Over the last 100 years, aluminium alloys have taken the world by storm. In mineral form (usually bauxite or cryolite), aluminium is actually the Earth’s most abundant metal and the third most abundant material (after silicon and oxygen). So it’s probably no surprise that aluminium has found its way into almost every nook and cranny of our lives.
Today, however, we’re going to get under the wing of Big Al and his mates to find out how and why they’ve come to be the backbone of the industry that makes those big metal birds in the sky.
So buckle up, stow away your luggage, and let’s get ready for take-off.
The aeroplane
Before we look at aluminium and begin to understand why it’s used so much in aerospace, I think it’d be wise to familiarise ourselves with the aeroplane first. I don’t mean in the sense that it has two wings, windows and wheels, although that is important. But rather, what it’s exposed to and what’s demanded of it. This will help illustrate the proverbial holes aluminium has come to fill in our planes.
Over their life, aeroplanes are exposed to extreme environmental, mechanical and operational conditions. Some of those are on the ground. And others are in the sky. So, material selection is obviously important - as is design - and I could write articles exploring each type of condition because they can become very complex topics. For today, though, let’s group and focus on a select few.
High stress
I’m not talking about long-haul armrest hoggers or loud snorers. But high mechanical loads (over a surface area). Stress. Components like landing gear, engine mounts and wing spars experience some hefty forces during take-off, landing and flight. Their material needs to withstand these loads.
Corrosive environments
Fish sandwiches. Musky shoes. In-flight gravy. They might be corrosive for you and me but less so for the plane. Instead, that has to deal with humidity, UV radiation, temperature extremes, pollutants, salty air (airports by the sea!) and chemical treatments (like de-icing liquids). Plane components must either protect themselves or be protectable against varying corrosive environments.
Temperature extremes
The hottest airstrip on record was a sweltering 54°C. If you get closer to (or inside) one of the engines, you could be exposed to temperatures over 1200°C. And outside, at 42,000ft, you could be confronted with a spine-tingling -60°C chill. So, components need to keep their composure in the face of hot and cold. Routinely.
Fatigue
And as if high stress isn’t enough. Some of it is cyclic… those pesky, repeat fish-sandwich-eating, armrest stealers.
Fatigue can occur from cyclic loads during take-off, landing, turbulence, repeat pressurisation and depressurisation, aerodynamics, frequent vibrations and probably hundreds of other ways. When these are combined with corrosive environments and cyclic thermal changes, stress can induce fatigue cracks, which can then lead to components failing unpredictably below their ultimate tensile strength.
That’s like letting a paper clip bathe in salt water, bending it a few times and then sneezing so it snaps. So, yes, fatigue resistance is important.
Fortunately, aluminium ticks or, sorry, can tick these boxes. It’s an impressive metal with a lot of great properties, and I’d like to share with you what they are - and why.
Aluminium’s good side
Raw, unalloyed aluminium has lots of benefits. You probably know them already, so I’ll rattle them off quickly.
- It’s low density and lightweight (~a third of stainless steel)
- It’s corrosion resistant, thanks to the natural protective (and restorative) aluminium oxide surface layer
- And it’s also highly ductile, making it great for intricately-shaped components
These properties alone make it an unsurprisingly good starting point for aeroplane components. But starting point is the key word here. Because, unalloyed, aluminium is actually a terrible choice for most aeroplanes. It has one major ‘flaw’ that I’ll share next.
Aluminium’s dark side
Big Al might be lightweight, ductile, and resistant to corrosion, but he doesn’t like stressful environments (you and me both, Al… you and me both). Generally speaking, ductility and strength don’t go hand in hand. So, Al will bend and keep bending - without breaking - but it won’t take much to make him bend. Exaggerated example: Raw aluminium is like having landing gear made out of wallpaper.
Al also doesn’t like the heat. Because, unlike copper (1084°C) or cast iron (1204°C), he’ll start to loosen up and melt at 660°C.
