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avoid heating up and contamination of the ultrasonic vibrating surfaces. The USWA technique has been demonstrated to be capable of atomizing extremely viscous fluids (such as synthetic resins and highly concentrated suspensions) or fluids of high surface tension (such as molten metals/alloys) into narrow-sized, very fine, spherical droplets/powders.

2.2.16 Steam Atomization

Atomization of melts was first conducted for lead and tin via steam or air in the early 1920’s. Steam atomization provides a choice of conditions between those of gas and water atomization. Lowpressure steam can produce relatively coarse droplets of less spherical shape (irregular, spheroidal). On average, relatively high quench rates, somewhat greater than 103 °C/s, can be achieved for finer droplets. Steam atomization has been successfully applied to most carbon steels and low alloy steels, stainless steels, Co-base alloys and superalloys, non-Al-Ti Ni-base alloys. Typical oxygen levels for untreated powders are 500 to 1500 ppm. Self-milling of such powders can remove 80%–90% of the surface oxides. To further lower the oxygen level, inexpensive chemical treatments can be used. Typically, cleaned stainless steel powders exhibit oxygen levels of 0.02%–0.04%, which are extremely clean according to ASTM standards.

2.2.17 Other Atomization Methods

In addition to the methods described above, there are other lesser known but interesting atomization techniques. In the eighteenth century, lead shot was made by pouring lead melt through a perforated basket down a shot tower, similar to the techniques used today for granulating precious metals. During World War II, a novel atomization process was invented for iron powder production in Germany, i.e., the DPG spinning disc method. In the DPG process, a partially water-atomized iron melt was directed to a rotating disc and further atomized by passing through slot orifices on the disc. Reynolds

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Metals Company used a commercial centrifugal atomizer to produce elongated droplets quenched with air. This atomizer has a spinner consisting of a steel cylinder with numerous fine holes along the barrel. The atomization system, while generating large output and high yield, allows variation of the spinner speed and hole size, adjustment of the melt temperature, and control of the atmosphere if necessary.

A simple experimental apparatus[196] has been used to produce individual mercury and sodium amalgam droplets in the size range of 0.3–2.5 mm diameter. A high resistance to the flow of the liquids is provided by a capillary tube consisting of a 0.20 mm OD tungsten wire placed in a 0.25 mm capillary bore. Flow rates of 1 µl/s and lower can be obtained, enabling the formation of droplets as small as 1 mm diameter in a time interval of 20 s. The droplets are formed at a plastic tip and stripped off by a stream of argon when reaching the desired size. The variation in the droplet size can be controlled within +/-2% of the mean value.

In pressure atomization, a liquid metal is pressurized by an inert gas through a very small diameter nozzle at the bottom of a melt crucible to form droplets. The process is relatively gas efficient and energy efficient. Using a swirl nozzle, alloys of melting temperatures up to 1300 K have been atomized, generating a mass median size as small as 50 µm and a narrow size range (a standard deviation of 1.4) with good sphericity. The nozzles used may be made from stainless steel, graphite, or ceramics allowing sufficient nozzle lives. Gas pressures of about 2 MPa are feasible for the production of fine particles. To achieve a large throughput, pressures in excess of 10 MPa or multiple arrays of small nozzles may be used. The method can process low melting-temperature metals at a rate over 1 ton/hour and make particles of 100–1000 µm. The two-fluid prefilming version of pressure atomization technique[159] is even more energy efficient due to the internal-mixing effect, as discussed previously. Another variation involves the generation of droplets by forcing liquid metal through a porous ceramic filter. The pore size of the filter may range from 40 to 150 µm, generating aluminum droplets

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with a mean size of 500–900 µm. Backup pressure above the melt can speed up the process. The use of gas blast or vibrations can increase the cooling rates of droplets and result in smaller droplets by detaching droplets from the filter pores earlier. This technique can generate fairly spherical droplets and control droplet size by changing filter pore geometry and size. Oxides and foreign particles may be filtered out during the process. Moreover, the associated apparatus is simple.

