Metals

47 pages, 16114 KB

Open AccessReview

Mechanical and Wear Properties of Additive Manufactured Metal Matrix Composites: A Review

by Haris Farooq Kiani, Nan Xiao, Zan Li and Shaofan Ge

Metals 2026, 16(3), 260; https://doi.org/10.3390/met16030260 - 26 Feb 2026

In critical sectors such as energy, transportation, and high-end manufacturing, components must endure simultaneous exposure to high temperatures, heavy loads, and severe wear, necessitating materials with balanced strength, toughness, and durability. Metal matrix composites (MMCs), enhanced with ceramic reinforcements, offer a promising solution [...] Read more.

In critical sectors such as energy, transportation, and high-end manufacturing, components must endure simultaneous exposure to high temperatures, heavy loads, and severe wear, necessitating materials with balanced strength, toughness, and durability. Metal matrix composites (MMCs), enhanced with ceramic reinforcements, offer a promising solution to these multifaceted demands. While conventional techniques like casting and powder metallurgy often struggle with limited design freedom and uniform reinforcement distribution, additive manufacturing (AM) enables the production of complex, graded components with tailored microstructures and unlocks new possibilities for materials operating under extreme service conditions. This review systematically examines recent advances in AM-processed MMCs—focusing on aluminum-, titanium-, nickel-, and steel-based systems—for applications in coupled extreme environments. It provides a detailed analysis of their high-temperature mechanical performance and wear resistance, emphasizing the roles of reinforcement selection, microstructural design, and AM processing parameters in governing key properties. Furthermore, the underlying strengthening and wear mechanisms are discussed, along with current challenges and future opportunities. This work aims to serve as a foundational reference for the development of next-generation AM MMCs tailored for high-performance engineering applications. Full article

(This article belongs to the Special Issue Microstructure and Characterization of Metal Matrix Composites)

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28 pages, 11887 KB

Open AccessArticle

Effect of Layer Thickness and Scanning Parameters on Melt Pool Geometry and Track Continuity in Powder-Bed Arc Additive Manufacturing

by Arif Balci and Fatih Alibeyoglu

Metals 2026, 16(3), 259; https://doi.org/10.3390/met16030259 - 26 Feb 2026

Abstract

Powder-bed arc additive manufacturing (PBAAM) may reduce the cost of powder-bed metal additive manufacturing and enable thicker layers than laser powder bed fusion (LPBF), but melt-track stability limits are not well established. Here, 316L stainless steel powder (15–53 µm) was melted by a [...] Read more.

Powder-bed arc additive manufacturing (PBAAM) may reduce the cost of powder-bed metal additive manufacturing and enable thicker layers than laser powder bed fusion (LPBF), but melt-track stability limits are not well established. Here, 316L stainless steel powder (15–53 µm) was melted by a TIG-based arc in a custom powder-bed system while varying current, travel speed, layer thickness and hatch distance. Single tracks on an inclined bed (≈0–0.4 mm thickness) were used to identify continuity loss and melt-pool width, quantified from top-view images via width profiles, a gap-based continuity metric and the coefficient of variation. Parallel-track tests at 0.15, 0.20 and 0.25 mm layer thickness with hatch distances set to 25%, 50% and 75% of the measured melt-pool width assessed inter-track bonding and lack of fusion, and selected parameters were validated in five-layer builds. Higher current with low-to-moderate travel speeds produced wider, more stable melt pools on the inclined bed. Hatch ratios of 25–50% were the most effective for sustaining fusion in single layers and multi-layer builds, whereas 75% promoted unbonded regions and narrow-track morphologies. Overall, PBAAM can process substantially thicker layers with relatively simple equipment, but requires a narrow, carefully tuned window to balance continuity, fusion and heat accumulation. Full article

(This article belongs to the Special Issue Additive Manufacturing of Metallic Materials: Experiments and Modelling)

27 pages, 10852 KB

Open AccessArticle

Microstructure Evolution and Wear Resistance of TiC-Reinforced H13 Alloy Coatings Fabricated by Laser Cladding on H13 Steel

by Xu Jiang, Shan Gao, Xintian Zhao, Hongyu Zheng, Yongling Wu, Xiaoli Cui and Zongshen Wang

Metals 2026, 16(3), 258; https://doi.org/10.3390/met16030258 - 26 Feb 2026

Abstract

With the growing demand for high-performance die materials under harsh service conditions, the development of composite coatings with enhanced hardness and wear resistance has attracted significant attention. In this study, homogeneous laser cladding was employed to fabricate H13 alloy coatings reinforced with varying [...] Read more.

