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the very fine droplet size, the cooling rates of droplets are approximately 105 °C/s, yielding highly refined microstructures in the solidified particles. An important application of USGA is the production of continuous strip by spray deposition using the linear USGA atomizer. It has been planned that a batch operation will have a capacity of about 2400 kg for carbon steel strip, and about 450 kg for aluminum strip.[4]

Although the primary goal in gas atomization of liquid metals is to produce smooth, spheroidal (spherical) particles, large variations from smoothness and roundness are usual in practice, particularly for aluminum-base alloys, zinc and copper-zinc alloys. This is due primarily to cooling and solidification of droplets. Fine satellites observed on coarser particles are formed due to collisions among droplets in the turbulent two-phase flow in the spray. This condition makes the separation of fine and coarse particles impossible, exerting negative effects on the microstructure and properties by incorporating powders with significantly different dendrite sizes and phase distributions into final products. The second major problem associated with gas atomization is the entrapment and dissolution of the atomization gas in droplets that eventually remains in solidified particles, i.e., powders.

2.2.2Water Atomization

The earliest vestiges of water atomization technology can be recognized in the process invented by Marriott in England in 1872,[4] in which a steam injector was used to atomize metal melts. This process established the basis for the development of the technology and system for metal powder production from hundreds of grams to tonnage quantities in a wide range of chemical and metallurgical applications. With the development of the iron and steel industry and the growing need for shot to clean castings, water granulation with large capacity and pressures of 1–10 bar evolved for the production of shot in the size range between 0.5 and 5 mm. Following the idea of water granulation, high pressure water atomization was developed

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and applied first on copper and its alloys between the 1930’s and 1940’s, and later on steels in the 1950’s. During this time period, Powder Metallurgy Ltd. in England developed a process for nonferrous metals and alloys which established the basis for the modern commercial water atomization technology. This process involves an annular nozzle from which a high velocity conical jet of water is issued to impact the downward flowing metal stream. The process was further applied to high melting temperature alloys (iron, low alloy steels, stainless steels, high speed steels and superalloys) by B.S.A. Co. Ltd. in England and Höganäs in Sweden. To date, this technology has been used for making metal and alloy powders for a variety of metallurgical and chemical applications. It is the dominant method for the production of metal powders in terms of tonnage. The largest commercial application of water atomization is iron powder production for pressing and sintering. Water atomization is also used for the commercial production of copper, copper alloy, stainless steel, tool steel, and magnetic powders for press and sinter applications. In addition, water atomization is employed for the production of nickel alloy powders widely used in thermal spray coating and brazing, and precious metal powders used in brazing, sintering and dental amalgams.

Extensive reviews of water atomization processes have been made by Gummeson,[145] Klar and Fesko,[148] Lawley,[4] Beddow,[146] Yule and Dunkley,[5] and Reinshagen and Neupaver.[150] Similar to gas atomization, water atomization of melt is also a two-fluid atomization process involving the interaction of the melt and water. The significant difference is that water possesses a higher quench capability than gas due to its high thermal conductivity and capacity. A water atomization unit typically consists of a melting facility, a water pressurization system, an atomizer, an atomization chamber (tank) and a powder collection and drying system. In water atomization, metal or alloy charge is usually induction-melted in air, inert gas, or under vacuum, depending on the metal or alloy involved. Other melting techniques, such as arc melting and fuel heating, may also be used. In commercial-scale water atomization facilities, the molten

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metal is mostly poured into a tundish that provides a uniform and controlled temperature and flow of liquid metal through a circular delivery nozzle at the bottom of the tundish. The liquid metal exits from the delivery nozzle and falls a certain distance under gravity or pressure before impacted by water jets. The delivery nozzle serves as a controller of the diameter and shape, and to some extent, temperature and flow velocity of the melt stream. The melt stream is directed into the atomization zone where it is disintegrated into droplets by water jets. While the droplets fly downward in the atomization chamber they are rapidly cooled and solidified as a result of the interaction with water. The solidified powders are subsequently transported into the collection and drying system for post-process- ing. The overall process scheme of water atomization is similar to that of gas atomization with some minor differences. For example, the high-pressure gas supply in gas atomization is replaced by highpressure water pump in water atomization, a cyclone particle separator and filter are replaced by a decant water line.

