Question 4 of 7: Powder Metallurgy — Atomization Processing Steps, Particle Size Ranges, and Major Processing Stages

Question 4: Powder Metallurgy — Atomization Processing Steps, Particle Size Ranges, and Major Processing Stages (20 marks: 7, 3, 3, 7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Four processing steps of liquid/gas atomization

1. Melt the metal. The feedstock metal (or alloy) is heated in a furnace/tundish until fully molten and superheated slightly above its melting point, so it stays liquid all the way to the atomizing nozzle.
2. Stream the melt. The molten metal is fed as a thin, continuous stream through a small orifice/nozzle at a controlled flow rate.
3. Atomize the stream. A high-velocity jet of gas (inert gas — nitrogen, argon — for gas atomization) or liquid (high-pressure water, for water/liquid atomization) impinges on the falling molten stream, breaking it up into a spray of very fine liquid droplets.
4. Solidify (and collect) the droplets. Surface tension pulls each still-liquid droplet toward a spherical shape as it falls through the atomizing chamber and cools; the droplets solidify in flight and are collected as a fine, largely spherical metal powder.

(ii) Typical particle-size range — liquid (water) atomization

Water atomization uses a much higher-velocity, more aggressive quenching medium than gas, which produces a coarser, WIDER, and less perfectly spherical (more irregular) particle-size distribution than gas atomization — the rapid, uneven quenching partially freezes droplets before surface tension has fully rounded them. Typical water-atomized powder falls in roughly the 20–500 μm range, with the distribution's centre commonly cited around 100 μm, depending on water pressure, nozzle geometry, and melt superheat.

(iii) Typical particle-size range — gas atomization

Gas atomization uses a gentler, more controllable quenching medium (an inert gas jet rather than water), giving the droplets more time in flight for surface tension to pull them into a near-perfect sphere before they solidify, and producing a finer, narrower size distribution than water atomization. Typical gas-atomized powder falls in roughly the 10–150 μm range, which is why gas atomization is the preferred route when fine, highly spherical powder (e.g. for powder-injection molding or additive manufacturing feedstock) is required.

(iv) Two major processing steps of metal powder (powder metallurgy) technology

Once the powder itself has been produced (part (i)/(ii)/(iii) above), powder-metallurgy (PM) part production centres on two major, defining unit operations (a preliminary blending/mixing of the powder with lubricant/alloying additions is also normally performed, but it is a preparatory step rather than one of the two major process stages):

1. Compaction. The (blended) loose powder is pressed in a rigid die under high pressure, forming a "green" compact that already has the part's final shape and near-final dimensions, held together at this stage only by mechanical interlocking and cold welding between particles — it has little strength until sintered.
2. Sintering. The green compact is heated in a controlled-atmosphere (or vacuum) furnace to a temperature below the metal's melting point, allowing solid-state atomic diffusion to metallurgically bond the powder particles together, which develops the part's final strength, hardness, and density.