In the cycling world, alloy means aluminum. Pedants with nothing to contribute will whinge that calling it alloy is wrong and makes no sense because steel is an alloy and so is titanium used in bicycles. Yes, steel is an alloy, it’s a special single syllable word for iron-carbon alloy with a specific range of carbon content because we use so much of it, and we don’t call it iron alloy. Aluminum alloy is a term that’s used. Aluminum can’t be shortened. Al is a name, short for Albert. Alum is a chemical compound (related to aluminum) that has been used since antiquity and can be bought at the grocery store. -inium or -inum is the suffix for multiple elements and a strange abbreviation that different parts of the Anglosphere won’t agree on because British people will say aluminium is correct because -ium is the correct suffix for elements like for the element “platinium”. Alloy also happens to cover named aluminum alloys like Duralumin, Hiduminium or Scandium alloy, making it a shorthand blanket term for parts made out of aluminum alloy, even if those parts were never advertised as being made out of “aluminum” alloy. Titanium came later, and has an exotic cachet, it would be insulting to call titanium merely alloy. So alloy parts are technically and pedantically made of alloys, even if not all alloys are aluminum, and it’s a historically useful shorthand.
Most of the alloys in cycling (crmo included) are old WWI or WWII technology, and haven’t really changed because they’re a known quantity and readily available because multiple manufacturers make the same material to the same standards.
These alloy designations are often followed by something like T6 which is the heat treatment state. If it’s missing the T, that doesn’t mean it isn’t heat treated, it just means the information was omitted. Again, beware the false pedant, 6061 is not a different material from 6061-T6, nor does 6061 without T6 mean without heat treatment. 6061-O is dead soft annealed and has to be ordered that way by companies that want to form it before hardening. Anything you buy as a end-user product will probably be appropriately heat treated, although not always to maximum strength because of manufacturing considerations. Bar stock purchased off the shelf will be heat treated. Sometimes the same alloy will have multiple heat treatment variants to optimize different characteristics, and sometimes equivalent heat treatments will have different designations depending on if the original material was sheet or bar.
There are some manufacturing concerns, like weldability, formability, extrudability and how well it takes anodization, but those won’t be mentioned in detail since that doesn’t really matter to the end-user. It does explain why certain parts are made out of certain alloys though. There are also some alloys designed for casting (or “melt forging” when that term is used), but they will often not advertise the alloy. In general, the fact that a part was cast makes it an inferior part anyways.
6061 – (T6 – 14 ksi endurance limit) Introduced in 1935, this is the most common “structural” or “aircraft” grade aluminum and if you buy a bar of aluminum at the hardware store, it’s probably this. False pedants might claim “structural” or “aircraft” doesn’t mean anything, but they are wrong. Common uses for aluminum include soda cans which use a very low strength malleable aluminum alloy, as well as other aluminum cast items, so not all aluminum alloys are “structural” or “aircraft” grade. It in the general purpose grade of aluminum, has decent strength, good corrosion resistance, and easy to weld, but benefits from a post-weld heat treatment. It’s a good no-nonsense base level alloy that won’t get you the lightest parts but will be perfectly functional. However, 6061, even in T6 condition, is too soft for durable chainrings though and they will wear excessively quickly. They’re used to cheaply “upgrade” from steel chainrings. For some reason people think 6061 chainrings are cheaper because they are easier to machine than hard 7075. The opposite it true. 7075 and 6061 are both relatively soft, but 7075 machines better because it is less gummy. 6061 is easier to stamp and form however. It is rarely used or supplied in anything but T6, unless it needs to be bent like handlebars. It is favored because this is the standard grade of structural aluminum you can from buy from any metal supplier off the shelf without any special orders.
6082 – (T6 – 14 ksi endurance limit) Less common. A European formulation that is roughly equivalent to 6061, it just uses a different mix of alloying elements that are more common in Europe’s dirt to get there.
6066 – (T6 – 16 ksi endurance limit) Less common. Used in Europe and Asia as a stronger substitute for 6061, but with less corrosion resistance than 6061 and less resistance to stress corrosion cracking, but still better than 2000 series and similar to 7000 series in terms of corrosion resistance and stress corrosion cracking. It can be considered an upgrade, but a relatively minor upgrade.
6069 -(No comparable data on fatigue) Less common. Used for tubes and extrusions and doesn’t seem readily available except for Asian manufacture even though it was developed in Oregon. Another even higher strength upgrade of the 6000 series alloys that can approach 2000 series alloys. It suffers from the marketing limitation of only being 8 more than 6061.
