures, which require a combination of force and sensitivity. Human–robot cooperation is a viable solution to overcome existing barriers, by synergistically combining human and robot abilities in the sense of a human-centered Industry 5.0. In this sense, robotics solution should be designed in a way to integrate and meet the end-users’ demands in a common development process for successfully implementation and widely acceptance. The paper presents the outcomes of the field evaluation of a robotic workstation, which was designed for a complex maintenance operation that is daily performed in the steel shop. The system derives from a co-creation process, where workers were involved since the beginning in the design process, according to the paradigm of social innovation combining technological and social development. Therefore, the evaluation aimed at assessing both system reliability and end-users’ satisfaction. The results show that the human-centered robotic workstations are successful in reducing cumbersome operations and improving workers’ health and safety conditions, and that this fact is clearly perceived by system users and developers. Full article

Springback is a common defect caused by elastic recovery in the plastic forming process, which can cause the formed workpiece to deviate from the target shape. The purpose of this paper is to grasp the influence law of process parameters on springback deviation so as to optimize process parameters to reduce springback. Taking the Y-profile as the research object, the influence of process parameters such as horizontal bending radius, vertical bending radius, transition zone length, radius of roller dies, and wall thickness of the profile on springback deviation is discussed by numerical simulation. Additionally, the validity of the numerical simulation is verified through experiments. The springback law obtained by numerical simulation is applied to practical production, and the large-scale production of the Y-profile is realized. Full article

The effect of high-temperature thermomechanical treatment on the structural transformations and mechanical properties of metastable austenitic steel of the AISI 321 type is investigated. The features of the grain and defect microstructure of steel were studied by scanning electron microscopy with electron back-scatter diffraction (SEM EBSD) and transmission electron microscopy (TEM). It is shown that in the initial state after solution treatment the average grain size is 18 μm. A high (≈50%) fraction of twin boundaries (annealing twins) was found. In the course of hot (with heating up to 1100 °C) plastic deformation by rolling to moderate strain (e = 1.6, where e is true strain) the grain structure undergoes fragmentation, which gives rise to grain refining (the average grain size is 8 μm). Partial recovery and recrystallization also occur. The fraction of low-angle misorientation boundaries increases up to ≈46%, and that of twin boundaries decreases to ≈25%, compared to the initial state. The yield strength after this treatment reaches up to 477 MPa with elongation-to-failure of 26%. The combination of plastic deformation with heating up to 1100 °C (e = 0.8) and subsequent deformation with heating up to 600 °C (e = 0.7) reduces the average grain size to 1.4 μm and forms submicrocrystalline fragments. The fraction of low-angle misorientation boundaries is ≈60%, and that of twin boundaries is ≈3%. The structural states formed after this treatment provide an increase in the strength properties of steel (yield strength reaches up to 677 MPa) with ductility values of 12%. The mechanisms of plastic deformation and strengthening of metastable austenitic steel under the above high-temperature thermomechanical treatments are discussed. Full article

The feasibility for friction stir welding (FSW) surface-treated AA2099-T83 aluminium extrusions with AA2060-T8E30 aluminium sheets in the overlap configuration and using a sealant at the interface was investigated in this work. New Cr-free surface treatments such as thin film sulphuric acid anodising (TFSAA) and sol–gel were applied to the parent materials, and a sealant was applied before applying the FSW process. FSW welds were produced using several combinations of surface treatments and sealant application with no significant influence on FSW process stability and performance. The metallographic examination of the welds showed that a good protection of the crevice was achieved with some sealant accumulation at the edges of the overlapping region. The microstructural analysis showed no sealant remnants but the presence of some oxide remnants in the stir zone (SZ) of the welds, especially in the TFSAA treated parent material cases. However, these remnants did not show any significant effect in the static pull-out strength of the joints and failures at the most stressed zone of the AA2099-T83 extrusions outside the FSW weld region were consistently obtained. Full article

