Aluminium Extruded 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: we are doing extrusion every time we squeeze toothpaste from a tube. In the industrial version, plastics, foodstuffs or metals like aluminium can be made into very long objects with a fixed profile including square rods, round bars and the I-beams that hold up many of our buildings. This is done by pushing material through a die, the hard metal tool which gives the extruded material the desired profile (just like bronze dies turn pasta dough into fusilli, linguine or penne shapes). 

It might seem a little mind-bending that solid metal can be squeezed into shape just like toothpaste or pasta. Yet, blacksmiths can hammer metals into shape: if metals are malleable, it means that somehow, they can flow. In fact, metals behave more like super-strong plastics than brittle rocks. At the microscopic level, they look like a collection of tiny crystals, called grains. Whether you are casually bending a paperclip, or an industrial hydraulic press is forcing a massive aluminium billet through a die, these tiny grains are stretching, bending, and sliding past one another. This internal shifting results in a permanent change of shape known as plastic flow. Since plastic deformation is easier at high temperatures, metals are typically heated as they are extruded: not so much that they melt into a liquid, but rather to give them the consistency of thick clay while remaining completely solid.

This particular sample came to us from the lab of Dr. Mike Clode at King’s College London, where his team studied how the conditions under which aluminium samples are extruded affect their microscopic structure and mechanical properties. 

In this sample, 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 the billet through. This part is called the ‘butt end’ of the billet – the very final bit of material to pass through the die – and is generally not used due to the enormous force required when the metal must awkwardly flow almost perpendicular to the die to pass through. 

If you look closely at the flat surface of this cross-section, you will see the aluminium crystal grains, which beautifully show how the solid metal flows through the extruder. 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 like this.