Splat Diameter Research Articles

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.

On surface area coverage by an electrostatic rotating bell atomizer

An electrostatic rotating bell atomizer was used to produce a spray of a 20 wt% aqueous glycerol solution. Droplet size distributions were measured by photographing droplets in-flight and using image analysis software to measure their diameters. Increasing the rotational speed of the bell-cup resulted in a decrease in the average droplet diameter. The fraction of area covered by spray droplets was measured for various bell-cup speeds using a high-speed camera positioned behind a vertical glass substrate on which the liquid was sprayed. An analytical model, based on probability theory, was used to predict the fraction of surface covered solely using the droplet flux and the droplet size. Results from the model were compared to experimentally measured fractions of surface coverage and showed good agreement. The minimum theoretical film thickness assuming full coverage was derived from the probabilistic model. When the minimum theoretical film thickness was divided by the droplet diameter, it was found to be only dependent on the spread factor. Experiments to measure the fraction of area covered were also conducted using an automotive paint. Increasing the paint flow rate increased the fraction of area covered. Increasing rotational speed reduced splat size but had little effect on coverage. Increasing shaping air 1 flow rate reduced splat size and increased surface coverage significantly. Application of an electrostatic potential increased splat diameter but enhanced surface coverage only slightly.

Efficacy of graphene nanoplatelets on splat morphology and microstructure of plasma sprayed alumina coatings

Splat morphology and its microstructure are of significance to affect the comprehensive properties of plasma sprayed ceramic coatings. This paper studies the effect of graphene nanoplatelets (GNPs) on the splat morphology and microstructure evolution of plasma sprayed alumina coatings. With an increasing GNP content, splat morphology became more disk-shaped and the average diameter of the splat decreased from 120 ± 28 μm for pure Al2O3 to 83 ± 17 μm for 1.0 wt% GNP/Al2O3. Simulated splat morphology presented a good agreement with the experimental observations. GNP agglomeration at the periphery of a droplet accelerate the formation of solid ring resulting in a decrease in splat diameter, and homogeneously distributed GNPs inside a droplet is expected to simultaneously promote rapid solidification of inward melt following the formation of a solid ring, which suppresses splat fragment. Meanwhile, columnar grain refinement and GNP pull-out are responsible for the composite splats free of microcracks.

Plasma sprayed carbon nanotube reinforced splats and coatings

Plasma sprayed composite coatings prepared from powders doped with 1wt% CNT were characterized by their scratch resistance. Additionally, the scratch resistance of the splats obtained from one powder, namely, crushed alumina, was also investigated. The average splat diameter of the splat containing CNT was larger. CNT reinforced splats were more damage resistant as compared to those produced from the pristine powder. The estimated increase in the work of adhesion, while scratching the CNT reinforced coatings was 59–176% higher than their counterpart without CNT. This is attributed to CNT bridging across splats resulting in an improvement in coating properties. Other factors promoting coating adhesion were CNT anchorage in the asperities of the bond coat and enhancement of coating stiffness in the presence of CNT. In general, coating failure during scratching occurred owing to the formation of tensile crack and large area spallation ahead of the indenter.

Formation of Splats from Suspension Particles with Solid Inclusions Finely Dispersed in a Melted Metal Matrix

A theoretical model has been developed to describe the splats formation from composite particles of several tens of micrometers in size whose liquid metal binder contains a high volume concentration of ultra-fine refractory solid inclusions uniformly distributed in the binder. A theoretical solution was derived, enabling evaluation of splat thickness and diameter, and also the contact temperature at the particle-substrate interface, under complete control of key physical parameters (KPPs) of the spray process (impact velocity, temperature, and size of the particle, and substrate temperature) versus the concentration of solid inclusions suspended in the metal-binder melt. Using the solution obtained, the calculations performed demonstrate the possibility of formulating adequate requirements on the KPPs of particle-substrate interaction providing a deposition of ceramic-metal coatings with predictable splat thickness and degree of particle flattening on the substrate, and also with desired contact temperature during the formation of the first coating monolayer.

