Aluminium Section

This stunning metal object is a cross-section of a cylindrical billet of aluminium, caught and collected in the middle of the process of extrusion. Extrusion is the familiar everyday process of pushing material through a hole – whenever we squeeze toothpaste from a tube we are doing extrusion. In the industrial version, plastics, foodstuffs or metals like aluminium can be made into very long objects with a fixed profile such as square rods, round bars and the I-beams that hold up many of our buildings. This is done by pushing material through a die of the chosen cross-sectional shape. The process for extruding metals was first patented by Joseph Bramah, an inventor and locksmith from Yorkshire, in 1797. It was later paired with a hydraulic press, also of Bramah’s invention. In this object, the narrow section at the top is the material which has already been extruded through holes in the die, to form a rod. The wide, central part is the billet of aluminium which is being pushed and forced through the die. The stubby section at the base is likely there to aid the grip of the ram which pushes against the billet. This part is the so-called ‘butt end’ of the billet – the very final material to pass through the die – and this end is generally not used due to the enormous force required when the metal must awkwardly flow almost perpendicular to the die in order to pass through. If you look closely at the flat surface of this cross-section, you will see grains of miniscule aluminium crystals, which beautifully shows how the solid metal flows through the extruder. Although extrusion is done in the solid state (the metal is not molten), aluminium is heated to around 300-600°C to keep it soft and avoid work hardening the metal. The outside surfaces of this object are streaked with black marks; this is likely from the graphite which is used as a solid lubricant in processes such as this.