Metals

23 pages, 14978 KB

Open AccessArticle

A Dual-Physics-Informed Neural Network with Incremental Learning for Corrosion Fatigue Crack Growth Prediction in Aluminum Alloys

by Yongzhen Zhang, Xinyu Feng, Dongxu Zhang, Haitao Wang, Leijiang Yao and Zhenshuang Wu

Metals 2026, 16(9), 1009; https://doi.org/10.3390/met16091009 - 10 Sep 2026

Abstract

Aluminum alloys used in aircraft structures are susceptible to corrosion fatigue cracking under combined aggressive environments and cyclic loading, threatening structural integrity. Pure data-driven models often fail under distribution shifts, while single-physics-informed neural networks (PINNs) lack flexibility in complex conditions. This paper proposes [...] Read more.

Aluminum alloys used in aircraft structures are susceptible to corrosion fatigue cracking under combined aggressive environments and cyclic loading, threatening structural integrity. Pure data-driven models often fail under distribution shifts, while single-physics-informed neural networks (PINNs) lack flexibility in complex conditions. This paper proposes a dual-physics-informed neural network (DPINN) that integrates Walker and Forman crack growth models into a deep residual network. The model adaptively fuses both physical formulas via a trainable weight α and predicts material constants. A hybrid loss function with α regularization ensures physically consistent predictions. Using comprehensive corrosion fatigue data covering eight aluminum alloys, we evaluate the model on an internal test set and, more importantly, on an independent external test set simulating real-world distribution shifts. We further investigate an incremental learning scenario where the model is sequentially fine-tuned with increasing fractions of the external set. Results demonstrate that DPINN rapidly rectifies initial distribution mismatch, crossing the engineering reliability threshold (R² > 0.90) at an early incremental stage, and achieves superior performance after fine-tuning, significantly outperforming both a single Walker-PINN and gradient boosting regressors. SHAP feature importance analysis identifies ΔK and stress ratio as dominant drivers, confirming mechanistic consistency. The proposed architecture offers a data-efficient and interpretable tool for corrosion fatigue crack growth prediction in aluminum alloy structures. Full article

(This article belongs to the Section Corrosion and Protection)

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12 pages, 15665 KB

Open AccessArticle

Simulation and Experimental Study on Electrochemical Machining for Nickel-Based High-Temperature Alloy Turbine Blades

by Yaowu Zhou, Yang Liu, Mingzhu Ren and Zhaozhi Wu

Metals 2026, 16(9), 1008; https://doi.org/10.3390/met16091008 - 10 Sep 2026

Abstract

Electrochemical machining is widely recognized as a high-efficiency, low-cost and high-precision non-traditional machining technology for the manufacturing of turbine blade components. Nevertheless, in the practical electrochemical machining of turbine blades, the uneven spatial distribution of electric field intensity within the inter-electrode machining gap [...] Read more.

Electrochemical machining is widely recognized as a high-efficiency, low-cost and high-precision non-traditional machining technology for the manufacturing of turbine blade components. Nevertheless, in the practical electrochemical machining of turbine blades, the uneven spatial distribution of electric field intensity within the inter-electrode machining gap inevitably causes inconsistent anodic dissolution, which significantly deteriorates the final surface quality of machined blades. A set of comparative machining experiments were conducted on Inconel 625 superalloy. The experimental results fully verified that pulsed current machining could effectively improve the surface integrity and surface quality, and the optimal matching electrical parameters were successfully determined through systematic data analysis. The essential improvement mechanism lied in the effective suppression of stray current-induced scattered dissolution under pulsed power supply, which was highly consistent with the numerical simulation conclusions. Full article

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

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36 pages, 22579 KB

Open AccessReview

From Conventional Dispersion Strengthening to Core–Shell Reinforcement Architectures in Iron-Based Metal Matrix Composites: A Review

by Petr M. Korusenko, Vladimir K. Kudymov, Vladimir E. Gaishun and Elena G. Zemtsova

Metals 2026, 16(9), 1007; https://doi.org/10.3390/met16091007 - 10 Sep 2026

Abstract

Despite extensive research on iron-based metal matrix composites (Fe-based MMCs), an integrated assessment linking fabrication methods, reinforcement architectures, interface engineering, and strengthening mechanisms remains limited. This review summarizes recent advances in Fe-based MMCs, with emphasis on the relationships among processing routes, microstructural evolution, [...] Read more.

