Metal Extrusion: Hot and Cold Extrusion Processes, Dies, and Defects

Metal extrusion forces a heated or room-temperature billet through a shaped die orifice under high compressive ram pressure, producing a continuous product whose cross-section is defined entirely by the die geometry. The process can generate profiles of extraordinary complexity — solid rods, multi-void hollow sections, asymmetric structural shapes, and near-net-form tubes — in a single stroke, at extrusion ratios from 4:1 to over 400:1. This article covers the mechanics of direct and indirect extrusion, extrusion pressure and force calculation, die types, hot and cold extrusion parameters, the metallurgical effects of extrusion on microstructure and properties, and the principal defects that arise from incorrect process parameters or die design.

Extrusion Mechanics and Pressure Equations

The fundamental quantity governing extrusion is the extrusion ratio R, which defines both the geometry of the process and the equivalent plastic strain imposed on the workpiece material. All pressure and force predictions are anchored to R and to the material’s flow stress at the extrusion temperature and strain rate.

Extrusion Ratio and True Strain:
  R = A₀ / Aₓ
  ε = ln R = ln(A₀/Aₓ) = 2 × ln(D₀/Dₓ)   [for round billet → round bar]
  where:
    A₀ = billet cross-sectional area (mm²)
    Aₓ = extrudate cross-sectional area (mm²)
    D₀ = billet diameter (mm);  Dₓ = extrudate diameter (mm)

Johnson Empirical Pressure Formula (flat-face die):
  p = σ₀ × (a + b × ln R)
  where:
    p   = mean ram pressure (MPa)
    σ₀  = flow stress at extrusion temperature and strain rate (MPa)
    a   = 0.8 (empirical constant, flat-face die, lubricated)
    b   = 1.5 (empirical constant, flat-face die, lubricated)
    Note: a ranges 0.8–1.2; b ranges 1.2–1.5 depending on die geometry and friction

Total Ram Force:
  F = p × A₀   (N or MN)

Adiabatic Temperature Rise (approximate):
  ΔT ≈ (0.9 × σ₀ × ε) / (ρ × Cₚ)
  where:
    ΔT  = temperature rise from deformation (°C)
    ρ   = billet density (kg/m³)  e.g. 2700 for Al, 8900 for Cu, 7850 for steel
    Cₚ  = specific heat (J/kg·°C) e.g. 900 for Al, 385 for Cu, 500 for steel
    0.9 = fraction of deformation energy converted to heat

Extrusion Speed (extrudate):
  vₓ = vᵣ × R
  where vᵣ = ram velocity (mm/s)

The Johnson formula is an empirical approximation that accounts for internal deformation work and friction in a single combined expression. It tends to underpredict actual press requirements by 20–40% because it does not separately account for container-wall friction in direct extrusion, die-face friction, or inhomogeneous deformation. Industrial press tonnage selection typically applies a safety factor of 1.3–1.5 over the Johnson prediction.

Avitzur Model for Conical Dies and the Chevron Cracking Criterion

For conical dies with die half-angle α, the upper-bound Avitzur model provides a more physically grounded pressure estimate and, critically, a criterion for when central burst (chevron cracking) will occur:

Avitzur Upper-Bound Pressure (conical die):
  p/(2k) = f(α, m, R)
  where:
    k   = shear yield strength = σ₀/√3   (von Mises)
    m   = friction factor (0 = frictionless; 1 = sticking friction)
    α   = die half-angle (degrees or radians)

Simplified form (Avitzur, no friction):
  p = 2k × ln R × [1 + (2m/3√3) × cot α]

Chevron Cracking (Central Burst) Criterion:
  Central burst occurs when hydrostatic tension at centreline exceeds material fracture stress.
  Safe zone boundary (Avitzur): 
    R_crit = exp[α_crit / (1 − m × cosec α)]
  Practical rule:
    → Increase R (higher reduction) moves process into safe zone
    → Reduce α (shallower die angle) suppresses burst at same R
    → Increase friction (use lubricants with higher m) raises hydrostatic
      compression at centreline, suppressing void nucleation
    → Apply back-pressure to extrudate to maintain compressive stress state

Process Variants

Direct (Forward) Extrusion

In direct extrusion, the ram and die are on opposite ends of the container. As the ram advances, the billet is forced toward the stationary die and extruded through the orifice. The billet slides against the container wall throughout the stroke, generating friction that opposes billet movement. This friction has two significant effects: it adds to the ram force requirement (the “friction hill” in the pressure profile); and it creates a velocity gradient between the billet surface (which tends to stick to the container) and the billet interior (which moves toward the die). This velocity gradient is the origin of the dead metal zone (DMZ) at the die corners, and of the piping defect during the final stage of the stroke when the surface layer is drawn inward.

