Abstract ：

The droplet state (temperature, mass and velocity) has a very important influence on the shape accuracy and material quality of the manufacturing components in wire-arc directed energy deposition (DED). The heat input of the wire end and base metal can be controlled freely by independently adjusting the main and bypass currents under a bypass hybrid plasma arc (BHPA). This approach offers great potential for regulating the base metal thermal state and the droplet state from the wire. However, the modulation capability and mechanism for the droplet state under BHPA are still unclear, which hinders the widespread application of this process. Therefore, in an innovative approach, an auxiliary magnetic field is utilized to deflect the hybrid arc tail flame based on the solid copper calorimeter, effectively avoiding interference. The droplet temperature can be measured using a solid calorimeter, while droplet mass and velocity can be determined using a high-speed camera and subsequent image processing. Based on this, the regulation rules of the wire feeding speed, main current and bypass current on the droplet state are studied. The results reveal that the droplet temperature ranges between 1138 and 1726 ℃ under BHPA, the droplet velocity varies from 0.02 to 1.75 m/s, and the droplet mass spans from 1.4 to 159 mg. Second, the heat flux at the wire end was quantified via numerical simulation. A positive correlation is identified between the droplet temperature and the heat flux at the wire end for unit mass transfer. The utilization efficiency of the droplet temperature for heat at the wire end increases with increasing X-coordinate. Subsequently, an analysis of droplet acceleration was performed to determine the distribution of the plasma flow force, revealing a Gaussian distribution with a characteristic radius of 2 mm. The contribution of the main current to the plasma flow force is approximately three times that of the bypass current. Finally, the distribution along the X-axis and the mathematical model of the droplet state were studied. X = -2 mm is a characteristic point of the distribution of the droplet state. This work reveals the regulation law of the droplet state under BHPA, and lays a foundation for further systematic study of the control mechanism of deposition layer morphology under this process. © 2024 Elsevier B.V.

Keyword ：

High speed cameras Plasma arc melting Plasma arc cutting Enameling Gaussian distribution Hard facing Positive temperature coefficient Calorimetry Plasma flow

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Abstract ：

Conductive hydrogels based on liquid metal microdroplets are widely used as wearable electronic devices. Droplet uniformity affects sensor sensitivity for weak signals, such as heart rate and pulse rate. Surface acoustic waves at micrometer wavelengths allow precise control of a single droplet, and have the potential to make uniformly discrete liquid metal droplets and distribute them in hydrogels. But the control law of liquid metal droplet size and its spatial configuration by acoustic surface waves is not clear. The aim of this paper is to present an analysis of the acoustic regulation mechanism in the interfacial evolution of fluids with high interfacial tension coefficients, and to investigate the influence of microdroplet generation characteristics (size and spacing) on the conductive and mechanical properties of conductive hydrogels. The results showed that the combined action of acoustic radiation force, shear force and pressure difference force helped to overcome interfacial tension and speed up the interfacial necking process during the filling and squeezing stages. The use of acoustic surface waves serves to diminish the influence of droplet size on the two-phase flow rate. This provides an effective approach for achieving decoupled control of microdroplet size and spacing, alongside the formation of a homogenous array of liquid metal droplets. The acoustic surface wave effect makes the liquid metal microdroplets more uniform in size and spacing. As the liquid metal content relative to the hydrogel substrate solution increases, the liquid metal size decreases. The hydrogel's initial conductivity and conductivity after self-healing increase by 10% and 25%, respectively, which can realize the effective monitoring of ECG and EMG signals. This study helps to reveal the evolution mechanism of liquid-metal interfaces induced by acoustic surface waves, elucidate the effects of microdroplet size and spacing on the conductive and mechanical properties of hydrogels, and provide theoretical guidance for the high-precision preparation of wearable electronic devices.

