Friction stir welding (FSW) is a suitable method for joining dissimilar materials. However, with respect to Al/steel joints with unequal thicknesses, conventional butt joining results in a limited effective bonding area, whereas conventional lap joining causes severe pin wear, a high risk of tool fracture, and increased bending moment and space occupation. In this study, a new combined butt−lap joint design for joining a 6 mm 6061-T6 Al alloy to 3 mm 304 stainless steel was proposed. Four different types of combined butt–lap joints for unequal-thickness Al/steel were developed and compared with the two conventional joints. The results indicated significant variations in the microstructures and mechanical properties with different joint types because the combined joints enabled effective bonding at both the butt and lap interfaces. In particular, the Type-C joint was characterized by reinforced mechanical interlocking between Al and steel, a thin single layer of IMCs at the lap interface, and continuous intercalated structures at the butt interface. As a result, the fracture load of the Type-C combined butt–lap joint was 892.03 N/mm, which was 43 and 79 pct greater than those of the conventional butt joints and conventional lap joints, respectively.

Weld pool behavior in underwater wet flux-cored arc welding (UWFCAW) plays a crucial role in determining the overall quality and efficiency of the welding process. In this study, a numerical analysis was conducted to investigate the effect of ultrasonic frequency pulse current (UFPC) on the dynamic behavior of the weld pool. A mathematical model of “water-droplet-weld pool” was established using the finite volume method, which considered the interaction between ultrasonic waves generated by UFPC and the molten metal. The study analyzed differences in energy, temperature, fluid flow, and pressure fields between conventional UWFCAW (C-UWFCAW) and UFPC-assisted UWFCAW (UFPC-UWFCAW). The results revealed that the application of UFPC significantly altered the weld pool behavior. Specifically, UFPC induced ultrasonic oscillations that generated a distinct counterclockwise vortex in the weld pool. This vortex promoted the downward flow of high-temperature molten metal from the surface to the bottom of the weld pool. The maximum temperature on the surface of the weld pool in UFPC-UWFCAW was higher than that of C-UWFCAW, and the temperature distribution became more uniform. Additionally, UFPC eliminated the small bulge at the front of the weld pool, which was beneficial in preventing undercut and improving welding quality. The simulated weld penetration, reinforcement, fusion line, and thermal cycle curve were in good agreement with the experimental results, demonstrating the accuracy of the computational model. These findings provide valuable insights into the underlying mechanism of UFPC-UWFCAW weld pool behavior and form a foundation for further optimizing welding processes in underwater environments.

The conventional metal inert gas (C-MIG) welding of 316L stainless steel under short-circuit transfer mode often encounters problems such as an unstable welding process and grain coarsening, which will deteriorate its mechanical properties and corrosion resistance. To address these issues, this study employed the ultrasonic frequency pulsed current assisted-MIG (UFPC-MIG) welding method to increase arc stability and facilitate microstructure refinement. The study investigated the welding process stability, weld formation, microstructure evolution, as well as the mechanical properties and corrosion resistance of the joints at various UFPC frequencies. Due to the skin effect and self-magnetic contraction effect of UFPC, the maximum width of the arc was reduced by 29.51

The beneficial effects of ultrasonic frequency pulse current (UFPC)–assisted underwater wet flux-cored arc welding (UWFCAW) on the welded joint performance have been experimentally confirmed. However, the mechanism by which UFPC parameters influence the temperature and flow fields of the arc-bubble system remains unclear, limiting further application prospects of this technology. To overcome these limitations, this study developed a multiphysics-coupled model to investigate heat and mass transfer processes in the arc-bubble system under varying UFPC parameters. By analyzing the UFPC waveform via Fourier series expansion and applying electromagnetic field theory, a novel approach was developed to elucidate the ultrasonic wave generation mechanism induced by the arc. This mechanism was subsequently modeled by incorporating the resulting ultrasonic radiation force as a time-varying source term in the momentum conservation equation. Simulation results indicated that increasing peak current and UFPC frequency significantly enhanced momentum output and pressure concentration in the arc core region, promoting stable bubble evolution. This synergistic effect improved arc thermal characteristics, reduced temperature and pressure fluctuations, and ensured continuous energy delivery to the weld pool. Moreover, higher peak currents expanded the high-temperature plasma region, while elevated UFPC frequencies enhanced arc stability and thermal symmetry. The combination of high peak current and UFPC frequency further improved momentum transport and flow stability by increasing plasma velocity and vortex strength. Experimental observations of bubble morphology, evolution period, and volume closely matched the simulation results, confirming the model’s accuracy. These findings provide valuable theoretical support for optimizing process parameters to improve weld quality and stability in UWFCAW.