The characteristic pressure-stroke curve for direct extrusion shows a high breakthrough peak (overcoming initial billet-container static friction and filling the die), followed by a declining steady-state pressure as the billet length decreases and the total frictional force reduces. The discard (butt end) left in the container after extrusion is typically 15–20% of original billet length and is cut off before the piped zone enters the product.

Indirect (Backward) Extrusion

In indirect extrusion, the die is mounted on the front face of a hollow ram that moves toward the sealed end of the container. The billet is stationary relative to the container walls throughout the stroke, eliminating container-wall friction entirely. Consequences: required force is 25–30% lower at the same extrusion ratio and flow stress; the pressure-stroke curve is essentially flat throughout the stroke (no friction decay); piping is eliminated because the billet surface layer does not slide toward the die; and surface quality of the extrudate is superior because no oxidised billet surface is drawn into the product stream. The practical limitations are that the hollow ram limits the extrudate length to the container length, and the structural complexity of the hollow-ram tooling increases capital and maintenance cost.

Hydrostatic Extrusion

In hydrostatic extrusion, the billet is surrounded by a pressurised fluid (typically castor oil or a glycerine-water mixture) that transmits the ram force hydrostatically to every surface of the billet simultaneously. The result is that: container-wall friction is eliminated (no billet-container contact); the conical die surface is also lubricated; and the compressive hydrostatic stress state significantly reduces the tendency for cracking during deformation — enabling extrusion of materials that crack under conventional direct extrusion (brittle metals, intermetallics, powder-consolidated billets). Hydrostatic extrusion can achieve extrusion ratios of 100:1 to 2000:1 for ductile metals in research settings. Industrial limitations include the sealing challenges of the high-pressure fluid (up to 3 GPa for some materials) and the limited stroke achievable before seal failure.

Impact Extrusion

Impact extrusion is a high-speed cold-forming process in which a punch descends rapidly onto a slug placed in a die cavity. The material flows up around the punch (backward impact) or through a die orifice at the base (forward impact). The process is used for aluminium collapsible tubes (toothpaste, pharmaceutical packaging), aluminium and zinc closures, and copper electrical connector sleeves. Typical cycle time is under 1 second; dimensional tolerances of ±0.1 mm are achievable on wall thickness. The extreme deformation rate produces significant work hardening in the product.

Die Design and Geometry

Flat-Face (Square) Die

The flat-face die is the standard die for direct aluminium extrusion. It presents a flat surface perpendicular to the extrusion direction, with the profile orifice machined into the flat face. When the billet contacts the die face under pressure, material in the corners adjacent to the die entry does not flow — it sticks in place, forming the dead metal zone (DMZ). The DMZ acts geometrically as a self-forming conical die at approximately 40–45° to the extrusion axis, guiding the flowing metal toward the orifice. The surface of the DMZ is continuously scraped by the flowing material, regenerating a fresh contact surface throughout the stroke. This self-cleaning action is responsible for the excellent surface quality of aluminium extrusions despite the apparently harsh die geometry. For steels, the flat-face die is also standard because die wear at extrusion temperatures makes conical geometry impractical to maintain; the DMZ provides an effective conical guide without precision machining requirements on the worn die face.

Conical (Converging) Die

Conical dies with a smoothly converging entry profile (semi-angle α typically 30–60°) are used for copper, brass, lead, and some aluminium alloys where the converging geometry reduces pressure, improves surface finish by guiding metal smoothly to the orifice, and reduces the tendency for inhomogeneous deformation. The die land (the short cylindrical section following the conical entry) controls dimensional accuracy and surface finish of the extrudate. Die half-angles below about 15° tend toward central burst because the low angle redirects flow without sufficient compressive constraint at the centreline; die angles above about 60–70° approach the flat-face geometry and produce DMZ behaviour regardless of the nominal cone angle.

Porthole (Spider/Bridge) Die

The porthole die enables hollow aluminium profiles to be extruded without a separate mandrel and without requiring a tube as starting stock. The die assembly consists of three elements: the porthole plate (also called the spider or bridge) which divides the incoming metal flow through 3–8 radial ports around a central mandrel; the welding chamber, a recess in which the divided metal streams reunite and weld under the elevated temperature and pressure of the extrusion process; and the die bearing plate with the final hollow profile shape.

