Powder metallurgy: powder production, compaction and sintering

Why it matters

Powder metallurgy (P/M) makes near-net-shape parts — gears, cams, self-lubricating bushes, tungsten-carbide inserts, filters — with almost no machining and 95–97 % material utilisation. It is also the only practical route for metals that are hard to melt or cast (tungsten, molybdenum), for metal–ceramic mixtures (cemented carbides, cermets) and for parts whose controlled porosity is the whole point (oil-impregnated bearings, filters).

Key ideas

The P/M chain. Powder production → blending/mixing (with lubricant such as zinc stearate) → compaction in a die → sintering → optional secondary operations (sizing/coining, re-pressing, infiltration, oil impregnation, heat treatment, machining).

Powder production.

• Atomization (water, gas or centrifugal): a molten stream is broken into droplets that freeze. Most common route for iron, steel, copper and aluminium alloys. Gas atomization gives spherical particles; water atomization gives irregular particles that interlock and give better green strength.
• Chemical reduction of oxides (e.g. iron oxide by H₂ or CO) gives porous "sponge" powder with good compressibility.
• Electrolytic deposition gives very high-purity, dendritic powders (copper, iron).
• Mechanical comminution (ball milling, crushing) for brittle materials; mechanical alloying repeatedly cold-welds and fractures particles in a high-energy mill to make dispersion-strengthened alloys.
• Carbonyl decomposition (Fe(CO)₅, Ni(CO)₄) gives very fine, pure spherical powder.

Powder characteristics decide flow, packing and sintering: particle size and size distribution (sieve mesh number), shape (spherical, irregular, flaky, dendritic), specific surface area, apparent density (loose-fill density), tap density, flow rate (Hall flowmeter) and compressibility. Finer powder has more surface area per unit volume, so it sinters faster, but it flows poorly and picks up more oxide. A spread of sizes packs better because fine particles fill the voids between coarse ones.

Compaction. Powder is pressed in a rigid die (typically 100–800 MPa, depending on the material) to a green compact with enough strength to handle. Density rises quickly with pressure at first (particle rearrangement), then slowly (plastic deformation of particles). Friction at the die wall makes pressure, and hence density, fall with distance from the moving punch, so tall thin parts are pressed from both ends (double-action) or by isostatic pressing. Cold isostatic pressing (CIP) uses fluid pressure on a flexible mould for uniform density; hot isostatic pressing (HIP) applies gas pressure at high temperature and reaches almost full density. The press force is set by the projected area of the part in the pressing direction, not by its height.

Sintering is heating the green compact in a protective atmosphere (H₂, dissociated ammonia, endothermic gas or vacuum) to about 0.7–0.9 of the absolute melting temperature of the main constituent. Atoms diffuse to the contact points, forming necks that grow; pores round off and shrink; the part gains strength and usually shrinks. The driving force is the reduction of surface energy. Stages: (1) bonding at contact points, (2) neck growth and pore channel closure, (3) pore rounding and shrinkage. In liquid-phase sintering (e.g. WC–Co, bronze from Cu + Sn) a minor constituent melts and speeds densification. Mass is conserved during sintering (apart from burnt-off lubricant), so any density rise appears as shrinkage that the die designer must allow for.

Secondary operations. Sizing/coining improves dimensional accuracy; infiltration fills pores with a lower-melting metal (copper into iron); impregnation fills interconnected pores with oil (self-lubricating bushes) or resin.

Limits. Powders are expensive; die cost needs large batch sizes; part size is limited by press capacity; undercuts, cross-holes and threads perpendicular to the pressing direction cannot be pressed; residual porosity lowers ductility and fatigue strength; density gradients appear in tall parts.

Formulas

ρ = m / V ρ = bulk density of the compact (kg/m³ or g/cm³), m = mass (kg), V = external (envelope) volume including pores (m³).

ρ_r = ρ / ρ_th and porosity P = 1 − ρ / ρ_th ρ_th = theoretical (pore-free) density of the material. ρ_r is the relative density, a fraction.

1 / ρ_th = Σ wᵢ / ρᵢ Theoretical density of a powder mix from mass fractions wᵢ and pore-free densities ρᵢ (rule of mixtures, assumes no volume change on alloying).

F = p · A_p F = press force (N), p = compaction pressure (Pa), A_p = projected area normal to the punch motion (m²).

ρ_g · V_g = ρ_s · V_s (mass conserved) L_s / L_g = (ρ_g / ρ_s)^(1/3) for uniform (isotropic) shrinkage Subscripts g = green, s = sintered; L = any linear dimension. Linear shrinkage = 1 − L_s/L_g.

p_x = p₀ · exp(−4 μ k x / D) Pressure at depth x below a single-action punch in a cylindrical die: p₀ = punch pressure, μ = die-wall friction coefficient, k = ratio of radial to axial pressure in the powder, D = die diameter (m). Shows why density falls along a tall compact.

