Gas Atomization

Gas atomization is the process where the liquid metal is disrupted by a high-velocity gas such as air, nitrogen, argon, or helium. Atomization occurs by kinetic energy transfer from the atomizing medium to the metal.

Gas atomization differs from water atomization in many respects. Rather than being dominated by the pressure of the medium like water atomization, it is found that the gas-to-metal ratio is the dominant factor in controlling particle size, with median particle size being related inversely to the square root of the gas-to-metal ratio. The reasons for this fundamental difference are easily explained. In the case of gases, increases in pressure above 0.1 MPa (the pressure at which sonic velocity is reached) give only very small increments in gas velocity. In contrast, to reach sonic velocity (in air/nitrogen) with a water jet, a pressure of nearly 40 MPa is needed and the velocity increases uniformly as the square root of the pressure. Also, gas atomization takes place by the action of a continuum on another, while in water atomization a stream of droplets (in an entrained gas flow) acts on a continuum. The density of the water medium is about a thousand times higher than typical gases, giving much greater "punch" or short-range forces.

Gas-atomizing units also come in a much wider range of designs than water atomizers and are classified as either "confined" or "free-fall" nozzle configurations (Fig. 18). There is also a third type, "internal mixing," where the gas and metal are mixed together before expanding into the atomizing chamber (discussed in the following sections). Free-fall gas units are very similar in design to water-atomizing units. However, due to the rapid velocity decay as the gas moves away from the jet, in free-fall gas units it is very difficult to bring mean diameter of powder below 50 to 60 /'m on iron-base material. High efficiency is thus difficult to obtain in free-fall systems, although special design and configuration of nozzle arrangements can produce relatively fine powder at reasonable gas-to-metal ratios for high-velocity oxyfuel thermal spray, plasma tungsten arc (PTA) welding, and hot isostatic pressing applications. As well as vertical designs, resembling water atomizing designs, there are a number of asymmetric horizontal designs where a vertical, inclined, or sometimes horizontal melt stream is atomized by essentially horizontal gas jets. These designs are widely used in zinc, aluminum, and copper alloy air atomizers.

Gas Atomization

Fig. 18 Two-fluid atomization with (a) free-fall design (gas or water) and (b) continued nozzle design (gas only). Design characteristics: ot, angle formed by free-falling molten metal and atomizing medium; a, distance between molten metal and nozzle; d, diameter of confined molten metal nozzle; p, protrusion length of metal nozzle

Fig. 18 Two-fluid atomization with (a) free-fall design (gas or water) and (b) continued nozzle design (gas only). Design characteristics: ot, angle formed by free-falling molten metal and atomizing medium; a, distance between molten metal and nozzle; d, diameter of confined molten metal nozzle; p, protrusion length of metal nozzle

Closed or "confined" nozzle designs enhance the yield of fine powder particles (~10 /Jm) by maximizing gas velocity and density on contact with the metal. However, although confined designs are more efficient, they can be prone to freezing of the molten metal at the end of the tundish nozzle, which rapidly blocks the nozzle. Also, the interaction of the gas stream with the nozzle tip can generate either suction or positive pressure, varying from suction that can triple metal flow rate to back pressure sufficient to stop it and blow gas back into the tundish. Thus, great care is needed in setting up close-coupled nozzles, and the closer the coupling, the greater the care (as well as the efficiency). Water-bench test techniques (Ref 20), where the melt is substituted by water as a model liquid, have allowed consistent hot performance to be achieved by cold testing. Tests of suction alone have proved poor predictors of hot performance because the gas stream is considerably affected by its interaction with the liquid being atomized.

Confined designs are of two types: conventional (Fig. 18) or ultrasonic (Fig. 19). The ultrasonic design uses the Hartman tube principle to apply high-frequency pulsation to the gas stream, with gas exit velocities reported to be Mach 2 to 2.5 and the major pulsation frequency at about 100,000 Hz.

Model Metal Powder Atomization
Fig. 19 Ultrasonic gas atomizer (U.S. patent 2,997,245)

In many confined designs, the circulation created by the gas flowing down the side of the tundish nozzle causes the molten metal to flow across the face of the ceramic nozzle to its edge, where it is sheared by the flowing gas (Ref 21, 22, 23). These nozzles are referred to as "prefilming" and are quite widely used (see Fig. 20).

Fig. 20 Prefilming operation for gas atomization. (a) The prefilming operation of a closed nozzle, (b) The atomization of aluminum powder (25 /Jm). Source: Ref 22, 23

Fig. 20 Prefilming operation for gas atomization. (a) The prefilming operation of a closed nozzle, (b) The atomization of aluminum powder (25 /Jm). Source: Ref 22, 23

Worldwide annual tonnage of inert gas-atomized powder is much less than that of water-atomized powders, probably amounting to no more than 50,000 tons/year. Metal feed rates are lower than in water atomization, and melt size is smaller. However tonnage of air-atomized powders, especially zinc and aluminum, but also tin, lead, and copper alloys, probably exceeds 300,000 tons/year. Most of these air atomizers operate continuously for many hours or even days. Multinozzle units are often used to boost output on aluminum and zinc.

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