Drawing is the process that converts cast or extruded aluminium stock into wire, bar, tube or a shaped shell by pulling the material through a die, or by pushing a punch through a clamped blank. Both variants rely on the same principle: the metal is forced to flow plastically at room temperature, so the tool geometry, the reduction per pass and the lubricant film determine whether the part comes out sound or tears.
Cold Drawing of Rod, Wire and Tube
Cold drawing is used to reach dimensional accuracy and surface finish that extrusion cannot deliver. Rod and wire are drawn to close diameter tolerance and improved straightness, while seamless tube is drawn over a mandrel, which controls both the outside diameter and the wall thickness in a single pass. Drawn seamless aluminium tube is specified under ASTM B210, and rolled or cold-finished bar, rod and wire under ASTM B211. The typical total reduction from the extruded or hot-rolled stock to the finished size is achieved in several passes rather than one, because the tensile load on the exiting product must stay well below its yield strength or the material will neck and break at the die exit.
Die Geometry and Reduction per Pass
Die design controls the balance between drawing force and surface quality. A die approach angle that is too steep raises the drawing stress and increases the risk of centre cracking; one that is too shallow increases friction and die wear. For aluminium tube, an approach half-angle in the range of 12 to 24 degrees with a bearing land of roughly 30 to 50 % of the finished diameter is a common starting point. Reduction per pass is normally held between about 15 and 25 % of area for tube and wire drawing, with higher reductions acceptable on softer tempers. Mandrel-drawn tube can take larger reductions but requires careful control of the mandrel-to-tube clearance to avoid wall thinning and internal scoring.
| Parameter | Typical starting range | Effect if exceeded |
|---|---|---|
| Reduction per pass | 15 - 25 % of area | Necking or break at die exit |
| Die approach half-angle | 12 - 24 degrees | Centre cracking or high friction |
| Bearing land length | 30 - 50 % of diameter | Die pick-up, scoring, excessive pull force |
| Drawing speed | Set by lubricant film stability | Film breakdown and surface tearing |
Lubrication and Surface Condition
Aluminium has a strong tendency to adhere to steel tooling, so the lubricant must form a stable film that separates the two surfaces at high pressure. Straight mineral oils with fatty acid additives, emulsifiable oils and synthetic lubricants are all used, and their viscosity is matched to the drawing speed and to the exit temperature. Surface preparation before drawing matters as much as the lubricant: the stock is normally descaled chemically or mechanically, and a conversion coating may be applied to carry lubricant into the die. After drawing, the lubricant must be removed completely, because residual oil on tube and wire becomes a source of porosity during subsequent welding and of lacquer adhesion failure during coating.
Intermediate Annealing and Temper Control
Every cold drawing pass increases dislocation density and therefore hardness. Once the material has been reduced by roughly 50 to 70 % in total, it must be process annealed before further drawing, otherwise it will crack. Process annealing of aluminium is carried out at moderate temperatures, generally in the region of 340 to 380 C for the common non-heat-treatable alloys, and the annealed O temper is the softest fully recrystallised condition. The final pass determines the delivered temper, from O through the H-series work-hardened designations. Mechanical properties of cold-drawn tube and bar are specified in the relevant parts of the product standards, and EN 754-2 covers the mechanical properties of cold-drawn aluminium rod, bar and tube in the European system.
Deep Drawing of Sheet into Containers and Shells
Deep drawing is a separate discipline in which a punch pushes a blank through a die to form a cup. The alloys used differ from those used for wire and tube: 1100, 1200, 3003 and 8011 are the workhorses for foil containers and cookware because they combine low strength with high elongation. Tooling design focuses on the draw ratio, punch and die radius, blank holder force and lubrication, all of which control the balance between thinning and wrinkling. Earing, the formation of uneven walls caused by crystallographic texture, is controlled by the rolling schedule and by the final anneal. Tensile testing of the finished part follows ASTM B557 or ASTM E8/E8M, and formability is often monitored through r-value and Erichsen-type cup tests during development.
Frequently Asked Questions
Q: How much reduction can be taken in a single drawing pass?
A: For tube and wire drawing a reduction of roughly 15 to 25 % of cross-sectional area per pass is normal. The practical limit is the point at which the drawing stress approaches the yield strength of the exiting product.
Q: Why does drawn aluminium need intermediate annealing?
A: Cold work accumulates dislocations and raises hardness until ductility is exhausted. A process anneal at roughly 340 to 380 C restores the ability to take further reduction without cracking.
Q: What standard covers drawn seamless aluminium tube?
A: ASTM B210 covers drawn seamless aluminium-alloy tube, and ASTM B211 covers rolled or cold-finished aluminium bar, rod and wire. EN 754-2 gives the mechanical property requirements for cold-drawn rod, bar and tube in Europe.
Q: Which grades are used for deep-drawn foil containers?
A: Alloys 1100, 1200, 3003 and 8011 dominate because they combine low flow stress with high elongation. Heat-treatable grades are generally avoided for severe draws.
Q: What causes scoring on the drawn surface?
A: Scoring is usually caused by lubricant film breakdown, die pick-up of aluminium, or an over-long bearing land. Increasing lubricant viscosity or reducing the land length normally resolves it.
Q: How is the finished temper of drawn wire or tube controlled?
A: The total amount of cold reduction after the last anneal sets the temper. The final pass is sized to land in the required H-temper band, and hardness or tensile tests confirm the result.