Aluminum Droplets Research Articles

Effects of surface oxide layer on the impact dynamic behavior of molten aluminum droplets

Oxide layer is easily formed on the surface of some metal droplets, which affects the dynamic behavior of droplet impact. In this paper, experiments were conducted for the impacting process of a molten aluminum droplet on a Teflon substrate. The thickness of surface oxide layer of aluminum droplets was controlled by varying oxygen concentrations in the gas atmosphere. The effects of oxygen concentration and Weber number on droplet spread, rebound, and splash behavior were investigated. The results show that the oxygen concentration in the environment directly affects the droplet impact behavior. The differences in the droplet retraction process at different oxygen concentrations are more significant than the spread process. Strong retraction causes droplet rebound (We = 17.11) and splash (We = 102.3) in hypoxic environments, whereas only weak retraction in air. The viscous dissipation of the droplet impact in different oxygen concentration environment is estimated by counting the droplet bounce height, and the dissipation increases with the increase in the oxygen concentration. The thickness of the oxide layer measured by scanning electron microscope became thinner as the oxygen concentration decreases and, accordingly, the aluminum droplet collisions show different dynamic behavior. The experimental results are analyzed and explained regarding the reduction in surface tension, the increase in viscous forces, and the shear-thinning properties which increase the viscous dissipation during retraction. Ryan model was used to predict the droplet maximum spreading factor ξmax with the Weber number to the power of 0.5, better than the energy conservation-based Pasandideh-Fard model.

Improving ignition and combustion performance of Al@Ni in CMDB Propellants: Effect of nickel coating

Micron-sized aluminum particles tend to melt into agglomerates, leading to incomplete combustion, two-phase flow losses, and energy decrease of propellants. The higher ignition performance of core–shell Al@Ni makes it a promising candidate fuel with high combustion efficiency. However, little work has been done to study the combustion of Al@Ni in propellants. Here, we report a first attempt to investigate the combustion mechanism of core–shell Al@Ni in the double-based propellant containing high energy explosive HMX, and evaluate its potential as a fuel. Nickel in Al@Ni is determined to play a critical role during its intrinsic combustion and the combustion in propellants. In the condensed phase, the presence of Ni enhances the oxidation process by forming a porous oxide layer on the aluminum surface that promotes oxygen diffusion. Subsequently the phase transition and expansion of Ni break down this oxide layer, leading to the earlier ignition on the burning surface. Upon Al melting, the alloying of Ni and Al accelerates the volume expansion and surface tension reduction of the fuel, giving rise to vigorous melt-dispersion combustion and enhanced energy release. Enhanced thermal feedback and thermal decomposition exotherm combined to increase the burning rate of propellant. The resulting higher flame temperature allows the evaporation of small aluminum droplets into vapor to form smoke oxide productions, further reducing the size distribution of the condensed combustion products (CCPs). This study demonstrates the potential of Al@Ni as an effective fuel for double-based propellants, and provides the insight into its combustion mechanism in HMX-CMDB propellants, which is believed to be important for the development of novel aluminum-based fuels for solid propellants.

Crystallization of Aluminum droplet at low undercooling: homogeneous nucleation approach

Crystallization in an undercooled Aluminum (Al) droplet occurs via the formation of crystalline nuclei by homogeneous or heterogeneous nucleation and successive growth of nuclei. Clusters of a new phase appear in the system due to fluctuations and after reaching a critical size grow up to microscopic size. Crystallization event at low undercooling 6.4°C of 9.91 mg in compact Al sample was detected using isothermal differential scanning calorimetry (DSC) after several tenths of minutes. In this case, standard analysis based on Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is not appropriate as Avrami parameter n obtained by fitting of DSC data is too low (n ≤ 1 and thus the dimensionality of the system d = n – 1 ≤ 0). Al growth rate is extremely high and that is why we presume that the time delay of crystallization, detected by DSC, is slightly higher than nucleation time delay. A homogeneous nucleation model is applied to determine the basic characteristics of nucleation: the size distribution of nuclei, nucleation rate, total number of nuclei and crystallization fraction at the nucleation process. The number of atoms in a liquid Aluminum droplet decreases with time as the formation of a new crystalline phase occurs. As a consequence, a decrease in stationary nucleation occurs. It is shown that decrease in the number of atoms in a liquid Al droplet is predominantly caused by formation of subcritical clusters.