Soft magnetic materials are characterized by achieving a high magnetic induction value in the presence of a small magnetic field. Common applications of these materials, such as transformers or sensors, are in constant evolution and new requirements are becoming more demanding. Nickel and its alloys are employed as smart materials taking advantage of their superior magnetoelastic properties. A metal injection molding (MIM) technique provides high-quality complex-shaped parts with a good density and controlled impurity levels, which are necessary for these applications, by carefully adjusting the sintering stage. Previous investigations have established a sintering cycle for pure nickel consisting of 1325 ${}^{\circ }$C for 12 h within an N${}_2$-5%H${}_2$ atmosphere. Nevertheless, microstructural, mechanical and magnetoelastic responses can still be greatly enhanced. In this context, the effects of hot isostatic pressing (HIP), and sintering atmosphere have been investigated. The application of an adequate HIP treatment leads to significant improvements in comparison to the reference sintering process. It achieves almost complete densification while increasing field-dependent elastic modulus from 8.1% up to 9.6%. Additionally, the sintering atmosphere has been proven to be a key factor in reducing impurities and hence facilitating magnetic domain motion. Three different atmospheres have been studied: N${}_2$-5%H${}_2$ (with a higher gas flow), N${}_2$-10%H${}_2$-0.1%CH${}_4$ and low vacuum. Minimum carbon contents have been registered using more reducing atmospheres (N${}_2$-5%H${}_2$ and N${}_2$-10%H${}_2$-0.1%CH${}_4$) which has led to values of field-dependent elastic modulus higher than 10%. This value is 2.5 times higher than that obtained when nickel parts are processed via conventional techniques. Moreover, although minimizing carbon content has been shown to be easier and more beneficial than achieving complete densification, both strategies could be used in combination to improve and maximize magnetoelastic performance. Full article

The steel run-off plates attached at the start and end of a weld usually have the same material, shape, and groove as the base metal according to the rules for designing and constructing ship steel structures. However, rather than using the same grade of base steel, readily available steel is often used for these plates to increase material utilization. When using dissimilar grade steel as a run-off plate, the physical properties of the base metal may be diluted if the weld joint of the base plate is penetrated, thereby lowering the physical property value of the joint. If the physical properties of the welded part of the real structure are lowered because of the dilution of the properties of the dissimilar grade steel material, the safety of the entire structure can be affected. Therefore, in this study, analyses of the metal microstructure, hardness, impact, and chemical composition were performed on surfaces where the run-off plate was removed after welding to examine the effect of dissimilar run-off plate. The results of this study reveal that the properties of the run-off plate were not diluted toward the base metal and the Charpy impact value at the weld end was up to 48% greater than the impact value at the start of the weld at a temperature of −40 °C. Full article

In this study, the plastic anisotropy distribution of an extruded 7075 aluminum alloy thick plate was evaluated through small-cube compression tests. The extruded plate with a thickness of 15 mm was divided into five layers in order to verify the difference in plastic anisotropy along the thickness direction of the extruded thick plate. Small-cube specimens with a side length of 1 mm were extracted from each layer and subjected to compression tests in each direction to evaluate the directional r-values of the extruded material. The r-values were applied to Hill’s quadratic yield criterion to calculate the six coefficients for each layer. To consider the plastic anisotropy in the thickness direction, a finite element model divided into five layers in the thickness direction was applied. Upsetting tests were conducted to verify the accuracy of the finite element analysis using cube specimens with a side length of 15 and 10.6 mm, and the results of the finite element analysis and the upsetting test were compared and analyzed against each other. Consequently, the finite element analyses were precisely simulated the upsetting test results. Full article

Joining metallic alloys can be an intricate task, being necessary to take into account the material characteristics and the application in order to select the appropriate welding process. Among the variety of welding methods, pulsed laser technology is being successfully used in the industrial sector due to its beneficial aspects, for which most of them are related to the energy involved. Since the laser beam is focused in a concentrated area, a narrow and precise weld bead is created, with a reduced heat affected zone. This characteristic stands out for thinner material applications. As a non-contact process, the technique delivers flexibility and precision with high joining quality. In this sense, the present review addresses the most representative investigations developed in this welding process. A summary of these technological achievements in metallic metals, including steel, titanium, aluminium, and superalloys, is reported. Special attention is paid to the microstructural formation in the weld zone. Particular emphasis is given to the mechanical behaviour of the joints reported in terms of microhardness and strength performance. The main purpose of this work was to provide an overview of the results obtained with pulsed laser welding technology in diverse materials, including similar and dissimilar joints. In addition, outlook and remarks are addressed regarding the process characteristics and the state of knowledge. Full article

