Powder metallurgy

In short

Powder metallurgy (PM) makes solid components from metal or ceramic powders by compacting them to shape and then bonding the particles by heating below the melting point (sintering). It is the only economic route for materials that cannot be melted and cast — tungsten, cemented carbides, refractory and immiscible combinations — and the cheapest route for high-volume small precision parts such as self-lubricating bushes, oil-pump rotors, sprockets and cam lobes. Because porosity is deliberately controlled, PM also gives products impossible otherwise: oil-retaining bearings, metallic filters and friction linings. In India it is examined in every manufacturing-technology paper, in GATE/ESE production engineering, and in interviews at Kennametal India and Sandvik (carbides), Hoganas India (powders), auto-component suppliers to Bajaj, TVS, Maruti and Tata Motors, and research bodies such as DMRL, ARCI and MIDHANI, Hyderabad.

Explained

What PM is, and the basic route

• Definition: shaping and consolidating fine particulate material into a coherent solid by pressure and/or heat, without fully melting it.
• Standard route (five steps): powder production → blending/mixing (alloy additions + lubricant) → compaction into a green compact → sintering in a controlled atmosphere → secondary/finishing operations.
• Green compact: the as-pressed part; it holds shape only by mechanical interlocking and cold welding, so its strength (green strength) is a few MPa — enough to handle, not to use.
• Driving force for sintering: reduction of total surface free energy of the powder; the system removes surface area by growing necks between particles and shrinking pores.
• Why it is used: near-net shape (little or no machining), 95–97% material utilisation, controlled porosity, uniform fine microstructure, ability to mix metals with ceramics or with mutually insoluble metals (W–Cu, Cu–graphite).
• Limits: very high tooling cost (justified only above roughly 10,000–20,000 pieces a year), part size restricted by press tonnage (mostly under about 2 kg), no re-entrant features/undercuts or side holes as-pressed, residual porosity lowers strength, ductility, fatigue life and impact energy, and fine powders are costly and explosive.

Powder production methods

• Atomization — the dominant industrial method. A liquid metal stream is broken into droplets that solidify in flight.
  • Water atomization: cheap, high cooling rate, irregular particles → good green strength; used for iron, low-alloy and stainless steels.
  • Gas atomization (N₂, Ar, air): spherical, clean, free-flowing particles → poor green strength but ideal for MIM, HIP and additive routes; used for superalloys, tool steels, Al.
  • Centrifugal / rotating-electrode (REP), ultrasonic, soluble-gas: specialised, high-purity, reactive alloys (Ti, superalloys).
• Chemical reduction of oxides: solid-state reduction of mill scale or ore with coke/H₂ gives sponge iron powder (Höganäs process) — porous, irregular, very compressible; WO₃ + H₂ gives tungsten powder; also Cu, Mo, Co.
• Electrolytic deposition: brittle dendritic deposits are crushed; gives the highest purity Cu and Fe powders, but is costly and energy-intensive.
• Carbonyl process: Fe(CO)₅ or Ni(CO)₄ decomposes thermally to very fine (1–10 µm), spherical, high-purity Fe/Ni powders — used for MIM and soft-magnetic cores. Carbonyls are highly toxic.
• Mechanical comminution: crushing, ball/rod milling, attritor and vibratory milling, hammer milling — suited to brittle materials (oxides, carbides, Si, intermetallics); gives flake or angular particles.
• Mechanical alloying: prolonged high-energy milling repeatedly fractures and cold-welds dissimilar powders to produce true alloy or oxide-dispersion-strengthened (ODS) powders.
• Others: thermal decomposition, precipitation from solution (hydrometallurgy), shotting and graining, machining/grinding swarf, vapour condensation (Zn dust), and atomization + annealing to restore compressibility.