Quantitatively characterizing the evolution of hole defects between overlapped aluminum droplets by a two-dimensional solidification model

Quantitatively characterizing the effect of process parameters on the evolution of holes in metal droplet 3D printing enables effective prediction and elimination of hole defects in printed parts. In this work, a two-dimensional solidification model is proposed to quantitatively characterize the influence of substrate temperature on hole morphologies between overlapped aluminum droplets. First, a phenomenon is observed in experiments that the surfaces of overlapped droplets show typical "L"-shaped solidification ripples, which indicates that the solidification of overlapped droplets involves two-dimensional solidification. Hole defects are formed in the transition area of two-dimensional solidification. In addition, the appearance of the first L-shaped ripple signifies that the hole is no longer changing. Based on this discovery, a two-dimensional solidification prediction model for the hole size is constructed by combining a hyperbola and the one-dimensional heat transfer model to trace the ripple, which can accurately demonstrate the relationship between the hole size and the parameters (i.e., the thickness of solidified layer X(t) , the two-dimensional solidification angle β , the solidification angle θ ). The hole formed between the two overlapped droplets can be divided into two parts: the hole Ⅰ caused by the two-dimensional solidification and the hole Ⅱ caused by the morphology of the previously deposited droplet. When X(t) ≤ 0 at the end of the spreading of the overlapping droplets on the substrate, the hole can be completely eliminated. This work provides effective theoretical guidance for the prediction and elimination of hole defects in metal droplets printed parts.

Effect of impact velocity on molten aluminum and copper droplets igniting expanded polystyrene foam

Hot molten metal droplets with high fire hazard can easily ignite combustibles. Molten metal kinetics on the surface of a combustible material directly affects the ignition likelihood. Existing research focuses on the behavior of droplets during collisions, rarely addressing the ignition of combustibles by hot metal droplets. Here, the mechanisms of aluminum and copper droplets impinging on and igniting extended polystyrene (EPS) foam boards at different velocities were investigated. The relationship between the critical ignition temperature and impact velocity of droplets with diameters of 6 and 8 mm was experimentally studied for aluminum droplets; the critical ignition temperature non-monotonically depended on the impact velocity. For copper droplets, the relationship between the ignition probability and the impact velocity of droplets with diameters in the 3.5–7 mm range was experimentally studied. The most obvious difference between the two droplet ignition types was that the impact of copper droplets was accompanied by intense splashing, and the fragmentation extent positively correlated with the impact velocity. It was challenging to ignite using completely broken copper droplets. Droplets with the diameter of 5 mm were the most dangerous under the experimental conditions of this study, because the foam could still be ignited at higher impact velocities. Numerical simulations suggested that the main factors explaining the critical ignition temperature of aluminum droplets were gas mixing and splat cooling. The main factor affecting the ignition of copper droplets was fragmentation, and experimental observations were explained using non-dimensional droplet fragmentation theories.

Study of the process of metal droplets with high surface tension impinging on wall

The behavior of aluminum droplets impacting a wall critically affects the operation of solid rocket motors. In the present study, the flow and rebound of aluminum droplets with a high surface tension are studied experimentally and numerically. In addition, the impact of aluminum droplets at different inclined angles is monitored experimentally. When the droplet is about to rebound away from the wall, it is stretched to its maximum length. An empirical correlation formula is proposed to predict the maximum length when a droplet is about to bounce off a flat plane. The velocity and pressure distributions of a droplet flowing over a flat plane and an inclined plane are compared by using the volume of fluid method. Furthermore, the restitution coefficient of the droplet is discussed in detail. When normal Weber number Wen ranges from 0 to 20, normal restitution coefficient en ranges from 0.3 to 0.6. When tangential Weber number Wet <10, tangential restitution coefficient et is subject to great uncertainty. When 10 < Wet < 80, et is maintained at 0.75. The formulas for the total restitution coefficient, normal restitution coefficient, and tangential restitution coefficient of the aluminum droplets are also presented. Based on the principle of conservation of energy, we calculate the remaining energy of the aluminum droplets impinging on a wall and provide the relationship between the restitution coefficient and the dissipated energy. The results elucidate the mechanisms at work when aluminum droplets collide with a wall.