So yes, in its raw form, aluminium might have some great benefits and favourable properties, but its lack of strength writes it out of aerospace applications. So big Al needs his mates - his alloys - to build on his foundations and make him suitable for flight.
Aluminium’s FCC foundations
You’re probably familiar with atoms and how their organisation differs in solids, liquids and gases. If you’re not, the image below should help. (Notice how they’re packed tighter in solids, a little less in liquids and even less in gases)
Basic particle packing structures
Well, the above is true. Just in reality, it’s a little more complex than that. You see, when a metal solidifies (crystallises) it produces a repeating structure of atoms. This structure varies from metal to metal depending on factors like atomic size and bonding - and as you’d expect, affects the metal’s properties.
There are three main types of crystal structures: body-centred cubic (BCC), face-centred cubic (FCC) and hexagonal-close packed (HCP). BCC and FCC are below.
Body Centred Cubic and Face Centred Cubic crystal structure
Aluminium has an FCC crystal structure. So, looking at the image above, you’ll see the cell has an atom in each corner and another atom at the centre of each face (versus the cell body in BCC). This is a highly symmetrical crystal structure - and is actually the most densely packed of the three. (Aluminium’s low atomic mass, radius and weaker bonding bring down its overall density)
Anyway, thanks to its symmetry, when this crystal structure is under load, the atoms can slide past one another in a lot of places - at least comparatively. This movement is known as slip, and the crystallographic plane they move along, the one with the highest density, is known as the slip plane.
(FYI: The more closely packed the atoms are on a plane, the less energy that is required for them to move past each other. So, slip occurs more easily on these planes because there is less resistance to the movement.)
Other crystal structures aren’t so symmetrical, nor as densely packed, so they have fewer slip planes. This makes them less ductile and more brittle. So, aluminium boasts its ductility because it has so many slip planes.
Within crystal structures, you also have imperfections. These are known as dislocations (and there are a few types, but that’s a topic for another day). Because aluminium’s FCC structure has so many slip planes, the dislocations can move somewhat freely when you bend them. Hence the ductility.
But when you bend a metal, you actually induce more dislocations. And they begin to accumulate, gradually restricting their movement. This, on the surface, sounds bad. And I guess in some contexts, it might be. But for weak ol’ Al, this is a good thing. Because by inducing dislocations, you increase the metal’s strength.
As an example, think about cutting open a drink’s can and repeatedly bending it. What happens? It gets stiffer and stiffer.
You can do this at a larger scale. It’s what’s known as work hardening. You can also apply heat and do it too - although you can’t with every aluminium alloy. More on this soon.
For now, let’s leave the land of material science and return to the land of alloys.
Land alloy! (Oh wait, that’s boats…)
Upgrading Al
Raw aluminium is lightweight, corrosion-resistant and ductile. But, unfortunately, weak. So, to make it stronger and capitalise on these great foundations, we introduce other metals into the mix.
Here’s what’s commonly added to aluminium to make popular aluminium alloys:
Copper to improve strength, particularly compressive strength and hardness. Too much copper reduces corrosion resistance and ductility.
Magnesium to improve strength and work hardening outcomes. It can also improve corrosion resistance and weldability but comes at the cost of machinability.
Nickel to improve hardness, high-temperature strength and corrosion resistance. But it does make the alloy more brittle and harder to machine and weld.
Zinc improves the alloy’s strength (quite drastically) and produces heat-treatable alloys. It does, however, reduce corrosion resistance.
Silicon makes the alloy more castable (ideal for complex shapes) by reducing its melting point. Generally, this makes the alloy more brittle.
Manganese improves corrosion resistance and alloy strength. But, adding too much manganese can actually reduce strength.
Iron improves strength and wear-resistance but risks introducing unfavourable inter metallic compounds that damage mechanical properties. It also reduces the alloy’s corrosion resistance.
Chromium improves corrosion resistance and helps nurture a favourable microstructure (it promotes grain refinement so the microstructure is more uniform, meaning mechanical properties get better). However, it can reduce ductility and also lead to those pesky inter metallic compounds, which can damage mechanical properties.
