How Do You Make Metal Without Melting It? Press the Powder and Wait

Atoms at the surface of a particle have neighbours on one side only, which leaves them in a higher energy state than atoms buried inside, so a mass of fine powder holds a great deal of excess energy simply by having so much surface. Heating gives atoms enough mobility to move, and they move in whatever direction reduces that surface, which means towards the contact points between touching particles. Necks form at those contacts and thicken, particles merge, and the pores between them shrink and eventually close. Nothing has melted at any stage, and the driving force is entirely the reduction of surface area.

A typical run follows a consistent sequence:

• •Mix powder with a binder that holds the pressed shape together
• •Press it in a die, which gives a fragile part of roughly the right shape
• •Heat gently to burn off the binder without cracking the part
• •Raise the temperature to perhaps three quarters of the melting point
• •Hold while necks form, pores close and the part shrinks
• •Cool under a controlled atmosphere to prevent oxidation

Closing the pores removes volume, so the part comes out considerably smaller than it went in, typically by ten to twenty per cent in linear dimension, and the whole craft of the process is making that shrinkage predictable. Uneven density in the pressed part shrinks unevenly and warps or cracks it, so pressing must produce uniform compaction, which is difficult in a complex shape. Temperature must be uniform across the furnace, since a hotter region shrinks faster. The tooling is therefore cut oversize by a calculated allowance, and for precision work a final machining operation is still needed. Getting a complicated part to come out the right size is the main difficulty.

A variant deliberately includes a component that does melt, and it solves problems the fully solid version cannot. A small proportion of a lower-melting material becomes liquid at the process temperature, flows into the gaps between particles by capillary action, and draws them together far faster than solid diffusion can, after which it may dissolve and redeposit the main material to fill the remaining space. Tungsten carbide tools work this way, with cobalt melting and binding the hard carbide grains. Most porcelain works this way too, with a glassy phase forming and filling the pores, which is what makes it translucent and impermeable. The trade is speed and density against a weaker material at the boundaries.

The technique is far older and far more widespread than the name suggests. Every fired brick, tile and porcelain object is made this way, since clay particles join below their melting point and the material would be ruined if actually melted. Tungsten cannot practically be melted at all, so filaments and carbide cutting tools are made from powder. Self-lubricating bearings are made deliberately porous and filled with oil, which is possible only because the pores can be left partly open. Magnets, brake pads and gears are routine products. And metal printing works by fusing successive layers of powder with a laser, which is the same physics applied point by point.

Fine powder holds excess energy in its enormous surface area, and heating lets atoms move to reduce that, growing necks between touching particles until the pores close, with nothing melting. The part shrinks by ten to twenty per cent, so uneven pressing warps it and tooling is cut oversize. Bricks, porcelain, tungsten carbide tools, porous oil-filled bearings and laser-printed metal all rely on it.