For friction stir welding (FSW) of dissimilar components, the underlying correlation between in-process thermal-material flow and intermixing behavior and joint performance is crucial for process optimization and application in industry, but it is still far from being fully understood. In this study, a 3D model based on computational fluid dynamics (CFD) approach with a novel heat source and a self-adaptive boundary condition was proposed to simulate the multi-physics coupling fields during FSW of Al/Cu dissimilar alloys. Then, the numerical results were combined with the experimental observations to elucidate the significance of dissimilar Al/Cu flow and intermixing on joint formation mechanism and intermetallic compounds (IMCs) distribution. First, it was shown that both heat input and temperature increased with changing tool offset from Al-AS to Cu-RS, but decreased with increasing welding speed. Second, the dissimilar Al/Cu flow pattern around the pin was varied completely since the flow channel for Al-AS at the RS was changed from opening to closing, so that the dissimilar Al/Cu intermixing and interlocking was enhanced with changing tool offset from Al-AS to Cu-RS. However, there were limited variations of both flow pattern and mechanical interlocking of Al/Cu with increasing welding speed. Third, as an integrated result of the thermal-material flow behavior during Al/Cu FSW, the IMCs thickness at the bonding interface decreased with changing tool offset from Al-AS to Cu-RS, and also with increasing welding speed. Finally, it was demonstrated that numerical simulation well predicted the coupled multi-physics characteristics in FSW of Al/Cu with various welding conditions, and the significance of dissimilar material flow and intermixing on the FSWed Al/Cu joint performance was particularly elucidated.

The effects of dissimilar Al/Cu plate position on thermal, material flow and deposition behavior in FSW are studied in detail based on computational fluid dynamics (CFD) simulation with bonding interface tracking by volume of fluid (VOF) technique. A novel 3D steady-state model is proposed to compare the coupled multi-physics fields between Type-I (Al-AS/Cu-RS) and Type-II (Al-RS/Cu-AS) positions. A 2D transient-state model with refined grid is established for the dynamic evolution of the periodic feature with Type-I position by considering tool eccentricity. It is found that the total heat input is identical with different plate positions, but the temperature around the pin is about 50-100 K higher with Type-II than that with Type-I. The flow streamline analyses indicate that direct-through flow pattern is dominated with Type-I, whereas both direct-through and multi-circle flow patterns are subsistent with Type-II. The tool eccentricity is confirmed to be the primary factor for the formation of the periodic feature in dissimilar Al/Cu FSW, since it changes velocity field around the tool and causes reciprocating block-open of the flow channel at the RS. The modeling results are in good agreements with abundant experimental observations.

The optimal plate position of dissimilar AA6061 and AA2024 alloys during friction stir welding (FSW) is one of the most critical parameters, which affects the mechanical properties of the joints. Although extensive experimental research has been conducted on this issue, the underlying mechanism remains unclear so far. In the present study, numerical simulation is employed to predict temperature distribution and material flow between different plate positions by proposing a 3D model based on computational fluid dynamics (CFD) method. Numerical results, including heat generation, temperature, horizontal and transverse material flow during welding between different plate positions, are quantitatively analyzed, and validated with corresponding experimental observations. It is revealed that both total heat generation and peak temperature with 6A/2R condition are lower than those with 2A/6R condition. However, material evolution in both horizontal and transverse sections indicates more effective material mixing and stronger mechanical locking of the joint with 6A/2R condition than that with 2A/6R condition. Consequently, tunnel defects are more prone to appear at joint bottom with 2A/6R condition, thus higher joint tensile strength is achieved with 6A/2R condition. By integrating numerical results with experimental observations, the superiority of 6A/2R over 2A/6R plate position during dissimilar FSW is demonstrated.

