Direct Drop Formation Research Articles

Preparation of Sn–Pb Spherical Fine Metal Powders by Centrifugal Atomization Based on Mono-Sized Droplets

In order to meet the industrial manufacturing requirements for high-quality powders, a novel hybrid method for the preparation of spherical fine metal powders by centrifugal atomization of mono-sized droplets obtained by the pulsated orifice ejection method was proposed. The Sn–Pb powders with small size, narrow size distribution, high sphericity, and no satellite powders were obtained by this method. The impact of different process parameters on the powder size, particle size distribution, and powder morphology was investigated. The powder size decreased with the increase of the disk diameter, melting temperature, and rotation speed, and the size of the powder particles exhibited a normal bimodal distribution. The width of the liquid flow lines on the rotation disk was expected to be proportional to the size of the powder. The smallest median particle size D50 of the powders obtained was 21.3 μm with the disk diameter of 15 mm, the melting temperature was 573 K, and the rotation speed was 48000 r/min. The difference between the experimental and theoretical values of the particle sizes was analyzed as well. The median diameter D50 is smaller by a considerable margin compared to traditional theoretical values. In addition, the disintegration mode of the liquid film during the atomization process was also discussed. The direct drop formation mode was considered to exist in this novel method that suggests the possibility to implement the microrefinement and improve the controllability of metal powder preparation.

A brief introduction to the selective laser melting of Ti6Al4V powders by supreme-speed plasma rotating electrode process

In the present study, Ti6Al4V spherical powders were prepared by supreme-speed plasma rotating electrode process and the particle size fit log-normal distribution. The average diameter of the powders was successfully determined by a model developed in this work, suggesting that the particle size distribution is mainly affected by the rotating speed. The log-normal distribution factor of the particle size distribution maintains stable as the rotating speed ω varies. The particle size distribution indicates that the main atomization mode of Ti6Al4V under supreme-speed plasma rotating electrode process is of the characteristics of direct drop formation. The mechanical properties of the samples prepared by selective laser melting of Ti6Al4V powders were characterized, indicating that such Ti6Al4V samples with isotropy structure exhibit high yield strength and good ductility.

A novel model of calculating particle sizes in plasma rotating electrode process for superalloys

A novel PSD (particle size distribution) model to calculate the powder yields of plasma rotating electrode process (PREP) of nickel base superalloys powders was studied. The results showed that the meshing data of the nickel base superalloys powders like EP741NP and Inconel718 produced by PREP fitted the calculation of the novel PSD model well. The results also showed that not only rotating speed but also melting rate had significant influence on the fine powder yield in the centrifugal atomization system. The general PREP nickel base alloy powders showed characteristics of a bimodal Rosin-Rammler distribution mixed by the ligament disintegrating and direct drop formation. The fractions of the both mechanisms and the process factors of spraying were discussed. This fitting can help the engineers make the target powder yield increase by adjusting the process parameters.

Centrifugal granulation performance of liquid with various viscosities for heat recovery of blast furnace slag

Centrifugal granulation is a crucial step in the most promising heat recovery technique for molten blast furnace slag. Facing the fact that slag viscosity changes during the granulation process, in the present work, visualization experiments were conducted for centrifugal granulation of mixture liquids by a spinning disc. Then, the effect of liquid viscosity on granulation performance was discussed. It was found that the increase in liquid viscosity resulted in granulation mode translation under the same disc rotating speed and liquid flow rate, that is, from direct droplet formation mode to ligament formation mode. The Sauter Mean Diameter of the granulated droplets was increased and the droplets size distribution was narrowed with increasing liquid viscosity. Meanwhile, the ligament number increased in direct drop formation mode while decreased in ligament formation mode as the liquid viscosity was increased. Furthermore, for a specific mixture liquid, the increase in flow rate also resulted in the translation from direct droplet formation mode to the ligament formation mode and from ligament formation mode to the film formation mode in the end. The critical flow rate for this translation was found for various liquid mixtures. Finally, dimensionless correlations were developed to predict the average diameter and critical flow rate, and the analogy analysis was applied to the molten blast furnace slag.

