Atomization of melts

Atomization of melts has, in principle, some similarity to the atomization of normal liquids. The atomization processes originally developed for normal liquids, such as swirl jet atomization, two-fluid atomization, centrifugal atomization, effervescent atomization, ultrasonic piezoelectric vibratory atomization, and Hartmann-whistle acoustic atomization, have been deployed, modified, and/or further developed for the atomization of melts. However, water atomization used for melts is not a viable technique for normal liquids. Nevertheless, useful information and insights derived from the atomization of normal liquids, such as the fundamental knowledge of design and performance of atomizers, can be applied to the atomization of melts. 

Numerous atomization techniques have evolved for the production of metal/alloy powders or as a step in spray forming processes. Atomization of melts may be achieved by a variety of means such as aerodynamic, hydrodynamic, mechanical, ultrasonic, electrostatic, electromagnetic, or pressure effect, or a combination of some of these effects. Some of the atomization techniques have been extensively developed and applied to commercial productions, including (a) two-fluid atomization using gas, water, or oil (i.e., gas atomization, water atomization, oil atomization), (b) vacuum atomization, and (c) rotating electrode atomization. Two-fluid atomization 

In addition to the difference in the size distributions of droplets/particles produced, the overall production costs are apparently the point of differentiation among all atomization techniques under consideration. As the criteria for the evaluation of a specific atomization technique/system, the following factors are of importance high yields, minimum use of expensive gases, clean droplets/ particles, and high throughput. 

Method Droplet Size ( im) Metal/AUoy Cooling Rate (°C/s) Throughput (kg/mm) Capacity (Metric Tons) Advantage Limitation 

Gas Atomization 50-300 Standard deviation 1.9-2.5 10-50 at high gas pressures with close-coupled atomizer Solder materials. Precious metals, Cu, Fe, Al, Mg, Co, Ti, Zn, Al-6Cr-3Fe-2Zr alloy. Low-alloy steels. High speed steels. Stainless steels, Specialty alloys, Ni-base superalloys, Alumina, Intermetallics io3-.o5 1-70 Spherical smooth particles. Cleanliness, Rapidly-solidified structures, Acceptable production rates High cost, Low 1 volume, Low energy efficiency (EE), Gas-filled porosity in particles H 

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Atomization of melts has been used in two principal areas, i.e., powder production 4] and spray forming, 3] as shown in Figs. 1.4 and 1.5, respectively. It is not until recent years that the technologies for the atomization of melts have advanced sufficiently to ensure good yields of usable products over sustained periods of plant operation. In these two areas, some aspects such as melting of metals or alloys, basic atomizer designs, and atomization mechanisms are the same or similar. Metals or alloys melt at high temperatures to produce low viscosity but usually... 

A variety of atomizer designs have been developed in an effort to control the droplet size distribution and to increase the yield of fine powders. Gas atomizers used for the atomization of melts may be loosely classified into two primary categories in terms of the interaction mode between a liquid metal and an atomization gas during atomization, i.e., (1) internal mixing and (2) external mixing. 

Figure 2.13. Schematic of an internal-mixing atomizer for atomization of melts.

Figure 2.16. Design variations of close-coupled atomizers for atomization of melts.

In addition to the designs mentioned above, other atomizer configurations such as those used in spray combustion and spray drying processes may also be considered as an alternative in gas atomization of melts for special purposes after appropriate modifications. 

Controlling variables in gas atomization of melts include nozzle geometry parameters and many process parameters. An exhaustive list of these parameters is given in Tables 2.12 and 2.13, respectively, along with typical values and/or ranges of the parameters. 

Table 2.14. Geometry Parameters in Water Atomization of Melts[145l...

Figure 2.21. Schematic of rotating disk atomization of melt.

Figure 2.23. Schematic of single-stage spinning cup atomization of melt.

Figure 2.24. Ultrasonic standing wave atomization of melt. Top schematic. Bottom photograph. (Courtesy of Prof. Dr-Ing. Klaus Bauckhage at University of Bremen, Germany.)...

Figure 3.12. Schematic showing Liquid Jet-Ligament Breakup mode (left) and Liquid Film Sheet Breakup mode (right) in two-fluid atomization of melts.

Droplet Formation in Water Atomization. In water atomization of melts, liquid metal stream may be shattered by impact of water droplets, rather than by shear mechanism. When water droplets at high velocities strike the liquid metal stream, some liquid metal fragments are knocked out by the exploding steam packets originated from the water droplets and subsequently contract into spheroidal droplets under the effect of surface tension if spheroidization time is less than solidification time. It is assumed that each water droplet may be able to knock out one or more metal droplet. However, the actual number of metal droplets produced by each water droplet may vary, depending on operation conditions, material properties, and atomizer designs. 

Some quantitative studies1498115011 on droplet size distribution in water atomization of melts showed that the mean droplet size increases with metal flow rate and reduces with water flow rate, water velocity, or water pressure. From detailed experimental studies on the water atomization of steel, Grandzol and Tallmadge15011 observed that water velocity is a fundamental variable influencing the mean droplet size, and further, it is the velocity component normal to the molten metal stream Uw sin , rather than parallel to the metal stream, that governs the mean droplet size. This may be attributed to the hypothesis that water atomization is an impact and shattering process, while gas atomization is predominantly an aerodynamic shear process. 

Figure 4.3. Regimes in centrifugal atomization of melts Direct Droplet Formation, Ligament Disintegration, and Film/Sheet Disintegration.

In atomization of melts, the final droplet size also depends on the relative magnitude of the time th for a droplet to undergo deformation prior to secondary breakup, and the time tsol required for... 

Regarding the effects of process parameters on gas atomization of melts, modeling and experimental studies l63 l64 324l revealed that the mass median droplet diameter decreases with increasing atomization gas pressure and gas to metal mass flow rate ratio. The standard deviation decreases with increasing gas to metal mass flow rate ratio. As the melt superheat increases, both the mass median droplet diameter and the standard deviation decrease. 

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