Powder characterisation and testing

• Particle size and distribution: sieve analysis for > 45 µm (sieve designation: −100 +200 mesh means through a 100-mesh = 150 µm sieve and retained on 200 mesh = 75 µm; 325 mesh = 45 µm); sub-sieve sizes by sedimentation, laser diffraction, Fisher sub-sieve sizer, microscopy/image analysis.
• Particle shape: spherical, rounded, irregular, angular, dendritic, flake, acicular, nodular, sponge — governs flow, apparent density and green strength (irregular interlocks better; spherical flows better).
• Apparent density (ρ_a): mass per unit volume of loose powder, measured with a Hall flowmeter funnel into a 25 cm³ cup (ASTM B212/ISO 3923-1). Fixes die fill depth.
• Tap density: after standardised tapping (ASTM B527); Hausner ratio = ρ_tap/ρ_a — below about 1.25 means free-flowing, above about 1.4 poor flow needing granulation.
• Flow rate: seconds for 50 g to drain the Hall funnel (ASTM B213) — decides press speed and fill uniformity.
• Compressibility: green density achieved at a stated pressure (ASTM B331); green strength by transverse rupture of a pressed bar (ASTM B312/B528).
• Also specified: chemical composition, oxygen/carbon content, specific surface area (BET), sintered dimensional change, and compressive/friability behaviour.

Blending, mixing and lubricants

• Blending = combining powders of the same nominal composition (different sizes/lots); mixing = combining dissimilar powders. Industrially both words are used loosely.
• Additions: alloying elements (Cu, Ni, Mo, graphite), lubricant, binders, deflocculants; graphite is added as fine powder because it dissolves into iron only during sintering.
• Lubricant: 0.5–1.5% zinc stearate, lithium stearate, or synthetic wax (e.g. Acrawax/EBS). It reduces die-wall friction, improves density uniformity, lowers ejection force and tool wear; it must be burnt out (dewaxing, 400–600 °C) in the first furnace zone, or it leaves soot and blisters.
• Excess lubricant lowers green strength and sintered density; too little causes scoring, galling and even die seizure.
• Equipment: double-cone, V-cone, ribbon, planetary and plough-share mixers. Over-mixing work-hardens and segregates the powder.
• Safety: fine Al, Mg, Ti, Zr and Fe dusts are explosive — inert handling, earthing, no sparks, dust extraction, and compliance with dust/ventilation duties under the Occupational Safety, Health and Working Conditions Code, 2020 (OSH Code, in force 21 November 2025, which repealed the Factories Act, 1948) and the State Factories Rules saved under it; cobalt and nickel powders are respiratory hazards and classified carcinogens.

Compaction and consolidation

• Die (uniaxial) pressing — about 90% of PM tonnage. Tooling: die, core rod (for holes), upper and lower punches.
  • Single-action (one moving punch) gives a strong density gradient; double-action, floating die and withdrawal tooling press from both ends and give far more uniform density and multi-level shapes.
  • Cycle: fill → compact → eject (hold-down to avoid cracks) → recharge. Modern CNC presses run 10–50 strokes/min.
  • Typical pressures: Al 100–200 MPa; bronze 150–300 MPa; iron/steel 400–700 MPa; WC–Co 70–140 MPa; refractory metals 70–200 MPa.
  • Density gradient: pressure dies away from the punch face because of die-wall friction, so density falls with depth; parts with large length-to-diameter ratios (above about 3) cannot be pressed uniformly.
  • Part complexity classes (MPIF): Class I single-level thin; Class II single-level thicker; Class III two-level; Class IV multi-level — each needs more punch motions.
• Cold isostatic pressing (CIP): powder in an elastomer bag pressed by fluid at 100–400 MPa — uniform density, no die friction, high L/D parts, large sizes; wet-bag (flexible, low volume) and dry-bag (fixed, faster).
• Hot isostatic pressing (HIP): 100–200 MPa argon at 1,000–1,250 °C in a canned preform — gives essentially 100% density; used for superalloy turbine parts, tool steels, Ti, and for healing porosity in castings.
• Powder rolling: powder fed into a roll gap gives green strip, then sintered and re-rolled — strip and sheet of Ni, Cu, Fe.
• Powder extrusion: canned powder hot-extruded to bar/tube; used for Al-SiC composites and ODS alloys.
• Metal injection moulding (MIM): feedstock of about 60 vol% fine (< 20 µm) powder plus thermoplastic binder, injection-moulded like plastic, then debound (solvent/thermal/catalytic) and sintered to 95–99% density with 15–20% linear shrinkage. Gives complex 3-D shapes impossible by die pressing; part mass typically under ~100 g.
• Powder forging: a sintered preform is hot-forged in a closed die to near-full density — high-strength automotive connecting rods, gears.
• Others: slip casting, tape casting, explosive/high-velocity compaction, warm compaction, spark-plasma (field-assisted) sintering, roll compaction/granulation.