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Abstract ：

Treating oily wastewater by porous ceramic membranes derived from solid waste is of great significance. In the present work, porous ceramic membranes were fabricated using the dry-pressing and solid-state sintering method with red mud as the primary raw material and polymethyl methacrylate (PMMA) microspheres as pore-forming agents. The obtained membrane with a PMMA percentage of 35 % in the green body and sintering at 1100 °C (PCM35) possesses an open porosity of 57.22 %, a most probable pore size of 0.38 μm, an average pore size of 0.64 μm, and a flexural strength of 5.130 MPa, exhibiting amphiphilicity in air with the contact angle decreasing finally to 0° in 1.52 s for water and in 69.0 s for oil, and underwater oleophobicity with an oil contact angle of 148.9 ± 9.4°. The membrane was used for the separation of simulated oil-in-water emulsion. The separation performance depends on the oil concentration, the pH of the emulsion, and the transmembrane pressure. Membrane fouling occurs during the separation process, where four models, namely, complete blocking, intermediate blocking, standard blocking, and cake formation, are all involved in the fouling within 60 min. However, if the separation time is extended to 120 min, only the cake formation model will control the fouling. The membrane exhibits desirable long-term stability in separating the emulsion with an oil concentration of 500 ppm and a pH of 7 under a transmembrane pressure of 0.3 bar. The steady permeate fluxes remain relatively stable at approximate 131.4 L/(m2·h·bar), and the oil rejections exceed 95.8 % (higher than 99 % in the vast majority of cases) during the recycling process(totally for 25 cycles, 3000 min). The membrane was further adopted for real wastewater (with a high oil concentration of 21.5 g/L) treatment. The results show that the membrane can intercept most oil droplets within the wastewater with an oil rejection as high as 80.7 %, and with a steady permeate flux of 15.3 L/(m2·h·bar). Although the oil–water separation performance in this work is inferior to some of those reported previously, our ceramic membranes are highly competitive since they provide a promising strategy for comprehensively utilizing red mud. © 2025 Elsevier B.V.

Keyword ：

Pore size Wastewater reclamation Ceramic membranes Pressing (forming) Sintering Emulsions Emulsification Membrane fouling Wastewater treatment Water recycling Nafion membranes

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Abstract ：

As a conventional arc-depositing process, limited by the strong heat-mass coupling characteristics, gas metal arc welding (GMAW) is difficult to adapt to precision deposition requiring low heat input. To solve this problem, a novel high-dynamic locking-releasing wire feeding method driven by linear actuator was proposed and a compact mechanical structure was developed. The braking effect of the high-dynamic locking-releasing action of the wire feeding allows the molten droplets to acquire additional stronger drive forces, including the inertial force induced by the sudden locking of the wire and the elastic potential energy induced by the bending of the wire, both of which facilitate the droplet transition. The results show that the indirect 'energy storage' effect of the high-dynamic locking-releasing wire on the droplets enabled a stable one-droplet-per-pulse (ODPP) spray transfer with a transition frequency of 60 Hz at low heat input. The dynamic transition mechanism of the above ODPP process was revealed by kinetic simulations of the droplet. In addition, a new non-stationary re-locking (NSRL) control strategy was introduced, which further empowered droplets with greater inertial force through the sharp braking effect, thus significantly increased the transition frequency of the molten droplets (similar to 150 Hz). Further, by combining the above strategy with the growth characteristics of droplets under the specific pulsed currents, ODPP spray transfer with frequencies up to 190 Hz was achieved. These promising results indicates the promise of this method for precision arc deposition and even wire arc additive manufacturing at low heat input.

Keyword ：

One-droplet-per-pulse (ODPP) Gas metal arc welding (GMAW) Decoupling of heat and mass Spray transfer Droplet transition

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The core of programmable droplet transition is to realize the controlled directional deflection transition of the droplets in a certain area, and to regulate the droplet landing point by matching laser pulse program. On the basis of the symmetric arrangement of laser pulses, a programmable transition control strategy is proposed in the paper for the programmable transition of molten drops driven by the asymmetric arrangement of bilateral lasers. The effects of the pulsed laser process parameters on the transition behavior and deflection angle of the droplets under the asymmetric arrangement are investigated. The results show that, unlike the symmetric laser arrangement, one side of the asymmetric laser arrangement irradiates the solid-liquid interface and plays a cutting role at that point, while the other side of the laser irradiates the droplet body and mainly produces an impact effect on the droplet. The deflection of the droplet is mainly determined by the impact of the laser on the droplet on the right side, and the maximum deflection range of the droplet increases to 55°. A calculation model for the impact force of deflected droplets on the molten pool was established based on the force situation of the droplets. By using high-speed cameras to capture the transition process of the droplets, the impact force of the droplets on the molten pool can be calculated. © 2025 Harbin Research Institute of Welding. All rights reserved.