Splat morphology of plasma sprayed aluminum oxide reinforced with carbon nanotubes: A comparison between experiments and simulation

This study elucidates the effect of carbon nanotube (CNT) addition on the splat formation in plasma sprayed aluminum oxide (Al2O3) composite coating using experimental and computational methods. CNT content was varied as 0, 4 and 8wt.% in Al2O3 matrix. With an increasing CNT content, splat morphology became more circular and disk-shaped. The average diameter of disk-shaped splats increased from 28.6±1.4μm for Al2O3 to 43.2±1.3μm for Al2O3–8wt.% CNT. The population density of splats with fingers, fragments, and voids was the lowest for the highest (8wt.%) CNT content. The addition of CNTs resulted in two simultaneously competing phenomena viz. increased thermal capacity and increased viscosity of the melt. Increased thermal capacity delayed the localized solidification resulting in higher splat diameter while agglomeration of CNTs at the periphery of the splat results in higher viscosity of the melt which suppresses the splat fragmentation that leads to increased population density of disk shaped splats. Splat morphology of three compositions was also simulated using SIMDROP software, which showed a good agreement with the experimentally collected splats.

Processing parameter effect on the splat diameters of the droplets produced by liquid metal atomization using De Laval nozzle

Liquid metal atomization with De Laval nozzle is not a widely used technique to produce metallic powders. It appears as a versatile process to manufacture different particle sizes according to the processing parameters (gas pressure, melt nozzle diameter, De Laval nozzle diameter, etc.). However, few studies have focused on the understanding of the different stages of the process. During the atomization process two pressure stages can be distinguished: transition time (pressurization time) and stabilized stage (setting pressure). The aim of the work is to study the effect of the operating parameters on the splat diameter during these two stages. Copper (99.9 at. %) was atomized using different sets of operating parameters on smooth steel substrates. Different splat shapes and mean equivalent diameters are observed and have shown that they depended on both, the atomization parameters and the atomization time.

Splat Morphology of Yttria-Stabilized Zirconia Droplet Deposited Via Hybrid Plasma Spraying

The splat morphology of yttria-stabilized zirconia (YSZ) droplets deposited by dc-rf hybrid plasma spraying (HYPS) was studied. Two types of YSZ powder were used, namely fused and crushed powder (FC) and hollow spherical powder (HOSP). The three-dimensional shape of more than 600 disk-shaped splats on preheated substrates was evaluated using a laser-scanning microscope to determine the splat diameter, thickness, and their dimensionless forms. The HOSP showed a higher degree of flattening than the FC. Both the FC with a powder size distribution of 45-75 μm and the HOSP of 30-120 μm can be used as spray materials in the HYPS to achieve a coating design based on fully molten particles. The effect of the substrate temperature on the splat morphology was similar to that of atmospheric dc plasma spraying; however, the transition from a splashed shape to a disk shape gradually occurred at higher substrate temperatures.

On-Line Measurement of Plasma-Sprayed Ni-Particles during Impact on a Ti-Surface: Influence of Surface Oxidation

The objective of this study was to analyze the impact of plasma-sprayed Ni5%Al particles on polished and grit-blasted Ti6Al4V samples under oxidized and nonoxidized conditions. For this purpose, measurements of thermal radiation and velocity of individual plasma-sprayed particles were carried out. From the thermal radiation at impact, splat diameter during flattening and temperature evolution during cooling were evaluated. Characteristic parameters related to the quality of contact between the splat and the substrate were retrieved. The flattening speed was introduced to characterize wetting, while the cooling rate was used to characterize solidification. The idea was to get a signature of particle impact for a given surface roughness and oxidation state by identifying parameters which strongly affect the splat behavior. Sieved Ni5%Al powder in a narrow range (+65 −75 μm) was sprayed on four sets of titanium alloy surfaces, consisting of polished and grit-blasted samples, one set had a nonoxidized surface and the other one was oxidized in an oven at 600 °C for two hours. Resulting splats after impact were characterized by scanning electron microscopy, the splats on oxidized surface showed pores in their core and detached fingers at the periphery. The cooling rate and flattening degree significantly increased on the oxidized smooth surface compared to the nonoxidized one. This trend was not found in grit-blasted surfaces, which implies that impact phenomena are different on grit-blasted surfaces than on smooth surfaces thus further work is needed.

