High-strength steel, renowned for its optimal balance between strength and toughness as well as its exceptional weldability, has become a key structural material in critical applications such as long-distance oil and gas pipelines, offshore engineering structures, construction machinery, and hydropower facilities. The microstructure and mechanical properties of the weld metal—an integral component of the welded joint—directly impact the service safety and reliability of the entire welded structure in high-strength steel applications. Moreover, the solidification behavior and phase transformation processes of the weld metal play a pivotal role in determining its microstructure and mechanical properties. This study focuses on three crucial aspects: the synergistic effects of multi-component alloying, the design of welding process parameters, and the formulation of post-weld heat treatment regimes. It reviews recent advances in microstructure design and elucidates the strengthening and toughening mechanisms of weld metal in high-strength steel fusion welding. Furthermore, the correlation between strength and toughness inversion in weld metal is elucidated from the perspectives of chemical composition design and process parameters. Recent advancements and prospects in relevant fields are summarized, providing theoretical guidance for the development of welding materials and the preparation of weld metal in high-strength steels of 1000 MPa grade and above.

Cold spraying (CS), recognized for its solid-state deposition characteristics, holds significant potential for the fabrication of high-performance coatings, the repair of damaged components, and the additive manufacturing of metals and metal matrix composites. However, oxide films present on the surfaces of powder particles exert a profound impact on particle deformation during CS, as well as on the interfacial microstructure, bonding quality, and mechanical properties of the resulting coatings or deposits. The presence of oxide films increases the critical deposition velocity, reduces the plastic deformation capacity of particles, and promotes the formation of unbonded regions or brittle inclusions at interfaces, thereby compromising deposition efficiency and mechanical integrity. Nevertheless, under specific conditions, the oxide film on the powder surface can be fractured by particle collisions, and the resulting discontinuous oxide film may become evenly distributed, potentially contributing to the dispersion strengthening and enhancing the hardness of the coating. This study presents a comprehensive review of the deformation behavior of oxide films during the CS process and their influence on coating microstructure and properties, with particular focus on the mechanism where how oxide film influences interfacial bonding, coating microstructure and performance. Furthermore, the study discusses the importance of minimizing oxygen content in feedstock powders to achieve high-strength and high-ductility deposits, providing theoretical guidance for optimizing coating performance. Finally, the role of oxide films in CS-based additive manufacturing is explored, and prospective research directions are outlined.

Wire arc additive manufacturing (WAAM) has emerged as one of the most promising technologies for producing large and complex components due to its low cost, high deposition efficiency, and absence of size limitations. It is particularly suitable for Al-Mg-Sc alloys, which exhibit excellent weldability. This article provides a detailed review of studies from the past five years on WAAM Al-Mg-Sc alloys, focusing on metallurgical defects, microstructural evolution, and resulting performance. Existing researches indicated that in WAAM, optimizing wire compositions, process parameters, and introducing interlayer friction stir processing (FSP) can effectively reduce porosity, improve microstructure, and enhance performance. The lowest porosity was about 0.026%. Due to the strong microalloying effect of Sc, the microstructures were all equiaxed grains with an average grain size of about 10 μm. The alloys also exhibited excellent performance, achieving a highest tensile strength of approximately 470 MPa after direct aging, along with outstanding plasticity. However, the development of WAAM-specific Al-Mg-Sc wires, the mechanisms underlying metallurgical defect formation, the control of coarse and fine Al₃(Sc_{1 - x}, Zr _x ) precipitates, and the systematic evaluation of multi-property performance still need to be further addressed. Finally, considering the advantages of machine learning (ML) in the intelligent manufacturing, this review discussed its potential applications in WAAM, including forward performance prediction and reverse optimization of alloy compositions and processing parameters. Such ML-assisted approaches were expected to accelerate the development of high-strength Al-Mg-Sc filler wires, reduce manufacturing costs, and shorten alloy and process development cycles.