Mechanism of Aluminum Droplet Nucleation and Ripening on GaAs(001) Surface by Molecular Beam Epitaxy

Mechanism of Aluminum Droplet Nucleation and Ripening on GaAs(001) Surface by Molecular Beam Epitaxy

Erosion characteristics of molten aluminum droplets bouncing off solid walls in solid rocket motors

Based on the microscopic process of particles eroding the solid wall in the combustion chamber of solid rocket motors, the erosion mechanism of aluminum droplets in the process of vertical impact on solid walls is analyzed. Firstly, a method for measuring the impact force of the high-temperature molten-metal droplet is proposed on the basis of the pneumatic droplet generation method and piezoelectric force measurement technology. Then, the impact force of aluminum droplets with varing diameters and velocities is systematically studied. Additionally, the time-domain process of impact force in the rebound process of the aluminum droplets is obtained, and the coupling mechanism between the impact force and deformation process is analyzed and obtained. Then, based on the VOF interface tracking and the PLIC reconstruction method, the numerical model of the aluminum droplet impact force is established. On this basis, direct numerical simulations on the erosion effect of aluminum droplets on solid walls are carried out. Combining with the experimental results, the generation mechanism of the impact force and the influence mechanism of various parameters on the impact force during the rebound process of aluminum droplets are explored in depth. Ultimately, a semi-empirical prediction formula for the maximum value and duration of the impact force in the rebound process of the aluminum droplets is developed. Our research work can improve the cognition of the mechanism of the insulation materials in the combustion chamber of solid rocket motors, and thus expand the knowledge of engine structure design work.

A Primary-Auxiliary Coupled Neural Network for Three-Dimensional Holographic Particle Field Characterization

Particle field measurement is an important topic in many industrial branches. However, there are always complex imaging scenes in the engineering experiments, resulting in severe imaging artifact, noise, and blur, such as the optical holography. In this article, we propose a primary-auxiliary coupled neural network (PANet) for 3-D holographic particle field characterization, which can obtain a comprehensive particle measurement, including the identification, focus determination, segmentation, and size estimation. PANet is constituted by two subnets that are arranged in a coupled architecture, i.e., a Primary-Net (PNet) and an AuxiliaryNet (ANet). As the main frame, PNet is designed to accomplish the detection of most particles, while ANet aims to detect the tiny particles that PNet cannot identify. We exploit an alternative training method to realize their functional differentiation and complementation. PANet is evaluated on two kinds of holographic particle field data, i.e., high-energy laser shock aluminum target and droplet breakup in high Mach shock wave. By means of semisupervised learning and a specific loss function, the effect of deficient particle labeling can be alleviated. Experimental results demonstrate that PANet can achieve excellent performance in particle field characterization, especially for those with a wide size span and complex image background.

Three-dimensional numerical analysis and experimental confirmation for investigating the ground-based lateral droplet ejection toward microgravity simulation

Rapid in situ manufacturing is significant in space exploration. Droplet-based printing technology with micrometer accuracy has great potential in space due to the advantages of convenient transportation, customized metal material, and good environmental adaptability. It could achieve the ground microgravity simulation by a small Bond number (Bo < 1). The present work proposes a new method to evaluate the reliability of the ground microgravity simulation in the lateral metallic droplet-based ejection. The three-dimensional model is developed to numerically analyze the droplet ejection dynamic process coupled with the volume of fluid method and the k–ω shear stress transport model. The model accuracy and efficiency are improved by the local fine grid. In addition, the computation is validated by the cryogenic alloy droplet ejection experiments and theoretical analysis. The proposed theoretical analysis equation has good agreement with the SnPb alloy droplet ejection trajectory. Reynolds number (Re), Weber number (We), Froude number (Fr), Ohnesorge number (Oh), and breakup length (Lb) are used to analyze the gravity influences on the droplet ejection process of different materials, nozzle length–diameter ratios, and crucible fluid unfilled heights. The ejection direction has little effect on the aluminum droplet formation time and breakup length and the gravity effect increases with the length–diameter ratio and unfilled heights. In simulated results, the minimum We number of the aluminum droplet formation is 0.22 and the cryogenic alloy droplet formation is 0.19. The reliability of ground physical microgravity simulation is dependent on material selection, and aluminum is more suitable than the cryogenic and SnPb alloys.