2014 – (T4 – 20 ksi endurance limit) Duralumin is alloyed with copper and was developed in 1909. It results in a higher strength aluminum alloy, higher than 6000 series, but at the cost of corrosion resistance. This or similar alloys were used in early aluminum bicycle parts and also has a long history in aviation seeing widespread use in zeppelins and Junkers. In aircraft duralumin alloys are often clad with high purity aluminum for corrosion resistance, but this isn’t true of bike parts, making them prone to corrosion and stress corrosion cracking. They are generally used in T3 or T4 condition, higher numbers like T6 aren’t always better, it depends on the alloy. The high strength in the T4 (without artificial ageing) explains the early use as high strength aluminum alloy. It is sometimes used as a chainring material to offer improved performance over 6061, but reduce manufacturing costs over 7075-T6. Kaiser considers this best in class (of 2000 series), but it’s not clear why.
2017 – (T4 – 18 ksi endurance limit) Possibly the duralumin alloy used for French parts, although maybe not since other contemporary alloys like Hidunium used by Reynolds (specifically alloy Rolls Royce 56) had considerably lower copper content than the alloys listed here. Although most current duralumin parts are anodized for corrosion resistance, lots of the French ones were left bare and polished. Roughly equivalent to other Duralumins.
2024 – (T4 – 20 ksi endurance limit) Sometimes called super duralumin in Japan. It’s a little stronger in terms of UTS and has better resistance to stress corrosion cracking. This is the one commonly available for aviation. At least in some applications (spectrum/varied load fatigue and fatigue versus 7075-T73) 2024 has shown superior fatigue performance and Aerospace Structural Metals Handbook states that there is no advantage for 7075 in fatigue strength even though there is an advantage in ultimate strength.
7075 – (T6 – 23 ksi endurance limit) Invented in 1935 by the Japanese for the use on the Zero fighter, sometimes called extra super duralumin in Japan, it’s alloyed with zinc and has higher strength and somewhat better corrosion resistance, but still not as good as 6000 series. It still suffers from stress corrosion cracking. 2024 has shown better fatigue strength than 7075 at least in some applications however and Aerospace Structural Metals Handbook states that there is no advantage in fatigue strength even though there is an advantage in ultimate strength. It was a common material during the CNC MTB part craze because 7075 is a higher number than 6061, and 7075 has a higher strength, and stronger means you can make it lighter. Except you see a lot of failed 90’s CNC parts made from 7075 because it’s prone to cracking when engineered for minimum weight. T6 has higher ultimate strength, but T73 has better resistance to stress corrosion cracking, so is preferred for most bike parts except for chainrings, but is closer to 2024 strength. 7075-T6 can be baked to 7075-T73 even though T73 is less readily available, so parts with T73 generally show more attention to detail, except for chainrings which benefit from being maximum hardness since they will wear before they fail due to other reasons. Some false pedants will claim because it is harder that is it harder to machine and that is why it is more expensive, and they are wrong. It is harder and easier to machine because it is less gummy and any kind of aluminum is soft relative to a sharp carbide or HSS tool.
7050 – (No comparable data on fatigue) A later development on 7075. It has less ultimate strength, but improved toughness, corrosion resistance, resistance to stress corrosion cracking, and fatigue strength that I would consider a part made from 7050 an upgrade over 7075 even if it looks like a downgrade on paper based on UTS.
7005 – (No comparable data on fatigue) Used for tubes mostly because the post-weld heat treatment is less involved (or skippable for manufacturers wishing to cut corners) than 6061. It’s stronger than 6061, but not as strong as being a 7000 series alloy might imply and has an undeserved halo effect. It’s not as strong as the better 6000 series alloys. Since it is generally used in the T6 condition as delivered by the factory, it is not as favored for hydroforming since a major benefit is skipping heat treatment and hydroforming would have to be done by whoever is heat treating the tubes. That isn’t to say that 7005 can’t be hydroformed, just that there’s no longer a benefit to not having to do a full heat treatment to justify the premium for the manufacturer if they’re doing in-house hydroforming and it probably can’t be hydroformed as extensively as 6000 series. For frames with similarly shaped tubes, round tube versus round tube, ovalized vs ovalized, etc, 7005 is better than 6061. Once you add in hydroforming, the hydroforming can play a bigger role in frame quality and you’re more likely to get a more hydroformed frame out of 6000 series. It’s good, but not the best.
Kuromori 