In this study, a three-dimensional numerical model is established to investigate the dynamic behavior of the arc and bubble in ultrasonic frequency pulse current-assisted underwater wet flux cored arc welding (UFPC-UWFCAW). The aim is to deepen the understanding of the interaction between the arc and bubble in UFPC-UWFCAW process and evaluate its impact on welding quality. The mathematical model account for the effects of UFPC from both microscale and macroscale perspectives. On the one hand, a microscopic model of plasma under a non- equilibrium state is established to analyze the influence of UFPC on the thermophysical properties of plasma. This analysis reveals that UFPC influenced the plasma thermophysical parameters by altering the number density and velocity distribution function of particles in the plasma. On the other hand, the ultrasonic field excited by UFPC is added to the model as a momentum source term to investigate the influence of UFPC on the heat transfer and fluid flow process of the arc and bubble. To explore the effect of UFPC on the behavior of the arc and bubble, a comparison is conducted between conventional underwater wet flux cored arc welding (C-UWFCAW) and UFPC-UWFCAW. The simulation results demonstrate that UFPC effectively control the arc shape and bubble behavior, reducing instability during the welding process and improving the weld quality. This optimization effect is attributed to the high-frequency characteristics of UFPC, allowing for better control over the arc's heat input and affecting the bubble's growth, necking, and separation processes. The research provides a new theoretical basis for optimizing UFPC-UWFCAW and offers guidance for enhancing the welding process in practical industrial applications.

Investigating ultrasonic frequency pulsed current (UFPC)-assisted welding is crucial owing to its significant advantages over traditional welding methods, including enhanced weld quality, reduced defects, and increased efficiency. This study elucidated the generation mechanism of arc ultrasonic waves influenced by UFPC through theoretical derivation. A microscopic model of arc plasma under the influence of UFPC was developed, and the equations governing the thermophysical properties of the plasma were theoretically derived and calculated. A three-dimensional numerical simulation model was developed to analyze the dynamic behavior of arc plasma and molten metal in UFPC-assisted gas metal arc welding (UFPC-GMAW). The simulation results revealed that the incorporation of UFPC increased both the maximum arc temperature and plasma velocity, resulting in a conical shape of the arc. Additionally, the maximum arc temperature amplitude decreased from 13.6% in conventional GMAW (C-GMAW) to 9.6% in UFPC-GMAW, indicating improved arc stability. The increase in electromagnetic force led to a higher droplet transfer frequency. Moreover, the weld pool in UFPC-GMAW exhibited greater weld penetration than in C-GMAW. Notably, the variations in maximum arc temperature and velocity were synchronized with UFPC, resulting in localized thermal contraction and expansion within the arc. These changes affected the arc volume and shape, leading to the excitation of ultrasonic waves. The calculated droplet size and transfer frequency were consistent with the experimental results, confirming the reliability of the models. These simulation results provide valuable guidance for engineers in designing UFPC-GMAW technology to achieve high-quality welds.

More than ten ergot alkaloids comprising both natural and semi-synthetic products are used to treat various diseases1,2. The central C ring forms the core pharmacophore for ergot alkaloids, giving them structural similarity to neurotransmitters, thus enabling their modulation of neurotransmitter receptors3. The haem catalase chanoclavine synthase (EasC) catalyses the construction of this ring through complex radical oxidative cyclization4. Unlike canonical catalases, which catalyse H2O2 disproportionation5,6, EasC and its homologues represent a broader class of catalases that catalyse O2-dependent radical reactions4,7. We have elucidated the structure of EasC by cryo-electron microscopy, revealing a nicotinamide adenine dinucleotide phosphate (reduced) (NADPH)-binding pocket and a haem pocket common to all haem catalases, with a unique homodimeric architecture that is, to our knowledge, previously unobserved. The substrate prechanoclavine unprecedentedly binds in the NADPH-binding pocket, instead of the previously suspected haem-binding pocket, and two pockets were connected by a slender tunnel. Contrary to the established mechanisms, EasC uses superoxide rather than the more generally used transient haem iron-oxygen complexes (such as compounds I, II and III)8,9, to mediate substrate transformation through superoxide-mediated cooperative catalysis of the two distant pockets. We propose that this reactive oxygen species mechanism could be widespread in metalloenzyme-catalysed reactions.

