This demands a reproducible and high tap density to reduce shrinkage (typically ~10–15%) in pressing and excellent sphericity and smoothness are most desirable to assist in filling shaped cans. The HIP process ( Figure 2) consists of filling shaped metallic containers (‘cans’) with powder, which, after evacuation of air and sealing, are hot isostatically pressed, typically at pressures of the order of 1000–2000 bar and temperatures of 1000–1200☌. Water atomisation is widely used for stainless steels, but the superalloys demand vacuum inert gas atomisation (VIGA). Alloys include low alloy steels, a lot of stainless steels (~50% of the market) and some superalloys like IN718. Purity demands are moderate, for example oxygen content is not critical, as high temperature sintering allows reduction of most oxides.
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The shape of the powder is not a clear-cut demand high tap density is desirable to reduce shrinkage (typically ~15%) when sintering to full density, but excessively spherical and smooth particles can reduce ‘brown strength’ which is the strength of the components after debinding. Typical powders have (mass) median particle sizes of 5–15 μm with maximum particle sizes from 15 μm to 50 μm. This requires, above all, fine powders, in order to have the required high degree of sintering activity to allow the green part to be sintered to high density. However, production capacity for vacuum melted, gas atomised powders is only around 20,000–30,000 tonne yr –1. Thus there has been a major upsurge of investment in plants to produce such high value powders. The global volumes of powder consumed are still modest compared to the ‘classical’ PM parts business which accounts for ~1 Mt yr –1 of Fe powder ( 1), but they have growth rates in tonnage terms of 10–40% per annum and high powder values (typically in the range of €7–200 kg –1 compared with Fe powder’s €1–2 kg –1 ( 2). These newer branches of PM make quite different demands on the metal powders that they employ. These have, over the last ten years or so, led to a huge upsurge in research and industrial investment in PM all around the world. The ‘classical’ PM sintered parts business did not attract much research interest in the UK in the 1980s and 1990s, but the last twenty years have seen first, the advent of MIM and HIP and, more recently, AM or three-dimensional (3D) printing ( 1). Having spent many years working on powder metallurgy (PM) and atomisation, it is very heartening to see the current remarkable upsurge in interest in the field. Recent Developments in the Powder Metallurgy Industry