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A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Structural Integrity of Metals".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 17296

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Prof. Dr. Filippo Berto

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Department of Chemical Engineering, Materials and Environment, Sapienza University of Rome, 00184 Rome, Italy
Interests: fatigue and fracture behavior of materials; mechanical characterization; structural integrity of conventional and innovative materials
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Dr. Ricardo Branco

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Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal
Interests: Glass-fiber reinforced polymers; 3D printed polymers; fatigue of polymers; ageing effects
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Special Issue Information

Dear Colleagues,

Metals are widely used in various engineering sectors due to their superior properties. These materials are often subjected to complex static and cyclic loading histories, which can affect structural integrity. Therefore, designers must be able to account for the damage accumulation mechanisms and the associated durability to prevent unexpected failure. Thus, the development of safer and more durable components requires a deep understanding of the relationships between the microstructural features and the mechanical properties.

The goal of this Special Issue is to provide a comprehensive overview of the more recent advances in the structural integrity of metals. Potential topics include alloy design, processing techniques, microstructure features, chemical composition, advanced simulation methods, mechanical behavior, failure mechanisms, heat treatment strategies, forming and joining technologies, and environmental effects. Research and review papers addressing all aspects of the structural integrity of metals are welcome.

Prof. Dr. Filippo Berto
Prof. Dr. Ricardo Branco
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Metals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

structural integrity
metal alloys
fatigue and fracture
impact and wear
static loading
cyclic loading
microstructure features
alloy design
failure mechanisms
processing techniques
numerical methods
welding technologies

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Published Papers (11 papers)

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Research

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21 pages, 5683 KiB

Open AccessArticle

An Enhanced Approach for High-Strain Plasticity in Flat Anisotropic Specimens with Progressively Distorting Neck Sections

by Giuseppe Mirone, Raffaele Barbagallo, Giuseppe Bua, Pietro Licignano and Michele Maria Tedesco

Metals 2024, 14(5), 578; https://doi.org/10.3390/met14050578 - 14 May 2024

Viewed by 948

Abstract

Characterizing the behavior of ductile metals at high strains is essential in various fields. In the case of thin sheets, rectangular cross-section specimens are used to characterize these materials, typically by tensile tests. Unlike cylindrical specimens, flat ones pose additional challenges for the hardening characterization at high strains, especially in the post-necking phase, which, for many high-strength steels, may cover most of the plastic strain range. After the onset of global necking, the rectangular cross-sections tend to distort with respect to their original shape, as their edges progressively curve and bulge inward. The localized necking occurring after the global one in thinner specimens, further distorts the necked zone. Additionally, sheet metals usually exhibit anisotropic characteristics that affect the derivation of the stress–strain curve and need to be dealt with. No exact method exists for the stress–strain characterization of ductile thin sheets at high strains from tensile tests. Although several approximate methods are available in the literature, they either discard the post-necking range or require highly advanced and complex experimental setups not suitable for industrial applications (e.g., 3D DIC). Then, this work proposes a relatively simple methodology for the experimental characterization of anisotropic thin sheet metals through tensile tests on rectangular cross-section specimens that delivers the true stress–strain curve of the material, extended over the necking range and up to fracture, accurately assessing the anisotropy and the distortion of the neck section. The proposed methodology, employing a standard single-camera experimental setup, is illustrated here, referring to four different steels for automotive applications with reference to a single material orientation; it is intended as representative of the repeated procedure involving tensile tests along 3 or more material directions in order to describe the whole anisotropic plastic response. A detailed comparison between the novel methodology and four other common approaches is carried out, highlighting the differences and the enhanced capabilities of the novel one proposed. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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14 pages, 10169 KiB