With the growing demand for high-performance die materials under harsh service conditions, the development of composite coatings with enhanced hardness and wear resistance has attracted significant attention. In this study, homogeneous laser cladding was employed to fabricate H13 alloy coatings reinforced with varying TiC contents (0, 10, 20, and 30 in wt.%) on H13 steel, which minimizes compositional segregation and ensures strong metallurgical bonding. TiC particles acted as heterogeneous nucleation sites during solidification, refining the microstructure and enhancing phase stability. The coatings consisted of initial TiC residues, newly formed primary and eutectic TiC, as well as austenite and martensite phases. With increasing TiC addition, TiC morphology evolved from fine particles to complex fishbone-like and polygonal structures. The coating containing 30% TiC achieved the highest hardness of 1095.9 HV_0.5, approximately five times that of the as-annealed H13 steel substrate while the 20% TiC coating exhibited optimal high-temperature wear resistance. Under the sliding conditions at 600 °C, the friction coefficient decreased from 0.467 for the substrate to 0.367 for the 20% TiC coating, accompanied by a remarkable reduction in wear rate from 27.45 × 10⁻⁴ mm³ N⁻¹ m⁻¹ to 4.32 × 10⁻⁴ mm³ N⁻¹ m⁻¹. The superior performance was attributed to the multiscale TiC reinforcement mechanism: initial TiC promoted grain refinement and strong interfacial bonding, in situ formed primary TiC induced lattice distortion and dislocation strengthening, and eutectic TiC reinforced grain boundaries, jointly enhancing hardness, thermal stability, and wear resistance. Full article

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14 pages, 5746 KB

Open AccessArticle

Electroplating Nickel Coatings on Foam Nickel for Sand Control Screens

by Wenbo Wang, Xinyang Luo, Zhen Pei, Shengchi Bai, Wen Wen, Huidi Yu, Xiaoqi Wang and Xingzhong Guo

Metals 2026, 16(3), 257; https://doi.org/10.3390/met16030257 - 26 Feb 2026

Abstract

Nickel foam filtration layers used in sand control screens for petroleum extraction often suffer from insufficient mechanical strength and poor corrosion resistance and wear resistance. In this work, a two-stage electroplating strategy using the same metal was employed to construct hierarchical nickel coatings [...] Read more.

Nickel foam filtration layers used in sand control screens for petroleum extraction often suffer from insufficient mechanical strength and poor corrosion resistance and wear resistance. In this work, a two-stage electroplating strategy using the same metal was employed to construct hierarchical nickel coatings on nickel foam substrates. The effects of key process parameters, including electroplating time, temperature, and pretreatment, on the microstructure, mechanical properties, electrochemical corrosion behavior, and tribological performance of the coatings were systematically investigated. Electroplating time was found to directly regulate grain size and coating uniformity, while electroplating temperature significantly influenced nickel deposition behavior and electrolyte stability. In addition, UV pretreatment markedly improved the brightness and homogeneity of the deposited layers. Under optimized conditions (UV pretreatment for 10 min, electroplating at 60 °C for 8 min), a dense and uniform nickel coating with a well-ordered crystalline structure was obtained, leading to significantly enhanced hardness, wear resistance, and corrosion resistance. This study presents a practical and highly reliable approach for fabricating high-performance nickel-based coatings on nickel foam filter layers. Anticipated for application in the oil extraction industry, this method is set to enhance the performance of foam metal sand control layers. Full article

(This article belongs to the Special Issue Surface Modification of Metals for Corrosion Mitigation and Functionalization)

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18 pages, 4633 KB

Open AccessArticle

Fatigue Properties of Long-Term Thermally Aged Low-Alloy Steel

by Robert Magnusson, Thomas Damiani and Pål Efsing

Metals 2026, 16(3), 256; https://doi.org/10.3390/met16030256 - 26 Feb 2026

Abstract

Fatigue properties of low-alloy steels, LAS, are well defined in air and at the beginning of life. However, the potential influence from thermal ageing under conditions relevant for the nuclear industry is uncertain. In this study, the fatigue properties of LAS base and [...] Read more.

Fatigue properties of low-alloy steels, LAS, are well defined in air and at the beginning of life. However, the potential influence from thermal ageing under conditions relevant for the nuclear industry is uncertain. In this study, the fatigue properties of LAS base and weld metals, aged at 345 °C for 215,000 h, are compared to as-delivered archive reference materials. In the weld material, ageing appears as an increase in yield and ultimate tensile strength. Ageing also manifests as an inclined strain–cycle (ε-N) fatigue curve, where fatigue life decreases in the low-cycle fatigue region and conversely increases in the high-cycle fatigue region. The results further show that both as-delivered and aged weld metals exhibit a significantly shorter fatigue life in the low-cycle fatigue region and a longer fatigue life in the high-cycle fatigue region when compared to the ASME Code best-fit curve. Full article

(This article belongs to the Section Structural Integrity of Metals)

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17 pages, 27358 KB

Open AccessArticle

Structure and Mechanical Properties of Laves Phase Al_0.5Nb_0.5TiV₂Zr_x (x = 0–2) Refractory High-Entropy Alloys

by Wei Zhao, Shiliang Wu, Haitao Wang, Sujuan Wang and Huiming Wu

Metals 2026, 16(3), 255; https://doi.org/10.3390/met16030255 - 26 Feb 2026

Abstract

Refractory high-entropy alloys (RHEAs) have garnered attention for their exceptional high-temperature mechanical properties, making them suitable for aerospace and energy applications. However, balancing strength and ductility remains a challenge due to the presence of Laves phases. In this study, Al_0.5Nb_0.5 [...] Read more.