Molybdenum alloys, as high-performance refractory metals, possess significant potential for applications under extreme service conditions, including high temperatures and irradiation environments. Under such conditions, high-temperature creep resistance is a critical performance metric. However, welded joints of molybdenum alloys frequently exhibit substantial degradation in creep properties, which severely limits their structural applications. This review systematically summarizes the mechanisms governing creep strengthening in molybdenum alloy base metals and examines the primary factors contributing to the reduced creep resistance in welded joints. Based on these mechanisms and contributing factors, various strategies for enhancing joint performance reported in domestic and international studies are consolidated. Furthermore, the advantages, limitations, and applicability of current creep testing methods for welded joints are evaluated. Finally, future research directions and challenges in improving the high-temperature creep performance of molybdenum alloy welded joints are discussed.

In recent years, due to the rapid development of high-strength and high-toughness steels, the requirements for the strength and toughness of the steel and weld metal have increased. The development of welding materials, welding processes, and post-weld heat treatment systems compatible with high-strength, high-toughness steels remains a major challenge. To address the challenge of achieving both high-strength and high-toughness in high-strength steel weld metals, this paper systematically summarizes the research status of high-strength steel weld metals in composition design, phase transformation kinetics and behavior, and strengthening and toughening mechanisms. The summary is based on prior research on welding material composition optimization, welding process optimization, and post-weld heat treatment, combined with the work of our research group. Furthermore, the study reviews the key challenges and solutions related to the insufficient strength and toughness of current high-strength steel weld metals and summarizes the research directions and key points that must be considered in the future.

Underwater welding technology plays a vital role in the emergency or permanent repair of underwater structures, particularly in high-end sectors such as shipping, offshore engineering, and nuclear power plants. This paper reviews the progress and challenges associated with underwater welding technology from three key perspectives: equipment development, process research, and engineering applications. Regarding equipment development, innovations such as high-performance welding power supplies, micro drainage hoods, intelligent wire feeding devices, and intelligent welding robots have significantly enhanced welding quality and efficiency. In terms of process research, the focus is on dynamic monitoring of the welding process, exploring microstructural evolution and mechanical properties, and clarifying the relationship between parameters such as heat input and microstructure. Combining these with quality optimisation methods can further enhance welding reliability and efficiency. Engineering application practices have demonstrated the significant value of underwater welding in nuclear power maintenance, ship repair, and offshore wind power restoration. Future research should drive underwater welding technology toward breakthroughs in high reliability, high efficiency, and autonomy, thereby providing critical technical support for the repair of large underwater structures.

Metal additive manufacturing is highly valued in the industrial field due to its ability to rapidly produce complex lightweight structures. However, molten metal additive manufacturing technologies are prone to defects such as compositional segregation, internal holes, and thermal cracks, which has driven researchers to develop alternative approaches. In recent years, solid-state friction-based additive manufacturing, derived from the principle of friction stir welding, has attracted considerable attention. This technology combines friction stir welding with the additive manufacturing concept and offers several advantages, including the avoidance of material melting during processing, high deposition rates, and the elimination of the need for protective gas. These advantages suggest broad application prospects for this technology in the field of metal structural parts manufacturing. This study systematically reviews the progress of solid-state friction-based additive manufacturing technology, domestically and internationally, outlining its technical classification, process-microstructure-property relationships, technological advantages, equipment development, sample fabrication, and solid-state repair applications. Finally, the study summarizes the current challenges faced by the process and explores its future development potential, aiming to promote the industrialization of solid-state friction-based additive manufacturing technology and to serve as a reference for further research and applications.

