Aluminium Cold Forging

What is aluminium cold forging?

Cold forging is a family of bulk-forming operations in which a pre-cut aluminium slug is pressed into shape by punches and dies without heating it to forging temperature. The metal flows plastically into the die cavity, so the part takes its outline, bores, steps, fins or splines in one or a few strokes.

The term covers several closely related operations. Upsetting shortens and widens a slug to form heads and flanges. Forward extrusion pushes metal through a die opening to make shafts and stems. Backward (reverse) extrusion drives a punch into the slug so metal flows back around it, forming cups, sleeves and housings. Combined extrusion does both at once, and coining or sizing finishes faces to tight dimensions. Impact extrusion, used for aerosol cans and battery cases, is a high-speed form of backward extrusion and belongs to the same family.

Because the work is done cold, the part comes out with no scale, a fine surface and dimensions set by the tooling. The metal grain follows the contour of the part instead of being cut through as in machining, and the strain hardens the alloy. Aluminium is particularly well suited: its low flow stress and high ductility in the annealed condition mean it can accept large reductions in a single stroke, which harder metals such as steel can only achieve with intermediate anneals.

The cold forging process step by step

1. Select the stockWire rod, drawn bar, extruded bar, rolled plate or cast-and-rolled stock in the chosen alloy. Clean, consistent material is essential because defects are not forged out at room temperature.
2. Make the slugSaw-cut, sheared or punched slugs of controlled weight. Volume accuracy matters: too little metal leaves unfilled corners, too much overloads the die.
3. AnnealSlugs are fully annealed (O temper) to minimise flow stress and maximise ductility. Heat-treatable 6xxx and 7xxx alloys are annealed with slow cooling so they stay soft.
4. Surface treatment and lubricationSoft alloys (1xxx, 3xxx, 6xxx) are usually tumbled in zinc stearate or coated with a modern zinc-free lubricant. Harder alloys (2xxx, 5xxx, 7xxx) need a conversion coating such as a phosphate, calcium aluminate or aluminium fluoride layer to carry the lubricant.
5. FormOne or more strokes on a mechanical, servo or hydraulic press, or on a multi-station cold former for small parts: upsetting, extrusion, piercing and sizing.
6. Trim and finishRemoval of the bottom web or flash, deburring and, if needed, straightening.
7. Heat treatHeat-treatable alloys are solution treated, quenched and aged to T6 after forming. Non-heat-treatable parts keep their cold-worked strength or are stress relieved.
8. Machine and finishCNC machining of fits, threads and sealing faces, followed by anodising, blasting, coating or cleaning.

A key design decision is how many forming stages a part needs. Simple cups and heads may take one stroke; parts with deep bores and a long shaft may need two or three, sometimes with an intermediate anneal when the cumulative strain becomes too high for the alloy.

Which aluminium alloys can be cold forged?

Cold forgeability falls as alloy content and annealed strength rise. The lower the annealed strength and the higher the elongation, the larger the reduction the slug will take before cracking and the lower the press load. Typical annealed values from Aluminum Association data show the spread:

Annealed strengths are typical published values for orientation. Actual forgeability depends on slug quality, grain size and the strain path, so trials are normal for new geometries.

In practice, thermal-management parts are forged from 1050, 1070, 1100 or 6063 because they combine easy flow with high thermal conductivity. Mechanical parts are mostly 6061 or 6082, forged soft and then heat treated to T6 for strength. 5xxx alloys are chosen for marine or chemical exposure where welding is not needed, and 2xxx/7xxx alloys only where the strength is essential and the volumes justify more complex tooling.

Typical aluminium cold-forged parts

A typical cold-forged pin-fin heat sink in 1070 has a 60 – 100 mm base and dense round or elliptical pins; the forged base is then machined flat on the contact face to keep thermal resistance low. A typical steering or pump shaft in 6061 is forged soft, solution treated and aged, and then turned and ground only on bearing journals and seals.

Cold forging compared with hot forging, machining and die casting

In short: choose hot forging for large, thick, high-load parts and complex 3D shapes; machining from bar for prototypes, small batches and designs still changing; die casting for complex thin-walled housings where strength and pressure tightness are less critical; and cold forging for compact, repeatable parts where accuracy, strength and material cost all matter. Many programmes start with machined prototypes and move to cold forging once the design is frozen and volumes are clear.

