Other Gas Atomization Methods

Ultrasonic gas atomization (Fig. 1e and 19) is claimed to allow finer particles, but no published comparative work has clearly demonstrated significant advantage over similar close-coupled nozzles. No commercial operator is known to use the method, and the largest melter used has been in the 10 to 50 kg size range.

Internal Mixing Nozzles. Typical values of k in Eq 6 shown in Fig. 22 range from 40 to 60 /Jm for close-coupled gas nozzles. Far higher efficiencies are obtained in another type of nozzle, the internal mixing nozzle, where gas-to-metal ratios are of a totally different order of magnitude than in conventional gas atomization (Fig. 21). In an internal mixing atomizer, gas and metal are mixed under pressure, and expansion and atomization take place at the nozzle exit into the atomization chamber. Two such processes are used today for the production of superalloy powders on a commercial scale (Ref 24). The first of the two gas/mix processes is known as vacuum atomization (Fig. 1d), where dissolved gas in the liquid metal adds an extra atmosphere pressure difference in the beginning of the atomization to promote the expansion of the gas in solution in the metal. In addition to the dissolved gas, some gas is also added to the system to finally break up the liquid into fine droplets as the dissolved gas alone would not be sufficient to create the fine particles that can be achieved with this process.

The vacuum atomization process is only operated by one company and is highly proprietary. The lack of any other operators, even when the original patents have expired, may indicate that the rather complex plant may be too expensive to justify the gas savings.

In another type of internal mixing nozzle (Ref 36), very high efficiency was achieved (Fig. 27), but low outputs. The k value in Eq 6 was below 10. The fundamental drawback of the internal mixing concept is the engineering difficulty of arranging to pressurize the melt to the same pressure as the gas. There are also problems with erosion of the ceramic nozzles employed as the metal velocity, normally only 1 to 3m/s in a pouring nozzle, could be expected to rise to the same order of magnitude as the gas velocities, that is, perhaps to 100 m/s. Thus, applications are so far very limited indeed.

Fig. 27 Dependence of mean particle size on gas-to-metal ratio. Source: Ref 36
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