The rising demand for high-end equipment manufacturing across the energy, electronics, and aerospace industries has necessitated the development of effective methods for joining dissimilar materials, particularly metals. Dissimilar metal brazing is a notable joining technique owing to its advantages such as low joining temperatures, efficient sealing capability, and applicability to complex structures; it is therefore a central topic in materials processing research. However, dissimilar metal brazing faces significant technical challenges that impede its broader industrial use. Differences in the physical and chemical properties of the base materials, such as melting points, thermal expansion behaviors, and chemical reactivities, can cause problems such as poor wetting of filler metals, generation of residual stresses (leading to joint cracking), and the formation of brittle intermetallic compounds that degrade mechanical strength. Additionally, base materials with irregular shapes further complicate the brazing process, leading to uneven heat distribution, misalignment, and difficulties in achieving effective filler-metal wetting. In this review, the key challenges and underlying mechanisms in dissimilar metal brazing are examined. Current strategies for improving brazability, including surface modification techniques, innovations in high-performance filler metals, and optimization of process parameters, are also systematically reviewed. This review also addresses issues arising from component shape and size differences and proposes viable solutions. Finally, future directions for dissimilar metal brazing are explored, highlighting the potential of intelligent process control systems and the development of environmentally sustainable brazing materials.

With growing emphasis on energy saving and emission reduction, lightweight structures have become a key development focus in vehicle manufacturing. Mg and Al alloys, as lightweight materials with excellent properties, have broad applications in aerospace, automobile manufacturing, 3C products, and other fields. Mg/Al composite components can fully leverage the advantages of both alloys. Therefore, achieving high-quality and high-efficiency joints of Mg/Al dissimilar alloys has become a critical challenge in the manufacturing industry. Among the important structural types of Mg/Al thin-plate dissimilar welded joints, lap joints have attracted considerable attention. Friction stir welding (FSW), a solid-state joining process, offers distinct advantages for producing high-quality, defect-free Mg/Al joints. Ultrasonic-assisted FSW can further broaden the process window and enhance joint strength. However, the mechanism by which ultrasound influences joint formation during welding remains unclear. In this study, ultrasonic vibration enhanced friction stir lap welding experiments were conducted on Mg alloy and Al alloy sheets. The optimal process parameters were determined to be a rotation speed of 800 r/min and a welding speed of 90 mm/min. During welding, the keyhole region of the lap joint was subjected to a sudden stop + freezing treatment. Material flow behavior was characterized on vertical cross-sections at various angles around the keyhole and on horizontal cross-sections. Microstructures of the characteristic regions on the horizontal cross-section around the keyhole and along the weld centerline passing through keyhole center near the Mg alloy side were characterized. The influence of ultrasonic vibration on the mechanical properties of the lap joints was also evaluated. The results show that introducing ultrasonic vibration during friction stir lap welding enhanced both the tensile shear strength and the effective sheet thickness of the Mg/Al lap joints. Furthermore, ultrasonic vibration increased the volume of material driven by the tool, promoting enhanced material flow and mixing. During joint formation, the grain size distribution around the keyhole on the Mg alloy side became more uniform, and the grains behind the tool underwent a significantly higher degree of recrystallization.

Although friction stir welding (FSW) is widely employed in aerospace and other engineering fields owing to its ability to produce high-quality joints and low residual stresses, the understanding of its physical mechanisms remains limited by the intense material flow during welding, which lags behind engineering practice. This gap motivates researchers to develop thermo-mechanical coupling models to elucidate the underlying physics, aiming to expand its application boundaries. During FSW, the material undergoes a highly rapid flow-deposition process that is completed within a few seconds. In this study, we present a fully coupled thermo-mechanical-fluid model for friction-stir-welded 2219-T87 aluminum alloy to demonstrate the complete material transport path in the vicinity of the tool. A particle-tracking algorithm was employed to quantitatively analyze the flow and subsequent refilling trajectory of the tracer material. The predicted temperature history and flow-zone geometry showed close agreement with experimental data. At the axis Z = 7 mm, the peak downward vertical velocity of the material reached 16.57 mm/s at the advancing side of the pin, whereas the peak upward vertical velocity reached 10.47 mm/s at the retreating side. The analysis revealed that the flow-deposition behavior of the material was highly position dependent. The material adjacent to the shoulder followed a multi-loop spiral trajectory around the tool, during which the loop radius gradually decreased and the material migrated markedly downward in the vicinity of the pin. Ultimately, all deposits accumulated on the advancing side. In contrast, the material far from the shoulder exhibited a through-flow pattern, depositing behind the pin without completing a full revolution. Post-weld deposits originating from the advancing side remained on the advancing side, whereas those from the retreating side settled on the retreating side.