Despite extensive research on iron-based metal matrix composites (Fe-based MMCs), an integrated assessment linking fabrication methods, reinforcement architectures, interface engineering, and strengthening mechanisms remains limited. This review summarizes recent advances in Fe-based MMCs, with emphasis on the relationships among processing routes, microstructural evolution, reinforcement characteristics, and mechanical performance. Powder metallurgy, casting, and additive manufacturing are critically compared in terms of their processing characteristics, advantages, limitations, and suitability for iron-based systems. The effects of reinforcement size, morphology, distribution, and volume fraction on composite performance are discussed. Particular attention is given to interface engineering strategies and architectured core–shell reinforcements produced through in situ reactions and solid-state diffusion, infiltration, laser cladding, sol–gel coating combined with additive manufacturing, electrochemical synthesis, and high-energy ball milling. Recent studies indicate that core–shell architectures can offer enhanced control of reinforcement–matrix interactions by combining hard ceramic or carbide phases with more ductile metallic components. Rod-like Me@MeC/Fe (Me = Ta, Nb, W) architectures and dispersed core–shell particles show promising combinations of strength, toughness, and wear resistance, although their performance depends strongly on shell architecture, interface characteristics, and processing conditions. Remaining challenges include reproducible and scalable fabrication, shell architecture control, interface stability, and long-term performance. Further progress may benefit from advanced reinforcement design, additive manufacturing, modelling, and AI-assisted optimization of high-performance Fe-based MMCs. Full article

(This article belongs to the Section Metal Matrix Composites)

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18 pages, 3310 KB

Open AccessArticle

Investigation of Fluorine Distribution and Pre-Defluorination in Hydrometallurgical Recycling Processes for Lithium-Ion Batteries

by Matthias Markus Mandl, Eva Gerold and Helmut Antrekowitsch

Metals 2026, 16(9), 1006; https://doi.org/10.3390/met16091006 - 10 Sep 2026

Abstract

The growing demand for lithium-ion batteries (LIBs), particularly in the field of electromobility, is leading to an increasing need for efficient and sustainable recycling processes. In addition to the recovery of strategically important metals, such as lithium, nickel, cobalt, and manganese, the fluorine [...] Read more.

The growing demand for lithium-ion batteries (LIBs), particularly in the field of electromobility, is leading to an increasing need for efficient and sustainable recycling processes. In addition to the recovery of strategically important metals, such as lithium, nickel, cobalt, and manganese, the fluorine content also poses a challenge. Fluorine enters the battery system primarily via the conductive salt lithium hexafluorophosphate (LiPF₆) and via fluorine-containing binders, such as polyvinylidene fluoride (PVDF). During hydrometallurgical recycling, fluorine can transfer into the leaching solution, where it can negatively impact metal recovery and product quality, e.g., by forming stable fluoride compounds. Furthermore, this halogen exhibits environmentally problematic and health-hazardous properties. This study investigates the distribution of fluorine and the influence of defluorination strategies on hydrometallurgical recycling processes for end-of-life lithium-ion batteries. For this purpose, the fluorine content in each process step from two different black masses, one pyrolysed and one unpyrolysed sample, are analysed. To determine their behaviour and the effect of pretreatment, these experiments are also conducted with defluorinated black masses. Wet-chemical defluorination in an alcoholic potassium hydroxide solution was used as the defluorination method, which was then compared to the untreated variants as a reference. After pretreatment, the hydrometallurgical recycling process of the black masses starts by leaching with sulphuric acid, as well as the addition of hydrogen peroxide as a reducing agent. The pregnant leach solutions are further processed using multi-stage precipitation processes to selectively recover nickel, cobalt, manganese, and lithium. The fluorine content in the individual process steps is measured in order to evaluate the fluorine distribution throughout the process. In combination with the effects of the respective pretreatments on the fluorine distribution, a broader understanding of fluorine behaviour in hydrometallurgical recycling of lithium-ion batteries can be gained. Full article

(This article belongs to the Special Issue Advances in Sustainable Metal Extraction, Recovery, and Recycling)

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20 pages, 3694 KB

Open AccessArticle

Plasma Characteristics in Underwater Laser Welding via In Situ Observation and Optical Emission Spectroscopy

by Chengyong Ma, Jie Su, Yang Yang, Qiren Zhao, Qing Guo, Manpeng Wu and Zhen Luo

Metals 2026, 16(9), 1005; https://doi.org/10.3390/met16091005 - 9 Sep 2026

Abstract

During underwater laser welding, the coupling among water cooling, gas–liquid interface disturbances, and metal-vapor recoil pressure means that the relationships among plasma evolution, the thermal state of the keyhole, and spatter behavior remain insufficiently understood. In this study, 304NG stainless steel was welded [...] Read more.

During underwater laser welding, the coupling among water cooling, gas–liquid interface disturbances, and metal-vapor recoil pressure means that the relationships among plasma evolution, the thermal state of the keyhole, and spatter behavior remain insufficiently understood. In this study, 304NG stainless steel was welded in air and under local dry underwater conditions using a 10 kW continuous-wave fiber laser with a wavelength of 1070 nm to investigate the effects of laser power on the energy state and process stability of underwater welding. The underwater experiments were conducted in deionized water, with the workpiece positioned approximately 100 mm below the free water surface and without additional pressurization. High-speed imaging, infrared thermography, and spectroscopy were employed to characterize the transient evolution of the plasma and spatter, the apparent thermal state near the keyhole opening, and the spectral characteristics of the underwater plasma, respectively, while the plasma excitation temperature was calculated using the Boltzmann multi-line fitting method. The results showed that as the laser power increased from 2000 to 4000 W, the plasma size and temporal persistence increased significantly in both air and underwater environments; the transverse width of the underwater plasma at T₀ + 0.8 ms increased from approximately 2.7 to 5.8 mm, while the average maximum apparent temperature near the keyhole opening increased from approximately 2477 to 3071 °C. Meanwhile, the intensities of the characteristic Fe I lines increased overall, and the plasma excitation temperature increased from 5977 to 6510 K, consistent with the expansion of the plasma-emitting region, the enhanced persistence of the high-temperature core, and the increase in the apparent temperature near the keyhole opening. This study aims to provide a new systematic understanding and technical insights into plasma evolution during underwater laser welding. Full article

