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particularly with reactive metals and alloys, is a drawback associated with water atomization, although any metal or alloy melt that does not react violently with water may be processed with water atomization method.
2.2.3Oil Atomization
In addition to water atomization, oil atomization using various hydrocarbons (oils) is the only other liquid atomization process used in the atomization of liquid metals. Due to the similarity of these two atomization processes, water atomizers can be readily adapted to oil atomization. Some problems inherent in water atomization, such as powder oxidation, can be avoided in oil atomization. The cost-effectiveness is superior to gas atomization, but not as good as water atomization.
Oil atomization process was developed by Sumitomo Metal Industries[179] for commercial production of powders of low alloy, high strength, high quality steels in the early 1980’s. This is also the first time that Mn and Cr were successfully prealloyed in a P/M steel. In this process, an annular nozzle and a proprietary oil at a pressure of 14 MPa were used to produce injection-molding grade 4100 steel powder. Compared to water-atomized 4600 steel powders containing Ni and Mo, the oil-atomized, Mn and Cr prealloyed 4100 steel powders exhibited higher compressibility and hardenability, low level of carbon (0.002 w/o C) after annealing, and no significant oxidation (0.088 w/o O).
The drawback of oil atomization is that for high-melting- temperature alloys (for example, >800 °C), pyrolysis of the oil may occur leading to carbon pick-up so that powder must be decarburized by a wet hydrogen anneal followed by a dry hydrogen reduction to control the oxide level. Thus, the commercial application of oil atomization is limited to the production of steel powders with relatively high carbon content (~0.4 w/o).[4] Oil atomization process has the potential to be applied to high speed steels and bearing steels. This is because decarburization is not necessary for steels of >0.6
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w/o C and a low temperature heat treatment is sufficient for removing the oil.
IPS Steel Powder AB in Sweden, probably the only operating plant using oil atomization, routinely produces steel powders using oil atomization. The oil pressure used in atomization is low (0.5 MPa) and thus the resultant powder size is coarse (~500 µm). The oxygen content is, however, very low (<0.01 w/o O). High-pressure oil atomization (up to 10 MPa) is currently under evaluation by this company in an attempt to produce finer powders.
The oil-atomized powders have typically a mass median size of 50–70 µm, with comparable size distribution and similar cooling rates to the water-atomized powders. However, oil atomization of some low-melting-temperature metals such as lead and tin generates finer powders than using water atomization, due to reduced cooling rate by nucleate boiling.[5] Finer powders are also possible by using higher oil pressures. In addition, the shape of oil-atomized particles is generally more spherical than water-atomized ones, particularly for the alloys which form tenacious oxide films such as NiCr, Al, Zn, etc.
2.2.4Vacuum Atomization
Vacuum atomization is a commercial batch process.[180] The development of vacuum atomization started in the mid 1960’s, concurrent with the development of inert gas atomization. In 1970, a patent for the vacuum atomization method was issued to Homogeneous Metals, Inc. Using vacuum atomization, this company routinely produces superalloy powders of fine size without great consumption of argon, giving powders free of inert gas filled porosity. Wentzell[180] has made detailed description of this proprietary process.
Vacuum atomization is a method conceptually similar to effervescent atomization. As schematically depicted in Fig. 2.19, a vacuum atomization facility consists of two chambers, one above the other. The overall dimension is about 18 × 4 m. In the lower chamber, metal is first induction-melted under vacuum and subsequently
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pressurized to 1–3 MPa with a mixture of inert gas (normally argon) and soluble gas (typically H2). Thus, the volume of the lower chamber can be small but it must withstand vacuum conditions during melting and pressure conditions during pressurization. The upper chamber is under vacuum and serves as a spray and collection tank. During pressurization, the energy for the atomization is stored in the liquid metal. When the liquid metal supersaturated with the gas under pressure is suddenly exposed to vacuum by exhausting through a guide tube into the upper vacuum chamber, the gas expands and comes out of solution. The high velocity of the exploding melt and the gas desaturation lead to the atomization of the melt. Therefore, this process is sometimes also termed soluble gas atomization or melt explosion atomization. The considerable energy input in introducing gas into the flowing melt stream generates an internal-mixing nozzle effect.