For an atomization unit, the atomizer is the most critical part. It controls the flow and interaction of water and liquid metal, and hence affects the atomization efficiency. The design of water atomizers is similar to that of gas atomizers with respect to nozzle geometry and configuration. A variety of atomizer designs have been developed and are currently in use, including both asymmetric or axisymmetric discrete multiple jets (such as opposed V-shaped jets) and annular slit nozzle concentric with the metal stream (cone jet). A fan spray pattern can be generated by two opposed, V-shaped jets. Annular nozzles are less flexible than V-shaped nozzles with two or more discrete jets distributed symmetrically about the metal stream. Therefore, annular nozzles are primarily used in large capacity plants for iron powder production. The melt is almost always in the free-fall configuration. The free-fall distance varies with designs but ranges typically from 10 to 30 cm.[4] A converging cone arrangement for water jets is the preferred configuration. Many atomizer designs are proprietary and some are described in patent literature.

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In water atomization, a number of operation variables are to be considered in order to properly control the process. The variables include geometry parameters, process parameters, and thermophysical properties of metal/alloy and water. Each design and configuration of an atomization unit are unique and thus only some specific operation conditions may be employed. Many of the variables are interrelated. Therefore, there may exist more than one set of optimum variable combinations for a given atomization unit.

The geometry and process parameters of importance are not unlike those for gas atomization, as summarized in Tables 2.14 and 2.15, respectively. In conventional commercial water atomization, the water pressure is normally in the range of 5.5 to 21 MPa. The use of much higher water pressure from 50 to 150 MPa has been considered for the production of ferrous alloy powders in powder injection molding applications. Such high levels of water pressure can lead to very fine powders with a mass median size of 5 to 20 µm. In commercial water atomization, the powder size distributions are relatively broad (10–1000 µm) with a mass median size of about 100 µm and a standard deviation from 1.7 to 2.4. Based on the measurement of secondary dendrite arm spacing (SDAS) in particles, the cooling rates of droplets are estimated to be within the range of 104–106 °C/s, depending upon droplet size.

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

		Parameter	Value/Range
		Nozzle Number	1, 2, 4, or Annulus
		Relative Angle	40–60 (Annular Jet)
		between Water
		Nozzle and Metal	15–35 (V Jets)
	Water	Delivery Nozzle (°)
	Atomizer
			External Mixing & Free-Fall:
		Configuration	Annular Cone Jet
			Symmetric Discrete Fan-Spray Jets (V Jets)
			Asymmetric Cross Jets (Horizontal)
	Metal	Diameter (mm)	~7
	Delivery
		Configuration	Column
	Nozzle

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Table 2.15. Process Parameters in Water Atomization of Melts[5][148]

		Parameter	Value/Range
		Pressure at nozzle exit (MPa)	5.5–21
	Water	Velocity at nozzle exit (m/s)	70–230
		Mass Flow Rate (l/min)	110–380 (3000 for Annular Jet)
	Metal/	Superheat (°C)	75–150
			4.5–90 (400 for Annular Jet,
	Alloy	Mass Flow Rate (kg/min)
			500 for Asymmetric Jet)
		Tundish Material	Ceramics/Refractory
	Heating/		Induction
	Melting	Heating/Melting Method	Arc Melting
			Fuel Firing

Generally, the relative angle between water nozzle and metal delivery nozzle is much smaller in water atomization than in gas atomization. The efficiency of water atomization is between 4% and 5%. For achieving maximum efficiency, laminar flow of water jets is desired. The higher the water pressure, the finer the powder size and the higher the powder yield. For example, by increasing the water pressure about two times (from 60 to 125 atmospheres), the yield of mild steel powders of less than 500 µm can be enhanced from 60% to 80%. However, pressuring water to above 100 atmospheres, plus delivering it to atomization system and recycling it, is not an inexpensive operation. In addition, even intermediate levels of oxidizable elements in melt may form a refractory thin layer on liquid surface, preventing liquid from disintegrating and forming spheroids. Nevertheless, water atomization of liquid metals/alloys represents a far cheaper process than gas atomization with most gases except with air. Compared to the gas-atomized powders, the wateratomized powders are more irregular in shape with a rough dimpled surface texture (Fig. 1.7 b–d).

Water atomization is intrinsically a high volume, low cost process. Therefore, it is generally more cost-effective compared to other commercial atomization methods. However, powder purity,