This paper proposes a novel metal additive manufacturing process, which is a composition of gas tungsten arc (GTA) and droplet deposition manufacturing (DDM). Due to complex physical metallurgical processes involved, such as droplet impact, spreading, surface pre-melting, etc., defects, including lack of fusion, overflow and discontinuity of deposited layers always occur. To assure the quality of GTA-assisted DDM-ed parts, online monitoring based on visual sensing has been implemented. The current study also focuses on automated defect classification to avoid low efficiency and bias of manual recognition by the way of convolutional neural network-support vector machine (CNN-SVM). The best accuracy of 98.9%, with an execution time of about 12 milliseconds to handle an image, proved our model can be enough to use in real-time feedback control of the process. Full article

Anisotropic behavior is a key characteristic for understanding eccentricity in tubes. In this paper, the effect of using a tilted die during tube drawing on eccentricity, texture, dislocation density, and mechanical properties is shown. Copper tubes were drawn with a ±5° tilted die for two passes. The increase or decrease in eccentricity can be controlled by controlling the angle of the tilted die. Two types of textures have been developed during tube drawing, namely plane strain and uniaxial types. Plain strain type texture is mainly characterized by the β fiber with a dominant copper component {112}<111>. The uniaxial deformation type is dominated by the <111> fiber, as commonly found by wire drawing. Texture sharpness increases with increasing drawing strain, and the texture varies significantly between the maximum and minimum wall thickness. This texture variation between maximum and minimum wall thickness has no significant influence on mechanical properties, which are more or less similar, but the increase in strength after each drawing pass is apparent. The dislocation density is low for the as-received tubes due to recovery and recrystallization. This is consistent with the as-received texture dominated by the cube component {001}<100>. During tube drawing, dislocation density increases as a function of the deformation strain. The variation of dislocation density between the maximum and minimum wall thickness in the tube deformed with −5° tilted die is higher than the variation in the tube deformed with +5° tilted die. Full article

Hydrogen gas pressure is an important test parameter when considering materials for high-pressure hydrogen applications. A large set of data on the effect of hydrogen gas pressure on mechanical properties in gaseous hydrogen experiments was reviewed. The data were analyzed by converting pressures into fugacities (f) and by fitting the data using an $f^{\left|n\right|}$ power law. For 95% of the data sets, $\left|n\right|$ was smaller than 0.37, which was discussed in the context of (i) rate-limiting steps in the hydrogen reaction chain and (ii) statistical aspects. This analysis might contribute to defining the appropriate test fugacities (pressures) to qualify materials for gaseous hydrogen applications. Full article

Hydro-joining is composed of hydro-piercing, hole flanging and nut-inlaying processes. In this study, a new hydro-flanging process combining hydro-piercing and hydro-flanging is proposed. An internal pressured fluid is used as the supporting medium instead of a rigid die. Three kinds of punch head shapes are designed to explore the thickness distribution of the flanged tube and the fluid leakage effects between the punch head and the flanged tube in the hydro-flanging process. A finite element code DEFORM 3D is used to simulate the tube material deformation behavior and to investigate the formability of the hydro-flanging processes of aluminum alloy tubes. The effects of various forming parameters, such as punch shapes, internal pressure, die hole diameter, etc., on the hydro-flanged tube thickness distributions are discussed. Hydro-flanging experiments are also carried out. The die hole radius is designed to make the maximum internal forming pressure needed smaller than 70 MPa, so that a general hydraulic power unit can be used to implement the proposed hole flanging experiments. The flanged thickness distributions are compared with simulation results to verify the validity of the proposed models and the designed punch head shapes. Full article