The quality of the solid-state pressure weld formed in the welding chamber is the critical property of porthole die extrusions. Weld quality depends on: temperature (must be above the minimum self-welding temperature, approximately 450–490°C for 6xxx aluminium); specific pressure in the welding chamber (minimum 80–120 MPa for most 6xxx alloys); surface cleanliness of the dividing streams (any oxide layer thicker than the self-diffusion distance inhibits bonding); and residence time in the welding chamber. The weld seams in porthole-die extrusions run longitudinally along the entire length of the profile and are not detectable by naked-eye inspection. They can be revealed by etchants (Barker’s reagent, anodising) and should be characterised by tensile and peel testing for structural applications.

Hot Extrusion: Temperature, Lubrication, and Materials

Hot extrusion is defined as extrusion performed above the recrystallisation temperature of the workpiece material. The elevated temperature reduces flow stress, enables higher extrusion ratios, and allows the material to recover dynamically during deformation. The critical constraints are: (1) the extrusion temperature must remain below the solidus (incipient melting temperature) of the alloy throughout the billet, including the adiabatic temperature rise from deformation; and (2) the tooling must survive the contact stresses and thermal cycling.

The Ugine-Séjournet Glass Lubrication Process (Steel Extrusion)

Extruding steel requires a lubricant that is simultaneously fluid enough to fill the tooling gap, viscous enough to form a continuous film, and chemically stable at 1100–1280°C. Conventional oil, graphite, and molybdenum disulphide all decompose or burn away at these temperatures. The Ugine-Séjournet process, developed in France in the 1940s, solved this by using glass as the lubricant. A glass pad placed against the die entry melts and extrudes as a thin glass film between the steel billet and the die, providing exceptional lubrication and simultaneously insulating the die from the hot billet. Glass composition is tuned to match its viscosity window (10²–10⁴ Pa·s) to the extrusion temperature of the specific steel grade being processed. Post-extrusion glass removal requires pickling or blasting.

Cold Extrusion

Cold extrusion is performed below the recrystallisation temperature — in practice, at or near room temperature for most metals. The workpiece material undergoes work hardening during deformation, raising its yield and ultimate tensile strength relative to the annealed starting material. This is simultaneously an advantage (improved mechanical properties in the finished part) and a challenge (increasing die pressure as the stroke progresses and the material work-hardens).

Lubrication in Cold Extrusion

At cold extrusion pressures of 600–2500 MPa, conventional mineral oil lubricants are squeezed out of the contact zone before they can function. Two lubrication systems dominate:

Phosphate-soap conversion coating (steels): The steel billet is first zinc phosphated (producing a porous Zn₂Fe(PO₄)₂ layer of 5–15 μm), then soap-lubricated with sodium stearate. The phosphate layer mechanically keys the soap into the surface, retaining it under extreme contact pressure. This system can sustain extrusion pressures up to 2500 MPa with tool life measured in millions of parts. The phosphate layer must be clean, uniform, and free of bare metal spots; any bare steel directly contacting the tool causes galling and die pickup.

Zinc stearate / polymer film (aluminium): Aluminium billets for cold impact extrusion are typically zinc stearate-coated or press-lubricated with a PTFE or polymer film. Phosphating is not used on aluminium because the amorphous phosphate layer is insufficiently adherent to the non-ferrous surface under cold extrusion conditions.

Cold Extrusion Tooling

Cold extrusion dies operate at very high contact stresses and must resist: compressive crushing failure; radial hoop stress (which can split the die along a diametral plane); wear from repeated billet contact; and thermal fatigue from the modest frictional heating generated even at room temperature. Die materials for cold extrusion of steel include: cemented carbide (WC-6–15% Co) for the die insert (highest wear resistance, requires prestress by a shrink-fit steel ring to prevent tensile hoop failure); D2 cold-work tool steel (air-hardened, 58–62 HRC, for lower-pressure applications); and M2 high-speed steel (for punches subject to impact loading). Punch pressures up to 2500 MPa can be achieved with WC-Co inserts reinforced by double or triple shrink-fit steel rings generating compressive prestress in the carbide.

Microstructural Effects of Extrusion

The severe plastic deformation of extrusion imposes true strains of typically ε = 2–5 on the billet material, producing strong crystallographic texture, grain elongation, and either dynamic recovery (in aluminium at hot extrusion temperatures) or work hardening (in cold extrusion). The specific microstructural outcome depends on the metal system, extrusion temperature relative to T_m, strain rate, and post-extrusion cooling rate.