High-quality joining between Mg and Cu based alloys was still a huge challenge due to excessive formation of detrimental intermetallic compounds (IMCs) by most of the conventional welding and joining methods. In this study, with controlling heat input and intensifying dissimilar material deformation and intermixing, friction stir welding (FSW) technique was employed for the joining between AZ31 Mg and T2 copper, and the process mechanism was elucidated at both macro and micro scales by combining experimental and numerical approaches. First, a process-based contact boundary was proposed for a precise description of the condition at the interface between the tool and the redistributed dissimilar Mg/Cu materials. Second, defect-free Mg/Cu FSW joints were obtained by using Mg-RS/Cu-AS configuration. With the tool offset of 1.0 mm to Cu-AS, the maximum temperature in the stirring zone was 710 K, which was lower than the minimum eutectic temperature of Mg-Cu binary system. Third, over 5-times difference of flow stresses resulted in upward and downward materials transfer, and also different flow-deposition behaviors of Mg and Cu, respectively, with effect of the rotating tool, which finally caused the elongation of Mg/Cu interface. The thickness variation of IMCs layer ranging 0.5–2.5 µm was originated from the inconsistent evolution of temperature, velocity and strain rate at different locations. Both Mg2Cu and MgCu2 IMC phases were observed, and the Mg2Cu layer was first generated, grown faster and finally much thicker than the MgCu2 layer. Moreover, an elongated and continuous IMCs layer was advantageous for achieving Mg/Cu joining strength over 124 MPa, which was greatly improved comparing with the published results.

A comprehensive numerical model was developed to investigate the effects of varying ambient pressures on arc and molten metal behaviors in local dry underwater welding (LDUW). The model accounted for the influence of ambient pressure on the thermophysical properties of plasma (mass density, specific enthalpy, specific heat, electrical conductivity, thermal conductivity, and viscosity), ensuring high accuracy of simulation results. The results revealed that increasing ambient pressure significantly concentrated the arc shape and altered the spatial distribution of metal vapor. This constriction fundamentally modified the arc plasma properties by reducing the effective heat transfer area and intensifying the interactions between plasma and molten metal. Key parameters such as arc temperature, plasma velocity, current density, and electromagnetic force all decreased with increasing ambient pressure, leading to reduced energy input to the weld pool. Furthermore, the increased ambient pressure altered the droplet transfer behavior. Higher ambient pressures reduced the droplet detachment frequency, while increasing droplet size due to the enhanced constriction of the arc and the altered surface tension forces at the plasma-droplet interface. To validate the numerical model, experiments were conducted using high-speed imaging to capture the real-time droplet processes, and the arc temperature distribution wad measured using spectroscopic methods. The experiments results showed excellent agreement with the simulation data, confirming the reliability of the model. This study provides valuable insights into the impact of ambient pressure on LDUW, offering a solid foundation for optimizing welding parameters to improve process efficiency and weld quality under varying high ambient pressure conditions.

The pore defects and grain coarsening are the main reasons for performance deterioration in wire-arc directed energy deposition (DED) of ultra-high-strength aluminium alloy. In this study, experimentation and numerical simulation were performed to investigate the Zn element loss, bubble behaviours, microstructure and performance and effects of wire feeding rate (WFR) in wire-arc DED of 7075 aluminium alloy. The results suggested that both the Zn-bubble and H-bubble did not flow upward directly, but flowed with the molten pool convection. A low WFR decreased the sources of Zn vapour and hydrogen instead of promoting the bubble escape, thus the porosity was decreased (up to 95.8% decrease with WFR = 4 m/min). Meanwhile, it reduced the grain morphology difference, and promoted the dispersed distribution of precipitated phases. After the heat treatment, the optimal performance reached 527.80 MPa of ultimate tensile strength and 7.57% of elongation, approaching the performance level of a forged 7075-T6 plate.

