Powder metallurgy

Why it matters

Powder metallurgy (P/M) makes millions of near-net-shape parts every day: gears for oil pumps, sintered bronze self-lubricating bushes, carbide cutting inserts, soft-magnetic cores for motors and actuators. For a mechatronics engineer it is the route to cheap, accurate small parts and to materials (tungsten carbide, porous filters, tungsten heavy alloys) that casting or forging cannot make.

Key ideas

The process chain. Powder production → characterisation and blending → compaction → sintering → optional secondary operations.

1. Powder production.

• Atomisation — a molten stream broken up by high-pressure water or gas jets; the main route for iron, steel, aluminium and copper alloy powders. Gas atomisation gives near-spherical particles; water atomisation gives irregular particles that interlock and press to higher green strength.
• Reduction — oxides reduced by hydrogen or carbon monoxide (e.g. sponge iron, tungsten, molybdenum); spongy, porous particles.
• Electrolytic deposition — very pure, dendritic copper and iron powders.
• Mechanical methods — milling and mechanical alloying for brittle materials and oxide-dispersion-strengthened alloys.
• Carbonyl decomposition — very fine, pure iron and nickel powders.

2. Powder characteristics. Particle size and its distribution, particle shape, apparent (loose-fill) density, flow rate and compressibility. Fine particles pack less well loosely and flow worse, but they have more surface energy, so they sinter faster to a higher final density. A mix of sizes fills the voids between large particles and raises packing density.

3. Blending and mixing. Alloying additions (graphite for carbon, copper, nickel), a lubricant (zinc stearate or wax, about 0.5–1 % by mass) to cut die-wall friction and ease ejection, and sometimes a binder. Pre-alloyed powders (every particle already the full alloy) are an alternative to elemental blends.

4. Compaction. Powder is pressed in a closed die at typically 100–800 MPa (higher for harder powders). The pressed part is the green compact: it holds its shape but has low strength. Friction with the die wall makes density fall with distance from the moving punch, so double-action (top and bottom punch) pressing is used for taller parts. Isostatic pressing (cold, CIP; or hot, HIP) applies uniform fluid pressure and gives uniform density. Part height-to-diameter ratio is kept low (about 2–3 maximum) for this reason.

5. Sintering. The compact is heated in a protective atmosphere (endothermic gas, hydrogen, dissociated ammonia, nitrogen-based mixes or vacuum) to about 0.7–0.9 of the absolute melting temperature of the main constituent. Atoms diffuse to form necks between particles; pores round off and shrink; the part densifies and shrinks. Typical stages: burn-off of lubricant, high-temperature sintering, controlled cooling. Liquid-phase sintering uses a minor constituent that melts (cobalt in WC–Co carbides, copper in some iron parts) to speed densification.

6. Secondary operations. Sizing or coining (re-pressing for accuracy), repressing and resintering for density, infiltration (filling pores with a lower-melting metal such as copper), impregnation with oil (self-lubricating bearings) or resin, heat treatment, steam treatment, and light machining.

Advantages. Near-net shape with little scrap (material utilisation often above 95 %), close tolerances, controlled porosity, materials that cannot be melted and cast economically (W, Mo, WC–Co cermets), and combinations of metals and non-metals (friction materials, electrical contacts).

Limitations. Expensive dies and powders, so economical only for large batches; residual porosity lowers ductility, fatigue strength and impact strength; part size is limited by press tonnage; undercuts and transverse holes cannot be pressed; density gradients in tall parts.

Links. Sintering is diffusion-controlled, like the heat-treatment topics; the press-force and density relations are the same ones used in forging and sheet-metal press selection.

Formulas

ρ_rel = ρ / ρ_th

• ρ = actual density of compact (kg/m³), ρ_th = theoretical (fully dense) density of the material (kg/m³). Dimensionless; often quoted as a percentage.

Porosity = 1 − ρ / ρ_th

• Volume fraction of pores, assuming no closed impurity phases.

m = ρ_g · V_g = ρ_s · V_s

• Mass is conserved through sintering (lubricant burn-off neglected). ρ_g, V_g = green density and volume; ρ_s, V_s = sintered density and volume.