Offset impacting of a liquid aluminum droplet train on a molten pool on a horizontally moving substrate during TIG-assisted droplet deposition manufacturing

Internal metallurgical defects and poor mechanical properties and surface quality of the formed parts are the most important challenges of metal micro-droplets deposition manufacturing technology. Lack of strong coalescence between neighboring droplets and the blocking effects of periodic ripples on the fusion process are major sources of abovementioned problems. To address this issue, a tungsten inert gas (TIG) welding arc was introduced to pre-melt the material below the droplet impacting location, thus, one droplet train will be deposited on a curved molten pool surface to fabricate deposited layers in an additive approach. In the present work, a numerical model based on the computational fluid dynamics (CFD) method was developed to investigate the dynamic behaviors of successive droplets’ impact and coalescence on the molten pool created by a tilted TIG welding arc, and focuses on the effect of the offset distance between the impacting point of droplet and the longitudinal plane of molten pool on the deposition morphology and the impact-induced molten pool behaviors. The simulated tracks exhibit regular scaly shapes at different offset distances, which is validated against the corresponding experiments. The results indicated that as the lateral offset distance increases, the symmetric spreading and recoiling of the subsequent droplets on the pre-existing bead will be broken due to the inclined and curved solidification front morphologies, the inclined front morphologies are caused by the increased growth rate of the solidification front at the impact zone. With an increased lateral offset distance, the volume of the molten pool decreases, the width of the rear molten pool increases slightly. The calculated solidification parameters are found to be dependent of the offset distance, and vary significantly at the onset of solidification at different portions of the molten pool. The scientific findings provide useful insights for further researches of TIG-assisted DDM.

Drag and heat transfer of metal and oxide agglomerates in flow of combustion products of solid propellant

Modeling and simulation of the motion of agglomerates with a complex composition and shape in the flow of combustion products is of interest for problems related to design and optimization of solid propellant rocket motors. An approach is developed for modeling of unsteady processes that occur when a particle consisting of an aluminum droplet and a condensed oxide particle attached to it (aluminum droplet with oxide cap) is transported by a viscous incompressible fluid flow. Methods are proposed to take into account the sliding of an attached oxide particle over the surface of the aluminum droplet. The results of numerical computations are used to find the drag and heat transfer coefficients of a non-spherical particle at flow velocities corresponding to the formation of separation zones. The constructed model is one of the components of a more general mathematical model designed to calculate two-phase flows of combustion products based on a multi-scale approach to the simulation of two-phase flows in a complex environment.

Peculiarities of aluminum particle combustion in steam

This work experimentally addresses aluminum combustion in steam, pure or mixed with diluents, for aluminum particles in size range 40∼80 µm, using an electrodynamic levitator. High-speed videos unveil an unreported and complex mechanism in steam, not observed in other oxidizers. The detached flame is quite faint and very close to the surface. Alumina smoke around the droplet rapidly condenses and coalesces into a large, single orbiting alumina satellite. It eventually collides the main aluminum droplet while generating secondary alumina droplets. A unique feature is the presence of several distinct oxide lobes on the droplet, which merge only at the end of burning and encapsulate the remaining aluminum, possibly promoting an incomplete combustion. The measured burning times in pure water vapor are longer than expected and the efficiency of steam is found to be 30% that of oxygen, lower than the usually accepted value of 60%. A general correlation on burning time, including the major oxidizers, is proposed. Direct numerical simulations are conducted and are in line with experiments, in terms of burning rate or flame stand off ratio. Combustion in steam seems mostly supported by surface reactions, giving a faint flame with low gas temperatures and high hydrogen content. It is speculated that those two specific features could help explain the peculiarity of steam.