Zr alloys are widely used as cladding materials in light-water reactors because of their low neutron absorption and excellent corrosion resistance. However, achieving high-quality diffusion bonding of Zr alloys at conventional high temperatures is challenging, where grain coarsening and formation of interfacial secondary-phase particles (SPPs) degrade joint performance and may compromise the dimensional accuracy of precision components. This study develops a low-temperature, high-strength diffusion-bonding technique for Zr-4 alloy via surface nanocrystallization, and elucidates the associated microstructural evolution and strengthening mechanisms. A gradient nanostructure (GNS) with a thickness of approximately 70 μm was fabricated on the Zr-4 alloy surface via ultrasonic impact treatment (UIT). The GNS comprised nanograins, nanolamellae, and deformed grains, with high densities of grain boundaries, dislocations, and twins. This surface nanostructure was designed to enhance atomic diffusion, reduce bonding temperature, and improve joint properties. Diffusion-bonding experiments were performed for 30 min at temperatures ranging from 740 ^oC to 800 ^oC under a pressure of 10 MPa. The results revealed that the surface nanograins significantly accelerated interfacial void closure and suppressed SPPs overgrowth and aggregation, resulting in a more dispersed distribution of SPPs along the bonding interface. Abnormal grain growth appeared at 15-100 μm from the bonded interface, with the largest grains reaching up to 7.2 times the size of the matrix grains. This abnormal grain growth is attributed to the uneven distribution of strain energy within the GNS, which enables some grains with energy, orientation, or size advantages to grow preferentially by continuously consuming surrounding finer grains. Fracture behavior analysis revealed that cracks initiated neither at the bonded interface nor within the abnormally large grains, but in the Zr matrix region approximately 130 μm from the interface. These grains exhibited numerous deformation twins and acted as crack propagation barriers by coordinating deformation with the surrounding finer grains. Despite their lower yield strength, the abnormally large grains positively contributed to joint strength through a strengthening mechanism induced by hetero-deformation. The shear strength of the Zr/Zr and GNS-Zr/GNS-Zr joints improved as the bonding temperature increased. The GNS-Zr/GNS-Zr joint achieved the highest shear strength of 376.9 MPa at 800 ^oC. Under the same bonding conditions, GNS-Zr/GNS-Zr joints exhibited 1.2-1.6 times higher shear strength than the Zr/Zr joints, with greater improvements at lower temperatures.

316H steel is an important structural material candidate for Generation-IV advanced nuclear reactors. During service, the material operates under conditions of high temperature, irradiation, and complex stress for extended durations, and the working environment is extremely harsh. In particular, He bubbles, generated through nuclear transmutation under irradiation, can cause severe irradiation embrittlement and accelerate the failure of the material. To clarify the role of He bubbles in the degradation of 316H steel and weld metal, this study proposes a coupled computational framework that integrates the phase field model and crystal plasticity. Within this framework, the nucleation, growth, and coalescence of He bubbles in 316H steel and weld metal were simulated, and their mechanical responses were systematically analyzed. The research shows that He bubbles nucleate and grow by absorbing supersaturated vacancies and He atoms. In the later stage, they grow through coalescence and Ostwald ripening processes. As the He bubble size increases, the internal pressure gradually decreases until reaching an equilibrium state. The high density of dislocations in the weld metal, which preferentially absorb interstitial atoms, results in an increased vacancy concentration. Meanwhile, dislocations act as rapid diffusion channels. These two factors together contribute to the rapid growth of He bubbles. An increase in diffusion capacity does not change the final proportion of He bubbles; instead, it accelerates the nucleation and growth processes, thereby promoting the kinetic evolution of He bubbles in the weld metal. At small strains, the stress-strain response is governed by the effects of external strain and the internal pressure of He bubbles, with the macroscopic stress value being negative. As the external strain increases, the stress-strain response becomes influenced by the external strain. During tensile deformation, significant stress concentration arises at the He bubble-matrix interface, leading to considerable plastic deformation in these regions. At 4% applied strain, distinct plastic deformation bands form in both 316H steel and the weld metal. However, due to strain localization in the weld metal, the degree of plastic deformation is greater than that observed in 316H steel.

