Powder metallurgy

Why it matters

Oil-pump rotors, variable-valve-timing sprockets, synchroniser hubs, self-lubricating bushes and powder-forged connecting rods are made by powder metallurgy (PM) because it produces net-shape parts in very large numbers with almost no machining and little scrap. PM is also the only practical route for cemented-carbide cutting tools and for materials that cannot be melted and cast, so it links directly to the machining topics later in this subject.

Key ideas

The process chain. Powder production → characterisation and blending → compaction into a green compact → sintering → optional secondary operations.

Powder production.

• Atomisation – a molten stream is broken up by high-pressure water or gas jets; the most widely used method (water gives irregular particles that compact well, gas gives spherical ones).
• Reduction of oxides by hydrogen or carbon monoxide – spongy, porous particles (sponge iron).
• Electrolytic deposition – very pure, dendritic powders (Cu, Fe).
• Carbonyl decomposition – very fine, pure Ni and Fe powders.
• Mechanical comminution (milling, crushing) – for brittle materials; mechanical alloying in ball mills.

Powder characteristics. Particle size and distribution (by sieving, quoted as mesh number), particle shape, flow rate (time for a fixed mass to flow through a funnel – governs die fill speed), apparent density (loose fill) and tap density, compressibility (green density at a given pressure) and green strength. Irregular particles interlock and give higher green strength; spherical ones flow and pack better.

Blending. Powders are mixed for uniform composition, and lubricants such as zinc stearate are added to reduce die-wall friction and ejection force; they are burnt off early in sintering.

Compaction. Powder is filled into a die and pressed, typically at a few hundred MPa for iron powders.

• Friction with the die wall makes density uneven along the height; double-action pressing (both punches move) and limiting the length-to-diameter ratio reduce the gradient.
• Isostatic pressing applies fluid pressure from all sides: cold isostatic pressing (CIP) in rubber moulds, hot isostatic pressing (HIP) with gas at high temperature for near-full density.
• Other routes: powder rolling, powder extrusion, and metal injection moulding (MIM) – fine powder with a polymer binder is injection-moulded, debound and sintered for small complex parts.

Sintering. The green compact is heated to roughly 0.7–0.9 of the absolute melting temperature of the main constituent, in a protective (reducing or inert) atmosphere or vacuum to prevent oxidation. Driving force: reduction of surface energy. Diffusion builds necks between particles, pores round off and shrink, and the part densifies and shrinks dimensionally. Liquid-phase sintering uses a small amount of a lower-melting phase that wets the solid particles – the standard route for cemented carbides (WC with Co binder).

Secondary operations. Repressing or sizing/coining for accuracy and density; infiltration (pores filled with a lower-melting metal, e.g. Cu into Fe); impregnation with oil (self-lubricating bearings, which depend on interconnected porosity) or resin; heat treatment, steam treatment, plating and machining. Powder forging forges a sintered preform to near-full density – used for connecting rods.

Advantages and limits. Advantages: near-net shape, very high material utilisation, controlled porosity, close tolerances, combinations impossible by melting (W–Cu, WC–Co, metal–ceramic friction materials), refractory metals. Limits: costly powders and dies, size and height limited by press force and density gradients, undercuts and cross holes cannot be pressed, and residual porosity lowers toughness and fatigue strength unless densified further.

Formulas

ρ_r = ρ / ρ_th Porosity = 1 − ρ_r ρ = bulk density of the part (kg/m³ or g/cm³), ρ_th = theoretical (pore-free) density; ρ_r and porosity are dimensionless.

1 / ρ_th = Σ (w_i / ρ_i) Theoretical density of a powder mixture; w_i = mass fraction of each powder.

F = p·A_p Compaction press force (N); p = compaction pressure (Pa), A_p = projected area of the part normal to the pressing direction (m²).

h_fill = h_c·(ρ_c / ρ_app) Die fill height (m) needed for a compact of height h_c; ρ_c = green density, ρ_app = apparent (loose) density. Based on constant mass and constant cross-section.

V_s / V_g = ρ_g / ρ_s L_s / L_g = (ρ_g / ρ_s)^(1/3) Volume and linear change on sintering at constant mass with uniform (isotropic) shrinkage; g = green, s = sintered.

Worked examples

Example 1 (standard) – press force and fill height for a bush. An iron bush 30 mm OD × 20 mm ID × 25 mm long is pressed at 500 MPa. Apparent density 3.0 g/cm³, required green density 6.4 g/cm³ (ρ_th = 7.87 g/cm³).

1. Projected area: A_p = (π/4)(30² − 20²) = 392.7 mm².
2. F = p·A_p = 500 N/mm² × 392.7 mm² = 1.963 × 10⁵ N = 196 kN.
3. h_fill = h_c·ρ_c/ρ_app = 25 × 6.4/3.0 = 53.3 mm of loose powder.
4. Green porosity = 1 − 6.4/7.87 = 18.7 %.

Example 2 (GATE level) – sintering shrinkage and die size. A green compact of density 6.3 g/cm³ sinters to 7.1 g/cm³ (ρ_th = 7.87 g/cm³). The sintered part must be 50.00 mm long; assume isotropic shrinkage.

1. Volume ratio V_s/V_g = 6.3/7.1 = 0.8873 → volumetric shrinkage = 11.3 %.
2. Linear ratio = 0.8873^(1/3) = 0.9609 → linear shrinkage = 3.9 %.
3. Green length = 50.00/0.9609 = 52.03 mm, so the die cavity is made about 52.0 mm long.
4. Sintered porosity = 1 − 7.1/7.87 = 9.8 %.

Example 3 – theoretical density of a mix. Fe with 2 wt % Cu (ρ_Fe = 7.87, ρ_Cu = 8.96 g/cm³): 1/ρ_th = 0.98/7.87 + 0.02/8.96 ⇒ ρ_th = 7.89 g/cm³.

Common mistakes

• Averaging densities by mass fraction directly (ρ = Σ w_i·ρ_i). Densities combine through volumes: 1/ρ = Σ w_i/ρ_i.
• Taking linear shrinkage equal to volumetric shrinkage; linear ≈ one-third of volumetric for small changes.
• Using the full surface area instead of the projected area for press force.
• Assuming sintering melts the main constituent; only liquid-phase sintering has a (minor) liquid phase.
• Forgetting that finer powders sinter faster but flow and compact less easily (more inter-particle friction, lower apparent density).
• Expecting oil-impregnated bearings to work at high density – they need connected porosity (typically around 20–30 %).

For GATE ME

Expect process-sequence and matching questions (production method ↔ powder shape, secondary operation ↔ purpose, product ↔ process such as cemented carbides by liquid-phase sintering) and statements on advantages and limitations. Numericals cover density, porosity, theoretical density of mixtures, press force from projected area, fill height, and volumetric/linear shrinkage on sintering. Practise constant-mass reasoning.

Quick check

1. What is the porosity of a part at 92 % relative density?
2. Which powder production method gives spherical particles?
3. Why is zinc stearate added to iron powder?
4. A part shrinks 9 % in volume on sintering. Approximately what is the linear shrinkage?
5. Which secondary operation makes self-lubricating bushes?

Answers: 1. 8 %; 2. Gas atomisation; 3. As a die-wall lubricant to reduce friction and ejection force; 4. About 3.1 %; 5. Oil impregnation.