Effect of substrate temperature on the properties of coatings and splats deposited by wire arc spraying

Experiments were conducted to study the effect of impact velocity and substrate temperature on the properties of individual splats and coatings deposited with a wire arc spraying system. Aluminum was sprayed onto polished AISI304L coupons maintained at temperatures ranging from 25 °C to 450 °C. In-flight particle parameters (diameter, velocity, temperature) were measured using a DPV2000 system. Individual splats deposited on the substrate were photographed. At low substrate temperature droplets splashed, forming irregular splats; at higher temperatures there was no splashing and splats formed circular disks. The temperature at which the transition occurred decreased with increasing impact velocity. Raising substrate temperature increased both deposition efficiency and adhesion strength. Predictions from analytical models to calculate splat diameter and transition temperature were compared with experimental measurements and found to agree well.

Size distribution of particles from individual wires and the effects of nozzle geometry in twin wire arc spraying

By separating particles from the anode and the cathode wires, particle properties of individual electrodes of the arc spray process were investigated. Bimodal size distribution was observed for particles from each single wire. In addition, after atomisation and acceleration, average diameter difference between particles from the anode and cathode becomes quite small and the difference can be reduced by increasing atomising gas pressure and/or by using a closed nozzle system with a converging–diverging orifice (CD/CL). Microstructure analysis indicates that the CD/CL nozzle tends to produce coatings with finer microstructure, lower porosity and higher oxide content, while the open nozzle system leads to very coarse coating microstructure with higher porosity but less oxide content. CFD analysis of the gas dynamics was performed for different nozzles and led to numerical prediction of splat diameter distribution. Results show a smaller average splat diameter and a narrower splat diameter distribution for the CD/CL nozzle that is in close agreements with the microstructure analysis. It was found that a major disadvantage of the open nozzle system is its relatively longer distance between the wires interception point and the nozzle exit where the gas velocity attains its maximum value.

Optimization for plasma spraying processes by numerical simulation

Numerical simulation of plasma spraying processes from the injection of metallic particles to formation of the coating was conducted by integrating plasma thermofluid, and splat and coating formation models. The first model clarifies the electromagnetic thermofluid fields of a particle-laden plasma flow impinging onto the substrate, as well as the characteristics of the fine particles injected in the plasma flow. The second model clarifies the splat thickness and diameter of a particle using the melted particle impact velocity and temperature obtained by the first model. Furthermore, the optimal particle impact velocity and temperature were derived from the stable splat criterion given by the integrated model.

Simultaneous Independent Measurement of Splat Diameter and Cooling Time during Impact on a Substrate of Plasma-Sprayed Molybdenum Particles

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed two-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique, the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma-sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 µs after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 µs.

Impact, recoil and splashing of molten metal droplets

We studied the impact and solidification of molten tin droplets on a stainless steel surface. Droplet impact velocity was varied from 1.0 to 4.0 m/s and substrate temperature from 25 to 240°C (above the melting point of tin, 232°C). We photographed droplet impact and measured splat diameter and liquid–solid contact angle from these photographs. Substrate temperature under an impacting droplet was measured using a fast response thermocouple. Thermal contact resistance at the droplet–substrate interface was calculated by matching measured surface temperature variation with an analytical solution. A simple energy conservation model was used to predict the maximum spread of droplets during impact. Predictions agreed well with measured values. Instabilities were observed on the periphery of the droplet, which led to the formation of fingers. A model based on the Rayleigh–Taylor instability was used to predict the number of fingers around the periphery of the droplet.

