Hot Gas Atomisation
The development of the hot gas atomisation technique stems from the simple observation that gas exit velocities in the supersonic gas nozzles used in atomisers increase substantially as the stagnation temperature in the atomisation die increases. This increased velocity results in higher a kinetic energy available to disrupt the melt stream into finer droplets and hence powder median (d50) size decreases. This is particularly advantageous where high yields of lower size powders are required, as is the case for Metal Injection Moulding (MIM) applications. Alternatively, where process economies are required, the same kinetic energy can be imparted from a relatively expensive atomising gas at a lower mass flow rate.
Equipment to utilise hot gas atomisation is now being fitted to new installations supplied by PSI, where the demand for the capability has arisen specifically from the demand for increased yields of finer powders. However, the majority of existing plants also may be converted to operate with hot gas by the addition of the appropriate heating system, atomisation die and engineering changes to the plant to cope with increased heat loads.
The cost of powder of the required size distribution from an atomisation plant is a detailed function of several factors, the main ones being costs of raw materials (gas and metal), capital and labour. If production of fine sizes is the economic driver then gas heating reduces costs by savings in materials and increased plant utilisation through higher yields. Each case has to be treated on its merits and a detailed cost/benefit analysis is invariably worthwhile.
On the simplest level if relatively expensive argon is used to atomise and is not recycled, then reduced gas usage through gas heating, producing the same size distribution, may in itself provide the justification for conversion. Once argon has been selected for metallurgical reasons to atomise a given melt, at current energy costs it makes economic sense to make the best use of it by the utilisation of electrical energy to raise both its pressure and temperature to levels where it atomises most effectively.
Further improvements are expected by increasing gas temperatures still further and atomisation up to 800 degC is quite practicable before different technical solutions for heating and handling of the gas are required.