Effect of polymer substrate elasticity on splat formation during thermal spraying

In this study, we have numerically investigated the effect of elastic properties of polymer substrates on the spreading of the metal droplets. The spreading dynamics of an aluminum droplet on solid substrates with elastic modulus in the range of 50 MPa to 210 GPa has been studied keeping the effects of other physical phenomena like viscosity, surface tension, heat transfer, and solidification unchanged. The results show that the elastic modulus of the substrate has a significant impact on the droplet spread factor owing to the resilience of the substrate. While the substrate behaves like a rigid surface beyond E = 10 GPa, at lower values, the spread is impeded owing to absorption of the energy by the substrate on impact leading to lower spread factors. The introduction of an air jet for cooling the substrate shows that the convective losses are minimal and conduction from the droplet to the substrate remains the dominant mode of heat transfer. As a result, deposition of a bond coat of copper was considered for protection of the substrate from localized heating. It was found that droplet spread was invariant to bond coat thickness in the range of 20 μm and 40 μm as well as any change in surface wettability for the material and range of velocities considered thereby establishing the effectiveness of inserting a Cu bond coat in protecting the substrate without compromising the spreading characteristics of the droplet.

Homogeneous nucleation and crystallization model of aluminum droplet based on isothermal DSC analysis

Homogeneous nucleation and crystallization model of aluminum droplet based on isothermal DSC analysis

Study on the mechanism of molten aluminum droplets igniting EPS foam

Molten metal droplets with higher fire risk may cause large fires. The ignition by molten metal droplets is more complicated than by solid inert spots due to the latent heat release of solidification and fluid spread involved. There are few studies on the ignition by molten metal with liquid behavior at present. The mechanism of molten aluminum droplets igniting EPS foam was studied in this work. A small spherical aluminum particle (4-12 mm in diameter) was heated and melted to a pre-set temperature (740-1000 °C) and was dropped to a foam sample at a fixed height (20 cm). It was observed that the droplet became a splat after a collision due to the flow of liquid aluminum. The splat diameter was recorded to analyze heat transfer during the collision process. Results show that as the diameter increased from 4 to 12 mm, the critical temperature to ignite decreased from 975 to 760 °C. Compared with solid hot particles, the critical temperatures for ignition and fire spread by molten aluminum droplets were much lower. The internal energy of droplets was also investigated, and the energy per unit contact area was found to be a critical parameter (about 2.5 MJ/m2), which determines whether the ignition will occur. The diffusion of combustible gas was numerically studied and was found consistent with the trend of experimental results. Moreover, the effects of temperature and the initial size of droplets on gas mixing time were also theoretically analyzed. The competition between the gas mixing time and the splat residence time explained the variation of critical ignition temperature with droplet size.

Sonoprocessing of wetting of SiC by liquid Al: A thermodynamic and kinetic study

The sonoprocessing of droplet spreading during the wetting process of molten aluminum droplets on SiC ceramic substrates at 700 °C is investigated in this paper. When wetting is assisted by a 20 kHz frequency ultrasonic field, the wettability of liquid metal gets enhanced, which has been determined by the variations in thermodynamic energy and wetting kinetics. Wetting kinetic characteristics are divided into two stages according to pinning and depinning states of substrate/droplet contact lines. The droplet is static when the contact line is pinning, while it is forced to move when the contact line is depinning. When analyzing the pinning stage, high-speed photography reveals the evidence of oxide films being rapidly crushed outside the aluminum droplet. In this work, atomic models of spherical Al core being wrapped by alumina shell are tentatively built, whose dioxide microstructures are being transformed from face-centered cubic into liquid at the atomic scale. At the same time, the wetting experiment reveals that the oxide films show changes in the period of sonoprocessing from 3rd to 5th second.During the ultrasonic spreading behavior in the late stage, there is a trend of evident expansion of the base contact area. The entire ultrasonic process lasts for no longer than 10 s. With the aid of ultrasonic sinusoidal waves, the wettability of metal Al gets a rapid improvement. Both molecular dynamic (MD) investigations and the experiments results reveal that the precursor film phenomenon is never found unless wetting is assisted by ultrasonic treatments. However, the precursor film appears near the triple line after using ultrasonics in the droplet wetting process, whose formation is driven by ultrasonic oscillations. Due to the precursor film, the ultrasonic wetting contact angle is lower than the non-ultrasonic contact angle. In addition, the time-variant effective ultrasonic energy has been quantitatively evaluated. The numerical expressions of thermodynamic variables are well verified by former ultrasonic spreading test results, which altogether provide an intrinsic explanation of the fast-decreasing contact angle of Al/SiC.