Submerged arc welding (SAW) is a widely used technique for joining oil and gas pipelines and shipbuilding steels. During SAW, fluxes are critical for atmospheric shielding, weld metal refinement, and heat loss prevention. Their electrical conductivity—a key temperature- and composition-dependent property—dictates arc stability and weld pool heat distribution. However, the mechanisms governing ionic and electronic conduction remain inadequately understood. This study explores the conductivity of CaF₂-SiO₂-Al₂O₃-MgO(MnO) fluxes using a combination of four-electrode experimental measurements and ab initio molecular dynamics simulations, systematically analyzing contributions from ionic and electronic conduction. The results demonstrate that substituting MgO with MnO induces fluctuations in ionic conductivity, primarily owing to competing effects of structural polymerization and ion diffusion. Specifically, the degree of polymerization in Si and Al polyhedral structures and the proportion of bridging oxygens initially increase but later decrease with MnO addition. A general trend is observed in ionic diffusion coefficients, reflecting a balance between structural rigidity and ion mobility. However, MnO consistently enhances electronical conductivity. As MnO content increases, the partial density of states of Mn near the Fermi level rises considerably, indicating improved electron mobility. MnO facilitates electron migration by reducing electron localization around O and F atoms. Furthermore, the increased Bader charge on Mn atoms suggests enhanced charge transfer between Mn and O, thereby creating additional pathways for electron hopping. Consequently, the overall conductivity increases markedly with elevated MnO content, enabled by a consistent rise in electronical conductivity that outweighs fluctuating ionic contributions. These findings underscore the dominant role of electronic conduction in enhancing slag conductivity and the need for future efforts toward optimizing electronic transport through the rational design of transition metal oxides.

Residual stress generated during the welding of 34CrNi1Mo gear steel can lead to stress corrosion cracking and reduced fatigue strength. Therefore, the accurate prediction and effective control of residual stress in welded joints are of critical importance. In this study, a multipass butt joint with a plate thickness of 40 mm was fabricated using 34CrNi1Mo and Q355B steels. The residual stresses after welding and post-weld heat treatment were measured using the hole-drilling method. Based on the MSC.Marc software platform and the solid-state phase transformation characteristics of medium-carbon quenched and tempered steel (34CrNi1Mo steel), a thermal-metallurgical-mechanical multifield coupled finite element model was developed to simulate welding-induced residual stress. Additionally, a thermal-elastic/plastic finite element model that accounts for creep effects was developed to simulate stress evolution during post-weld heat treatment. This work primarily investigates the effects of solid-state phase transformation during welding on the distribution of residual stress as well as the effects of creep behavior during heat treatment on the degree of residual stress relaxation. A comparison of simulation results and experimental measurements indicates that solid-state phase transformation considerably affects the magnitude and distribution of longitudinal and transverse residual stresses in the heat-affected zone on the side of the medium-carbon quenched and tempered steel. When simulating heat treatment, the consideration of only the temperature-dependent variations in yield strength leads to considerable discrepancies between the predicted and experimental residual stress values. However, simulation results that incorporate the creep effect exhibit excellent agreement with the experimental data.