Keyword ：

Laser beam cutting Laser materials processing Drop formation Deflection (structures) High speed cameras

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The decoupling characteristics of multi-electrode arc heat and mass provide a reliable approach for achieving efficient and high-quality welding and additive manufacturing. However, the presence of polar effect reduces the stability of the main arc, limiting its application. To address this issue, this paper proposes the skew-coupling arc (SCA) and conducts in-depth research on the mass transfer (MT) mechanism. The paper conducts comparative experiments to evaluate the MT capacity (MTC) of the inter-wire arc (IWA) and analyzes how characteristic parameters of the SCA influence the MT performance of the IWA. The results indicate that the MTC primarily depends on the heat generated by the IWA itself. Compared to situation without the IWA, the deposition speed can reach 15.9 m/min with a growth rate of 356 % at an input voltage of 30 V. The IWA current is linearly related to the input voltage, with a growth rate of 13.3 A/V. At the same current, the MTC of the IWA is three times greater than that of the direct current gas metal arc, and can reach 21 m/min at a current of 280 A. Notably, a segment of molten liquid is suspended at the end of the cathode wire, with its behavior intimately linked to the transition dynamics of cathode droplets. The input voltage varies from 21 V to 27 V, and the temperature of the suspension liquid increases, resulting in a decrease in surface tension and a significant reduction in size from 4.6 mm to 1.9 mm. Raising the input voltage can significantly enhance the MTC of the IWA and promote anode droplet transfer, but the spot force at the base of the cathode droplet leads to an excessive increase in droplet size, hindering effective transfer. The cathode droplet size reaches 3.9 mm at an input voltage of 30 V. By adjusting the plasma arc current and plasma gas flow rate to regulate the plasma fluid force acting on the cathode droplets, the adverse effects of the spot force can be mitigated, thereby facilitating the droplet transfer process and simultaneously improving weld formation quality. These findings are expected to establish a theoretical basis for the MT and provide methodological guidance for the practical application of SCA heat sources. © 2025 The Society of Manufacturing Engineers

Keyword ：

Plasma arc melting Aerodynamics Vortex flow Electric arcs Flow of gases Hard facing Carbon dioxide arc welding Levitation melting Flux-cored arc welding (FCAW) Wire Plasma arc cutting

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Abstract ：

Microlubrication technology significantly improves lubrication performance and heat dissipation during metal cutting by spraying atomized oil droplets around the surface of the workpiece to form a uniform lubrication film. Therefore, it is of great significance to thoroughly investigate the effects of the viscosity characteristics of oil droplets and the inclination angle of the wall on the dynamic behavior of oil droplets impacting on the inclined wall. The dynamic process of viscous oil droplets impacting the inclined wetted wall was observed and analyzed through high-speed camera experiments, with the aim of investigating the effects of droplet viscosity, impact velocity, and wall inclination on the morphological characteristics and distribution pattern of the oil droplets. The results show that the morphological characteristics of oil droplets are affected by the coupling of droplet viscosity, wall inclination, impact velocity, etc. and that deposition spreading, jet slipping, and jet fracture will occur. The increased viscosity inhibits the generation of jet fracture phenomena, causing the critical angle at which jet fracture occurs to increase, leading to a shift in the critical kinematic morphology critical point. Increasing the Weber number and the inclination of the wall will be more favorable for the occurrence of the jet fracture phenomenon. The ratio of deposition spreading at low inclination increases with an increasing viscosity of oil droplets in a certain range, and the spreading characteristics of oil droplets with different viscosities are more significantly affected by the angle of the wall. The fracture time of the jet decreases with the increase of impact velocity, and the volume of the droplet increases with the increase of wall inclination.