Experimental investigation on ligament formation for molten slag granulation

During the dry granulation for molten blast furnace slag, rotary cup is used to atomize the molten slag. In this study, the mechanism of ligament formation for molten slag granulation was investigated. The results indicated that the transition equations from direct drop formation to ligament formation and ligament formation to sheet formation, obtained from glycerol/water mixture, can identify the type of disintegration for molten blast furnace slag granulation. Due to short wavelength of dilational wave along molten slag ligaments, the diameter of slag particles decreased with an increase in angular speed, and more and more slag particles were far away from the center of rotary cup. The slag particles diameter decreased with an increase in rotary cup diameter. The empirical equation can be used to predict the diameter of slag particles obtained by ligament formation for molten slag granulation at high angular speed. Because of slight change in viscosity and surface tension when the temperature of molten slag was over than 1300 °C, there was no change in diameter and mass fraction of slag particles with an increase in molten slag temperature. All the results could provide guidance for the design of industrial plant for molten blast furnace slag granulation.

Ligament-type liquid disintegration by a spinning wheel

In this paper, liquid disintegration by a spinning wheel was investigated experimentally. The mechanism of ligament formation on a spinning wheel was studied using photographs taken by a high-speed camera. Three different liquids with Newtonian properties were used at various flow rates and the wheel rotational speed was varied in a wide range. The atomization process was found to be significantly different from the spinning disc and cups atomization due to the highly non-uniform circumferential ligament distribution and the absence of the direct drop formation mode. Nevertheless, the dependence of ligament number and diameter on the input process parameters is similar as with other types of centrifugal atomizers due to the same underlying hydrodynamic instabilities.

Experimental investigation of liquid disintegration by rotary cups

The disintegration of liquid by rotary cups has been investigated experimentally. The mechanism of direct drop formation and ligament formation in a rotary cup was studied using photos taken by a high-speed camera. Two kinds of liquids and three kinds of cups were used in the experiment. In the study, we found that three different types of disintegration formation occur as the angular speed increases in a given flow rate. The equations have been derived theoretically for calculating diameter of liquid drop produced by direct drop formation and ligament formation and the dimensionless equations obtained from the experimental results are applicable for calculating diameter of drops in a wide range of operating conditions. In the process of disintegration by ligament formation, the diameter of liquid drop is determined by the diameter of tail end of ligament, which was constant in diameter. The transition equations from direct drop formation to ligament formation and ligament formation to sheet formation arranged by three dimensionless parameters and compared with various experimental results. The transition equations fit quite well in the trend shown by the experimental date and they are applicable in a particular range of We, i.e. 103<We<105.

Hybrid Atomization Method for Manufacturing Fine Spherical Metal Powder

Hybrid atomization is a new atomization technique that combines gas atomization with centrifugal atomization. This process can produce fine, spherical powders economically with a mean size of about 10 μm diameter and a tight size distribution. Experiments on the process were carried out using a Sn-9 mass% Zn alloy to investigate the influence of processing parameters on powder characteristics in hybrid atomization. The primary atomization mechanism under normal hybrid atomization conditions is predicted to be direct drop formation mode.

ハイブリッド噴霧法による金属微細球状粉末の創製

Hybrid atomization is a new atomization technique that combines gas atomization with centrifugal atomization. This process can produce fine, spherical powders economically with a mean size of about 10μm diameter and a tight size distribution. Experiments on the process were carried out using a Sn-9mass%Zn alloy to investigate the influence of processing parameters on powder characteristics in hybrid atomization. The primary atomization mechanism under normal hybrid atomization conditions is predicted to be direct drop formation mode.

プラズマ回転電極法による均一球形粉末製造の検討とその金属間化合物への応用

Atomization mechanism and production of atomized high quality powder by Plasma Rotating Electrode Process (PREP) were discussed. The atomization mechanism changed from direct drop formation (DDF) to film disintegration (FD), through ligament disintegration (LD) depending on either the changes in material parameters or those in the operation parameters. DDF mechanism yielded the highest quality powder particles consisting of exceedingly spherical shape, uniform size and clean without contamination. Therfore, this powder seemed to be suitable for compaction by Wing or near-net-shape forming. The results of the study were applied for producing PREPed powders of some intermetallics such as Ti-aluminides and TiNi. The compacts of those powders showed improved mechanical properties compared with the conventionally processed materials.

