The most ductile metal on our planet is gold. It is said that a single gram of gold can be stretched, or drawn, into a wire that is more than two kilometers long – longer than a mile. Without breaking. This is ductility.
Ductility is important when it comes to formability, because materials that are considered brittle – the opposite of ductile – cannot be formed successfully, such as for the bumper beams on your car. Metals that can be formed or stretched or pressed into another shape without fracturing, such as your bumper beams, are ductile.
Tensile testing and measuring ductility
Most metals are ductile, particularly when heated to elevated temperatures. Aluminium is one. This has been proven through testing and measurement.
Tensile testing is what we generally use to establish levels of ductility. A tensile test for aluminium involves pulling a metal sample, normally shaped like a dog bone, in a Universal Testing Machine until it fractures. This measures operational load limits and elongation to ascertain, amongst other things, ductility. A stress-strain curve generated from the test is used to determine the point where the reversible elastic strain is exceeded and plastic deformation occurs.
Ductility can be measured in two different ways. One is by percentage elongation, the other percentage reduction.
- Percentage elongation measures the length that a metal deforms as a percentage of its original length, after it is pulled to failure during a tensile test
- Percentage reduction measures the narrowest part of the cross-section of a metal specimen after a tensile test-induced fracture
The percentage elongation of aluminium is 50-to-70 percent. This is quite high, relative to other metals. It can be easily drawn, extruded, machined, stamped and cast, also due in part to its malleability. Aluminium is ductile and malleable.
By the way, metals that only can receive a small amount of deformation before fracture are classified as brittle.
Aluminium’s high ductility
I already mentioned that aluminium has high ductility.
You can find a great example of this ductility in your kitchen, when you wrap sandwiches for lunch or leftovers from dinner in aluminium foil. You are taking advantage of the ductility of aluminium because the foil is shaped around the food and remains in that form without breaking. This would not work with a thin sheet of glass.
Pure aluminium boasts the highest level of ductility in the aluminium family. Each of the more than 540 registered alloy compositions are ductile, although the percentages differ because adding other elements to the pure aluminium has an impact on ductility.
The 6061 aluminium alloy, thermally treated, has 17 percent ductility, compared with an annealed 3003 alloy, which offers 40 percent. It is important to note that the same alloy will have a considerably different ductility percentage depending on its heat treatment. This is because heat treatments usually increase the hardness of the metal, thereby decreasing its ductility.
Go visit Hydro's design manual to find out which alloy is best suited for your project.
Strong and tough in the cold
One of my former colleagues, Stanislaw Zajac, has written that aluminium does not become brittle at low temperature, and that the metal instead gets stronger and that its ductility and toughness increases. This is true. The fracture toughness for aluminium is essentially independent of temperature.
Steel, in comparison, does not get stronger when temperatures fall, not even when a stronger steel is being used. And the consequences could be fatal.
ScienceDirect, which is an online platform for scientific literature, writes about the Liberty cargo ships that Allied forces built in vast numbers during World War II. The website points out that early versions of the ships suffered hull and deck cracks, and that 19 of them actually broke in half. According to researchers, the problem was that the steel used for the hulls of the ships was subject to fracture when exposed to temperatures below a certain level. The material went from ductile to brittle, and that is when the hulls began to crack.
The ductility of aluminium is relatively constant within the temperature range -60°C to 20°C.
Here is another illustration: India’s Bharati Research Station in Antarctica was built with Wicona-brand aluminium windows and facades so that it could withstand some of the most extreme climatic conditions on earth. It stands up to hurricane-like winds, abnormally high thermal and mechanical loads caused by blizzards, huge quantities of snow and temperatures of -60°C and below. And it has been standing up to these extreme conditions since 2012.
Ductility an important design consideration
This ability to remain strong and flex without breaking is essential for applications like bicycle frames, ladders, the structure of airplane wings, and plenty more.
When thinking about the best metal for a project, one would find it natural to choose strength as the most important physical property. But it may not be, because metals and their properties are much more nuanced than that. Ductility is one of those properties that also deserves a close look.
Ductile materials are able to sustain more stress due to their ability to absorb more energy prior to failure than brittle materials are.
Long list of products benefiting from aluminium’s ductility
The extrusion process works well with highly ductile metals because it can form virtually an infinite variety and complexity of shapes. It is also a cost-effective process, for instance, with aluminium.
I’ve put together a short list of the products and market segments that are benefiting from the ductility of aluminium.
In the automotive and transportation segments, ductility is a key physical property for body panels and closures, such as the hood of your car, and for wheels and suspension parts. Crash management components – I spend a lot of time improving the alloys used in this area – include bumper beams and crash boxes. This is also because aluminium can absorb the energy in a crash by bending and deforming, rather than fracturing.
Otherwise, ductile aluminium is critical for structural parts for airplanes and in the building and construction industry, in facades and in door and window frames.
Again, aluminium is a ductile material and this is incredibly useful. It may not be as ductile as gold, but it is a lot more abundant – and less expensive, too.
