Blown to atoms: how to make metal powders

Before anything can be manufactured from metal powder, the powder itself has to be made. Most powders are made by the atomisation of molten metal John Dunkley looks at the major atomisation methods in use...

p owt)Ek Metallurgy depends on the economic sup- ply of consistent quality metal powders. The majority of metal powders are now produced from

molten metals by atomisation. In water atomisation, high-pressure water jets are used to break up, and quench the molten metal. Particle size is closely related to pres- sure. While many units operate at 100-200 bars and make powders around 30-100 pro, some special units in Japan operate at up to 1500 bars and make powders as fine as 10 ~+Inl+

hi terms of tonnage, water atomisation is n o w the pre- eminent mode of atomisation for metal, especially ferrous metal, powders. Over 500 kt/yr of iron powder is atom- ised. Any metal o r alloy that does not react violently with water can be water atomised, provided it can be

melted and poured satisfactorily. However, it is tbund that metals melting below about 500"C give extremely irregular particles due to ultra-rapid freezing, which is often undesirable. Thus zinc is the lowest melting metal produced in this way commercially.

In general, water atomisation is cheaper than other methods because of the low cost of the medium (water), low energy use fi~r pressurisation compared with gas or air, and the very high productivity that can be achieved - up to ~0t/hour or 500kg/min. The main limitations of water atomisation are powder purity and particle shape, partic- ularly with more reactive metals and alloys. Melting units from a few kilos to over 100 tons are in use.

A schematic flow sheet of water atomization is shown in Fig. 1. Melting of metals follows standard procedures. Air induction melting, arc melting, and fuel heating are

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Figure t - Schematic Flow Sheet of Water Atomisat ion

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Figure 2 - Schematic How Sheet of Gas Atomisation

all suitable. Typically, the molten metal is poured into a tundish, which is essentially a reservoir that supplies a uniform and controlled head of molten metal to the tundish nozzle. The nozzle controls the shape and size of the metal stream and directs it through an atomising noz- zle system in which the metal stream is atomised by high- velocity water jets.

The slurry of powder and water is pumped to a first- stage dewatering device (e.g. cyclone, magnetic system etc) which often feeds a second stage (e.g. vacuum filter) dewatering unit to reduce drying energy use. The pow- der is then dried and may be sieved or further processed as required. In the case of iron powder, annealing in a reducing atmosphere is necessary to achieve good compressibility.

Gas atomisation is the process where the liquid metal is disrupted by a high-velocity gas such as air, nitrogen, argon, or helium (Fig 2). Operating pressures range from 5-100bar. The higher pressures, often in the 15 - 60-bar range, are used to make inert gas-atomised iron, nickel and cobalt alloys, for example. Air-atomised non-ferrous metals are normally made with pressures ranging from 3- 15 bars.

Gas atomisation differs from water atomisation in many respects. Rather than being dominated by pressure like water atomisation, the gas flow rate is the dominant fac- tor controlling particle size. Gas atomisers also come in a wider range of designs than water atomisers, and are most- ly classified as either "confined" or "free-fall" nozzle con- figurations.

Free-fall gas units are very similar in design to water atomising units. The metal falls into gas atomising jets some distance from the ceramic nozzle with the result that it is very difficult to bring the mean diameter of powder

below 50 to 60 pm on iron- base material. However, they are fairly simple and reliable. As well as vertical designs resembling water atomising designs, there are a number of horizontal free-fall designs, where a melt stream is atom- ised by essentially horizontal gas jets. These designs are widely used in zinc, aluminium and copper alloy air atomisers. Closed or "confined" nozzle designs enhance the yield of fine powder particles (~ 10 pro) by maximising gas velocity and density on contact with the metal. The gas contacts the metal as it leaves the ceramic nozzle. However, although confined designs are more effi- cient, they can be prone to freezing of the molten metal at the end of the tundish nozzle, which rapidly blocks it. Great care is needed in setting up

close-coupled nozzles, and the closer the coupling, the greater the care (as well as the efficiency).

Inert gas-atomised powders are normally spherical, while air-atomised powders are often irregular, due to an oxide fihn forming on the molten droplets and inhibiting spherodisation.

World-wide production of inert gas-atomised powder is much less than that of water atomised powders, at around 50,000t/yr. Metal feed rates are lower than in water atomisation (typically 10-50kg/min), and melt size is smaller, ranging up to six tons. The largest units are used to produce High Speed and other special alloy steels fur subsequent HIP compaction to produce either PMHSS bar steel or special HiPped shapes (e.g. in dual- phase stainless steels).

The second largest market is probably for thermal spraying using plasma, flame and high-velocity oxygen fuel torches. More modest amounts are vacuum melt- ed superalloys for gas turbine applications. However tonnage of air-atomised powders, especially zinc and aluminium, but also tin, lead, and copper alloys, prob- ably exceeds 300,000 t/yr. Most of these air atomisers operate continuously for many hours or even days. Multi-nozzle units are often used to boost output on aluminium and zinc. Applications for air atomised powders include non-ferrous powder metallurgy (e.g. bronze filters, PM brass, tin for bronze bearings) but are predominantly outside PM in fields such as chemistry, smelting, pyrotechnics, paints and batteries.

The auth~

John Dunkley is the founder of Atomising Systems Limited, based in Sheffield, UK.

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