Titanium acts similarly to chromium and controls grain growth, so the microstructure is more uniform and mechanical properties improve. It’s also great at improving fatigue resistance. But it’s more expensive than other alloying elements, and adding too much can lead to more brittleness.
So, as you can see, there are lots of potential combinations. You’ll have your main alloying elements and then others to almost counteract side effects (or further improve others), and then more to counteract those. It’s a bit like whack-a-mole. Actually... err... it’s nothing like that.
Anyway… to help us identify alloys here’s how we group aluminium alloys.
Wrought aluminium series
Side note 1: To view the designations for cast aluminium alloys, click here. Spoiler: they’re pretty similar but have an “.0” on the end.
Side note 2: To make things more confusing, heat treatments and work hardening processes change the coding. For more, read the article here from Righton Blackburns.
Aerospace Aluminium alloys
As you know, or can probably guess, there are A LOT of aluminium alloys. And more are finding their way into the aerospace industry. For now, let’s focus on the already well-established alloys and where you might find them on a plane.
Aluminium alloy 2024
This is the most used aluminium alloy in aerospace - and has been for some time. It provides great stress resistance and high tensile strength and can be heat treated to improve its properties further. It also has particularly good fatigue resistance. The downside, however, is that it can be susceptible to corrosion. So, it’s often coated or treated to help protect it when it’s working in aeroplane wings or fuselage structures.
Aluminium alloy 5052
5052 is very corrosion-resistant, has impressive fatigue strength and boasts great ductility, making it very shapeable. Unlike 2024, it’s not heat-treatable, so only cold-working will improve its properties further. Its corrosion resistance means it’s often used to make plane fuel tanks. (It’s also used in marine applications and even cooking utensils!)
Aluminium alloy 6061
Now a 6000-series alloy. 6061 also has great corrosion resistance but, unlike others, can be welded and brazed a little easier, so it’s great for fast prototyping. It’s typically used for wings and fuselage components, especially in smaller planes (because it’s not as strong as others). Tempered variants are sometimes used as scuba tanks.
Aluminium alloy 7050
Up first from the 7000-series alloys: 7050. This Al-Zn alloy has great corrosion resistance and strength, often finding itself as part of the wing skin and fuselage components. It’s popular among military crafts and was allegedly developed from 7075 because it lacked corrosion resistance.
Aluminium alloy 7068
7068 Aluminium is one of the strongest aluminium alloys on the market. Its strength brings a real zen to high-stress environments (which is maybe why it’s sometimes used in top-end archery bows?...). Like all 7000-series alloys, it’s heat treatable too.
Aluminium alloy 7075
The 7075 alloy is the second most popular aluminium alloy in aerospace. It was originally used during the Second World War because its strength is similar to steel (thanks to the zinc) but is a heck of a lot lighter. It’s also very fatigue-resistant, which we know is important in aerospace. And it’s also easy to machine. So, its popularity comes as no surprise. 7075's biggest letdown? Its resistance to corrosion. (See 7050...)
Origins of aerospace aluminium
Aluminium compounds have been used for thousands of years - some of the earliest date back to over 5000 years ago! Back then, typical applications included leather tanning or textile dyeing. And while they might still work as they did, aluminium serves a very different purpose now.
In 1808, Sir Humphry Davy, the British electrochemist who discovered several elements, including sodium, potassium and calcium, theorised and identified that aluminium existed. But despite his efforts, wasn’t able to produce any.
17 years later, however, in 1825, Danish chemist Hans Christian Ørsted did manage to isolate aluminium and 'produce' some… a small, impure (and unusable) pellet.
Friedrich Wöhler, a German chemist, stepped up in 1827 and isolated aluminium again. This time, by mixing anhydrous aluminium chloride with potassium. Arguably more importantly, however, was the fact that Wöhler calculated aluminium's specific gravity, identifying just how lightweight the metal is.