(This article belongs to the Special Issue Research Progress of Laser Welding Technology of Metals and Alloys)

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19 pages, 28555 KB

Open AccessArticle

Tribological Behavior of Binder-Jetted STS316L Sintered Bodies and Cu-Infiltrated Composites

by Taehwa Kim, Minji Kim, Kyung Il Kim and Kyung-Taek Kim

Metals 2026, 16(9), 1004; https://doi.org/10.3390/met16091004 - 9 Sep 2026

Abstract

Binder-jetted metallic components retain residual porosity after sintering, which can substantially affect their mechanical and tribological performance. This study clarifies how sintering-induced pore evolution and Cu infiltration influence the wear mechanisms of binder-jetted STS316L. STS316L specimens were sintered at 1100, 1200, and 1300 [...] Read more.

Binder-jetted metallic components retain residual porosity after sintering, which can substantially affect their mechanical and tribological performance. This study clarifies how sintering-induced pore evolution and Cu infiltration influence the wear mechanisms of binder-jetted STS316L. STS316L specimens were sintered at 1100, 1200, and 1300 °C, and corresponding Cu/STS316L composites were fabricated by molten Cu infiltration. Their microstructures and wear behavior were analyzed using Filed-emission scanning electron microscopy (FE-SEM), Energy-dispersive X-ray spectroscopy (EDS), Confocal laser scanning microscopy (CLSM), X-ray diffraction (XRD), and cross-sectional Transmission electron microscopy (TEM). As the sintering temperature increased, the relative density and hardness of the sintered specimens increased from 72.75% to 80.69% and from 194 to 243.8 HV, respectively. Nevertheless, their average coefficient of friction increased from approximately 0.25 to 0.71, while the wear width and maximum depth increased from 932.8 to 1492.0 μm and from 24.7 to 60.3 μm, respectively. Abrasive and adhesive wear predominated at 1100 and 1200 °C, whereas repeated formation, cracking, and delamination of an approximately 2.5 μm thick oxygen-rich tribolayer caused severe oxidative delamination at 1300 °C. Cu infiltration increased the relative density to 90.68–95.69%. The composites exhibited similar friction behavior, with an average coefficient of friction of approximately 0.6, and were governed primarily by adhesive wear associated with Cu transfer and plastic deformation. These results demonstrate that increased densification does not necessarily improve wear resistance and that pore structure and Cu transfer critically determine the dominant wear mechanisms. Full article

(This article belongs to the Section Additive Manufacturing)

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24 pages, 4593 KB

Open AccessArticle

Selective Lithium Pre-Leaching from Pyrolyzed Industrial Lithium-Ion Battery Black Masses Using Dilute Organic and Inorganic Acids

by Eva Gerold, Manuel Schmid and Helmut Antrekowitsch

Metals 2026, 16(9), 1003; https://doi.org/10.3390/met16091003 - 9 Sep 2026

Abstract

Selective lithium pre-leaching can separate an accessible lithium fraction before complete black-mass dissolution while limiting the co-dissolution of accompanying elements. However, comparative evidence across compositionally different, industrially pyrolyzed black masses under a consistent operating framework remains limited. To address this gap, three such [...] Read more.

Selective lithium pre-leaching can separate an accessible lithium fraction before complete black-mass dissolution while limiting the co-dissolution of accompanying elements. However, comparative evidence across compositionally different, industrially pyrolyzed black masses under a consistent operating framework remains limited. To address this gap, three such black masses were systematically investigated using water and dilute inorganic and organic acids (0.1–0.5 mol·L⁻¹, 25–80 °C) without an external reducing agent. Water-leaching experiments were additionally performed to quantify the readily soluble lithium fraction. Lithium extraction after 120 min varied strongly with black mass origin and leaching conditions. Water leaching extracted 12–16% Li from BM1, 24–28% from BM2, and 36–47% from BM3. Under the most favourable water-leaching conditions, transition-metal dissolution remained negligible, resulting in substantially higher selectivity than in most acidic systems. Dilute acids increased lithium extraction in selected experiments, reaching up to 98%, but this was generally accompanied by increased dissolution of Co, Ni, Mn, Cu, and Al. The results demonstrate that lithium accessibility is governed more strongly by feed-specific phase composition and material history than by total lithium content. Selective pre-leaching should therefore be considered a feed-dependent conditioning step whose operating conditions must balance lithium removal against preservation of the transition-metal-rich solid fraction. Full article