Production of micro-gel beads by a rotating disk atomizer

Micro-gel beads (200–1,200 μm in diameter) were produced by atomization of sodium alginate solution or a sodium alginate-perfluorocarbon mixture with a rotating disk. At any rotation speed of the disk ( N) and low volumetric flow rate of the alginate solution ( Q), direct drop formation was observed at the edge of the disk. However, with increases in the volumetric flow rate, there was a transition from the direct drop formation to a ligament type of disintegration. The length of the ligaments increased with an increase in Q. Although the diameter of the micro-gel beads showed a bimodal distribution with the number of satellite beads increasing with an increase in Q and a decrease in N, the volume percentage of the satellite beads was negligible. The mean volume diameter of the micro-gel beads ( d v) was represented by a dimensionless equation: d v D = 22.4N We −0.43 N Re 0.08 N Z 0.07 , where D is the diameter of the disk, N We, N Re and N Z are the Weber, Reynolds, and Ohnesorge numbers, respectively. By immobilizing viable baker's yeast cells in micro-gel beads 500 μm in diameter, the effectiveness factor for oxygen consumption at a DO concentration of 0.022 mol/m 3 was 6 times higher, while the apparent K m value was 50% lower, than when they were immobilized in gel beads 3.2 mm in diameter.

遠心噴霧粉末製造の水モデル解析

Some of the processing parameters for centrifugal atomization of molten metal were studied by means of a model-experiment using water.The shape and revolution velocity of the high speed rotating stages affected the size and the scattering distribution of disintigrated droplets. In the atomization using a flat stage, liquid disintegrated into smaller particles but sticked considerably to the wall of the apparatus. Conical type stages lessened the sticking and increased the portion of the larger drops slightly. Higher velocity revolution made finer particles and narrowed the scattering range of drops at the bottom of the chamber closely to the center. The wettability of the liquid on the stage surface was found to be an important factor for the disintegration of the liquid. Enhanced wettability brought direct drop formation, while poor wettability brought ligament disintegration.

Rotary atomization in the ligament formation mode

A rotary atomizer can produce sprays with narrow ranges of drop size either by forming drops directly at its edge, or by the disintegration of liquid ligaments formed from the edge. Ligament formation occurs at higher flow rates than direct drop formation and can produce enough spray to make the use of spinning discs for applying 50 1/ha from a tractor-mounted sprayer a practical possibility. A study of the process is reported, including criteria for its occurrence and an expression for the prediction of drop size is developed.

Atomization of Liquids by Means of a Rotating Cup

Abstract Liquid, supplied through a stationary tube to the inner part of a rotating cup widening toward a brim, flows viscously in a thin layer toward this brim and is then flung off, all by centrifugal action. The flow within this layer and the disintegration phenomena occurring beyond the brim have been studied, experimentally as well as theoretically. A formula has been derived for the thickness and for the radial velocity of the liquid layer within the cup, which proved to agree reasonably well with experimental results. Three essentially different types of disintegration may take place around and beyond the edge of the cup designated, respectively, by: (a) the state of direct drop formation; (b) the state of ligament formation; and (c) the state of film formation. Which one of these is realized depends upon working conditions. Transition from state (a) into (b), or of state (b) into state (c) is promoted by an increased quantity of supply, an increased angular speed, a decreased diameter of the cup, an increased density, an increased viscosity, and a decreased surface tension of the liquid. The experimental results have been expressed in relationships between relevant dimensionless groups. For the state of ligament formation a semiempirical relationship has been derived between the number of ligaments and dimensionless groups determining the working conditions of the cup. Results of drop-size measurements made for the state of ligament formation as well as for the state of film formation show that atomization by mere rotation of the cup is much more uniform than commonly achieved with pressure atomizers.