But, up until then, producing aluminium was a difficult and inefficient process. So, the metal was rare and allegedly more expensive than gold and silver. In fact, it was so rare that Napoleon III used aluminium cutlery at royal banquets - banquets where the less distinguished were given gold and silver instead. (Can you imagine if time travel existed?! I'd be hand-delivering rolls of Tesco foil to him!)
It wasn’t until 1886 that the Hall-Héroult Process was invented. This now very well-established process uses electrolysis to extract aluminium from alumina. Which is great… if you have alumina to use. Thankfully, in 1888, Karl Bayer developed the Bayer process, which enabled you to extract alumina from the very abundant bauxite ore. These two processes made aluminium accessible for industrial use. And use it, the industry (eventually) did.
From fancy cutlery to flying engines
The first use of aluminium in aerospace dates back to the Wright brothers, who, in 1903, completed the first-ever manned flight - and used an aluminium cylinder block alongside some other aluminium engine parts.
This is quite the breakthrough. Not only because it was the first-ever manned flight (duh!) but because, at the time, car engine blocks (and their almighty 8hp performance) were made from cast iron - and weighed a small tonne. Although the Wright brothers' engine only had 12hp, its new-and-improved power-to-weight ratio helped them take off (and land) successfully with a pilot onboard. Click here to see the “Wright Flyer”.
So, yes, by 1903, the Hall-Héroult and Bayer process had been invented - and worked - but aluminium was still embedding its roots. It was expensive and hard to come by. So, the rest of the Wright brothers’ plane was made using spruce, bamboo and canvas - applications that would eventually become those of aluminium.
A flying friendship
During WW1, aluminium started to replace wood as the key material in aerospace manufacturing. German aircraft designer Hugo Junkers built the first fully metal aeroplane in 1915, where the main metal was an Al-Cu alloy (likely what is now a 2000-series alloy). It was this alloy, that over the coming 40-odd years, would make aeroplanes and Big Al best mates.
Before WW2, those ‘40 odd years’ were known as the Golden Age of Aviation. Aeroplane racing became popular in the US and Europe and led to a lot of innovations, improving efficiency and performance. Monoplanes replaced biplanes. Landing gear became retractable. The skin became load-bearing. And all-metal aluminium alloy frames became the norm.
With these improvements (and World War 2), the demand for aluminium and its alloys skyrocketed. The US alone made over 290,000 aircraft between 1940 and 1945! (Including the P-51 Mustang)
But, of course, progress never stops.
Despite 2024 and 7075 being already well-established aerospace aluminium alloys, Concorde's development prompted new 7000-series alloys to hit the market. These targeted even better strength, corrosion resistance and fatigue properties. From the humble 7075, out came 7150, 7050 and 7055.
And with money as fuel, aluminium’s potential continues to reach new heights. Its alloys continue to make flights more efficient, reduce components and allow airlines to carry more people over longer distances. It’s no wonder the Boeing 737 – the best-selling commercial aeroplane – is now made from 80% aluminium. I think it's fair to say that aluminium has well and truly taken off, and it doesn’t look like it will be landing any time soon.
Aeroplanes, drivers and wrenches
Whilst aluminium has changed the aviation game, you should see its impact in the world of screwdrivers, drivers and wrenches… (sorry, shameless plug incoming)
The MetMo Grip’s adjuster uses anodised 6061. It’s lightweight, durable and adds function without weight. Fiddle. Open bottles. Or, simply, clamp until your heart's content. Learn more about the Grip here.
The MetMo Driver and Pocket Driver, on the other hand, use aerospace-grade 7075 and 2024 aluminium. They’re as strong as steel, weigh a lot less and are much easier to machine than titanium. That way, you can experience the drive of your life - and then stow it away in your pocket.
Learn more about our Driver and Pocket Driver.
I hope you’ve enjoyed reading this. A lot of materials have changed - or been developed - for aviation. So, who knows, maybe in the future we’ll share more. If you have any fun facts or creative uses for aluminium, we’d love to hear from you in our CubeClub forum.