Aluminium stampings: alloy choice and galling

Why aluminium behaves differently

Aluminium has a lower melting point, a lower modulus and a different surface than steel. It also forms a thin, hard oxide almost immediately, which is good for corrosion and awkward for forming. Under pressure, the oxide can break and the fresh metal can weld to the die surface. That is the start of galling.

The lower modulus means aluminium springs back differently. A bend that is correct in steel may open more or less in aluminium, and the difference changes with temper. The lower melting point means the heat from friction and the heat from a high stroke rate can soften the surface and make pickup worse.

Aluminium is also lighter, which changes the way the strip feeds and the way the part behaves after it leaves the die. A light part can be blown by air, can stick to a punch and can be marked by a vacuum cup. The handling and the tooling have to account for that.

The alloy families used in stamping

The 1xxx series, such as 1050A or 1100, is commercially pure aluminium. It has excellent formability and conductivity, low strength and good corrosion resistance. It is used for heat sinks, shielding, decorative parts and electrical conductors where strength is not the main requirement.

The 3xxx series, with manganese as the main addition, gives moderate strength and good formability. 3003 is a common general-purpose stamping alloy. It is stronger than 1050A, welds well and is widely available. It is a sensible default when the part does not need high strength or a marine finish.

The 5xxx series, with magnesium, gives higher strength and good corrosion resistance. 5052 and 5754 are common. 5052 is stronger and is used for brackets, panels and parts that need fatigue resistance. 5754 is often chosen for marine and transport applications because of its corrosion behaviour. Both form well in the right temper, but they spring back more than 1050A.

The 6xxx series, with magnesium and silicon, can be heat treated. 6082 and 6061 are common in machined parts, and some 6xxx sheet is used in stamping. The higher strength comes with lower formability and more springback, so a 6xxx part needs generous radii and a careful bend sequence.

Temper and the forming limit

The temper tells you how much the alloy has been worked. O is annealed and softest, H14 is strain hardened to a quarter-hard condition, H32 is strain hardened and stabilised, and H111 is annealed and slightly hardened. T4 is solution heat treated and naturally aged, which applies to the heat-treatable alloys.

A soft temper forms deep shapes and tight radii, but it is weak and can mark easily. A harder temper gives strength and a better surface, but it cracks at a tight bend and springs back more. The drawing should state the temper and the minimum bend radius, and the supplier should confirm that the pair is achievable.

Bend direction matters. A bend along the rolling direction behaves differently from a bend across it, and the difference is larger in a hard temper. If the part has a critical bend, the drawing should show the grain direction or the supplier should be allowed to choose the layout that gives the best result.

How galling starts and spreads

Galling is a transfer of material from the workpiece to the tool. In aluminium stamping, a small particle of aluminium adheres to the punch or the die, then picks up more material on the next stroke. The particle becomes a lump, the lump scores the strip, and the score becomes a scratch or a tear. In a severe case, the part seizes in the die.

The conditions that cause it are high contact pressure, poor lubrication, a rough tool surface, a tight clearance and a high sliding speed. Aluminium is prone because its oxide breaks under pressure and its fresh surface is reactive. A die that runs cleanly in steel may gall in aluminium with the same clearance and lubricant.

The first sign is often a dull patch on the die or a scratch on the part. By the time the scratch is visible, the die already needs attention. The process should include a die inspection interval and a way to clean or polish the tool before the damage becomes a scrap event.

Die design for aluminium

The die surface should be smooth and free of tool marks. A polished or coated punch reduces friction and gives the aluminium less to grip. A radius on the die entry helps the material flow and reduces the pressure at the edge. A sharp die edge is a good place for a pickup to start.

Clearance should be set for aluminium, not copied from a steel tool. Too little clearance raises the pressure and the heat; too much leaves a heavy burr and a rough edge. The right value depends on the alloy, the temper and the thickness, and it should be confirmed at the trial.

The die should be designed for cleaning. If the part is prone to galling, the operator needs access to the punch and the die without a full strip-down. A die with removable inserts or a quick-release punch is easier to maintain and less likely to run in a damaged state.

Lubrication and tool coatings

Lubrication does three jobs: it separates the strip from the die, it carries heat away and it reduces friction. A lubricant that is too thin can let the surfaces touch; one that is too thick can trap air, mark the surface or leave a residue that interferes with the finish. The choice should match the alloy, the form and the downstream process.

Water-based lubricants are common for aluminium because they cool well, but they need rust control for the tool and a clean water supply. Neat oils give better film strength but can leave a residue that has to be removed before anodising or coating. A dry film or a solid lubricant can help in a difficult form where a liquid cannot reach.

Tool coatings can reduce pickup. A hard coating on the punch or the die lowers friction and provides a barrier between the aluminium and the tool steel. The coating has to be compatible with the die material and the sharpening process. A coated punch that is ground back without recoating loses the benefit.

Finishing and the surface you start with

Anodising is the most common finish for aluminium, and it is sensitive to the surface it receives. A scratch, a die mark or a galling patch will show through the anodic layer. The stamping process should produce a surface that the anodiser can work with, or the part should be specified as painted or bead blasted.

Anodising also changes the dimensions. The layer grows into and out of the surface, and a masked area or a thread needs protection. A part with a tight fit should account for the coating thickness. A hard anodised part can be more wear resistant but also more brittle at the edge.

Powder coating and painting cover the surface but add thickness and can bridge a small gap. A coated part needs a pretreatment that suits aluminium, and the coating should be tested for adhesion at a bend or a cut edge. A coating that chips at the edge exposes the aluminium and starts a corrosion site.

What to put on the drawing

State the alloy and the temper, not just aluminium. Give the thickness with a tolerance, the bend radii, the grain direction if it matters, the surface condition and the finish. Add the flatness and the edge condition if the part is visible or if it seals.

Ask the supplier to confirm the forming limit at the quoted temper and to propose a lubricant and a die coating if the form is difficult. Ask how the die is cleaned and how galling is detected. A supplier that has a galling procedure is more useful than one that promises no scratches.

For a part that will be anodised, ask for samples before anodising and after. The two surfaces can look very different, and a defect that is invisible on the raw part can be obvious after the finish. The approval should cover the finished appearance, not only the bare metal.