Aluminium Alloy Extrusion Microstructure

In hot-extruded 6xxx-series aluminium alloys (6061, 6063, 6082), dynamic recovery during extrusion produces a sub-grain structure with well-defined low-angle boundaries. On exit from the die, if the extrudate is slowly air-cooled, static recrystallisation produces a coarse, heterogeneous grain structure with grain sizes of 100–500 μm — this is the recrystallised surface layer commonly seen in 6063-T5 profiles. If the extrudate is immediately quenched at the press exit (press quench — typically by a water spray or water bath within 1 metre of the die), recrystallisation is suppressed, the supersaturated solid solution (Mg₂Si in 6xxx) is retained, and the material can subsequently achieve full T6 temper properties after artificial ageing at 165–185°C for 6–12 hours.

In 7xxx-series alloys (7075, 7050), the press quench after extrusion is mandatory for achieving T6 temper properties: these alloys have very high quench sensitivity, and even 30 seconds of slow cooling can precipitate coarse MgZn₂ at grain boundaries, reducing both strength and stress-corrosion cracking resistance.

Copper and Brass Extrusion Microstructure

Copper and brass undergo dynamic recrystallisation during hot extrusion, producing an equiaxed grain structure at the die exit. The grain size in the extrudate depends on the post-extrusion cooling rate: water quenching retains a fine grain size from the dynamic recrystallisation event; slow air cooling allows grain growth. For electrical conductor copper (ETP), grain size affects electrical conductivity only marginally, but for structural brass tubes and rod, grain size significantly influences cold formability and fatigue resistance in downstream operations.

Extrusion Defects

Extrusion defects arise from incorrect process parameters, die design errors, billet metallurgical problems, or lubrication failure. Each defect has a specific mechanism and a targeted remedy.

Post-Extrusion Operations

Stretching and Straightening

Extruded aluminium profiles exit the press with a curvature and residual stress imparted by differential cooling across the profile cross-section. Before ageing, the extrusion is stretched 0.5–3% of its length on a stretch-straightening machine. Stretching simultaneously straightens the profile, relieves longitudinal residual stress, and increases the dislocation density slightly (improving subsequent precipitation hardening response). Over-stretching (>3%) is detrimental: it lowers elongation and can fracture thin-wall sections.

Heat Treatment After Extrusion

For precipitation-hardenable aluminium alloys (6xxx, 7xxx), the post-extrusion heat treatment sequence determines the final mechanical properties:

Die Correction and Extrusion Balancing

Complex asymmetric aluminium profiles extruded through flat-face dies rarely flow perfectly on the first die trial. Metal velocity through the die is governed by the local bearing length — the short cylindrical land zone at the die exit. A short bearing gives fast local metal flow; a long bearing gives slow local flow. Die correction involves selectively increasing bearing length in fast-flowing regions and reducing it in slow regions until the profile exits straight and without twist. For hollow profiles through porthole dies, the bridge geometry, porthole port area, and welding chamber depth all additionally affect metal distribution and must be optimised for each profile geometry.

Industrial Applications

Extrusion is the primary production method for aluminium structural shapes (I-beams, T-sections, box sections, curtain wall profiles), aluminium tubes and pipe, copper and brass rod and tube, steel seamless tube (by hot extrusion on mandrel or Ehrhardt push-bench process), titanium and superalloy turbine disc preforms, and a vast range of cold-extruded steel fastener preforms, gear blanks, and pressure-vessel liners.

In the automotive sector, aluminium crash management structures (bumper beams, crash boxes, door sill reinforcements) are hot-extruded 6xxx alloys in T5 or T6 temper, optimised for controlled progressive buckling under axial crash loads. The extrusion process is uniquely suited to this application because it enables multi-cell hollow cross-sections — through porthole die design — that provide both high second moment of area for bending stiffness and controlled fold initiation features that manage crush energy absorption.

In aerospace, 7050 and 7075 aluminium extrusions in T73 temper provide the spars, stringers, and frame sections of commercial aircraft fuselages and wing boxes. The combination of high specific strength (UTS/density), excellent fatigue crack growth resistance, and the ability to produce near-net cross-sections with minimal machining stock makes extrusion the preferred primary structure manufacturing route over rolling and forging for elongated prismatic sections.

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