Conventional gas metal arc welding (C-GMAW) in short-circuit transition mode was often plagued by spatter, grain coarsening, and element segregation, which might degrade weld quality. To overcome these limitations, this study explored an ultrasonic-frequency pulsed current-assisted GMAW (UFPC-GMAW) technique for stainless steel welding. In this work, the underlying mechanisms of UFPC-GMAW, particularly the synergistic effects of arc shrinkage and microstructure evolution, were investigated. This study explained the principle by which the skin effect and self-magnetic contraction effect at the UFPC frequency of 20 kHz could enhance the arc compression, thereby increasing the droplet transition frequency and improving the process stability. Furthermore, a theoretical framework was developed to elucidate the role of UFPC in promoting ferrite dendrite fragmentation and modulating the molten pool temperature gradient. The electron backscatter diffraction (EBSD) analysis revealed that under the influence of UFPC, the grain size decreased by 15.08 %, and the residual stress was reduced. At the same time, the microhardness, tensile strength, and elongation were also enhanced. While UFPC-GMAW demonstrated potential for welding thin 316L stainless steel, further research was needed to comprehensively evaluate its broader applicability and reliability.

In this study, the ultrasonic frequency pulse current was used in underwater wet flux-cored welding (UFPC-UWFCAW) to improve the stability of the welding process and the mechanical properties of weld joints. The effects of ultrasonic frequency on bubble evolution characteristics, droplet transfer behavior and porosity in the weld seam were systematically studied by high-speed camera and x-ray inspection. The microstructure and grain characteristics of weld performed were analyzed by optical microscope and electron backscatter diffraction. The results showed that UFPC not only reduced the frequency of arc extinguishing but also distinctly inhibited the number of pores in welds. The average grain size and the amount of martensite in the welded joints decreased with increasing ultrasonic frequency, which reduced the maximum hardness of heat-affected zone. The fine microstructure and numerous high-angle grain boundaries appeared at an ultrasonic frequency of 40 kHz, which benefited the toughness value of the weld from 30.3 to 49.7 J/cm2. The research is meaningful to promote the development and application of UFPC-UWFCAW technology in marine engineering.

Tungsten alloys are widely employed in the fields of optics, medicine, and high-energy physics due to their exceptional physical properties. However, their inherent hardness, brittleness, and significant phase disparity present substantial challenges for precision & ultra-precision machining, including severe tool wear and surface defects. This research introduces ultrasonic elliptical vibration cutting (UEVC) with cemented carbide tools. It draws from the experience of UEVC of tungsten alloys with natural diamond tools in successfully achieving nanoscale surface. Comparative experiments involving cutting processes with and without the application of ultrasonic elliptical vibration were conducted to evaluate tool wear, chip formation, surface integrity, and the evolution of subsurface microstructures. The findings reveal that UEVC significantly suppresses tool wear and enables the formation of defect-free surfaces (Sa = 115 nm) compared to conventional cutting. The subsurface features a uniform, nanocrystalline layer (similar to 1000 nm in depth, with grain sizes ranging from 50 to 100 nm) and a broader dislocation distribution. This research corroborates the beneficial effects of ultrasonic vibrations in UEVC. It attributes the suppression of surface defects during the material removal process to the continuous ultrasonic impacts exerted by the tool. These impacts promote the proliferation, long-range motion, and interaction of dislocations, leading to a transition from brittle fracture to ductile removal modes, thereby supporting the prevailing "ultrasonic theory."

Biomimetic nicotinamide coenzymes, including nicotinamide mononucleotide (NMN+), have been demonstrated as promising low-cost alternatives to nicotinamide adenine dinucleotide (phosphate) (NAD(P)+) in biocatalysis. Herein, to efficiently regenerate NMNH from NMN+ in vitro powered by biomass sugars, a thermophilic NADP+-dependent glucose 6-phosphate dehydrogenase from Thermotoga maritima (TmG6PDH) was engineered to increase the activity toward NMN+. The catalytic efficiency (kcat/Km) of optimal mutant (TmG6PDH-R7) toward NMN+ increased by 71.7-fold than TmG6PDH-WT. As a result, compared to the wild type, the coenzyme specificity ([kcat/Km]NMN +/[kcat/Km]NADP +) of TmG6PDH-R7 increased by ~2.0×105-fold. The structural analysis revealed that the introduced hydrophobic and bulky residues lead to the formation of a smaller binding pocket, which resulting in a higher affinity for NMN+ with small size than NADP+. Then several in vitro synthetic enzymatic biosystems (ivSEBs) comprising this thermophilic TmG6PDH-R7 and a previously engineered thermophilic 6-phosphogluconate dehydrogenase were constructed. These ivSEBs harnessed the complete oxidation of renewable biomass sugars to facilitate the stoichiometric regeneration of 12 molecules of NMNH from 1 molecule of glucose, thereafter producing various products such as levodione, 2,3-butanediol or bioelectricity, over a wide temperature range. This study could pave the way for using stable and low-cost biomimetic coenzymes in ivSEBs for industrial biomanufacturing.