(This article belongs to the Section Extractive Metallurgy)

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21 pages, 23141 KB

Open AccessArticle

by Jessica Rawles, Mohammed Abdelmaola, Kai Hubbard, Svitlana Fialkova, Christopher Hale, Zhigang Xu, Jagannathan Sankar and Glenn Daehn

Metals 2026, 16(9), 1002; https://doi.org/10.3390/met16091002 - 9 Sep 2026

Abstract

Joining dissimilar lightweight metals, such as magnesium (Mg) and titanium (Ti), presents critical challenges due to their differing physical properties and limited mutual solubility. This research explores the potential of laser impact welding (LIW) to overcome these barriers by investigating the effects of [...] Read more.

Joining dissimilar lightweight metals, such as magnesium (Mg) and titanium (Ti), presents critical challenges due to their differing physical properties and limited mutual solubility. This research explores the potential of laser impact welding (LIW) to overcome these barriers by investigating the effects of aluminum (Al) content in Mg-Al alloys, interlayer integration, and the formation of intermetallic phases at the bond interface. Emphasis is placed on understanding how variations in Al composition and the inclusion of an Al interlayer influence the bonding mechanisms between Mg and Ti. Through detailed microstructural analysis and phase identification, this study characterizes the intermetallic phases present at the weld interface, examining their size, distribution, and location. The presence of an Al interlayer was found to significantly improve bonding success, promoting favorable interface morphology and minimizing detrimental intermetallic formation. These findings provide key insights into the underlying mechanisms enabling successful LIW of dissimilar lightweight metals and offer valuable guidance for industrial applications seeking to advance joining strategies in high-performance alloy systems. Full article

(This article belongs to the Section Welding and Joining)

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23 pages, 7371 KB

Open AccessArticle

Multi-Objective Optimization of the Hot Stamping Process for B1500HS Ultra-High-Strength Steel Based on a Four-Dimensional Quality Evaluation Framework

by Wei Li and Liming Zhou

Metals 2026, 16(9), 1001; https://doi.org/10.3390/met16091001 - 9 Sep 2026

Abstract

This study establishes a closed-loop chain of evaluation, optimization, and capability assessment for the hot stamping of 1.2 mm B1500HS ultra-high-strength steel. A four-dimensional quality framework (appearance, geometry, mechanics and microstructure) was established, and FMEA identified hardness non-uniformity, necking and cracking as priority [...] Read more.

This study establishes a closed-loop chain of evaluation, optimization, and capability assessment for the hot stamping of 1.2 mm B1500HS ultra-high-strength steel. A four-dimensional quality framework (appearance, geometry, mechanics and microstructure) was established, and FMEA identified hardness non-uniformity, necking and cracking as priority risks. An L₁₆(4⁵) orthogonal dataset (heating 870–930 °C; forming 720–780 °C; speed 40–70 mm/s) was re-analyzed by ANOVA. Heating temperature dominated all responses (contribution > 91%), whereas stamping speed was insignificant. Run 16 (930 °C, 780 °C, 40 mm/s) ranked first by Min–Max weighting and by equal-/entropy-weight GRA (Kendall τ ≥ 0.650, p < 0.001); because speed was insignificant, 70 mm/s is equally acceptable and preferable for productivity. Two-stage validation (merged n = 10) gave R_m = 1515.6 MPa, R_p0.2 = 1145.8 MPa, A = 7.8%, HV = 568.4 and ≈ 97.5% martensite. Preliminary capability indices were excellent for R_m (Cpk = 2.12), A (2.46) and HV (2.29), and acceptable for R_p0.2 (1.13). The framework links laboratory optimization with batch-production quality control. Full article

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

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43 pages, 14558 KB

Open AccessReview

A Comprehensive Review of Bridgman Solidification of High-Entropy Alloys

by Shuai Chen, Guangzeng Zhang, Jianzhong Jiang, Peter K. Liaw and Yong Zhang

Metals 2026, 16(9), 1000; https://doi.org/10.3390/met16091000 - 8 Sep 2026

Abstract

Conventional casting of high-entropy alloys was constrained by multi-principal element solidification behavior and was prone to severe grain boundary segregation and microstructural inhomogeneity, limiting their service performance in extreme environments. Bridgman directional solidification can help to achieve oriented columnar grain growth and mitigate [...] Read more.