As discussed in the next chapter, a liquid stream may spontaneously break up into droplets. Superimposition of vibrations as longitudinal oscillations (50–1000 Hz) on a stream through a nozzle orifice can promote the breakup and make it occur at a certain time, giving rise to almost identical droplet size and excellent uniformity (a standard deviation less than 1.05). This is the basic idea of the vibrating-orifice atomization technique. Since it makes one droplet at a time, the throughput is low and falls roughly with the square of orifice diameter or droplet diameter. The nozzle diameter and melt stream speed are important process parameters influencing droplet size. The melt density and surface tension are also of importance to the size distribution of droplets generated.

Electromagnetic forces may be used to disrupt a liquid metal. This requires passing a current through the liquid metal. Apparently, the current will cease once the breakup of the liquid metal begins. In electromagnetic atomization,[197] the current is passed from the liquid metal through a nozzle to an electrode. The gap between the nozzle and the electrode is placed in a strong magnet field. Intensive atomization then takes place due to very strong expulsive forces generated during pouring the liquid metal against the electrode. Expulsion velocities may exceed 100 m/s, depending on the strength of the magnetic field. The breakup of the liquid metal stream is controlled by the excitation of the Rayleigh instability. Very uniform tin droplets of 600–850 µm have been produced by pulsing the current every 27 ms and using a 0.5 mm diameter nozzle at a flow rate of 0.05–0.12 kg/min.[5] The atomization can be conducted in a

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vacuum, making the process cost lower than that using other gases such as argon. Another process, termed magnetohydrodynamic atomization, is conceptually similar to electromagnetic atomization in terms of the breakup mechanism. It is also similar to vibratingorifice atomization because it involves vibrations. However, the vibrations are imposed on the metal stream by pulsing a current through the liquid metal in the tundish placed in a magnetic field. Impulses and waveform can be well controlled and altered to allow optimum performance and control over droplet size distribution. The atomized lead droplets are 2–4 mm in diameter with a narrow size distribution within 10%.[5]

Impaction atomization, referred to asGranshot process, has been used by Uddeholm in Sweden to generate large particles of iron and other alloys. In this process, a liquid metal stream is poured onto a flat, circular refractory brick in air. The liquid metal splashes and ejects off the brick surface into a thin sheet and breaks up via ligaments into droplets during flight. It is followed by water quenching of droplets/particles. The atomization regime of the Granshot process is the same as the sheet regime for a rotating cup or disc. This technique is a major industrial method for granulation of liquid metals at large throughput up to several tons per minute. It has been used for the atomization of steels, cast iron, ferroalloys and mattes in large quantities. Melt stream can be over 20 mm in diameter. The particle size ranges from 3 to 15 mm, and smaller ones are spherical while larger ones are flat with a thickness of 4–7 mm. Droplet size and shape can not be readily changed because they are mainly determined by physical and chemical properties of the liquid metal/ alloy of interest. For some alloys, however, particle shape tends to be largely influenced by oxide film formation. In addition, the process requires much higher stream velocity and is limited to smaller scales for the production of powders, making the advantages of the method no longer apparent. Therefore, this technique does not appear to have established significant commercial applications in powder production.

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Electrostatic atomization technique has not been developed suitably for atomizing liquid metals/alloys at sufficiently high metal flow rates. Both the throughput and capacity are low. In addition, it is difficult to raise a liquid metal to over 10 kV. Thus, despite its potential advantages such as narrow size range and low velocity spray, this method is difficult to establish industrial applications for powder production. Recently, attempts have been made to scale-up electrostatic atomization using linear arrays of Taylor cones.[198] The feasibility of increasing the liquid throughput rate in electrostatic atomizers has been evaluated, and conditions for high-quality powders, composed of dense, spheroidal, submicrometer, and nanocrystalline oxide particles, have been found. The method has been applied to the synthesis of yttria powders.