Industrial pharmaceutical wastewater usually contains butyl acetate (BA) with a concentration of 1 wt%–7 wt%, and the traditional method for BA recovery is distillation with high energy consumption. Adsorption method is developed to recover BA with low concentration for the high efficiency and low energy consumption. Medium polar polyacrylate resins with macroporous structure of 233.1 nm and average particle size of about 526.5 μm are successfully synthesized by suspension polymerization and used for the BA adsorption and desorption. The maximum adsorption capacity reaches 171.1 mg g−1 with relative standard deviation (RSD) value of 0.2%, which is more than twice the results in the literature. The BA desorption rate is 97.0% at 100 °C with RSD value of 0.4%, and the resins are beneficial to the reuse in the adsorption-desorption cycle. The adsorption thermodynamics and kinetics are investigated, and the BA adsorption is a spontaneous and endothermic process with the increase of disorder degree. This process is mainly contributed by physical absorption and agree well with Freundlich model and pseudo-first-order adsorption kinetic model. The adsorption method avoids boiling a large amount of wastewater and hopefully provides a novel alternative technology for the BA recovery.

Indole monooxygenases (IMOs) are enzymes from the family of Group E monooxygenases, requiring flavin adenine dinucleotide (FAD) for their activities. IMOs play important roles in both sulfoxidation and epoxidation reactions. The broad substrate range and high selectivity of IMOs make them promising biocatalytic tools for synthesizing chiral compounds. In the present study, quantum chemical calculations using the cluster approach were performed to investigate the reaction mechanism and the enantioselectivity of the IMO from Variovorax paradoxus EPS (VpIndA1). The sulfoxidation of methyl phenyl sulfide (MPS) and the epoxidation of indene were chosen as the representative reactions. The calculations confirmed that the FADOOH intermediate is the catalytic species in the VpIndA1 reactions. The oxidation of MPS adopts a one-step mechanism involving the direct oxygen-transfer from FADOOH to the substrate and the proton transfer from the -OH group back to FAD, while the oxidation of indene follows a stepwise mechanism involving a carbocation intermediate. It was computationally predicted that VpIndA1 prefers the formation of (S)-product for the MPS sulfoxidation and (1S,2R)-product for the indene epoxidation, consistent with the experimental observations. Importantly, the factors controlling the stereo-preference of the two reactions are identified. The findings in the present study provide valuable insights into the VpIndA1-catalyzed reactions, which are essential for the rational design of this enzyme and other IMOs for industrial applications. It is also worth emphasizing that the quantum chemical cluster approach is again demonstrated to be powerful in studying the enantioselectivity of enzymatic reactions.

Wire-arc directed energy deposition technology was employed to manufacture ultra-high-strength Al-Zn-Mg-Cu alloy taking advantage of variable polarity cold metal transfer (CMT) process. The grain characteristics and precipitation behavior under as-deposited (AD) and post-processing heat treatment (PHT) states were comparatively analyzed. The relationship between the microstructure evolution and mechanical performances was fully disclosed. The obtained results show that the grain structure of AD sample exhibited a nearly fully-equiaxed feature (average size: ∼45 μm), which was caused by the low heat input and variable polarity of CMT advance mode. The microhardness distribution of AD sample consisted of slight fluctuation and stable areas along the building direction due to the different thermal history and precipitation process. After the PHT process, the average grain size and morphology almost remained unchanged. However, compared with the size and type of precipitation phases under the AD state, high-density nanoscale (∼5 nm) metastable η′ phases were precipitated in the grain interior of the PHT sample, which contributed to the crucial precipitation strengthening effect. The homogeneous microhardness distribution was achieved with a high level (∼ 180 HV0.2). Meanwhile, tensile properties and elongation significantly increased to 524.68 ± 7.31 MPa and 7.10 ± 2.09 % after the PHT process, respectively.