Conventional casting of high-entropy alloys was constrained by multi-principal element solidification behavior and was prone to severe grain boundary segregation and microstructural inhomogeneity, limiting their service performance in extreme environments. Bridgman directional solidification can help to achieve oriented columnar grain growth and mitigate micro-segregation via tailored matching of a temperature gradient, G, and a growth rate, V, yet its stable solidification relied on a high G/V ratio, imposing stringent requirements on equipment and process control. Trace rare earth elements are suggested to potentially stabilize the interfacial morphology and effectively broaden the processing window of Bridgman directional solidification through melt purification and modulation of the solid–liquid interfacial energy based on extrapolation from conventional casting and thermodynamic principles; however, direct experimental confirmation in Bridgman-processed HEAs remains scarce. This review summarizes the solidification microstructure evolution of high-entropy alloys fabricated by the Bridgman method, elucidates the regulatory mechanisms of rare earth microalloying on phase selection, solute partitioning behavior, and interface stability, and reveals the strengthening effects and corrosion performance variations under the synergistic interaction of processing parameters and chemical compositions. Finally, future perspectives are provided regarding interfacial reactions, compositional homogeneity control, and the lack of design criteria in Bridgman-based rare earth composite fabrication systems. Full article

(This article belongs to the Section Entropic Alloys and Meta-Metals)

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22 pages, 5763 KB

Open AccessArticle

Study on Abnormal Winding Behaviors of Hoisting Steel Wire Rope in Ultra-Deep Vertical Shaft

by Wenbo Fan, Shirong Ge, Dagang Wang, Yinhe Sun and Xiansong Deng

Metals 2026, 16(9), 999; https://doi.org/10.3390/met16090999 - 8 Sep 2026

Abstract

During multi-layer winding of hoisting steel wire ropes for ultra-deep vertical shafts on double broken-line drums, abnormal winding behaviors such as rope interlocking, rope jumping and disordered rope arrangement may occur, accelerating abrasion and wire breakage, reducing load-bearing capacity and service life, and [...] Read more.

During multi-layer winding of hoisting steel wire ropes for ultra-deep vertical shafts on double broken-line drums, abnormal winding behaviors such as rope interlocking, rope jumping and disordered rope arrangement may occur, accelerating abrasion and wire breakage, reducing load-bearing capacity and service life, and compromising operational safety. A dynamic rope-jumping discrimination approach considering transverse-vibration-induced fleet-angle variation was developed and evaluated through field tests. Meanwhile, based on the spatial trajectory model of multi-layer wound hoisting steel wire ropes and a quantitative criterion for rope interlocking, the effects of key drum structural parameters on rope interlocking were investigated. Results show that the broken-line zone is the main high-risk region for rope jumping, with the rightmost position of the third layer after the second-to-third-layer transition being the most critical location. Transverse rope vibration increases the fleet angle, and rope jumping occurs when the critical threshold is exceeded. At the three representative winding positions, the relative errors between the calculated and measured fleet angles are below 7.0%, and the predicted high-risk rope-jumping location is consistent with the field observation, providing field-based support for the model under the examined operating condition. Rope-interlocking risk is significantly higher in the broken-line zone and increases with larger fleet angles, smaller rope groove clearance coefficients and larger drum-to-rope diameter ratios. Full article

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

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9 pages, 4979 KB

Open AccessPerspective

Expanding the Design Space of Nb–Ti–Al-Related Lightweight Alloys: From High-Nb TiAl Intermetallics to Zr–Ti–Nb–(Al)-Concentrated BCC Matrices

by Jiasheng Wang and Yong Zhang

Metals 2026, 16(9), 998; https://doi.org/10.3390/met16090998 - 8 Sep 2026

Abstract

Nb-containing TiAl alloys are among the most important lightweight intermetallics for high-temperature applications because they combine low density with useful strength, oxidation resistance and creep resistance. Their long-standing difficulty is equally clear: the ordered γ/α₂ matrix and lamellar hierarchy that support thermal [...] Read more.

Nb-containing TiAl alloys are among the most important lightweight intermetallics for high-temperature applications because they combine low density with useful strength, oxidation resistance and creep resistance. Their long-standing difficulty is equally clear: the ordered γ/α₂ matrix and lamellar hierarchy that support thermal stability also restrict plastic accommodation and manufacturing tolerance. This Perspective addresses a specific question: how can Nb–Ti–Al-related alloy design expand from stabilizing ordered TiAl matrices toward matrices and architectures that also enable scalable deformation and processing? The discussion is organized around three connected routes. High-Nb TiAl alloys established a durable service-stability platform; lamellar, colony, and orientation engineering then created deformation pathways within ordered matrices, and compositionally adjacent Zr–Ti–Nb–(Al)-concentrated BCC alloys introduced a different matrix-selection strategy in which chemical disorder and BCC stability are used to build processability at an earlier stage of design. These routes solve different parts of the same design problem rather than representing direct competitors. They indicate that future lightweight high-temperature alloys should be designed by linking composition selection, phase architecture, thermomechanical processing and environmental validation within a processability–stability framework. Full article

(This article belongs to the Section Entropic Alloys and Meta-Metals)

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14 pages, 14156 KB

Open AccessFeature PaperArticle

Tool Force Monitoring for Efficient Friction Stir Welding of AA5754 Aluminum Alloy Joints with Enhanced Mechanical Performance

by Hakan Kalkan and Ozan Oflaz

Metals 2026, 16(9), 997; https://doi.org/10.3390/met16090997 - 8 Sep 2026

Abstract

Friction stir welding (FSW) is a solid-state joining process widely used for aluminum alloys due to its low heat input, reduced distortion, and ability to produce high-quality joints. However, excessive tool forces generated during the welding process can increase machine loading, accelerate tool [...] Read more.