Sintering

• Definition: heating a green compact to 0.7–0.8 of the absolute melting temperature of the base metal in a controlled atmosphere so that atomic diffusion bonds the particles.
• Three stages: (1) initial — point contacts grow into necks, little densification; (2) intermediate — necks thicken, pore channels become cylindrical and interconnected, grain growth and major shrinkage; (3) final — isolated, rounded, closed pores shrink slowly; full density is rarely reached.
• Mechanisms: surface diffusion, volume (lattice) diffusion, grain-boundary diffusion, evaporation–condensation, viscous/plastic flow. Only those sourcing material from the particle volume or grain boundaries cause shrinkage; surface diffusion and evaporation–condensation only round the pores.
• Types:
  • Solid-state sintering — all phases remain solid (Fe, Cu, stainless, alumina).
  • Liquid-phase sintering — a lower-melting constituent melts and wets the skeleton, so capillary forces pull particles together and densification is rapid: WC–Co cemented carbides (Co-rich liquid, 1,350–1,500 °C), Fe–Cu, W–Ni–Fe heavy alloys, Cu–Sn bronze.
  • Activated sintering (trace additive, e.g. Ni on W), supersolidus sintering of prealloyed powder, reactive sintering.
  • Infiltration: molten copper is drawn into the open pores of a sintered iron skeleton, giving pressure-tight, higher-strength parts.
• Typical temperatures/times: bronze 800–850 °C; iron and steel 1,120 °C (high-temperature 1,250 °C); stainless 1,150–1,300 °C; WC–Co 1,400–1,450 °C; alumina 1,600–1,700 °C; holding 15–60 min at temperature.
• Atmospheres and their jobs — exclude air, reduce surface oxides, remove the lubricant, and control carbon:
  • Hydrogen / dissociated ammonia (75H₂–25N₂): strongly reducing, for stainless, W, Mo, carbides.
  • Endothermic gas (CO–H₂–N₂, carbon-controlled) for steels; exothermic gas for copper and bronze.
  • Nitrogen-based synthetic atmospheres (N₂ + 3–10% H₂) — now the industry norm, safer and cheaper.
  • Vacuum for reactive and high-purity work (Ti, Be, superalloys, some carbides).
  • Wrong atmosphere = decarburisation (soft, weak part) or carburisation/sooting; dew point is monitored.
• Furnaces: continuous mesh-belt (to ~1,150 °C), pusher, walking-beam, roller-hearth, and vacuum/batch for higher temperatures; zones are dewax/preheat → high-heat → controlled cooling (slow cooling or sinter-hardening).
• Dimensional change: iron–carbon shrinks slightly; iron–copper grows (Cu diffuses into Fe); the premix and sintering schedule are chosen to make dimensional change nearly zero and repeatable, which is what makes PM a precision process.