Open AccessArticle

Distinct Evidence of Hydrogen-Enhanced Defect Formation on Pre-Strained Nickel Alloy 625 during In Situ Electrochemical Nanoindentation Test

by Chandrahaasan K. Soundararajan, Xu Lu, Dong Wang and Alexei Vinogradov

Metals 2024, 14(2), 161; https://doi.org/10.3390/met14020161 - 28 Jan 2024

Viewed by 993

Abstract

In the present work, in situ electrochemical nanoindentation was utilized to investigate the hydrogen effect on the nanomechanical properties of tensile pre-strained nickel alloy (0%, 5% and 20%). The study reveals that hydrogen-induced hardening occurs during cathodic polarization due to hydrogen incorporation and softening behavior during anodic polarization; this is due to the irreversible microstructure modification induced in the presence of hydrogen solutes. Their respective contributions were quantified by fitting the elastoplastic part of the load-displacement data. In addition, the differences in their plastic behaviors were investigated in detail by examining the dislocation structure underneath the indents. This study aims to shed light on hydrogen’s interaction with pre-existing defects. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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12 pages, 15824 KiB

Open AccessArticle

The Influence of Rapid Tempering on the Mechanical and Microstructural Characteristics of 51CrV4 Steel

by Antti Kaijalainen, Oskari Haiko, Saeed Sadeghpour, Vahid Javaheri and Jukka Kömi

Metals 2024, 14(1), 60; https://doi.org/10.3390/met14010060 - 3 Jan 2024

Viewed by 1172

Abstract

The microstructure and mechanical properties of a low-alloy medium carbon steel (Fe-0.5C-0.9Mn-1Cr-0.16V, in wt.%) were investigated after rapid tempering and compared with a conventionally tempered counterpart. The conventional thermal cycle was performed in a laboratory-scale box furnace while rapid heat treatments were carried out using the Gleeble 3800 thermomechanical simulator machine. In the rapid heat treatments, the heating rate was 50 °C/s for austenitizing and 60 °C/s for the tempering process, with a cooling rate of 60 °C/s for both treatments. Austenitization was performed at 900 °C for 3 s and tempering was conducted at 300 °C and 500 °C for 2 s. For conventional routes, the heating rate for both austenitization and tempering was 5 °C/s. Likewise, the austenitization was carried out at 900 °C for 45 min and tempering was carried out at 300 °C and 500 °C for 30 min. The results revealed that rapid tempering resulted in a significantly increased impact toughness compared to conventional tempering, while maintaining a consistent high strength level. The quenched samples showed the highest hardness and tensile strength but obtained the lowest toughness values. The optimum combination of strength and toughness was achieved with the sample rapidly tempered at 300 °C, resulting in a tensile strength of 2050 MPa and impact energy of 14 J for sub-sized CVN samples. These desirable mechanical properties were achieved throughout the tempered martensitic microstructure with a minor fraction of pearlitic strings. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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11 pages, 2689 KiB

Open AccessArticle

Selective Laser Melting of Non-Weldable Nickel Superalloy: Microstructure, Cracks and Texture

by Kirill Starikov, Igor Polozov, Evgenii Borisov, Artem Kim, Daniil Voevodenko, Anna Gracheva, Alexey Shamshurin and Anatoly Popovich

Metals 2023, 13(11), 1886; https://doi.org/10.3390/met13111886 - 13 Nov 2023

Viewed by 1363

Abstract

Additive manufacturing, particularly selective laser melting, presents exciting possibilities for fabricating components from high-temperature nickel-based superalloys. Controlling microstructural features and minimizing defects in fabricated specimens are critical challenges. This study explores the influence of process parameters on microstructure and defect formation in directionally solidified nickel-based superalloy specimens. We conducted a comprehensive analysis of selective laser melting process variables, including interdendritic spacing, crystallization times, and volumetric energy density. Electron backscatter diffraction analysis was employed to assess the feasibility of obtaining a directional structure in single-crystal nickel-based heat-resistant alloy specimens using selective laser melting. The study shows a significant correlation between reduced interdendritic spacing and increased defect formation. Longer crystallization times and higher volumetric energy density lead to decreased defect volumes and sizes. Electron backscatter diffraction analysis confirms the maintenance of preferential growth direction across subsequent layers. Our research underscores the importance of optimizing selective laser melting parameters, balancing refractory elements in alloy composition, and adopting strategies for enhancing crystallization times to minimize structural defects. This comprehensive approach ensures both heat resistance and minimal defects, facilitating the production of high-quality components. These findings contribute to advancing selective laser melting applications in critical industries like aerospace and power generation, where heat-resistant materials are paramount. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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17 pages, 11108 KiB

