Powder metallurgy (P/M): compact fine metal powders in a die, then sinter (heat without melting). Common metals: Fe, Cu, Al, Sn, Ni, Ti.

Why P/M: net-shape forming, wide range of compositions, good economics. Parts are typically < 2.5 kg, net shape, low surface roughness. Products: gears, cams, bushings, cutting tools, automotive parts.

Process Steps

1. Powder Production

• Atomization: spray molten metal through an orifice.
• Mechanical comminution: roll crushing, ball milling, hammer crushing.

Particle size 0.1–1000 µm; shape and size distribution affect processing and quality.

2. Blending

Uniform size/shape, mix different powders, add lubricants (flow).

3. Compaction

Press blended powder in dies (hydraulic/mechanical) into a green compact:

• Purposes: shape, density, particle contact, green strength.
• ↑Compaction pressure → ↑density → approaches bulk density; ↑density → ↑strength & $E$.
• Apparent density = loose-powder density (zero pressure).
• Equal-size particles leave ~24% porosity; smaller particles fill gaps → higher density.
• Pressures: 70–800 MPa (acts on projected area → sets press force).
• Cold isostatic pressing (CIP): rubber mold pressurized hydrostatically (~400 MPa); density not quite 100%.
• Hot isostatic pressing (HIP): gas pressure ~100 MPa at ~1100°C → ~100% density and good properties.
• Roll compaction: powder fed into a rolling mill.

4. Sintering

Heat green compacts below the melting point (oxygen-free atmosphere) to bond particles. Before sintering the compact is brittle (low green strength). Properties aren’t as good as forgings, but net shape + good finish make P/M competitive with small forgings.

5. Finishing

Heat treat for hardness/strength; machine (mill/drill) holes and threads.

Shrinkage on Sintering

A compact shrinks during sintering. For a linear shrinkage of 5%, the volume shrinkage is

$\frac{V_f}{V_0} = (1 - 0.05)^3 = 0.857 \;\Rightarrow\; \approx 14\%\ \text{volume reduction}$

Design Considerations

• Avoid sharp radii/corners and abrupt thickness changes.
• Threads and transverse holes need separate machining.
• Dimensional tolerances ~±0.05 mm.
• Mechanical & physical properties (conductivity included) fall with porosity (voids = less material + stress concentrations).