Friction stir welding (FSW) is a solid-state joining process widely used for aluminum alloys due to its low heat input, reduced distortion, and ability to produce high-quality joints. However, excessive tool forces generated during the welding process can increase machine loading, accelerate tool wear, and negatively affect the process efficiency. Therefore, understanding the relationship between welding parameters, tool forces, and the joint performance is essential for achieving high-quality welds while avoiding unnecessary mechanical loads. In this study, 4 mm thick AA5754 aluminum alloy plates were joined using the FSW process, and the feasibility of using tool force measurements for process optimization was investigated. A comprehensive experimental matrix consisting of nine different rotational speeds and ten different tool travel speeds was established based on preliminary studies and previous literature. During each welding operation, forces acting on the tool in the F_x, F_y, and F_z directions were continuously recorded. The welded joints were evaluated through tensile testing (Zwick Z300 universal testing machine, ZwickRoell, Ulm, Germany), hardness measurements, and microstructural characterization using scanning electron microscopy (SEM) (ZEISS Merlin scanning electron microscope, Carl Zeiss Microscopy GmbH, Oberkochen, Jena, and Göttingen, Germany). A Pearson correlation analysis and a two-way analysis of variance (ANOVA) were performed at a 95% confidence level to quantify the relationships and statistical significance of the process parameters. The results showed that F_z was the dominant force component during welding. The rotational speed had a statistically significant effect on the tensile strength, yield strength and hardness (p < 0.05), accounting for 99.39% of the total variation in hardness. For the mean tool force, both the rotational speed and the tool travel speed were statistically significant (p < 0.0001), contributing 38.48% and 47.16% of the total variation, respectively. The rotational speed also accounted for 81.55% of the variation in the maximum axial force. The Pearson correlation analysis showed a strong negative correlation between the rotational speed and hardness (r = −0.73), whereas the tool travel speed showed positive correlations with F_x (r = 0.61), F_z (r = 0.62), and the mean tool force (r = 0.68). Despite the increased tool loading associated with higher travel speeds, no corresponding improvement in the mechanical performance was observed. The results demonstrated that appropriately selected welding conditions produced joints with a yield strength and hardness exceeding 90% of the corresponding base material properties while maintaining relatively lower tool forces. SEM observations confirmed grain refinement in the stir zone. Overall, the combined correlation and ANOVA results demonstrate that real-time tool force monitoring can provide a quantitative basis for selecting FSW parameters that achieve an adequate mechanical performance while minimizing unnecessary machine and tool loading. Full article

(This article belongs to the Special Issue Friction Stir Welding/Processing of High-Temperature Non-Ferrous Alloys and Advanced Metallic Systems)

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25 pages, 5382 KB

Open AccessArticle

Constraint-Driven Fracture Toughness Assessment of Unequal-Wall-Thickness X80-X60 Girth Welded Pipelines Using SENT Specimens

by Ke Wang, Min Zhang, Dan Chen, Weifeng Ma, Weizhe Hao and Xuan Yang

Metals 2026, 16(9), 996; https://doi.org/10.3390/met16090996 - 7 Sep 2026

Abstract

Unequal-wall-thickness X80-X60 girth welded joints used in pipeline transition sections exhibit strong local mechanical heterogeneity, while the wall-thickness transition also introduces a separate structural geometry effect. Conventional homogeneous or weld-metal-only descriptions may therefore be insufficient for interpreting the fracture response of single-edge-notched tension [...] Read more.

Unequal-wall-thickness X80-X60 girth welded joints used in pipeline transition sections exhibit strong local mechanical heterogeneity, while the wall-thickness transition also introduces a separate structural geometry effect. Conventional homogeneous or weld-metal-only descriptions may therefore be insufficient for interpreting the fracture response of single-edge-notched tension (SENT) specimens sampled from such joints. In this study, a constraint-driven fracture toughness assessment was conducted for single-edge-notched tension (SENT) specimens representing the X80 base metal, X80 heat-affected zone (HAZ), weld metal, X60 HAZ, and X60 base metal as five distinct material regions. Miniature tensile tests provided the local constitutive input for these regions. The numerical procedure was verified against published SENT force versus crack-mouth-opening-displacement (CMOD) data; because direct SENT fracture-toughness tests for the present X80-X60 target joint are not yet available, the literature comparison is treated as verification of the modeling procedure rather than direct validation of the target joint. Parametric analyses were then performed for an initial crack ratio of a₀/W = 0.10–0.30, a thickness-to-width ratio of B/W = 0.50–1.50, weld width = 10–30 mm, HAZ width = 1–9 mm, and different weld/HAZ strength combinations. The simulations show asymmetric crack-tip plastic deformation toward the lower-strength X60 side. Increasing the crack length increases crack-tip opening displacement (CTOD), whereas increasing B/W, weld width, or HAZ width generally reduces CTOD by increasing local constraint. The influence of weld strength is strongly coupled with the strength level of the adjacent HAZs. The results are therefore interpreted as numerical CTOD-response trends for a heterogeneous SENT specimen; direct experimental fracture-toughness measurements of the target X80-X60 joint remain an important subject of follow-up work. Full article