Open AccessArticle

Improved Mechanical Properties of Biocompatible Zn-1.7%Mg and Zn1.7%Mg-0.2%Zr Alloys Deformed with High-Pressure Torsion

by Natalia Martynenko, Natalia Anisimova, Natalia Tabachkova, Georgy Rybalchenko, Igor Shchetinin, Olga Rybalchenko, Maria Shinkareva, Dmitry Prosvirnin, Elena Lukyanova, Diana Temralieva, Andrey Koltygin, Mikhail Kiselevskiy and Sergey Dobatkin

Metals 2023, 13(11), 1817; https://doi.org/10.3390/met13111817 - 27 Oct 2023

Cited by 1 | Viewed by 1028

Abstract

The potential medical Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys strengthened using high-pressure torsion (HPT) were investigated in this work. HPT led to a significant refinement of the microstructure of both alloys with the formation of an ultrafine-grained structure (UFG). The average grain size after HPT was ~700–800 nm for both alloys. The formation of the UFG structure led to an increase in the ultimate tensile strength of up to 401 ± 16 and 482 ± 12 MPa for the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys, respectively. Additionally, a variation in ductility of the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys of up to 56.3 ± 16.9% and 4.4 ± 0.6%, respectively, was also observed, apparently due to textural changes. HPT led to a small increase in the degradation rate of the alloys after 1 day of incubation in the medium. However, an increase in the incubation period of up to 30 days slowed down the degradation process and leveled the difference between the initial and HPT-treated state of the alloys. HPT did not affect the cytotoxicity of the Zn-1.7%Mg-0.2%Zr alloy and contributed to the reduction of hemolysis. Thus, the processing of the Zn-1.7%Mg and Zn-1.7%Mg-0.2%Zr alloys using HPT accelerated their biodegradation without compromising their biocompatibility. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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14 pages, 6586 KiB

Open AccessArticle

Effect of Al5TiB Master Alloy with P on Microstructure and Mechanical Properties of AlSi7Mg Alloy

by Tomasz Lipiński

Metals 2023, 13(9), 1560; https://doi.org/10.3390/met13091560 - 6 Sep 2023

Cited by 1 | Viewed by 1029

Abstract

Aluminum-silicon alloys are popular casting alloys. In its raw state, the microstructure of the hypoeutectic silumin consists of a large eutectic β phase against the background of dendritic eutectic α. Due to its large microstructure components, mainly the eutectic β phase, this alloy has low mechanical properties. The unfavorable properties of hypoeutectic silumin can be improved by changing the size and shape of the alloy’s microstructure components. There are several possibilities for controlling the microstructure and the resulting mechanical properties of the alloy. One possibility is to modify the alloy with elements and chemical compounds. This paper presents the effect of phosphorus with Al-Ti-B on the microstructure and mechanical properties of hypoeutectic silumin AlSi7Mg. The proportions of Ti to B were selected on the basis of the results presented in the literature, recognizing the optimal ratio of 5:1. The modifier was introduced into the alloy in the form of an AlTiBP master alloy with a variable content of titanium, boron, and phosphorus. Phosphorus was added at the levels of 0.1, 0.2, and 0.3% of the weight of the modified casting. As a result of the tests carried out, the modifying effect of the introduced master alloy was confirmed. A different morphology of microstructures was obtained for the different chemical compositions of the modifier. The most favorable modification effect, whose measurable parameter is the highest (out of the obtained) mechanical properties, was found for the modifier containing 0.25% Ti + 0.03% B + 0.2% P. It was also found that phosphorus, in the presence of titanium and boron, affects the microstructure and mechanical properties of hypoeutectic silumin AlSi7Mg. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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19 pages, 16939 KiB