Refractory high-entropy alloys (RHEAs) have garnered attention for their exceptional high-temperature mechanical properties, making them suitable for aerospace and energy applications. However, balancing strength and ductility remains a challenge due to the presence of Laves phases. In this study, Al_0.5Nb_0.5TiV₂Zr_x (x = 0–2.0) alloys were prepared using vacuum arc melting, and their microstructural evolution and mechanical properties were analyzed. At room temperature, the Al_0.5Nb_0.5TiV₂Zr_0.5 alloy exhibits the highest yield strength (1658.1 MPa), which is primarily attributed to strong lattice distortion induced by Zr and moderate precipitation strengthening from Laves phases. In contrast, at higher Zr contents, excessive Laves phase precipitation promotes stress concentration, leading to a marked reduction in both strength and ductility. High-temperature compression tests revealed that the Al_0.5Nb_0.5TiV₂Zr_0.5 and Al_0.5Nb_0.5TiV₂Zr_1.5 alloys still exhibited over 50% compressive plasticity at 800 °C and 1000 °C. However, when the temperature reached 1000 °C, the instability of the Laves phase led to a reduction in the yield strength to below 160 MPa, indicating that the effect of solid-solution strengthening was no longer significant under high-temperature conditions. These findings clarify the critical role of Zr content and temperature in governing the microstructural and mechanical evolution of the Al–Nb–Ti–V–Zr system and provide a theoretical basis for achieving an optimized strength–ductility balance in RHEAs through compositional control. Full article

(This article belongs to the Special Issue Advances in High-Entropy Alloys’ Microstructure, Properties and Preparation)

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22 pages, 2246 KB

Open AccessReview

Environmental Impact of Extraction of Rare Earth Elements from Primary Sources and NiMH Batteries: A Literature Review

by Daniel Sánchez Piloto, Denise Crocce Romano Espinosa and Amilton Barbosa Botelho Junior

Metals 2026, 16(3), 254; https://doi.org/10.3390/met16030254 - 26 Feb 2026

Abstract

Rare earth elements (REEs) hold great importance in the transition to a low-carbon economy. However, their increased exploitation, supply risks, low recyclability, and limited substitution by other elements have led to their classification as critical and strategic materials. The extraction of REEs from [...] Read more.

Rare earth elements (REEs) hold great importance in the transition to a low-carbon economy. However, their increased exploitation, supply risks, low recyclability, and limited substitution by other elements have led to their classification as critical and strategic materials. The extraction of REEs from primary mining sources generates several negative environmental impacts, with greenhouse gas emissions being among the most significant. These emissions are quantified through Life Cycle Assessment (LCA) under the Global Warming Potential (GWP) category. Recycling REEs from secondary sources has emerged as a promising alternative to reduce mining dependence and environmental impacts. Nickel–metal hydride (NiMH) batteries contain approximately 5–10% REEs and represent a potential secondary source through urban mining. Our literature review presents a comparative analysis of the carbon footprint associated with the extraction of REEs from primary sources (bastnäsite and monazite), expressed per tonne of rare earth oxides (REO) produced, and with industrial-scale recycling processes of NiMH batteries, expressed per tonne of recovered REE mixture. The analysis indicates that CO₂ emissions associated with recycling processes (85–179 kg CO₂-eq per tonne of REO) are approximately 4 to 9 times higher than those reported for primary extraction routes; however, this comparison should be interpreted with cautiously, as recycling systems are multifunctional and involve the simultaneous recovery of additional metals such as Ni and Co, whereas primary mining operations are typically focused exclusively on REEs. Furthermore, differences in functional units, energy mixes, and geographical contexts limit the strict comparability of the results. Accordingly, a direct comparison based solely on REEs may overestimate the environmental burden of recycling. Consequently, the reported emission ranges provide an indicative perspective on relative magnitudes under current technological and regional conditions rather than a definitive comparative assessment. Despite the higher reported emissions, recycling should not be regarded as environmentally detrimental; it also plays a vital role in mitigating supply risks and reducing dependence on primary extraction. By diversifying supply sources, recycling enhances resource security and resilience. Full article

(This article belongs to the Special Issue Green Solvent Extraction for Critical Metal Recovery)

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23 pages, 7358 KB

Open AccessArticle

Effect of Cr–Ni Co-Alloying on Corrosion Behavior and Rust-Layer Evolution of HRB500 Rebar in Chloride-Containing Environments

by Shasha Zhang, Jing Liu, Weiyong Yang, Xiaotan Zuo, Tianqi Chen, Xiaogang Li and Chao Liu

Metals 2026, 16(3), 253; https://doi.org/10.3390/met16030253 - 26 Feb 2026

Abstract

This study investigated how increased Cr and Ni contents affect the corrosion behavior and rust layer evolution of HRB500 rebar in chloride-containing environments. Corrosion of the Cr- and Ni-alloyed rebars was characterized by distinct stages: in the initial stage, before a stable rust [...] Read more.