Volumetric shrinkage = (V_g − V_s) / V_g = 1 − ρ_g / ρ_s

Linear shrinkage = (L_g − L_s) / L_g = 1 − (ρ_g / ρ_s)^(1/3)

• Applies only if shrinkage is isotropic (same in all directions). L_g, L_s in m.

F = p · A

• F = press force (N), p = compaction pressure (Pa), A = projected area of the part normal to the pressing direction (m²). Use the area at the parting plane, not the surface area.

Worked examples

Example 1 (standard) — press tonnage. A sintered iron bush has outer diameter 30 mm and inner diameter 18 mm. The powder needs a compaction pressure of 600 MPa. Find the press force.

1. Projected area: A = (π/4)(D² − d²).
2. A = (π/4)(30² − 18²) = (π/4)(576) = 452.4 mm² = 452.4 × 10⁻⁶ m².
3. F = p · A = 600 × 10⁶ Pa × 452.4 × 10⁻⁶ m² = 271.4 × 10³ N.
4. Final: F ≈ 271 kN (about 27.7 tonnes-force). A press with margin, say 30–40 t, would be chosen.

Example 2 (GATE level) — sintering shrinkage and die size. A 50 g iron compact is pressed to a green density of 6.2 g/cm³ and sinters to 7.0 g/cm³. The fully dense iron is 7.87 g/cm³. Assume isotropic shrinkage. Find (a) green and sintered volumes, (b) volumetric and linear shrinkage, (c) porosity before and after sintering, (d) the green length needed for a sintered length of 25 mm.

1. (a) V_g = m/ρ_g = 50/6.2 = 8.065 cm³; V_s = m/ρ_s = 50/7.0 = 7.143 cm³.
2. (b) Volumetric shrinkage = 1 − ρ_g/ρ_s = 1 − 6.2/7.0 = 0.1143 → 11.4 %. Linear shrinkage = 1 − (6.2/7.0)^(1/3) = 1 − 0.9604 = 0.0396 → about 3.96 %.
3. (c) Green porosity = 1 − 6.2/7.87 = 0.212 → 21.2 %; sintered porosity = 1 − 7.0/7.87 = 0.111 → 11.1 %.
4. (d) L_g = L_s / (ρ_g/ρ_s)^(1/3) = 25 / 0.9604 = 26.03 mm. The die must produce a green length of about 26.0 mm.

Check: 8.065 × (1 − 0.1143) = 7.143 cm³ ✓.

Common mistakes

• Using the part's surface area, or the full cylinder area for a bush, instead of the projected area in the pressing direction when sizing the press.
• Treating linear shrinkage as equal to volumetric shrinkage. For small shrinkage, linear ≈ one third of volumetric, not equal to it.
• Saying sintering melts the powder. Solid-state sintering stays below the melting point of the main constituent; only the minor phase melts in liquid-phase sintering.
• Mixing g/cm³ and kg/m³ (1 g/cm³ = 1000 kg/m³).
• Assuming P/M parts are as strong and tough as wrought parts of the same alloy; residual porosity reduces ductility and fatigue strength.
• Forgetting that mass, not volume, is conserved in sintering.

For GATE ME

Mostly concept questions: the sequence of steps, what sintering does and at what temperature, powder-production methods matched to powders, the role of lubricant and atmosphere, infiltration and impregnation, and typical products (self-lubricating bearings, carbide tips, filters). Numerical questions use density, porosity, shrinkage and press-force relations. Practise converting between green and sintered dimensions and computing press tonnage from projected area.

Quick check

1. Name two powder-production methods and one powder each typically makes.
2. Why is a lubricant added to the powder blend?
3. A compact goes from 6.4 to 7.2 g/cm³ on sintering. What is the volumetric shrinkage?
4. What secondary operation produces self-lubricating bronze bearings?
5. Why are P/M parts with a large height-to-diameter ratio avoided?

Answers: 1. Atomisation (steel, Al, Cu powders), reduction (sponge iron, W), electrolysis (pure Cu); 2. To reduce die-wall friction, give more uniform density and ease ejection; 3. 1 − 6.4/7.2 = 11.1 %; 4. Oil impregnation of the porous sintered part; 5. Die-wall friction makes density very non-uniform along the height.