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Aluminum in Cryogenic Applications

Aluminum in Cryogenic Applications
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Many people wonder what happens to aluminum at extremely cold temperatures.  The branch of physics known as cryogenics looks at physical properties of materials at low temperature.  This is an important area for many industries, where it is essential that materials and applications perform well beyond adverse environmental conditions.

It should come as no surprise that aluminum and aluminum alloys will behave differently when the temperature drops below a certain level.  Moreover, different alloys will each respond distinctly as well, meaning that if you have a low temperature application, it’s very important to research and find the alloy that best matches your requirements.

With that in mind, today’s post will look at how different aluminum alloys respond to extreme cold and which industries are most likely to have such applications.

What is cryogenics?

In pure scientific terms, cryogenics is the production and behavior of materials at extremely low temperatures.  However, there is no definitely agreed upon dividing line between refrigeration and cryogenics.  One way of measuring the difference is in terms of gases.  Scientists generally designate a gas as cryogenic if it can be liquefied at −150°C or below.

Another determination is based on the U.S. National Institute of Standards and Technology, which considers the threshold of cryogenics to be −180°C, since all of the permanent gases (these include hydrogen, helium, nitrogen, and oxygen) have a boiling point that is colder than this.  On the other hand, common refrigerants (such as Freon and hydrocarbons) have boiling points warmer than this.

Many branches of industry and manufacturing require an understanding of how materials perform at cryogenic temperatures.  Aluminum is one of those materials.  Aluminum can be worked at freezing temperatures to alter or augment its mechanical properties for specific applications through a process known as cryogenic forming (https://www.clintonaluminum.com/cryogenic-forming-of-aluminum/).

How is aluminum affected by cryogenic temperatures?

An important factor regarding temperature fluctuations is that aluminum tends to have less variance from hot to cold compared to other metals, such as steel.  This means that aluminum offers better stability when it comes to extreme temperature environments.  Aluminum is still impacted by extremely cold temperatures and it’s important to understand how if your application will perform when exposed to a cryogenic environment.

Aluminum alloys offer manufacturers many advantages when it comes to cryogenic temperatures.  It is not unusual for aluminum to be used in applications that involve temperatures as low as -270°C.  When talking about subzero temperatures, a typical aluminum alloy will show very little change in its physical properties.  The yield and tensile strengths will increase slightly, while elongation will go down.  Impact strength will not be changed much at all.

Another consideration is how well a material retains its toughness at low temperatures. Aluminum does not exhibit any ductile-to-brittle transition.  This means that you will not need to conduct any Charpy or Izod tests for aluminum alloys.  If you are going to be welding aluminum alloys for sub-freezing temperatures, you will likely need to conduct notch-tensile and tear tests.

The main consideration when deciding whether aluminum is right for your cryogenic application is the lowering of the alloy’s elongation.  This is more pronounced in aluminum alloys than in certain other metals, such as austenitic stainless steel.  Two alloys that have managed to avoid this weakness are 5083 and 5456.

Which aluminum alloys are most used in cryogenic applications?

For the above-mentioned reason, alloy 5083 is the most common choice for extreme cold scenarios.  When this alloy is cooled to as low as -195°C, it will see an increase in ultimate tensile strength of 40% and in yield strength of 10%.  Alloy 5083 also exhibits excellent fracture toughness at such temperatures.

Another alloy that might see use at cold temperatures is alloy 6061.  It displays good fracture toughness in extreme cold, although its yield strength is not as good as some other options, like 5083.  Another option is alloy 7039, which is a weldable alloy and performs well in terms of both strength and fracture toughness.  Both 2024 and 2124 have similar tensile properties at subzero temperatures, but 2124 has a higher purity base, which leads to improved fracture toughness.

Other possibilities for cryogenic applications include 2214, 2419, 7050 and 7475, although they have not been studied as extensively.  They were developed so that they would have improved fracture toughness at room temperature, so it is likely they will also perform well at low temperatures.  If you are thinking of using one of these alloys, seek expert advice or do more research.

Aluminum for superconductors

Superconductivity is defined as the property of zero electrical resistance of certain substances at extremely cold temperatures.  In other words, a superconductor is a material that can conduct electricity and allow an electric current to flow without any resistance at cold temperatures.  Such materials might also repel magnetic fields as they are cooled (this is known as the Meissner Effect).  Superconductivity is a result of quantum mechanics and so takes place at atomic and subatomic levels.  It has become an important component of computing technology and there is a great demand for materials that can be used as superconductors.

Aluminum is one of the elements capable of superconductivity.  It has a superconducting critical temperature of 1.2 kelvin (colder than outer space!) and a critical magnetic field of about 100 gauss.

Superconductivity has already made possible several applications, and the prospective areas of research continue to increase.  Common examples that many people are aware of include MRI machines, NMR machines, high-speed maglev trains, super-fast digital circuits, low-loss power cables, RF and microwave filters found in mobile phone base stations, railgun and coilgun magnets, and electrical motors and generators.

As quantum computing and other applications that involve cryogenics and superconductivity continue to advance, aluminum alloys that are suitable for such use scenarios become increasingly important.  Preferred Alloys can help you with your aluminum supply needs for cryogenic applications.  Contact one of our friendly and knowledgeable customer service professionals today to learn more.

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