Secondary operations, materials and standards

• Repressing / coining / sizing: a second pressing raises density and corrects dimensions; as-sintered tolerances of about ±0.05–0.1 mm improve to ±0.01–0.025 mm, and surface finish improves.
• Double press–double sinter (DPDS): press, pre-sinter, repress, re-sinter — 7.3–7.5 g/cm³ in ferrous parts.
• Impregnation: oil (self-lubricating bearings, minimum ~18% oil by volume in standard grades) or resin/plastic (to seal pores before plating or for pressure tightness).
• Heat treatment: possible but hardenability and thermal conductivity are reduced by porosity, so quench severity and case depth differ from wrought steel; carburising, carbonitriding, induction hardening and sinter-hardening are used.
• Steam treatment: Fe₃O₄ grown in the pores at ~540 °C for corrosion resistance, sealing and wear resistance.
• Finishing: tumbling/deburring, shot peening (fatigue), machining (interrupted cut because of pores — tools wear fast, use sharp carbide/PCBN and no coolant that fills pores), grinding, plating after sealing.
• Common PM materials: plain and alloyed PM steels (Fe–C, Fe–Cu–C, Fe–Ni–Mo–C), sintered bronze (90Cu–10Sn) bushes, Cu–graphite brushes, cemented carbides (WC + 6–12% Co), ceramics (Al₂O₃, ZrO₂, SiC, sintered UO₂ nuclear pellets), refractory metals (W, Mo, Ta), magnets (Alnico, ferrites, SmCo, NdFeB), friction materials, metal filters, electrical contacts (W–Ag, W–Cu), tungsten heavy alloys, Al–SiC composites.
• Designations and codes: MPIF Standard 35 material codes are the industry language — e.g. FC-0208 is iron + 1.5–3.9% Cu + 0.6–0.9% combined carbon; FN-0205 is an Fe–Ni–C grade. ISO 5755 specifies sintered metal materials, ISO 2738/3369 density, oil content and open porosity, ISO 3327 transverse rupture strength of hardmetals, ISO 4499 hardmetal microstructure; ASTM B-series covers the powder tests above. BIS has adopted several of the ISO powder-testing and sintered-bearing standards, but IS numbers are revised periodically — check the current BIS catalogue rather than quoting a number from memory.

Key points

• Sintering - bonding particles by diffusion below the melting point, driven by reduction of surface energy; typically 0.7–0.8 T_m (absolute).
• Green compact - as-pressed part held by cold welding and interlocking; green strength only a few MPa.
• Apparent density - loose powder density; decides die fill depth and the compression ratio.
• Compression ratio - green density ÷ apparent density = fill height ÷ compact height; usually 2–3 for ferrous powders.
• Atomization - the main commercial powder route: water-atomized = irregular and compressible, gas-atomized = spherical and free-flowing.
• Mechanical alloying - high-energy milling that cold-welds dissimilar powders into true alloy/ODS powders.
• Lubricant (0.5–1.5% zinc stearate) - cuts die-wall friction and ejection force; must be dewaxed out at 400–600 °C.
• Die-wall friction - causes axial pressure decay and a density gradient; limits length-to-diameter ratio to about 3.
• Double-action pressing / CIP / HIP - progressively more uniform density; HIP reaches essentially full density.
• Liquid-phase sintering - a melting minor constituent wets and pulls the skeleton together by capillary force (WC–Co, Fe–Cu).
• Sintering atmosphere - excludes air, reduces oxides, removes lubricant and controls carbon; wrong dew point decarburises or soots the part.
• Porosity - total = open (interconnected, holds oil) + closed; strength, ductility, fatigue and impact fall steeply with porosity.
• Infiltration vs impregnation - infiltration fills pores with molten metal (Cu in Fe) for strength; impregnation fills them with oil or resin.
• Sizing and coining - cold secondary pressing for tolerance, finish and local density.
• Self-lubricating bearing - sintered bronze with 20–25% interconnected porosity holding oil released by shaft heat and capillary action.
• Cemented carbide - WC + 6–12% Co, pressed and liquid-phase sintered; the hardness comes from WC, the toughness from Co.
• MIM - powder + polymer injection-moulded, debound and sintered; complex shapes with 15–20% linear shrinkage.
• Economics - high tooling cost, very low cost per piece; PM wins above roughly 10,000–20,000 parts/year of small precision shapes.

Formulas and examples

Example 1 — die fill and press capacity for a bronze bush. Finished bush OD 20 mm, ID 12 mm, length 15 mm; premix apparent density 3.1 g/cm³; green density 6.5 g/cm³; compaction pressure 250 MPa.

• Volume = (π/4)(20² − 12²) × 15 = 0.7854 × 256 × 15 = 3,016 mm³ = 3.016 cm³
• Part mass at 6.6 g/cm³ sintered = 3.016 × 6.6 ≈ 19.9 g
• CR = 6.5/3.1 = 2.10 → fill depth = 2.10 × 15 ≈ 31.5 mm of die cavity above the core rod.
• Projected area A = 0.7854 × 256 = 201.1 mm² → F = 250 × 201.1 = 50,265 N ≈ 50.3 kN ≈ 5.1 tonf; a 15-tonne press is ample.
• Contrast: a plain Fe disc 50 mm diameter at 600 MPa needs 600 × 1,963.5 = 1.18 MN ≈ 120 tonf — this is why press tonnage, not geometry, limits PM part size.