This study investigated how increased Cr and Ni contents affect the corrosion behavior and rust layer evolution of HRB500 rebar in chloride-containing environments. Corrosion of the Cr- and Ni-alloyed rebars was characterized by distinct stages: in the initial stage, before a stable rust layer formed, the corrosion rate increased; with continued immersion, corrosion products progressively covered the surface and became more compact, and the overall corrosion rate decreased. Higher Cr and Ni contents were found to mitigate overall corrosion damage, markedly suppress localized corrosion, and shift the corrosion morphology toward a more uniform attack. Electrochemical measurements showed a noble shift in corrosion potential, a reduction in corrosion current density, and significant increases in low-frequency impedance and charge transfer resistance, indicating enhanced barrier properties against charge transfer and ionic migration. With corrosion progression, rust layer phases evolved from an Fe₃O₄-dominated assemblage to enrichment in stable iron oxyhydroxides; the fraction of α-FeOOH increased, raising the α/γ* index and suggesting improved rust layer stability and protectiveness. Mechanistically, Cr and Ni enrichment was found to facilitate the conversion of metastable products to α-FeOOH and to promote the formation of compact spinel oxides FeCr₂O₄ and NiFe₂O₄, thereby hindering chloride ion ingress and interfacial corrosion reactions and markedly improving corrosion resistance. Overall, this work elucidated the Cr–Ni co-alloying mechanism for rust layer stabilization and pitting suppression. At 504 h, the high Cr–Ni rebar reduced the maximum pit depth by approximately 61.8% and lowered i_corr to approximately 43% of that of the low Cr–Ni rebar, thereby providing quantitative guidance for marine-grade rebar design. Full article

(This article belongs to the Special Issue Advances in Corrosion and Protection of Materials (Third Edition))

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16 pages, 4650 KB

Open AccessArticle

Micro-Tensile Characterization of Heterogeneous Girth Welds in Unequal Wall Thickness X80/X60 Pipelines

by Ke Wang, Min Zhang, Junfeng Cao, Chaocheng Tan, Jihong Li, Weifeng Ma, Hailiang Nie and Junjie Ren

Metals 2026, 16(3), 252; https://doi.org/10.3390/met16030252 - 26 Feb 2026

Abstract

The structural integrity of pipeline girth welds is critical, especially when the welds involve heterogeneous materials and non-uniform wall thicknesses. Current evaluation methods that compare the strength of the weld to that of the base metal (BM) are inadequate for such complex welds. [...] Read more.

The structural integrity of pipeline girth welds is critical, especially when the welds involve heterogeneous materials and non-uniform wall thicknesses. Current evaluation methods that compare the strength of the weld to that of the base metal (BM) are inadequate for such complex welds. This paper addresses this gap by applying micro-tensile specimen testing to characteristic zones within heterogeneous girth welds that exhibit non-uniform wall thicknesses. We conducted tensile performance tests on X80 and X60 steel pipes featuring unequal wall thickness butt joints. The analysis focused on the wall thickness direction of the girth weld as well as the transverse direction, examining differences and patterns in performance across various regions. The findings provide an improved understanding of property gradients in heterogeneous girth welds and offer practical guidance for more reliable safety evaluation of pipelines with unequal wall thickness joints. Full article

(This article belongs to the Special Issue Failure Analysis and Evaluation of Metallic Materials)

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16 pages, 3569 KB

Open AccessArticle

Design and Dynamic Characteristics Analysis of Carbon Fiber-Reinforced Metal Composite Spindles with High Length-to-Diameter Ratio

by Ning Li, Haoling Wang, Mingkai Chi, Li Cui, Xin Wang and Jilong Zhao

Metals 2026, 16(3), 251; https://doi.org/10.3390/met16030251 - 26 Feb 2026

Abstract

This paper investigates deflection deformation and premature bearing failure in deep-hole machining spindles with high length-to-diameter ratios under eccentric loading. A contact stiffness model for angular contact ball bearings was developed based on Hertz contact theory. Combined with the finite element method (FEM), [...] Read more.