Preparing high relative density γ-TiAl alloy by pressure-less sintering at low-cost has always been a challenge. Therefore, a new kind of non-spherical pre-alloyed TiAl powder was prepared by the reaction of TiH₂ powder and Al powder at 800 °C to fabricate high-density Ti-48Al alloy via pressure-less sintering. The oxygen content was controlled to below 1800 ppm by using coarse Al powder (~120 μm). The sintered densities ranged from 92.1% to 97.5% with sintering temperature varying from 1300 °C to 1450 °C. The microstructure of the sintered compact was greatly influenced by the sintering temperature. The as-sintered samples had a near-γ structure at 1350 °C, a duplex structure at 1400 °C, and a nearly lamellar structure at 1450 °C. To achieve full densification, non-capsule hot isostatic pressing was performed on the 1350 °C and 1400 °C sintered samples. As a result, high compressive strengths of 2241 MPa and 1931MPa were obtained, which were higher than the existing Ti-48Al alloys. Full article

This study investigates the influence of resistive pre-heating of the feedstock wire (here called hot-wire) on the stability of laser-directed energy deposition of Duplex stainless steel. Data acquired online during depositions as well as metallographic investigations revealed the process characteristic and its stability window. The online data, such as electrical signals in the pre-heating circuit and images captured from side-view of the process interaction zone gave insight on the metal transfer between the molten wire and the melt pool. The results show that the characteristics of the process, like laser-wire and wire-melt pool interaction, vary depending on the level of the wire pre-heating. In addition, application of two independent energy sources, laser beam and electrical power, allows fine-tuning of the heat input and increases penetration depth, with little influence on the height and width of the beads. This allows for better process stability as well as elimination of lack of fusion defects. Electrical signals measured in the hot-wire circuit indicate the process stability such that the resistive pre-heating can be used for in-process monitoring. The conclusion is that the resistive pre-heating gives additional means for controlling the stability and the heat input of the laser-directed energy deposition. Full article

La_0.8Sr_0.2MnO₃ (LSM) catalyst is prepared via a sol-gel method and modified via a typical silver mirror reaction. Silver ammonia solution is reduced in a polyvinylpyrrolidone (PVP)-containing solution to obtain silver nanoparticles and sodium dodecyl sulfate (SDS) is added as a surfactant. The microstructure and morphology of the LSM-Ag composite powder are characterized. According to the results, the Ag particles precipitate on the LSM surface in elemental form and the grain size is about one hundred nanometers. The analysis of electrocatalytic performance of LSM-Ag cathodes with different amounts of silver loading reveals that the number of electrons transferred during the oxygen reduction reaction (ORR) of the cathode with an Ag content of 14% by weight reached 3.9, which is very close to that of commercial Pt/C catalysts. Similarly, the maximum power density of the air battery made of LSM-14%Ag is 73 mW/cm², which exceeds that of 63 mW/cm², found for the LSM battery. Finally, increasing the amount of silver loading allows one to improve the electrochemical performance of LSM catalysts. The best effect is achieved when the Ag loading exceeds 14%. Full article

The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration. In this study, the effects of Zr contents (0.1 to 0.25 wt.%) on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy were studied. The results showed that Zr element could refine grains, but when the Zr content was greater than 0.15 wt.%, the grain size was not uniform, the number of second phase particles increased, and the segregation of components became more serious. It was found that after ultrasonic treatment, the grain-size inhomogeneity was greatly improved, and the Zr content could be added up to 0.2 wt.%. When the Zr content is equal or lower than 0.2 wt.%, ultrasonic treatment can effectively improve the mechanical properties of materials by refining grains and weakening segregation. However, when the Zr content is up to 0.25 wt.%, the effect is getting worse. Full article

The formation of Zn and Mg segregations at a tilt Σ5{013} <100> grain boundary (GB) in Al and the effects of these solutes on deformation behavior of polycrystalline Al were investigated using ab initio total energy calculations. Using a step-by-step modeling of the segregation process, we found that the formation of a thick segregation layer of Zn at the GB is energetically preferable, while the formation of an atomically thin segregation layer is expected in the case of Mg. To reveal the effect of segregation on the cohesive properties of Al GBs, we calculated the energy of cleavage decohesion and the shear resistance for GB sliding. We show that the segregation of Zn results in a substantial decrease in barriers for GB sliding, while the segregation of Mg increases the barriers. The results obtained allow us to explain experimental findings and demonstrate a strong relationship between chemical bonding of solute atoms, their segregation ability, and GB strength. Full article