Example 2 — porosity and oil content of the same bush. Composition 90Cu–10Sn; ρ_Cu = 8.96, ρ_Sn = 7.31 g/cm³.

• 1/ρ_th = 0.90/8.96 + 0.10/7.31 = 0.10045 + 0.01368 = 0.11413 → ρ_th = 8.76 g/cm³
• Sintered density measured 6.60 g/cm³ → P = 1 − 6.60/8.76 = 24.7% total porosity
• Oil-impregnation test gives open porosity 22% → oil volume 0.22 cm³ per cm³; with ρ_oil = 0.88 g/cm³, oil mass = 0.194 g
• Oil content by mass = 0.194/(6.60 + 0.194) = 2.85%, i.e. 22% by volume — comfortably above the ~18 vol% minimum for standard self-lubricating grades.

Example 3 — why single-action pressing gives a density gradient. Punch pressure 500 MPa, die diameter 25 mm, effective μ = 0.15, compact height 20 mm.

• Exponent = 4 × 0.15 × 20/25 = 0.48 → p_x = 500 × e^−0.48 = 500 × 0.6188 = 309 MPa
• The far face sees only 62% of the applied pressure, so it ends up markedly less dense. Pressing from both ends halves the effective friction path and brings the two faces to similar density.

Example 4 — strength lost to porosity. A wrought steel of the same composition has UTS 600 MPa. Using X/X₀ = (1 − P)³:

• Single-pressed part, P = 12%: (0.88)³ = 0.681 → UTS ≈ 409 MPa
• Double press–double sinter, P = 6%: (0.94)³ = 0.831 → UTS ≈ 498 MPa
• Powder-forged, P ≈ 0: ≈ 600 MPa. Halving the porosity recovers roughly 90 MPa — the engineering reason for DPDS, HIP and powder forging. (The exponent is empirical; use the value given in the question.)

Example 5 — sintering temperature and die oversize for shrinkage. Iron, T_m = 1,538 °C = 1,811 K; green density 6.90, sintered 7.10 g/cm³; required finished OD 20.00 mm.

• 0.75 T_m = 0.75 × 1,811 = 1,358 K = 1,085 °C; the works practice of 1,120 °C = 1,393 K is 1,393/1,811 = 0.77 T_m — consistent.
• Linear shrinkage = 1 − (6.90/7.10)^1/3 = 1 − (0.97183)^1/3 = 1 − 0.99052 = 0.95%
• Green OD required = 20.00 × 1.00957 = 20.19 mm, so the die cavity is cut about 0.19 mm oversize (plus a small allowance for elastic springback on ejection).

Where it is used

• Automotive and two-wheeler components — sintered sprockets, oil-pump inner/outer rotors, shock-absorber valve plates, valve-seat inserts and guides, cam lobes, planetary gear carriers, powder-forged connecting rods; supplied to Bajaj, TVS, Hero, Maruti Suzuki, Tata Motors and their tier-1s.
• Self-lubricating bearings and filters — sintered bronze and iron bushes in fans, mixers, pumps, wipers and small motors; porous metal filters, flame arresters and silencer elements.
• Cutting-tool and hardmetal industry — WC–Co inserts, drills, rock bits and wear parts at Kennametal India (Bengaluru) and Sandvik Asia (Pune); grade selection by ISO P/M/K series.
• Electrical, magnetic and nuclear — W–Ag and W–Cu contacts, Cu–graphite brushes, soft-magnetic Fe–Si and Fe–P cores, ferrite and NdFeB magnets, and sintered UO₂ fuel pellets at Nuclear Fuel Complex, Hyderabad.
• Aerospace, defence and research — HIP superalloy and titanium parts, tungsten heavy alloys and ODS steels developed at DMRL and ARCI (Hyderabad) and MIDHANI; friction linings and brake pads.
• Job roles — PM process/tooling engineer, sinter-furnace and atmosphere engineer, powder metallurgist in QA (density, porosity, metallography), tool designer for compaction tooling, and R&D roles in powders and hardmetals.

