Overview Plasma Atomization: A New Process for the Production of Fine, Spherical Powders M. Entezarian, F. Allaire, P. Tsantrizos, and R.A.L. Drew A new atomization process, plasma at- omization (P.A.), has been developed to pro- duce fine, spherical powders. Unlike conven- tional high-pressure atomizers, P.A. utilizes multiple nontransferred direct-current arc plasmas to accelerate the atomization gas. In the P.A. process, metal wires are fed into the apex of the multiple plasmas, where they melt and are atomized in a single step. This pro- cess offers the unique ability to produce spherical powders of reactive metals with a typical average particle size of 40 pm. INTRODUCTION N ontransferred arc plasmas are widely used for spraying,! melting} gas heating} materials synthesis,3 and waste destruction. 4 The general advantages of using plasma in these applications are its high enthalpy and temperature in addition to high efficiency and inert gas environment; the latter enables the pro- cessing of reactive metals. The kinetic energy at the exit of the plasma torch has also been used in plasma spraying to accelerate and splat-quench molten par- ticles onto a substrate. A nontransferred arc plasma is gen- erated by introducing an electric dis- charge into a gas such as argon (Figure 1). Some of this argon is ionized and heated to temperatures as high as 11,000 K. Remaining gas is heated by the ion- ized argon and reaches 2,000-10,000 K. Gas atomization is used to break up a continuous molten stream into fine pow- der particles using the kinetic energy of the pressurized gas. Hot gases may be used to decrease the heat transfer from the atomized particles to the surround- ing gas; thus, particle solidification is delayed until surface tension completes the spheroidization process. Spherical powders are widely used in thermal spraying (due to their excellent flow- ability) and in powder metallurgy (due to the excellent reproducibility in manu- facturing parts with controlled poros- ity).5 The basic idea in a new approach, plasma atomization (P.A.), is to convert the electrical energy supplied to a plas- ma torch into kinetic energy. The high Plasma Torches To Vacuum Pump Powder Collection Figure 2. A schematic overview of the P.A. process. Figure 1. A typical nontransferred arc plasma torch. 1996 June • JOM kinetic energy generated by the plasma translates into a very high velocity at the exit of the plasma torch, which is essen- tial for the atomization process. Also, an inert gas such as argon is required to prevent a gaseous reaction with the at- omized materials. All of these require- ments can be fulfilled using multiple argon plasmas. Three nontransferred direct-current arc plasma torches oper- ating between 20 kW and 40 kW were used in this study. Argon was used as the plasma gas with a flow rate of 100- 120 1/ min. per torch. These torches were vertically inclined 30° into a common apex (Figure 2). The raw materials were fed either as wire or as a stream of mol- a L----.... I b Figure 3. Scanning electron microscopy mi- crographs of P.A. produced (a) copper and (b) titanium powders. 53