At present, the demand for hard-to-weld steels, such as high-nitrogen stainless steel, oxide-dispersion-strengthened steel, and twinning-induced plasticity steel, in the high-end equipment manufacturing industry has gradually increased. Low-cost and reliable joining is a prerequisite for meeting the diverse application requirements of these steels. Conventional fusion welding of hard-to-weld steels often produces metallurgical defects, including pores and cracks. In contrast, friction stir welding (FSW), a solid-state process performed entirely below the material's melting point, effectively avoids such defects. Moreover, its combined thermal-mechanical action promotes the formation of high-performance joints. However, the high temperatures and contact stresses associated with FSW of hard-to-weld steels can lead to tool wear and fracture. To address this limitation, this study proposes a novel prefabricated vortex flow-based FSW (PF-VFSW) process. A systematic investigation was conducted using 3-mm-thick Q195 steel to evaluate the effects of holder material, rotational speed, welding speed, and tool tilt angle on the joint's macroscopic morphology, microstructure, and mechanical properties. For a WC-Co holder with 0° tilt, the optimal rotation and welding speeds were 500 r/min and 20 mm/min. However, kissing-bond defects were observed at the bottom of the joint, with oxide distribution and unbonded regions increasing with distance from the top of the joint. Using a W-Re holder with a 1° tilt and adjusted process parameters eliminated these defects, with the optimal rotation and welding speeds being 300 r/min and 20 mm/min. Severe plastic deformation in the stir zone induced dynamic recovery and both continuous and discontinuous dynamic recrystallization, reducing the recrystallization fraction compared to the base material. The proportion of low-angle grain boundaries increased markedly, accompanied by pronounced grain refinement. The minimum average grain size in the stir zone was 3.8 μm, representing an 80.51% reduction relative to the base material. The microhardnesses of joints produced with WC-Co and W-Re holders increased by 6.44% and 18.90%, respectively, compared to the base material. Tensile strength improved by 1.74% to 317.26 MPa and 5.91% to 330.25 MPa, respectively, achieving a joint efficiency of 100% in terms of tensile strength relative to the base material. These results demonstrate that PF-VFSW is an effective, low-cost method for producing high-quality joints in Q195 low-carbon steel.

Narrow gap gas metal arc welding (GMAW) is increasingly applied in the manufacturing of thick-walled structures, such as large ships, offshore equipment, and pressure pipelines. Previous research focused on the improving weld formation and welding efficiency in this process but seldom addressed the correlations among the welding process, joint microstructure, and mechanical properties. The study aims to modify the microstructure and properties of the joint while overcoming the limitations of groove gap on cold wire swaying amplitude in deep-groove welding, thereby enhancing the practicality of the process. A rotating/swing arc narrow gap metal active gas welding assisted by a cold wire with variable swaying amplitude is proposed. Effects of arc rotation frequency, cold wire feeding speed, and the horizontal oscillation cooperative rate between the cold wire and the swing arc (η) on weld formation and welding efficiency are then investigated. Additionally, the evolution mechanisms of the microstructure and mechanical properties of cold wire-assisted rotating/swing arc narrow gap welding joints are clarified. Experimental results show that the cold wire-assisted rotating/swing arc processes yield stable weld formation and increase the welding efficiency by 25.7% and 44.2% at η ≤ 0.5, respectively. Compared to the rotating arc process, the swing arc process achieves greater penetrations into the groove sidewalls and weld bottom even at smaller swaying amplitudes of the cold wire; the swing arc has no the reheating effect on the rear of the molten pool, thereby narrowing the coarse-grain heat-affected zone (CGHAZ). This nonreheating effect, combined with the heat absorption effect of the cold wire, accelerates the molten pool cooling, substantially refining the weld grain size and toughening the CGHAZ. Owing to the dominant factors of the microstructure type and grain size, impact energy near the fusion line increases by 53.8% while weld strength rises by 6.0%. Consequently, the two cold wire-assisted processes concurrently improve welding efficiency and joint performance, advancing the application of high-quality, high-efficiency methods in narrow gap welding.