This paper investigates deflection deformation and premature bearing failure in deep-hole machining spindles with high length-to-diameter ratios under eccentric loading. A contact stiffness model for angular contact ball bearings was developed based on Hertz contact theory. Combined with the finite element method (FEM), a comprehensive mechanical analysis model of the spindle was established. The results show that spindles with high length-to-diameter ratios exhibit significant cantilever behavior, leading to considerable front-end deflection under eccentric loading. This deflection causes the inner and outer rings to incline, resulting in localized stress concentrations, which are the primary contributors to spindle fatigue failure. To improve the spindle’s stress distribution and dynamic performance, an optimized design replacing the metal housing with carbon fiber composite material is proposed. Static and modal analyses were performed using Abaqus and Romax. The analysis results demonstrate that the carbon fiber shell reduces self-weight deformation by 35.8%, decreases coupled deformation under self-weight and grinding loads by 28.6%, and increases modal fundamental frequencies by 20.88% to 47.41%. These improvements significantly enhance structural stiffness and dynamic stability. Experimental vibration monitoring during machine testing validated the accuracy of the modeling and simulation. Full article

(This article belongs to the Special Issue Advances in the Fatigue and Fracture Behaviour of Metallic Materials)

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22 pages, 4767 KB

Open AccessArticle

Study of the Microstructure and Properties of CoCrFeNiMn_x High-Entropy Alloys

by Zhengpeng Zhang, Shichen Yan, Jiankang Huang, Tianxiang Zhao, Chen Dong, Abdul Bari, Jiaojiao Xie, Xiaoquan Yu and Yingwei Chen

Metals 2026, 16(3), 250; https://doi.org/10.3390/met16030250 - 25 Feb 2026

Abstract

High-entropy alloys (HEAs) provide a broad compositional space for tuning phase stability and surface durability. CoCrFeNiMn_x (x = 0.5, 1.0, 1.5, and 2.0) alloys were fabricated by vacuum arc melting and characterized by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy [...] Read more.

High-entropy alloys (HEAs) provide a broad compositional space for tuning phase stability and surface durability. CoCrFeNiMn_x (x = 0.5, 1.0, 1.5, and 2.0) alloys were fabricated by vacuum arc melting and characterized by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), microhardness testing, electrochemical testing in 3.5 wt.% NaCl, and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations and first-principles molecular dynamics were further employed to analyze the Mn-dependent electronic structure and oxygen–metal bonding. The XRD results indicate a transition from a single FCC solid solution at x ≤ 1.0 to an FCC + BCC constitution at x ≥ 1.5. With increasing Mn, microstructures evolve from coarse dendrites toward higher fractions of equiaxed grains. Hardness decreases from 163.6 HV (x = 0.5) to 125.1 HV (x = 1.0) and then increases to 162.6 HV (x = 2.0), indicating competing solid-solution and phase/segregation effects. Electrochemical measurements show enhanced corrosion resistance with Mn addition; the x = 2.0 alloy exhibits the lowest fitted corrosion current density (icorr = 0.3482 × 10⁻⁶ μA·cm⁻²) and the most stable passivation response. XPS reveals passive films dominated by Fe₂O₃ together with Mn³⁺ oxides, whose synergistic formation promotes a denser barrier layer. DFT predicts a monotonic decrease in Fermi level and a narrowed conduction band range as Mn increases, consistent with reduced electron transfer activity during anodic dissolution. Interfacial simulations show that O preferentially bonds with Cr and Mn, while Ni–O bonds have the lowest estimated rupture barrier, rationalizing a tendency toward localized corrosion at Ni-associated sites. Full article

13 pages, 1289 KB

Open AccessArticle

Machine Learning-Based Prediction of High Cycle Fatigue and Fatigue Crack Growth Rate in LPBF Co-Cr-Mo Alloys Under Varying Scanning Strategies

by Vinod Kumar Jat, Roshan Udaram Patil, Manish Kumar and Denis Benasciutti

Metals 2026, 16(3), 249; https://doi.org/10.3390/met16030249 - 25 Feb 2026

Abstract

This study explores the use of machine learning to predict high-cycle fatigue (HCF) behavior and fatigue crack growth rate (FCGR) in Co-Cr-Mo alloys manufactured through laser powder bed fusion. Two machine learning (ML) models: extreme gradient boosting (XGB) and deep neural networks (DNN), [...] Read more.

This study explores the use of machine learning to predict high-cycle fatigue (HCF) behavior and fatigue crack growth rate (FCGR) in Co-Cr-Mo alloys manufactured through laser powder bed fusion. Two machine learning (ML) models: extreme gradient boosting (XGB) and deep neural networks (DNN), are implemented to estimate HCF and FCGR across three distinct scanning strategies. The raw datasets for HCF and FCGR are taken from previously performed experiments. The HCF dataset is augmented using a Gaussian Mixture Model, while the FCGR dataset is used in its raw form. Following hyperparameter optimization, both models exhibited quite similar accuracy on validation datasets. Their performance is assessed during testing using mean squared error (MSE) and R² scores. The DNN model demonstrated higher accuracy in HCF predictions by achieving higher R² scores. The DNN performs better because it can handle more complex patterns effectively due to its multiple neurons and deeper multilayer architecture. In contrast, the XGB model performed better in FCGR predictions and yielded higher R² scores compared to XGB. The good agreement with the experimental dataset shows that these two ML techniques are effective in predicting HCF and FCGR behavior. Full article

(This article belongs to the Special Issue Fatigue and Fracture Behavior of Metallic Components and Structures Under Various Loading Conditions)