Worked examples

Example 1 — iron bush (standard). A plain bush has outer diameter 30 mm, inner diameter 20 mm and height 25 mm. It is pressed from iron powder to a green density of 6.4 g/cm³ using 500 MPa. Pore-free iron density = 7.87 g/cm³. Sintering raises the density to 7.0 g/cm³. Find the powder mass, press force, green porosity and sintered dimensions.

1. Projected area A_p = (π/4)(D² − d²) = (π/4)(30² − 20²) = (π/4)(500) = 392.7 mm².
2. Volume V = A_p · h = 392.7 × 25 = 9817 mm³ = 9.817 cm³.
3. Mass m = ρ_g · V = 6.4 × 9.817 = 62.8 g of powder per part.
4. Press force F = p · A_p = 500 N/mm² × 392.7 mm² = 196 350 N ≈ 196 kN (about 20 t).
5. Green porosity P = 1 − ρ_g/ρ_th = 1 − 6.4/7.87 = 1 − 0.813 = 0.187 (18.7 %).
6. Linear ratio L_s/L_g = (ρ_g/ρ_s)^(1/3) = (6.4/7.0)^(1/3) = (0.9143)^(1/3) = 0.9706, so linear shrinkage ≈ 2.9 %.
7. Sintered height = 25 × 0.9706 = 24.26 mm; sintered OD = 30 × 0.9706 = 29.12 mm. The die must be made about 3 % oversize.

Example 2 — oil-impregnated bronze bearing (GATE level). A bush of OD 20 mm, ID 12 mm and length 15 mm is made from a 90 % Cu – 10 % Sn (by mass) mix and sintered to 6.6 g/cm³. Pore-free densities: Cu 8.96 g/cm³, Sn 7.29 g/cm³ (given data). If 90 % of the pore volume is interconnected and fills with oil, find the porosity and the oil held.

1. Theoretical density 1/ρ_th = w_Cu/ρ_Cu + w_Sn/ρ_Sn = 0.9/8.96 + 0.1/7.29 = 0.10045 + 0.01372 = 0.11417 cm³/g, so ρ_th = 8.76 g/cm³.
2. Porosity P = 1 − ρ/ρ_th = 1 − 6.6/8.76 = 0.247 (24.7 %).
3. Bush volume V = (π/4)(20² − 12²) × 15 = (π/4)(256)(15) = 3016 mm³ = 3.016 cm³.
4. Pore volume = 0.247 × 3.016 = 0.743 cm³; interconnected = 0.9 × 0.743 = 0.67 cm³ of oil.
5. Check mass: m = 6.6 × 3.016 = 19.9 g.

Example 3 — density gradient. For single-action pressing with μ = 0.3 and k = 0.4, the pressure at the bottom of a compact whose height equals its diameter (x/D = 1) is p₀ · exp(−4 × 0.3 × 0.4 × 1) = p₀ · exp(−0.48) = 0.62 p₀. The bottom is pressed only 62 % as hard as the top — the reason for double-action presses and lubricated dies.

Common mistakes

• Using the part's full surface area or volume instead of the projected area for press force.
• Treating sintering shrinkage as volumetric when a linear dimension is asked: linear ratio is the cube root of the density ratio.
• Mixing up apparent (loose-fill) density, green density, sintered density and theoretical density.
• Averaging densities by mass fraction directly; the correct rule adds specific volumes (wᵢ/ρᵢ).
• Saying sintering melts the powder. Only in liquid-phase sintering does a minor phase melt; the main constituent stays solid.
• Claiming P/M can press any shape — undercuts and transverse holes cannot be pressed.

For GATE PI

Expect conceptual MCQs on powder-production methods (which gives spherical, high-purity or sponge powder), the purpose of each step (sintering atmosphere, infiltration, impregnation, sizing), CIP vs HIP, and typical P/M products. Numericals use density–porosity relations, mixture density, press force from projected area and shrinkage from mass conservation. Practise carrying units between g/cm³, kg/m³, MPa and N/mm².

Quick check

1. Which powder-production method gives the highest purity powder?
2. A green compact is 80 % dense and sinters to 92 % dense. What is the linear shrinkage?
3. Why are tall thin P/M parts pressed from both ends?
4. A 50 mm diameter disc is pressed at 400 MPa. What press force is needed?
5. What is the driving force for sintering?

Answers: 1. Electrolytic deposition. 2. (80/92)^(1/3) = 0.9545, so about 4.5 %. 3. Die-wall friction lowers pressure and density away from the punch; double action evens out density. 4. 400 × (π/4)(50²) = 785 kN. 5. Reduction of total surface (interfacial) energy of the particles.