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In the process of oil-gas micro-lubrication, the lubricant oil liquids are disturbed by high-speed airflow and are prone to forming oil droplets containing micro-bubbles. Micro-bubbles have a significant influence on the kinetic characteristics of successive/synchronous oil droplet impingement on an oil film and its oil film formation mechanism. Numerical simulations of successive/synchronous oil droplet impinging oil film behavior are based on the coupled level set-volume fraction (CLSVOF) method. The effect of micro-bubbles on the flow dynamics characteristics of the successive/synchronous impinge oil film of double oil droplets is investigated. To investigate the mechanism of jet formation during the impingement process, the law of micro-bubbles influences the formation process of the central and neck jets and their morphology is analyzed. The results show that when micro-bubbles exist, the oil droplet impingement process produces the central jet phenomenon. During successive impingements, the micro-bubble is in the pilot or trailing oil droplet, respectively, and there is a large difference in the jet mechanism generated by these two cases. The energy loss in the successive impingement process is larger when the micro-bubble is inside the pilot oil droplet. The central jet hurts the velocity discontinuity effect of the fluid inside the oil film and plays an inhibiting role in the growth of the crown splash. When the micro-bubble is only within the trailing oil droplet, then the surface tension effect excited by the impingement between the subsequent oil droplet and the flat fluid surface inside the impingement crater will cause a transient sudden reduction in the inner diameter size of the crown splash. During the synchronous impingement, the symmetric micro-bubble impinge mode has a positive effect on both the final horizontal expansion inner diameter of the crown splash and its vertical jet height enhancement. The research method of numerical simulation is used in this study. Seven sets of impingement models under the influence of different micro-bubbles are systematically studied and analyzed under the premise of verifying the feasibility of the numerical method. The results of this study can provide theoretical references for the study of the impingement kinetic behavior and film-forming flow mechanism of highly viscous droplets.

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On the basis of double-sided dual pulse laser-driven programmable metal transfer in gas metal arc welding, a back propagation neural network prediction model was established with dual laser power and pulse width as inputs and molten droplet deflection angle as an output. The reliability of the model was verified through process experiments. Preliminary exploration was conducted on the control effect of droplet re-aggregation under the action of dual pulse laser, as well as the active creation of weld morphology. The results indicate that when the base material melts more and forms a high-temperature melt pool, the droplets entering the melt pool according to the set trajectory can actively regulate the weld morphology to a certain extent. When the base material melts very little, a programmable droplet path can form a droplet assembly manufacturing similar to metal droplet spray deposition. By matching the process parameters of double-sided laser pulses, the formation of "two welds in one direction" and "wave shaped" weld formation, as well as the distribution of "zigzag shaped" and "semicircular" molten droplets, were obtained, and the assembly type weld seam formation and molten droplet formation were preliminarily achieved.

Keyword ：

droplet deflection transfer droplet re-aggregation programmable transfer active creation of welds

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The transportation behaviors and dynamic characteristics of double emulsion droplets in a Y-junction are experimentally investigated to explore the influence of the interfacial tension. Based on the breakup behaviors, three flow patterns are identified: non-breakup (NB), once breakup of the outer droplet (OB), and twice breakup of the outer droplet (TB). Compared to the single emulsions, the inner droplet leads to new flow patterns for the double emulsions due to the coupling effect between interfaces. The flow pattern map can be built using the normalized droplet length and the capillary number. With the aid of the quantitative expression of the transition thresholds using a power law relation, the transitions are further found to rely on the junction structure and the physical properties of the double emulsion. It is indicated that the dynamic characteristics of double emulsions are shaped jointly by multiple controlling parameters, such as the droplet length, the capillary number, and the junction structure. A force model is proposed to estimate the quantitative evolutions of the working forces in different flow patterns. The evolution of the droplet morphology is consistent with the forces acting on it, and the prerequisite condition for the droplet breakup is confirmed to depend on the competition between the sum of the driving forces and the sum of the interfacial tensions. The variation of the transition thresholds in different liquid systems is further analyzed, which may provide useful methods to manipulate the double emulsions by changing the physical properties.

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