Numerical simulation of single aluminum droplet evaporation based on VOF method

In order to explore the law of single aluminum droplet evaporation process in argon, based on the VOF method, this paper carried out a corresponding two-dimensional transient multiphase flow numerical simulation. The evaporation process of droplet behavior in forced convection environment is compared and analyzed, and the influences of gas velocity on evaporation rate, the change of the droplet temperature and concentration boundary layer, and the variation of the velocity field around the droplet are studied. The research results showed that droplet evaporation can be divided into transient variation period and steady-state period. As the gas velocity increased from 2 m/s to 5 m/s, the evaporation rate at the transient variation period of aluminum droplets was increased by 22%–36%, and the evaporation rate at the steady-state stage was increased by 16%–33%; At 2 m/s, the temperature boundary layer on the windward and leeward sides of the droplet was thicker than the concentration boundary layer, indicating that the temperature diffusion was greater than the concentration diffusion; In addition, when the airflow around the droplet, the double row of vortices periodically fell off from the upper and lower sides of the droplet with opposite rotation direction and regular arrangement. With the mutual interference and influence between shedding vortices, Karman vortex street was formed, and the formation of the Karman vortex street became more obvious with the increase of the Reynolds number. The research results of this paper could provide reference for the unsteady numerical simulation of aluminum droplets combustion with chemical reaction in the future.

Experimental study on the rebound characteristics of high temperature alumina droplets impacting on an inclined cold wall

The rebound characteristics of high-temperature alumina droplets impacting on the cold wall of a solid rocket motor are of great significance for the accurate prediction of initial slag deposition and calculation of two-phase flow inside motors. In this paper, the impact experiment of alumina droplets on an inclined cold wall is carried out by a high-temperature alumina droplet impaction experiment system. The study shows that as the impact angle increases from 10° to 45°, three types of morphologies of rebound droplets are observed, namely multiple sharp corners mode, single “tail” mode, and “heart” mode, respectively. Under 10° impact angle condition, the first sharp angle is dominated by the tangential momentum component and the condensation on the wall; as the impact angle increases to 30°, the tangential momentum component has an increased influence on the rebound morphology, and the form of “tail” is influenced by the tangential momentum component and wall condensation; when the impact angle increases to 45°, the left contact surface partial detachment is dominated by rolling inertia. The study also found that the droplet size will profoundly affect the morphology of the droplet when it rebounds, and the phenomenon of “sharp corners” and “tail” gradually disappears as the droplet diameter decreases. As the impact angle increases, the dimensionless contact time of the droplet decreases, and the relationship between the dimensionless contact time and the normal Weber number of the droplet is obtained.

Local droplet etching on InAlAs/InP surfaces with InAl droplets

GaAs quantum dots (QDs) grown by local droplet etching (LDE) have been studied extensively in recent years. The LDE method allows for high crystallinity, as well as precise control of the density, morphology, and size of QDs. These properties make GaAs QDs an ideal candidate as single photon and entangled photon sources at short wavelengths (<800 nm). For technologically important telecom wavelengths, however, it is still unclear whether LDE grown QDs can be realized. Controlling the growth conditions does not enable shifting the wavelength of GaAs QDs to the telecom region. New recipes will have to be established. In this work, we study Indium–Aluminum (InAl) droplet etching on ultra-smooth In0.55Al0.45As surfaces on InP substrates, with a goal to lay the foundation for growing symmetrical and strain-free telecom QDs using the LDE method. We report that both droplets start to etch nanoholes at a substrate temperature above 415 °C, showing varying nanohole morphology and rapidly changing density (by more than one order of magnitude) at different temperatures. Al and In droplets are found to not intermix during etching, and instead etch nanoholes individually. The obtained nanoholes show a symmetric profile and very low densities, enabling infilling of lattice-matched InGaAs QDs on InxAl1−xAs/InP surfaces in further works.