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16 pages, 2789 KB

Open AccessArticle

Copper Recovery from Smelting Slags by Glycine Leaching: Influence of Slag Mineralogy and Ferromanganese Crusts

by Mauricio Mura, Norman Toro, Edelmira Gálvez, Sandra Gallegos, Felipe M. Galleguillos Madrid, Susana Leiva-Guajardo, Williams Leiva, Alessandro Navara, Pia Hernández and Jonathan Castillo

Metals 2026, 16(3), 248; https://doi.org/10.3390/met16030248 - 25 Feb 2026

Abstract

Copper smelting slags represent a growing environmental and metallurgical challenge due to their large volumes and their content of unrecovered critical metals such as copper. Although conventional treatment relies mainly on acidic leaching, more sustainable hydrometallurgical routes are required to valorise these residues. [...] Read more.

Copper smelting slags represent a growing environmental and metallurgical challenge due to their large volumes and their content of unrecovered critical metals such as copper. Although conventional treatment relies mainly on acidic leaching, more sustainable hydrometallurgical routes are required to valorise these residues. In this study, an alternative copper extraction process is proposed based on alkaline glycine complexation and the use of ferromanganese crusts as an unconventional oxidising agent. Leaching tests were performed using two slags (A and B) at ambient conditions. Copper recoveries up to 59.7% (slag A) and 25.7% (slag B) were achieved at 1 M glycine without external oxidants. The addition of ferromanganese crusts (1:1 and 2:1) resulted in marginal increases (up to 61.1% and 29.1%, respectively), attributed to the limited oxidative performance of MnO₂ at near-neutral pH. The results demonstrate that glycine is a viable lixiviant for copper recovery from slags at room temperature and highlights, for the first time, the use of naturally occurring Fe–Mn crusts as oxidants in alkaline leaching systems. This work contributes to the development of more sustainable valorisation strategies for metallurgical slags and offers a basis for future optimisation of alkaline complexation routes. Full article

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13 pages, 6820 KB

Open AccessArticle

Effect of Base Metal Microstructure on Softening Behavior of the Heat-Affected Zone of X80 GMAW Girth Weld

by Xueda Li, Zhangyi She, Xunyun Lv, Zeyang Zhang, Liying Li and Bin Han

Metals 2026, 16(3), 247; https://doi.org/10.3390/met16030247 - 25 Feb 2026

Abstract

Softening in the heat-affected zone (HAZ) of high-strength pipeline welds compromises its service safety but the corresponding softening mechanism is not well-understood. Softening behavior in the HAZ of two X80 pipeline girth welds with different base metal microstructures, i.e., acicular ferrite (AF)-dominated (X80-AF) [...] Read more.

Softening in the heat-affected zone (HAZ) of high-strength pipeline welds compromises its service safety but the corresponding softening mechanism is not well-understood. Softening behavior in the HAZ of two X80 pipeline girth welds with different base metal microstructures, i.e., acicular ferrite (AF)-dominated (X80-AF) and granular bainite (GB)-dominated (X80-GB), were investigated through microhardness tests and detailed microstructure characterization. The results showed that softening in the HAZ of two girth welds primarily occurred in the fine-grained (FG) HAZ, while hardening was found in the coarse-grained (CG) HAZ. X80-AF showed higher softening resistance than X80-GB, with softening ratios of 3.44% vs. 12.46%, and softened zone widths of 2.1 mm vs. 3.9 mm, respectively. Due to its high dislocation density and refined interlocking structure, AF could effectively inhibit phase transformation and grain coarsening during reheating, which resulted in smaller grains and a lower fraction of polygonal ferrite (PF) in the FGHAZ (28%). In contrast, coarse GB was more prone to grain coarsening and hence engendered higher PF proportion (68%). Therefore, for the microstructural design of high-strength pipeline steels, increasing the proportion of refined AF is beneficial to the softening resistance and thereby elevates the service safety of pipelines. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Welding and Joining” (2nd Edition))

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10 pages, 2212 KB

Open AccessArticle

The Relationship Between Hardness and Microstructure in Zn/Mg Ratio-Controlled Al–Zn–Mg Alloys Aged at 120 °C

by Wanlalak Sanphiboon, Seungwon Lee, Taiki Tsuchiya, Abrar Ahmed, Susumu Ikeno, Tomoo Yoshida and Kenji Matsuda

Metals 2026, 16(3), 246; https://doi.org/10.3390/met16030246 - 25 Feb 2026

Abstract

Al–Zn–Mg alloys are widely recognized for their high strength-to-weight ratio, with the primary strengthening precipitates being the η/η′ and T/T′ phases. In this study, Al–Zn–Mg alloys with Zn/Mg molar ratios of 0.17, 0.40, 0.75, 1.3, 2.5, and 6.0 were systematically investigated after aging [...] Read more.