Open AccessArticle

A Novel Lap-Butt Joint Design for FSW of Aluminum to Steel in Tee-Configuration: Joining Mechanism, Intermetallic Formation, and Fracture Behavior

by Reza Beygi, Amir Abbas Talkhabi, Majid Zarezadeh Mehrizi, Eduardo A. S. Marques, Ricardo J. C. Carbas and Lucas F. M. da Silva

Metals 2023, 13(6), 1027; https://doi.org/10.3390/met13061027 - 27 May 2023

Cited by 11 | Viewed by 1756

Abstract

The development of new joint configurations suitable for dissimilar materials enables a wider range of applications and allows for an accelerated replacement of traditional structural construction materials by lightweight materials. The T-configuration is a joint configuration that has not been sufficiently studied for use with dissimilar materials, especially when created using the friction stir welding (FSW) process. In this study, a combined lap/butt design was introduced and implemented, seeking to create a T-joint between aluminum and steel. Characterization of the joints showed that FSW could be successfully used to join aluminum and steel in a T-configuration. The formation of intermetallic bonds and kissing bonds was carefully analyzed, and their contribution to the fracture behavior during loading in the skin and stringer directions was studied. Finite element simulation was used to determine the stress state at the interface during loading. The characterization results showed that the intermetallic, as an indicator of metallurgical bonding, is formed when special features are observed in the pattern of material flow. The fractography images showed that the stress state has a major impact on the fracture. The results of the present study can be effectively used to design and fabricate dissimilar joints, taking into account the loading condition. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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13 pages, 10191 KiB

Open AccessArticle

Metallurgical Failure Analysis of Closed Water Circuit Containing Molybdate-Based Inhibitor

by Andrea Casaroli, Marco Virginio Boniardi, Barbara Rivolta, Riccardo Gerosa and Francesco Iacoviello

Metals 2023, 13(4), 723; https://doi.org/10.3390/met13040723 - 6 Apr 2023

Cited by 2 | Viewed by 1607

Abstract

In this work, two industrial heating/cooling circuits are compared. One of the two systems failed in a short time showing severe corrosion damage and a through thickness crack close to one of the welds. The main difference between the circuits is the presence of a sodium molybdate-based corrosion inhibitor in the damaged one. The addition of these substances is very frequent in such applications, and they generally work very well in preventing serious corrosion attacks. Nevertheless, the technical literature reports other cases in which systems working with fluids containing such inhibitors failed prematurely. The authors performed a failure analysis of the damaged circuit focusing their attention on the regions where fluid leaks were observed because of through thickness cracks. This damage was located close to the pipe–flange weld. These zones were investigated by visual examination, radiographic and scanning electron microscope (SEM) analyses, metallographic observations by light optical microscope (LOM), Vickers micro-hardness tests and optical emission spectroscopy (OES) chemical analysis. The failure was related to the presence of severe pitting and crevice corrosion in the welded areas with the final activation of a further critical corrosion mechanism, i.e., stress corrosion cracking (SCC). In order to explain the shorter working life of the failed system, a physical model of the corrosion mechanisms acting on the two circuits was proposed. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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19 pages, 13103 KiB