Interview questions

• Q: What exactly happens during sintering, and why does it work below the melting point? A: Atoms diffuse to the particle contacts and grow necks, reducing total surface free energy; solid-state diffusion is fast enough at 0.7–0.8 T_m, so no melting is needed.
• Q: Why is a lubricant added to the powder, and what happens if it is not burnt off? A: It cuts die-wall friction, evens out density and reduces ejection force and tool wear; if not dewaxed, it decomposes in the hot zone and leaves soot, blisters, cracks and poor bonding.
• Q: Why does a single-action pressed part have non-uniform density? A: Pressure is lost to die-wall friction with depth, so the face away from the punch is less densely packed; double-action or floating-die tooling corrects it.
• Q: What is compression ratio and why does it matter on the shop floor? A: Green density divided by apparent density — it sets the die fill depth and therefore the press stroke and tooling length for a given part height.
• Q: Difference between infiltration and impregnation? A: Infiltration draws a molten lower-melting metal (usually copper) into the pores to raise strength and pressure tightness; impregnation fills them with oil or resin for lubrication or sealing before plating.
• Q: Why are cemented carbides made only by powder metallurgy? A: WC melts above 2,800 °C and would decompose; the WC particles are instead bonded by liquid-phase sintering in a cobalt binder at 1,400–1,450 °C, keeping the carbide hardness with a tough matrix.
• Q: Which atmosphere would you use for sintering stainless steel and why? A: Hydrogen, dissociated ammonia or vacuum — a strongly reducing, very low dew point atmosphere is needed to reduce chromium oxides; nitrogen-rich atmospheres risk chromium nitride formation and loss of corrosion resistance.
• Q: An Fe–Cu PM part grew instead of shrinking after sintering. Why? A: Copper melts at 1,085 °C and diffuses into the iron lattice, causing dimensional growth; the copper and carbon contents and sintering schedule are adjusted to balance it against shrinkage.
• Q: Why is the fatigue strength of a PM part much lower than wrought steel of the same hardness? A: Pores act as internal stress concentrators and crack initiation sites, and reduce the load-bearing section; densification (DPDS, powder forging, HIP) or surface rolling/peening is used to recover it.
• Q: Why is machining a sintered part difficult? A: The cut is interrupted by pores, so the tool takes impact loading and wears rapidly; pores also absorb coolant and dull the finish, so sharp carbide tooling, light feeds and sealed parts are used.
• Q: When would you choose MIM over die pressing? A: For small, geometrically complex 3-D parts with undercuts, side holes or thin walls that cannot be pressed axially — accepting higher material cost and 15–20% shrinkage.
• Q: Why is PM called a near-net-shape process, and when is it not economical? A: Little or no machining is needed and 95%+ of the material ends up in the part; it is uneconomical for low volumes because of tooling cost, and unsuitable for large or very thick parts because of press tonnage and density gradients.

Common mistakes

• Saying the powder "melts" during sintering — it does not; only in liquid-phase sintering does a minor constituent melt.
• Calculating compaction force on the full cross-section instead of the projected area normal to the pressing direction, or forgetting the core-rod hole area in a bush.
• Mixing up apparent, tap, green, sintered and theoretical density — and so getting compression ratio and porosity wrong.
• Treating total porosity as available oil volume; only the open, interconnected porosity holds and releases oil.
• Forgetting the dewaxing stage, or assuming the lubricant does not need to be removed.
• Assuming shrinkage is always present and always isotropic — Fe–Cu grows, and die-pressed parts shrink unequally along and across the pressing axis.
• Quoting wrought-steel strength, hardenability or fatigue data for a sintered part without applying the porosity knock-down.
• Designing undercuts, side holes, re-entrant features, sharp corners or L/D above about 3 for a die-pressed part.

Related topics

• Casting · Additive manufacturing (3D printing) · Metal forming · Theory of metal cutting · Grinding and finishing processes · Metrology and quality control