Al–Zn–Mg alloys are widely recognized for their high strength-to-weight ratio, with the primary strengthening precipitates being the η/η′ and T/T′ phases. In this study, Al–Zn–Mg alloys with Zn/Mg molar ratios of 0.17, 0.40, 0.75, 1.3, 2.5, and 6.0 were systematically investigated after aging at 120 °C. η′/η precipitates predominantly strengthened alloys with high Zn/Mg ratios, whereas T′/T precipitates dominated those with low Zn/Mg ratios. In contrast, alloys with an intermediate Zn/Mg ratio (Zn/Mg ≈ 1.3) exhibited a balanced coexistence of η′/η and T′/T phases, resulting in the highest hardness among the six alloys. In addition, novel precipitates were observed, with their length increasing as the Zn/Mg ratio decreased. However, because these novel precipitates constitute only a small fraction of the total precipitate population, their direct contribution to the overall hardness remains unclear and warrants further investigation. Full article

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27 pages, 5691 KB

Open AccessArticle

Development and Characterization of the Performance of a Novel Machinability-Enhancing Additive for Powder Metallurgy Steels

by Amin Molavi Kakhki and Carl Blais

Metals 2026, 16(3), 245; https://doi.org/10.3390/met16030245 - 25 Feb 2026

Abstract

Although powder metallurgy (PM) is known as a near-net-shape fabrication process, a large number of PM parts need to be machined for dimensional conformance or to produce complex geometrical features that cannot be achieved through compaction. However, due mainly to the presence of [...] Read more.

Although powder metallurgy (PM) is known as a near-net-shape fabrication process, a large number of PM parts need to be machined for dimensional conformance or to produce complex geometrical features that cannot be achieved through compaction. However, due mainly to the presence of porosity, the machinability of PM steels is difficult compared to that of wrought steels and can add 20% or more to the overall fabrication cost of PM parts. Among the various measures known to improve the machinability of PM steels, the addition of machining aids, either as admixed or pre-alloyed constituents, is the most popular. Manganese sulfide (MnS) is by far the most common machinability-enhancing additive used in the PM steel industry. Although it is extremely efficient in improving the machining response of PM steels, MnS is known to have detrimental effects on mechanical properties and corrosion resistance. Thus, the use of MnS involves a compromise between obtaining good machinability at the expense of lower mechanical properties and corrosion resistance. In this study, free graphite particles are introduced as a new additive that not only noticeably improves the machinability of PM steel components but also does not affect their mechanical properties or corrosion resistance. It was found that it is possible to obtain free graphite particles in press-and-sintered PM steel components by coating graphite particles with a metallic layer. This coating prevents graphite from diffusing into the iron matrix while creating metallurgical bonds with the surrounding steel matrix during sintering. In this research, graphite particles were coated with nickel and copper through a cementation process. A heat treatment was then performed on this newly developed material to obtain a more uniform single-layer coating and achieve dimensional changes during sintering that are similar to those measured when MnS is used as a machinability enhancer. The results showed that the tensile properties as well as the fatigue resistance of components made of FC-0208-type PM steel containing admixed copper/nickel-coated graphite particles are not affected by the presence of the latter. Moreover, the corrosion resistance of the samples containing copper/nickel-coated graphite was found to be the same as that of samples without the additive, which is a significant improvement on the case where MnS is used. The performance of the newly developed additive in terms of machinability was also characterized in drilling. It was found that this new additive has an identical machinability-enhancing performance to admixed MnS. Full article

(This article belongs to the Section Powder Metallurgy)

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18 pages, 5866 KB

Open AccessArticle

Crystal Plasticity Simulation of the Effect of γ Lamellae on the Plastic Behavior of the Core–Shell-like Structured TiAl Alloy

by Zihe Xu, Meini Yuan, Yonghao Yu, Lezhang Yin, Judong Guo, Rui Wang and Meng Yuan

Metals 2026, 16(3), 244; https://doi.org/10.3390/met16030244 - 24 Feb 2026

Abstract

The preparation of the core–shell-like structured before hot working can significantly enhance the hot workability of the alloy. In order to research the properties of the alloy, the finite element method combined with the crystal plasticity constitutive theory was used to establish the [...] Read more.

The preparation of the core–shell-like structured before hot working can significantly enhance the hot workability of the alloy. In order to research the properties of the alloy, the finite element method combined with the crystal plasticity constitutive theory was used to establish the finite element model of the core–shell-like structured TiAl alloy with (α₂ + γ) lamellae colonies as the core and α₂ matrix as the shell. The research focuses on the influence of the length and number of γ lamellae on the stress–strain distribution and the contribution of slip systems in each phase to the plasticity of the alloy. The results show that when the γ lamella length increases from 12 μm to 16 μm, the overall stress decreases by 12.0%; when the number increases from 6 to 10, the stress decreases by 7.7%. The stress reduction is primarily influenced by the α₂ phase. Increasing the volume fraction of γ lamellae facilitates stress distribution within the α₂ phase and enhances the plasticity of the material. In the γ phase O4, S1 and S7 slip systems contribute the most to the plastic deformation of the γ phase. In the α₂ phase, the B1 slip system is the main contributor to the plasticity of the α₂ phase. And the B1 slip system contributes more significantly to the plastic deformation of the entire model. Full article