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

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16 pages, 6652 KB

Open AccessArticle

Interpretable Machine Learning for Mechanical Property Prediction of 5Cr-0.5Mo Steel: SHAP Explainability, Multi-Model Comparison, and Uncertainty Quantification

by Saurabh Tiwari, Hyoju Ahn, Jongwon Lee and Nokeun Park

Metals 2026, 16(9), 995; https://doi.org/10.3390/met16090995 - 7 Sep 2026

Abstract

5Cr-0.5Mo ferritic steels are widely used in high-temperature power-plant components. Although artificial neural network (ANN) models have shown good performance in predicting tensile properties, they provide limited insight into predictions and generally do not quantify the uncertainty. In this study, three tree-based machine [...] Read more.

5Cr-0.5Mo ferritic steels are widely used in high-temperature power-plant components. Although artificial neural network (ANN) models have shown good performance in predicting tensile properties, they provide limited insight into predictions and generally do not quantify the uncertainty. In this study, three tree-based machine learning models—Random Forest (RF), XGBoost (XGB), and Gradient Boosting (GB)—were developed using 36 unique alloy grade–temperature observations from a validated NIMS 5Cr-0.5Mo tensile dataset. The model performance was evaluated using leave-one-grade-out (LOGO) cross-validation, with pooled out-of-fold (OOF) predictions used to assess the overall performance. SHapley Additive exPlanations (SHAP) were used to examine feature contributions, whereas Gaussian Process Regression (GPR) was evaluated as a proof-of-concept for uncertainty quantification of yield strength (YS). GB showed the strongest performance for ultimate tensile strength (UTS) and reduction in area (RA), achieving pooled OOF R² values of 0.9698 and 0.9570, respectively. RF achieved corresponding R² values of 0.9406 and 0.9488, respectively. SHAP identified the test temperature as the most influential feature across all four properties, whereas the Cr content and austenite grain size contributed significantly to the strength predictions. For YS, the GPR achieved complete empirical coverage of the 95% predictive intervals, although the relatively large mean interval width indicated conservative uncertainty estimates. Given the limited dataset and feature correlations, the SHAP results should be regarded as exploratory, rather than mechanistic. Overall, this study demonstrates the potential of interpretable, uncertainty-aware ML for small alloy datasets, while emphasizing the need for larger, compositionally diverse datasets and independent validation. Full article

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

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12 pages, 5607 KB

Open AccessArticle

Effect of Solution Treatment on Microstructure and Properties of Rheo-Squeeze-Cast AA7075 Alloy

by Ke Zhou, Zhaoqiang Li and Yongkun Li

Metals 2026, 16(9), 994; https://doi.org/10.3390/met16090994 - 6 Sep 2026

Abstract

7xxx series aluminum alloys exhibit high strength and low density; however, their high degree of alloying results in a pronounced hot-cracking tendency, making direct casting forming challenging. In this study, a rheo-squeeze-cast AA7075 alloy was investigated. Scanning electron microscopy (SEM), transmission electron microscopy [...] Read more.

7xxx series aluminum alloys exhibit high strength and low density; however, their high degree of alloying results in a pronounced hot-cracking tendency, making direct casting forming challenging. In this study, a rheo-squeeze-cast AA7075 alloy was investigated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and room-temperature tensile testing were employed to systematically examine the effects of the solution treatment window on the microstructure and mechanical properties of the castings. The distinctive contribution of this work is the quantitative correlation of solution-time-dependent second-phase evolution and porosity with the tensile behavior of ECSC-assisted rheo-squeeze-cast AA7075. The results show that, with increasing solution treatment time, the intergranular eutectic phases gradually dissolved, and their morphology evolved from lamellar structures into fine rod-like features. When the solution treatment time was extended to 12 h, a small number of pore defects appeared in the microstructure. Under the conditions of solution treatment at 470 °C for 8 h followed by aging at 120 °C for 24 h, uniformly distributed Zn–Mg-rich aging precipitates were observed in the matrix, resulting in the best comprehensive mechanical properties, with an ultimate tensile strength, yield strength, and elongation of 498.1 MPa, 413.7 MPa, and 8.2%, respectively. The optimized heat-treatment condition provides a practical route for achieving a favorable strength–ductility balance in rheo-squeeze-cast AA7075 components. Full article

(This article belongs to the Special Issue Advances in Continuous Casting and Solidification of Metals)

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26 pages, 2570 KB

Open AccessReview

Research Status on the Corrosion Resistance and Protective Coating Technologies of Mg-Rare Earth Alloys

by Yueyue Yang, Yufeng Chen, Haoran Hong, Hongliang Zhang, Teng Liu and Zhisheng Nong

Metals 2026, 16(9), 993; https://doi.org/10.3390/met16090993 - 5 Sep 2026

Abstract

Mg alloys are among the lightest metallic structural materials used in engineering applications, possessing a range of favorable properties and holding broad application prospects. However, the corrosion resistance of Mg alloys is relatively poor compared to other metallic structural materials, which limits their [...] Read more.

