Search

The utilization of uniformly deposited metal droplets has attracted significant attention for diverse applications, such as rapid prototyping and manufacturing. Achieving flawless aluminum structures through droplet-based liquid metal 3D printing (LM3DP) is critical in aerospace and electronics. However, challenges persist in eliminating defects during deposition due to constrained temperature ranges and complex impact dynamics. This study introduces a 3D computational model using a volume of fluid technique to analyze the consecutive deposition of molten aluminum droplets on a heated substrate, protected by nitrogen gas. Simulations reproduce droplet shapes in agreement with experimental results. Molten aluminum droplets solidify layer by layer continuously in an upward direction due to high thermal conductivity, forming surface ripples. L-shaped ripples emerge on neighboring droplets due to combined effects of solidification and oscillation, which involve alternating spreading and recoiling of the droplets leading to defects like cold laps, whereas bottom-hole defects occur due to inadequate metal filling with the substrate. This investigation systematically explores shape, defect formation, temperature, solid fraction, and velocity evolution during continuous deposition. Insights establish a fundamental basis for LM3DP technology. [ABSTRACT FROM AUTHOR]

The determination of solid–liquid interfacial tension plays an important role in science and technology. Here, we propose a simple method to directly measure the contact angle between metal droplets and a graphite substrate for the determination of metal–graphite interfacial tension. The proposed method involves the synthesis of micro- and nanosized metal droplets on graphite by arc melting. Owing to its small volume, the rapid cooling of the prepared metal droplets on the graphite substrate leads to the freezing of equilibrium contact configuration after solidification. We observe that the measured contact angle between micro- and nanosized Au (or Ag) particles and the graphite substrate is almost size independent, even though the size of the particles synthesized herein is 1–3 orders of magnitude smaller than that studied in previous works. In addition, the interfacial tensions of Au and Ag on the step edges (edge plane) of graphite are found to be larger than that on the (0001) plane (basal plane). The proposed method provides a simple approach to determine the solid–liquid interfacial tension and may be effective in the study of interface related science and technology.Highlights: A simple method to directly measure the contact angle between metal droplets and graphite by the fast cooling of metal droplets on graphite. The contact angle between micro- and nanosized Au (Ag) particles and graphite is almost size independent. The contact angle and interfacial tension of Au (Ag) on the step edges of graphite are much larger than that on the basal plane of graphite. [ABSTRACT FROM AUTHOR]

Changes in the composition of groups of droplets (sprays) emitted by a drop of water with a diameter of D = 0.42 cm at a velocity of U = 3.3 m/s falling into a crucible with a metal melt (Rose alloy at a temperature of 200°C) are traced by photo and video recording for the first time. In the splash formation mode, groups of small water droplets are successively ejected from the tops of the spikes on the veil around the primary contact area and then from the spikes at the rim of the spreading layer. Further, water droplets fly out vertically and obliquely from the tops of short jets, the streamers. Groups of small droplets are formed when large bubbles burst. After a splash has been formed and its top with suspended gas bubbles has been detached, an emission of compound droplets consisting of a metal core and a water envelope is observed. At the last stage, in addition to water streamers, rarer streamers consisting of a melt alloy are observed and metal droplets fly out from the tops of the streamers. The geometry of the cavity and the layer of boiling water of the spreading drop is traced. [ABSTRACT FROM AUTHOR]

The oxygen-converter production of steel is determined by processes in the converter’s reaction zone, which consists of primary and secondary regions. The primary region is the crater formed by the collision of a supersonic gas jet with the molten-metal surface. It is filled with metal droplets (diameter 0.1-2 mm). The surrounding secondary region consists of melt with an enormous quantity of gas bubbles (diameter 0.2-4 mm). The total surface area of the droplets and bubbles is four orders of magnitude greater than the surface of the quiescent melt. That indicates the important role of processes at phase boundaries in steel production. The structure of the reaction zone and the corresponding temperature distribution are studied by hot simulation, when the molten metal is blown by oxygen in a transparent quartz crucible. The transparent walls permit photographic and video recording of the processes in the crucible. Besides the temperature distribution, the hydrodynamics of the bath may be studied directly in the injection zone. The most unexpected result of hot simulation is the motion of the bubbles in the secondary region. They move normal to the crater surface. In other words, their motion is almost horizontal, rather than vertical, as in cold simulation in water. This may be attributed to nonuniformity of the melt’s surface tension, resulting in motion of the bubbles toward higher temperatures. In liquid with a temperature gradient, the surface tension will be different ahead of and behind the bubbles. The forces pushing the bubbles from behind are greater than the forces at the front. Accordingly, they move toward the region of lower surface tension. The nonuniformity of the surface tension is due to the temperature gradient (up to 1200°C within the secondary region) and the change in concentration of the melt components, especially oxygen. The surface tension of the ferrocarbon melt changes in a complex manner with increase in temperature. The surface tension rises on heating to 1550°C, but begins to decrease beyond 1550-1600°C. With decrease in carbon content in the melt, the maximum value of the surface tension increases. The motion of gas bubbles and other phases toward lower surface tension begins at the 1550°C isotherm, which is therefore the external boundary of the secondary region, separating it from the remainder of the bath. Within this boundary, the resultant vector of the surface forces pushes the gas bubbles and slag particles, together with the molten metal, horizontally toward the crater, at increasing speed. This determines the hydrodynamics of the smelting bath and the associated redistribution of oxygen over different parts of the bath and hence the refining process as a whole. [ABSTRACT FROM AUTHOR]

This study presents a new, simple and robust, pneumatically actuated method for the generation of liquid metal micro droplets in the nano- to picoliter range. The so-called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes liquid plugs in its center by means of capillary forces. Single droplets of the liquid metal can be pneumatically generated by the interaction of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. For experimental validation, a print head was build consisting of silicon chips with a star-shaped nozzle geometry and a heated actuator (up to 280°C). The silicon chips are fabricated by Deep Reactive Ion Etching (DRIE). Chip designs with different star-shaped geometries were able to generate droplets with diameters in the range of the corresponding nozzle diameters. The StarJet can be operated in two modes: Either continuous droplet dispensing mode or drop on demand (DoD) mode. The continuous droplet generation mode for a nozzle with 183 μm diameter shows tear-off frequencies between 25 and 120 Hz, while droplet diameters remain constant at 210 μm for each pressure level. Metal columns were printed with a thickness of 0.5-1.0 mm and 30 mm height (aspect ratio >30), to demonstrate the directional stability of droplet ejection and its potential as a suitable tool for direct prototyping of the metal microstructures. [ABSTRACT FROM AUTHOR]