(This article belongs to the Special Issue Intermetallic Compounds and Their Composites Materials)

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22 pages, 4323 KB

Open AccessArticle

Effect of Tempering on Microstructure, Strength and Toughness Gradient in Quenched Low-Alloy Medium-Thickness Steel Plate

by Boyu Guan, Shaobin Bai, Yongqing Zhang, Peimao Fu, Haitao Lu, Hejia Zhu, Xingchi Chen, Kaikai Guo, Haonan Wang and Yongan Chen

Metals 2026, 16(3), 243; https://doi.org/10.3390/met16030243 - 24 Feb 2026

Abstract

To elucidate how tempering temperature influences through-thickness microstructure and strength–toughness gradients in an online direct-quenched (DQ) low-alloy medium-thick plate, a 25-mm plate was direct-quenched from 900 °C to <150 °C and tempered at 530 °C × 1.5 h or 580 °C × 1.5 [...] Read more.

To elucidate how tempering temperature influences through-thickness microstructure and strength–toughness gradients in an online direct-quenched (DQ) low-alloy medium-thick plate, a 25-mm plate was direct-quenched from 900 °C to <150 °C and tempered at 530 °C × 1.5 h or 580 °C × 1.5 h. Tensile and room-temperature Charpy V-notch impact testing and microstructure characterization were performed at the upper surface, mid-thickness, and lower surface. In the as-DQ state, the upper surface contained ferrite (F, ~60%) and granular bainite (GB, ~30%) with minor lath bainite (LB, ~10%) and a small amount of martensite/austenite (M/A). The mid-thickness and lower surface remained dominated by F + GB (mid-thickness: GB~50%, F~30%, M/A~20%; lower surface: F~85%, GB~15%); the mid-thickness showed the lowest yield strength/ultimate tensile strength (YS/UTS) of 498/675 MPa. In the as-DQ state, the room-temperature Charpy V-notch absorbed energies at the upper surface, mid-thickness, and lower surface were 223.23, 229.88, and 261.22 J, respectively, indicating a pronounced through-thickness variation (ΔE_(max–min) ≈ 38 J). After tempering at 530 °C, the upper surface and mid-thickness developed an F + tempered sorbite (TS) microstructure (upper surface: F~70%, TS~30%; mid-thickness: F~60%, TS~40%), whereas the lower surface was mainly ferrite with a small amount of spheroidized carbides/tempered cementite (SC). The mid-thickness YS/UTS increased to 619/805 MPa, and the impact energies at the upper surface and mid-thickness increased to 240.62 J and 235.56 J, respectively, resulting in a reduced through-thickness gradient. After 580 °C tempering, recovery and polygonal ferrite formation dominated; surface yield strength increased but mid-thickness yield improvement was limited. Full article

(This article belongs to the Special Issue Heat Treatment, Microstructures, and Mechanical Properties of Metallic Materials)

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19 pages, 7242 KB

Open AccessArticle

Artificial Neural Network-Based Optimisation of Geometric Characteristics in Laser Metal Deposition of TiC/Ti6Al4V

by Thabo Tlale, Peter Mashinini and Bathusile Masina

Metals 2026, 16(3), 242; https://doi.org/10.3390/met16030242 - 24 Feb 2026

Abstract

Laser metal deposition operates on the principle of layer-by-layer material addition, wherein each layer is formed by overlapping individual single tracks. Consequently, clads formed serve as the fundamental building blocks for this technology. Their quality directly affects the overall build quality, particularly the [...] Read more.

Laser metal deposition operates on the principle of layer-by-layer material addition, wherein each layer is formed by overlapping individual single tracks. Consequently, clads formed serve as the fundamental building blocks for this technology. Their quality directly affects the overall build quality, particularly the geometric characteristics, which are also critical to process productivity. In the present work, geometric characteristics of TiC/Ti6Al4V single tracks fabricated via laser metal deposition are optimised. An artificial neural network model was developed to predict the clad width, height, and dilution using processing parameters, laser power, scan speed, and powder feed rate, as model inputs. The Particle Swarm Optimisation algorithm was employed for hyperparameter selection. The hyperparameter-optimised model achieved a mean squared error of 0.00183 and an R² score of 0.979 during training, and a mean squared error of 0.00709 and an R² score of 0.887 during testing. Although the small discrepancy between training and testing metrics suggests slight overfitting, likely due to the size of the dataset, the model achieved a mean absolute percentage error of less than 10% during testing. Subsequently, process plots generated by the model predictions were used to identify suitable parameters, and a processing map was developed to highlight the window that achieves suitable dilution (14–24%), defect-free sound bonding, and thick and dense clads. Full article

(This article belongs to the Special Issue Additive Manufacturing and Characterization of Metallic Alloys and Composites)

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