Numerical Simulation of Liquid Droplet Solidification on Substrates.

A mathematical model describing the deformation and solidification behavior of liquid droplets impinging on substrates is presented. The mathematical model is numerically solved using a finite element method. In the experiment, a molten tin droplet (2.2–4.3 mm diameter) impacts copper, stainless steel and glass substrates at various preimpact velocities (1.4–4.0 m/s). The values of the heat transfer coefficient at the droplet/substrate interface are evaluated by comparing the calculated splat diameters to the experimental ones. The estimated values are within the previously reported ranges. The model almost predicts the Weber number dependence of the experimental splat diameters. The time variations of the numerical splat diameters also agree with the experimental results. The simulation reveals that the frozen layer at the splat edge, rather than at the center region, affects deceleration of the droplet spreading. The effect of the solidification on the splat diameter is explained from the freezing rate at the splat edge.

Impact of High Velocity Cold Spray Particles

This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.

Parameters controlling solidification of molten wax droplets falling on a solid surface

An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.

On the spreading and solidification of molten particles in a plasma spray process effect of thermal contact resistance

The spreading and simultaneous solidification of a liquid droplet upon its impingement onto a substrate permitting thermal contact resistance has been numerically simulated; the effect of contact resistance and the importance of solidification on droplet spreading are investigated. The numerical solution for the complete Navier-Stokes equations is based on the modified SOLA-VOF method using rectangular mesh in axisymmetric geometry. The solidification of the deforming droplet is considered by a one-dimensional heat conduction model. The predictions are in good agreement with the available experimental data and the model may be well suited for investigating droplet impact and simultaneous solidification permitting contact resistance at the substrate. We found that the final splat diameter could be extremely sensitive to the magnitude of the thermal contact resistance. The results also show that for the condition of higher Reynolds and/or higher Stefan numbers the effect of solidification on the final splat diameter is more important.

Influence of the velocity of plasma-sprayed particles on splat formation

The behavior of individual plasma- sprayed particles as they impinge on a surface was monitored using two high- speed two- color pyrometers, one focused 2 mm before the substrate and the other focused on the substrate surface. The influence of the velocity of the impinging particles (determined from the time of flight between the two focused points of the pyrometers) on the deformation and cooling processes was investigated. Results on zirconia particles impacting on a smooth bare steel substrate are presented in terms of apparent flattening time, splat diameter, and cooling rate determined from the pyrometer signals.

Numerical simulation of impingement of molten Ti, Ni, and W droplets on a flat substrate

Thermal spraying has been widely applied to process thin protective coatings on preshaped parts and to manufacture metallic preforms of a variety of geometries. The quality of materials produced by thermal spraying depends critically on the impact conditions of the droplets. In the present study, the deformation behavior and interaction of molten droplets impinging onto a flat substrate during thermal spraying have been numerically simulated. The calculated results reveal that a droplet spreads uniformly in the radial direction during impingement and eventually forms a thin splat with final diameter and thickness up to 11.3 times and down to 0.02 times the impact diameter, respectively. The final splat diameter increases rapidly with increasing impact velocity and melt density or decreasing melt viscosity. For the processing conditions of interest, the final splat diameter and the spreading time may be approximated by correlations: ds/do = l.04Re0.2 and ts/(do/uo) = 0.62Re0.2, where ds/do is the dimensionless splat diameter; ts/(do/uo) is the dimensionless spreading time; and Re is the Reynolds number. A fully liquid droplet impinging onto a flat solid substrate leads to good contact between the splat and the substrate. Multiple fully liquid droplets striking simultaneously onto other flattening, fully liquid splats cause ejection and rebounding of the liquid, as well as formation of voids within the liquid.