Is your part a good candidate for cold forging?

Three quick checks give a reliable first answer. If a part meets at least two of them, it is worth a feasibility study.

• Size: it fits within roughly 10 – 150 mm in diameter or outline and weighs from a few grams to a few kilograms. Very large housings, long beams and heavy flanges belong to hot forging or extrusion.
• Volume: there is a stable, recurring demand, usually several thousand pieces a year or more, so the die cost can be spread.
• Performance: strength, fatigue life, density (no porosity), dimensional consistency or material cost matter, not just making the shape.
• Geometry: the shape is mostly axisymmetric or has features aligned with the press stroke: steps, bores, fins, splines and flanges rather than undercuts and side holes.
• Material: the required properties can be met by a 1xxx, 3xxx or 6xxx alloy, or the strength justifies a more difficult 2xxx/5xxx/7xxx alloy.

Design guidelines for cold-forged aluminium parts

Volume constancy governs the whole design: the slug must contain exactly the metal the part needs plus any trimmed web. Designers who work with the forger at the drawing stage typically gain the most, because small changes in radii, wall thickness or bore depth can remove a forming stage or an anneal.

Tolerances, inspection and documentation

Cold-forged dimensions are controlled by the tooling, so diameters and heights produced in the die are typically held within a few tenths of a millimetre, and to tighter limits after a sizing stroke. Fits such as H7 – H9 bores, bearing journals and sealing faces are machined. Typical quality checks on a cold-forging programme are:

Common defects and their usual causes: folds or laps (poor flow path or sharp radii), underfill (slug too light or lubrication failure), surface galling (lubricant breakdown, especially in 5xxx and 7xxx alloys), and internal chevron cracks in forward-extruded shafts (excessive reduction in one stroke). Each is prevented at the design and process-planning stage rather than by inspection.

What drives the cost of a cold-forged part?

The cross-over point against machining depends on how much metal a machined part would waste and how many parts are needed. For a heat sink or sleeve where machining would remove most of the bar, cold forging can become cheaper at modest volumes; for a part that is close to bar shape anyway, machining may stay competitive for longer.

What to prepare for a cold forging enquiry

1. Drawings2D drawing with tolerances and GD&T, plus a 3D model if available.
2. MaterialAlloy and temper, or the target strength and hardness, and any conductivity or corrosion requirement.
3. FunctionWhether the part is pressure-bearing, fatigue-critical or a thermal-management part, and its operating environment.
4. VolumesEstimated annual usage, batch size and the timeline for samples, pilot run and series production.
5. ScopeSlugs or stock only, forged blanks, or blanks with heat treatment, machining and finishing, plus the documents required.

Metline supplies the input materials for cold forging, including slugs, wire rod, drawn and extruded bar in 1xxx, 3xxx, 5xxx, 6xxx and 7xxx alloys, and can supply forged blanks to drawing on request, with mill certificates and export packing for delivery worldwide.

Common mistakes when moving a part to cold forging

• Copying the machined design unchanged. Sharp corners, side holes and undercuts that cost nothing on a lathe make a forging die complex or impossible. A short design-for-forging review usually removes them.
• Specifying a temper the process cannot deliver directly. A drawing that calls for 6061-T6 is met by forging soft and heat treating afterwards; asking for T6 slugs to be forged leads to cracking and die wear.
• Choosing a stronger alloy than needed. Moving from 6061 to 7075 for a small strength gain can double the number of forming stages and require conversion coating. Check whether a 6xxx part in T6 already meets the load case.
• Ignoring slug quality. Seams, inclusions or coarse grain in the stock reappear as surface laps or orange peel on the part. Order slugs or bar with defined chemistry, grain size and surface condition.
• Underestimating volumes or ramp-up. Die life and tooling investment are planned around expected quantities; unrealistic forecasts make the piece price look better on paper than in practice.
• Leaving finishing to the end. Anodising response differs between alloys and between forged and machined surfaces. Agree the finish, colour and any masking before tooling is cut.

Related forging and slug materials

Aluminium (aluminum) cold forging: frequently asked questions