Figure 2.19. Schematic of vacuum atomization of melt.
Process parameters of importance include gas pressure, H2 concentration in solution in liquid metal, diameter of the guide tube, and melt superheat. The concentration of H2 gas in solution in the liquid metal ranges from 0.0001 to 0.001 w/o, proportional to the square root of H2 partial pressure that is in turn controlled by the composition of the gas mixture. For a given H2 partial pressure, the concentration of H2 gas in the liquid metal is dependent on the metal type and superheat.
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Normally, the vacuum-atomized droplets are clean. However, hydrogen may be entrapped or dissolved in the droplets. For powder metallurgy applications, powder treatment may be required to avoid porosity in final products. The vacuum-atomized droplets are normally spherical and smooth with few satellites. The cooling rates of droplets are lower than in gas atomization, typically in the order of magnitude of 102 °C/s. This is because the cooling of droplets is primarily by radiation heat transfer in vacuum atomization while it is by forced convection heat exchange with gas during gas atomization. Droplet size may range from 1 to 500 µm with a typical mass median diameter of 40–70 µm.
The arrangement of the melting and vacuum spray chambers is critical for guiding the liquid metal to eject into the vacuum chamber. Difficulties exist in precisely controlling the expulsion of the liquid metal into the vacuum chamber. Therefore, flaky droplets may be formed in vacuum atomization. Although vacuum atomization was developed mainly for the production of high-purity nickel and cobalt based superalloy powders, it is also applied to atomize the alloys of aluminum, copper and iron.
2.2.5Rotating Electrode Atomization
Rotating electrode atomization is a commercial centrifugal atomization process. This process, designated REP (Rotating Electrode Process) or PREP (Plasma Rotating Electrode Process), was invented by Nuclear Metals, Inc. in 1963.[181] The original short rod design was modified to long rod design in 1974.[182] Since its invention, REP has been widely used for producing clean spherical powders of low carbon steels as carrier for carbon black in office copier applications. Over the past ten years, this atomization technique has been applied to the production of cobalt-chromium and titanium alloy powders for prosthetic devices. It has also been adapted to the production of ultra clean titanium alloy powders for potential aerospace applications.
Rotating electrode atomization process has been described in detail by many researchers.[183][184] As illustrated in Fig. 2.20, in the
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long rod designs, although it may be increased up to 2100 radians/s in experiments. Melt superheat is limited in the process since the liquid metal is removed by the centrifugal force from the rod as soon as it is melted. This makes it difficult to atomize the alloys with a wide melting range.
Compared to those intrinsically energy-intensive processes, such as gas atomization and water atomization, the energy used in REP (more generally, centrifugal atomization) is low and atomization occurs by means of centrifugal forces. In addition, REP requires no containment of liquid metals. This eliminates the contamination from refractory oxides. Therefore, the rotating-electrode-atomized particles are usually very smooth, spherical, almost satellite-free, and of high purity. This is the remarkable feature of REP and other centrifugal atomization methods operating in inert gas or vacuum at low rates. This is also an indication that the atomization occurs in the Direct Droplet regime, as detailed in the next chapter. The droplet size is relatively coarse, typically ranging from 100 to 400 µm with a mass median diameter of about 200 µm. The powder yield is high, i.e., the droplet size distribution is narrow. The cooling rates of droplets are up to 102 °C/s, depending on the gas used in the chamber and the droplet size.
Rotating electrode atomization may be applied to almost all metals and alloys since it does not require a crucible for melting and/ or pouring. In particular, high melting-temperature metals and alloys, such as Ti and Zr, are well suited for the process. However, the production cost is still a drawback associated with the process, since electrode production is generally more expensive than a metal melt. In addition, production rates are relatively low compared to other atomization processes such as gas atomization and water atomization.
In a recently developed process, the consumable rods used in REP are replaced by disk-shaped electrodes, since such electrodes are easier to fabricate than long round rods. During atomization, a rotating consumable disk is melted at its periphery.
