Rotating Electrode Process
The rotating electrode process (REP) is a commercial form of centrifugal atomization that overcomes the material problem by spinning the metal before it is melted. Thus, the spinning element is a consumable electrode. Material in the form of a rod electrode is rotated at about 15,000 rpm while it is melted by an arc. The molten metal is ejected centrifugally in the form of molten-metal droplets that solidify before hitting the walls of the inert-gas-filled chamber. The REP concept was developed by Nuclear Metals, Inc., and resulted in the granting of patents (Ref 39).
Figure 29(a) shows the process carried out using a plasma torch process (PREP). The original method (REP) used a tungsten-tipped cathode as shown in Fig. 29(b). Use of a transferred arc helium plasma torch eliminates the tungsten contamination characteristic of the earlier design. This was a critical factor in titanium powder production.
- Fig. 29 Schematic of the REP. (a) Plasma arc rotating electrode process (PREP). (b) Tungsten tip rotating electrode process (REP)
The rotating electrode and plasma rotating electrode processes offer certain advantages over other powder-making processes. Titanium alloys are optimally produced by these methods, because the corrosive nature of molten titanium and the difficulty involved in containing this molten material are overcome. Generally, many high-duty materials to be made into powder benefit in that there is no liquid metal/container contact. This ensures that ceramic particles are not inadvertently added. When the addition of foreign particles from external sources is prevented, these processes provide a method for making powders to exact standards of cleanliness. However the bar stock must first be sufficiently clean itself.
As with other centrifugal methods, size distributions can be held within tighter ranges than is commonly achieved by gas atomization. A cumulative plot of weights passing through progressively finer screens versus screen size is shown in Fig. 30. The effect of rotation speed is illustrated, and the median particle size d50 is approximately defined by:
where Lc'is the rotation rate, D is the diameter of the electrode, and K is a constant for a given alloy for a limited range of arc power. Hollow gas-filled particles, which cause thermally induced porosity in compacts, are not generated. Powders made by these techniques have demonstrated a high degree of sphericity along with good surface quality. Particles also pour readily into molds of complex shape. They give consistent packing at approximately 65% full density so that near-net shapes can be obtained by HIP. However, a major drawback is the mechanical limitations on rotational speed, which limit the minimum median particle size to about 50 to 150 /'m (depending on alloy density, etc.). Also, the cost of making a high-quality bar of metal is very significant and productivity is low and energy consumption high compared with other techniques.
100 90
60 70 60 50
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Screen size, mesh 100 140 200
Screen size, mesh 100 140 200
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100 Screen siie, 200 Gf50 100 Screen siie, Fig. 30 Cumulative plot of REP particle size distributions. Weight percent of sample finer than a given screen size versus screen size. Electrode: C-1018 steel, 63.5 mm (2'i in.) diameter The process has only been commercially operated by one firm and is effectively a very "niche" one, with a few highly specialized industrial applications and total annual output not exceeding 1000 tons/year. | |||||
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