Process Variables

Conventional atomization pressures are typically in the range 0.5 to 4 MPa (70 to 600 psi), and gas velocities in the nozzles range from Mach 1 to 3. However, in free-fall atomizers, measured gas velocities in the impingement area normally have fallen to 50 to 150 m/s (for air/nitrogen). Typically, gas-atomized powder is spherical with a log normal size distribution. Mean particle size is usually in the range 10 to 300 /m with a standard deviation of about 2. Oxygen content is about 100 ppm. Powder cost is extremely sensitive to the economies of scale. Prealloyed ferrous, nonferrous, and specialty alloys are made by inert gas atomization.

For free-fall gas atomization, the key process variables are similar to those of water atomization (Fig. 4b). In a confined-nozzle design, the major parameters are the geometry of the tundish nozzle tip, the gas jet apex angle, and gas jet diameter (or width for an annular design), the number of jets, and the horizontal spacing between the jets and the tundish nozzle center line.

In conventional gas or air atomization, typical metal flow rates through single orifice nozzles range from about 1 to 90 kg/min (2 to 200 lb/min). Typical gas flow rate ranges from 1 to 50 m3/min (40 to 1600 ft3/min) at gas pressures in the range of 350 kPa to 4 MPa (50 to 600 psi). Effective gas velocities are very difficult to measure and depend on nozzle design, ranging from 20 m/s (66 ft/s) to supersonic velocities. The temperature differential between the melting point of the metal and the temperature at which the molten metal is atomized (superheat of the molten metal) is generally about 75 to 150 °C (135 to 270 °F). In gas atomization with argon or helium, the cost of gas consumption is significant, and a means of circulation to facilitate gas reuse is desirable in larger-scale facilities.

In practice, for a given gas nozzle design and size, average particle size is controlled by the pressure of the atomizing medium and the melt flow rate (controlled by nozzle diameter and nozzle suction). For all nozzles, the velocity of the gas usually "chokes" at sonic velocity (about 300 m/s, or 1000 ft/s, for nitrogen and argon) in the narrowest region of the nozzle if the upstream gas pressure is at least 1.9 times the external pressure.

Consequently, the amount of gas flow (A) depends on gas pressure, temperature, and nozzle area. For ideal conditions and zero velocity on the entrance side of the nozzle, gas flow is:

where a is the cross section of gas nozzle at exit, k equals Cp/Cv, the ratio of specific heat at constant pressure and volume, p is the gas pressure in gas reservoir, T is the temperature in gas reservoir, R is the gas constant, and g is the acceleration due to gravity. For nitrogen, with k = 1.4:

As a compressible fluid passes through a nozzle, a drop in pressure and a simultaneous increase in velocity result. If the pressure drops sufficiently, a point is reached where, in order to accommodate the increased volume due to expansion, the nozzle unit must diverge. Thus, nozzles for supersonic velocities must converge to a minimum section and diverge again.

Gas efficiency can be compared on the basis of how much powder surface is generated per unit volume of gas spent. This criterion accounts for higher gas consumption requirements when higher gas pressures are applied in producing finer powders. Confined- or "close-coupled-" nozzle designs give higher efficiencies at comparable gas/metal ratios (Fig. 21) (Ref 24). A more simple method is to use the equation:

Where k is a constant for the process and metal, and G/M is the gas/metal ratio, which is variously measured in kg/kg or cubic meters of gas per metal mass (m3/kg). A typical plot of dm versus G/M is shown in Fig. 22 with data from several published papers. For a given metal, the value of k can be taken as an index of efficiency in terms of fineness achieved per unit gas consumed (a lower value of k is a higher efficiency). If dm is in microns (/'m), k is the median size when using one cubic meter of gas per kilogram of metal, a not untypical level.

Gas/metal mix

Ui ro

O 50

Free fall

C ose couple

C ose couple

Fig. 21 Gas efficiency expressed as powder area produced per unit volume of gas consumption (m2/m3). Source: Ref 24

103 300

102 30

60 40

103 300

102 30

60 40

A

nnuy iu source from Ref 14 o Aluminum {Unal)

A

a Iron [Miller; * Cu-G.SAI (K □ Sn-5%Sb (

uibiii;

lar)

bidder)

i

A

o □

^ A

o

GasAnetal ratio, m3/kg

GasAnetal ratio, m3/kg

Fig. 22 Effect of gas-to-metal ratio on median particle size. Source: Ref 14

Typical values of k shown in Fig. 22 range from 40 to 60 /'m for close-coupled gas nozzles.

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