Open AccessArticle

Plasticity Resource of Cast Iron at Deforming Broaching

by Yakiv Nemyrovskyi, Ihor Shepelenko and Michael Storchak

Metals 2023, 13(3), 551; https://doi.org/10.3390/met13030551 - 9 Mar 2023

Viewed by 1503

Abstract

The contact interaction mechanics of deformation broaching in low-plasticity materials is studied. Particular attention is paid to the study of the stress–strain state parameters and the plasticity margin in the deformation zone during the machining of gray cast iron EN-GJL-200. The stress–strain state was analyzed using a finite-element model of the deforming broaching process for each area of the deformation zone. The model parameters of the machined material were determined experimentally by compressing specimens of gray cast iron EN-GJL-200. The changes in the parameters of accumulated strain, stress tensor components, stress triaxiality ratio, hydrostatic stress, and plasticity margin at different deformation zones along the machined specimen depth are analyzed. It is shown that there is a zone of local plastic deformation in conditions of critical contact stresses. This leads to the appearance of tensile stresses that reduce the plasticity margin in the surface layer. The impact of tool geometry on the stress–strain state of the surface layer is also discussed, and recommendations for the optimal working angle of the deforming element are provided based on plasticity margin minimization. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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16 pages, 19784 KiB

Open AccessArticle

Failure Analysis of a Femoral Cephalomedullary Nail

by Farah Hamandi, Stephen Whatley, Gerard Simon, Indresh Venkatarayappa and Tarun Goswami

Metals 2023, 13(3), 506; https://doi.org/10.3390/met13030506 - 2 Mar 2023

Cited by 3 | Viewed by 2373

Abstract

A fractured cephalomedullary femoral nailing system was investigated for the clinical and mechanical reasons responsible for its failure. Optical and scanning electron microscopes were utilized to investigate the fracture surface characteristics. Striations presented on the surface indicated mechanical fatigue. A qualitative material conformity test was conducted using available resources and found to be inconclusive, requiring more advanced testing of Ti-15Mo per ASTM standards in a third-party laboratory. In addition, the investigation showed that there is evidence of overloading failure once the fatigue-propagated crack reached a critical size. Based on the observed features, it is possible that nail and self-tapping helical screw interference may have occurred. The interior wall of the nail exhibited damage, allowing a surface crack to form. This surface crack was propagated due to cyclic loading occurring as a result of activities of daily living. The propagation of cracks formed the striations seen on the failed device. This continued for a period of time up until the crack grew to the point where the structure of the nail could no longer withstand the load and catastrophically failed by overloading. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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Review

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28 pages, 30073 KiB

Open AccessReview

Review on Laser Shock Peening Effect on Fatigue of Powder Bed Fusion Materials

by Francisco Bumba, Paulo Morais, Rodolfo Batalha, Vitor Anes and Luis Reis

Metals 2023, 13(10), 1762; https://doi.org/10.3390/met13101762 - 17 Oct 2023

Viewed by 2172

Abstract

The ability to manufacture parts with complex geometry by sending a model from CAD directly to the manufacturing machine has attracted much attention in the industry, driving the development of additive manufacturing technology. However, studies have shown that components manufactured using additive manufacturing technology have several problems, namely high tensile residual stresses, cracks, and voids, which are known to have a major impact on material performance (in service). Therefore, various post-treatment methods have been developed to address these drawbacks. Among the post-treatment techniques, laser shock peening (LSP) is currently considered one of the most efficient post-treatment technologies for improving the mechanical properties of materials. In practice, LSP is responsible for eliminating unfavorable tensile residual stresses and generating compressive residual stresses (CRS), which result in higher resistance to crack initiation and propagation, thus increasing component life. However, since CRS depends on many parameters, the optimization of LSP parameters remains a challenge. In this paper, a general overview of AM and LSP technology is first provided. It then describes which parameters have a greater influence during powder bed melting and LSP processing and how they affect the microstructure and mechanical properties of the material. Experimental, numerical, and analytical optimization approaches are also presented, and their results are discussed. Finally, a performance evaluation of the LSP technique in powder bed melting of metallic materials is presented. It is expected that the analysis presented in this review will stimulate further studies on the optimization of parameters via experimental, numerical, and perhaps analytical approaches that have not been well studied so far. Full article

(This article belongs to the Special Issue Feature Papers in Structural Integrity of Metals)

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