A skilled craftsman makes a difficult job look easy. Take barrel-makers (coopers) as an example, or consider glass-blowers. And then there are aluminum extruders, (both the people and the businesses who do it.) We’d put them in that category too. After all, extrusion looks simple enough: heat the metal billet and force it through a die. How hard can it be?
Very hard. The challenge is to get exactly the desired size, material properties and surface finish in the extruded metal. Harder still is achieving the same results time after time, at a speed that makes the operation profitable. While someone once said that extrusion isn’t rocket science, the reality is that despite an absence of rockets, there’s plenty of science.
Aluminum extrusion plays a big part in many metal fabrication projects and we’re in awe of the producers who can give us consistent properties and appearance in every batch. That prompted us to research how they do it. It turns out there’s one factor in particular that affects extruded section properties, quality and appearance.
Using extruded section saves time and money. It can also result in a stronger or more rigid fabrication, depending on the properties of the metal and the details of the extrusion process used. A wide range of metals, such as magnesium, copper, tin, lead, steel and even titanium, are extrudable but aluminum is the one that’s extruded in the highest volume.
Amazingly, the world consumes something like 30 million tons of extruded aluminum each year, and that number keeps growing. Where does it go and who uses it? We looked at that few weeks back in “Consumption of Aluminum Extrusion Just Keeps On Rising”.
To be suitable for extrusion a metal needs a degree of ductility, and you’ll notice that about those in the list above. With enough force, each can be bent or formed into a different shape. Without going deep into materials science, ductility requires that the metal crystals or grains can slide against one another, rather than being locked rigidly into place. (A material where the structure is locked tends to be brittle. Think of glass as an example.)
Hot Versus Cold Extrusion
Most metals are heated before being extruded. This improves grain mobility, which increases ductility. (Heat it too much though and it becomes liquid – perhaps the ultimate in ductility!) Greater ductility reduces the force needed to push metal through the die at a given rate.
Heating also affects a metallurgical behavior called “recrystallization”. This is when heat and pressure initiate the formation of new grains that swallow up the old grains as the material moves through the extrusion die. Recrystallization improves tensile and yield strength and elongation, all of which contribute to the usefulness of extruded aluminum.
The alternative to hot extrusion is cold extrusion. In this process the metal billet isn’t heated before loading into the extrusion press. As there’s less grain mobility it takes a lot more force to push it through the die, which begs the question, why extrude cold?
Most metals are extruded hot because it’s faster, uses less energy for a given section, and can form larger sections. However, there are some disadvantages.
As the metal is more fluid it tends not to fill corners and thin sections in the die, so accuracy is reduced. Hot metal, particularly aluminum, also suffers more surface oxidation and scaling. It may tear on the surface and it can take on a wave or twist as it passes through air quench on its way onto the runout table. Stretching straightens it out, but at the expense of some work hardening that alters the properties.
In contrast, cold extrusion provides better dimensional accuracy and surface finish. Cold working hardens the metal while distortion improves strength through grain elongation. However, the amount of deformation possible is far less than for hot extrusion.
Cold extrusion is generally used to form small metal parts like gear blanks and cup-shaped parts like fire extinguisher cases. It’s not typically used to make long lengths of extruded section. That’s the domain of hot extrusion.
Temperature and extrusion speed have a large impact on grain size and orientation as well as surface finish. The key to producing extruded section with consistent, predictable properties is to maintain a stable process with extrusion speed and temperature closely matched.
Most hot extrusion starts by preheating both the metal billet and the dies. The objective here is to get the metal to its recrystallization temperature so it can extrude quickly and with the least possible force. Every metal has a very specific temperature band. As an example, 6063 series aluminum should be within a range of 914 – 932°F.
An interesting “wrinkle” in this is that the work of extrusion heats up the metal. Thus the end of the billet leaves the extrusion die hotter than the beginning. As this changes extrusion speed and material properties, extruders try to prevent it from happening or minimize its impact. Their techniques include:
Tapered heating of the billet – have the back end cooler than the front
Control of press and die temperatures
Reducing extrusion speed, so less work means less temperature rise
Speed is controlled by measuring the temperature of the metal leaving the die, what’s called the exit temperature. For aluminum this is typically around 930 – 950°F, although every grade has specific requirements. Temperature readings are fed back to the controller which adjusts ram pressure to keep speed constant.
Consistency is Essential
Extruded section, and particularly aluminum extrusion is extremely useful in fabrication, but only if its properties are known and consistent. Achieving that takes precise control over the extrusion process, and in particular, the most important factor, temperature.
Measuring the temperature of a length of moving metal is quite a difficult thing to do. Thermocouples can give an indication by measuring the temperature of surfaces touched by the metal, but this is an indirect method.
In recent years extruders have adopted infrared pyrometery for temperature measurement. This is a non-contact way of measuring the heat being given off by the metal. It’s not easy because aluminum has low emissivity – it doesn’t radiate much heat – but technology advances have now made this practical. Pyrometry coupled with sophisticated methods of speed control, have brought new levels of consistency to extruded aluminum.
Aluminum for your Next Fabrication Project?
Consistency is one of the joys of working with aluminum extrusion: you know exactly what you’re going to get and how it will behave. As we’ve highlighted in previous Tiki Talk posts, there’s practically no limit to the uses to which it can be put, from frames and guards to structures of every kind imaginable.
If you have a project you think would benefit from using aluminum extrusion, talk to us. And if you’re unsure whether aluminum extrusion would be a good choice, yes, talk to us. We’d love to help turn your ideas into reality.