Mg alloys are among the lightest metallic structural materials used in engineering applications, possessing a range of favorable properties and holding broad application prospects. However, the corrosion resistance of Mg alloys is relatively poor compared to other metallic structural materials, which limits their use in various working environments. The addition of rare earth elements is one approach to improving the corrosion resistance of Mg alloys; nevertheless, the corrosion resistance of Mg-rare earth alloys still falls short of industrial application requirements. This paper aims to introduce the corrosion resistance of Mg-rare earth alloys and their applied protective coatings. Through coating characterization techniques, the protective effects of coatings on Mg-rare earth alloys are analyzed and elucidated. Finally, a summary and outlook on the corrosion resistance of Mg-rare earth alloys and coating protection technologies are provided. Full article

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22 pages, 21002 KB

Open AccessArticle

Weldability of Cold-Roll-Bonded Fe–Al Clad Sheets in Dissimilar Joining: Effects of Sheet Configuration and Internal Steel-Layer Melting

by Seung Cheol Shin, Yong Kim, Hye Chan Park and Hyun Jin Woo

Metals 2026, 16(9), 992; https://doi.org/10.3390/met16090992 - 4 Sep 2026

Abstract

Cold-roll-bonded (CRB) Fe–Al clad sheets, which incorporate an internal steel layer within the clad structure, are candidate materials for dissimilar joining in lightweight structural applications, yet systematic comparisons of their weldability across multiple joining processes remain limited. This study evaluates the process-dependent weldability [...] Read more.

Cold-roll-bonded (CRB) Fe–Al clad sheets, which incorporate an internal steel layer within the clad structure, are candidate materials for dissimilar joining in lightweight structural applications, yet systematic comparisons of their weldability across multiple joining processes remain limited. This study evaluates the process-dependent weldability of CRB Fe–Al clad sheets using three joining processes: laser lap welding, DC resistance spot welding, and cold metal transfer-pulse (CMT-P) arc welding. Weld appearance, cross-sectional morphology, tensile shear behavior, and fracture characteristics were assessed for multiple sheet configurations and clad-layer thicknesses. Across all three processes, configurations that placed the clad sheet directly toward the heat source were prone to cracking or strength degradation when the internal steel layer melted extensively, whereas limiting steel-layer involvement produced more stable joints. Among the three processes, resistance spot welding provided the widest workable process window, while laser and CMT-P arc welding achieved peak strengths only within a narrow heat-input range. These findings indicate that the role of the internal steel layer is process-dependent and provide practical guidance for sheet configuration and parameter selection when joining CRB Fe–Al clad sheets to steel or aluminum components. Full article

(This article belongs to the Section Welding and Joining)

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16 pages, 7506 KB

Open AccessArticle

Investigation of Hydrophobicity Enhancement of Molybdenum Surfaces by Micromilling

by Xian Meng, Hao Xu, Hui Zhang, Jinwen Cao, Ying Zhang, Jinyue Geng, Cong Yan and Heji Huang

Metals 2026, 16(9), 991; https://doi.org/10.3390/met16090991 - 4 Sep 2026

Abstract

Molybdenum (Mo), owing to its excellent high-temperature resistance and low sputtering yield, is widely used in advanced equipment such as extreme ultraviolet (EUV) lithography systems. To meet the demand for regulating the surface wettability of Mo, this study employs micromilling to fabricate microgroove [...] Read more.

Molybdenum (Mo), owing to its excellent high-temperature resistance and low sputtering yield, is widely used in advanced equipment such as extreme ultraviolet (EUV) lithography systems. To meet the demand for regulating the surface wettability of Mo, this study employs micromilling to fabricate microgroove arrays on Mo surfaces to enhance their hydrophobicity. First, comparative micromilling experiments are conducted under flood cooling and minimum quantity cooling lubrication (MQCL) conditions. The effects of process parameters, including axial depth of cut (a_p), spindle speed (n), and feed per tooth (f_z), on burr height (H) are investigated under the two cooling/lubrication conditions. The results show that the burr heights obtained under flood cooling are generally lower than those obtained under MQCL. Subsequently, the machining parameters under flood cooling are evaluated using an orthogonal experimental design. A mathematical model relating the microgroove array geometry to the contact angle is then established based on Gibbs free energy to guide the design of surface microgroove structures. Finally, microgroove arrays are fabricated on Mo surfaces using the preferred machining parameters. The results demonstrate that the microgroove arrays effectively enhance the water hydrophobicity of the Mo surface. The static water contact angle increases from 62.45 ± 0.25° for the untreated Mo surface to a maximum of 128.50 ± 0.12°, thereby achieving a transition from hydrophilic to hydrophobic behavior. Full article

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