It better not come as a surprise to you that your bike is made of something. That something is probably a metal. Metallurgists, the people who study metals, use their own language when they're talking about what they make.
For what it's worth, I'm one of those people. I am a metallurgist at Georgia Tech. If I talked to you about metals the way I talk to my peers, you probably wouldn't understand too much. Yet this is how a lot of bike companies talk to you through their ads. They use jargon because it sounds cool and looks good. You aren't really expected to know what it all means. But all those numbers and letters can tell you a lot about what youÇre riding - and what you're thinking about spending your money on.
Metallurgy can be a complicated subject but it's not impossible. Hopefully by reading this you'll gain an appreciation of aluminum and insight into some fundamental aspects of how your bike works. Let's start off easy. Pure metals are soft and bend easy. That's why you don't ride a pure aluminum or pure iron frame. By mixing other stuff with the pure metal, it gets stronger and stiffer. There are a couple of reasons for this. Most of them are technical, so we won't talk about them any more than we need to. Don't worry, just because we're not talking about something doesn't mean I'm not going to bore you.
Combinations of metals are called alloys. The most common bike materials are alloys of iron and carbon (steels) and alloys of aluminum. A system of four digits is used to identify an alloy composition. These numbers are called an alloy designation. When we talk about an alloy designation, the first number tells you the series. It's like an area code. Every alloy in the three-thousand series (also called 3XXX) contains aluminum and manganese. 3034 happens to be the aluminum alloy used in every can you drink out of. It's good for cans, but you wouldn't want to make a bike out of it.
Every alloy in the six-thousand (6XXX) series contains aluminum, silicon, and magnesium. Since 6061 belongs to the 6XXX series - they both start with 6 - so we know that it contains aluminum, silicon, and magnesium. Each alloy within a series contains the same major ingredients. The second number indicates the modification or version of the alloy: 0, 1, 2, and so on as the technology advances. The last two numbers identify the specific alloy.
For bikes, 6XXX and 7XXX series alloys are used the most often. As I said earlier, the 6XXX series consists of aluminum alloyed with silicon and magnesium (Al-Si-Mg). The 7XXX series consists of aluminum alloyed with zinc, copper, and magnesium (Al-Zn-Cu-Mg). Both the 6XXX and 7XXX alloys are heat treatable. That's why you see them used so often.
You've probably heard about heat treating before. Following each four digit alloy designation is a hyphen and a temper designation. This tells you how the alloy was heat treated. When you see a T in the temper designation, that means heat treated. Usually it means age hardened. WeÇll get to that in a minute.
Heat treatment consists of two parts. The first is done at high temperature. When you put the ingredients in an alloy together, it's a lot like baking a cake. You have to mix the ingredients somehow. You do it with a solution heat treatment. By heating the metal, the ingredients mix up through a process known as diffusion. You heat the metal up for a couple of reasons. First, the high temperature is going to allow you to supersaturate the metal with the ingredients youÇre adding. Let me illustrate this. If you take a glass of cold water and stir in a pile of sugar, you won't be able to dissolve too much sugar in the water. The stuff that dissolves makes the solution saturated. But if you put the sugar in hot water, you can dissolve extra. The water is still saturated, but there's a lot more sugar in it. This example is a liquid solution, but the same thing happens with aluminum. It just happens a little slower, and needs higher temperatures.
Because aluminum is a solid, a solution of stuff in aluminum is called a solid solution. The metal is then quenched. It starts off hot from the solution heat treatment. You cool it fast by dropping it in something cold like water. Quenching freezes in the uniformity resulting from the solution heat treatment and lets you keep all of your ingredients dissolved. If you were to cool slowly instead of quenching, you wouldnÇt be able to keep the metal supersaturated. It would be like setting your hot sugar-water on the counter to cool. You'd make big chunks of sugar. The amount of chunks is the difference in the amount of sugar between the supersaturated and saturated solutions. If this happens in the sugar water, you get yummy rock candy. If this happens in aluminum, it is bad and your bike will break.
Ageing is the second part of the heat treatment. Ageing is how the metal actually gets stronger. In 6XXX and 7XXX alloys, ageing results in precipitation hardening. Ageing takes place spontaneously at room temperature in some alloys. This is called natural ageing. Artificial ageing occurs at a higher temperature, usually between 100 and 190 °C. Supersaturation is what allows heat treatment to work, because it is supersaturation that lets you precipitation harden when you age. Did you follow me?
Here's the recipe:
Without getting too detailed, precipitation means a lot of very tiny pieces (precipitates) form in the metal. It's called hardening because the precipitates make the metal stronger. If you heat the metal too long, these pieces get bigger and combine. If they get too big, it is bad. Remember I said how each four digit alloy designation is a hyphen and a temper designation? This tells you how the alloy was heat treated. Heat treating is also known as tempering. The T means heat treated. Usually it means age hardened. When you solution heat treat, quench, then naturally age, itÇs called a T4 treatment. When you solution heat treat, quench, then artificially age, it's the more familiar T6. T6 ageing is known as peak ageing. It is the combination of time and temperature that maximize strength. Put another way, it's where the number, size, and distribution of precipitates gives you the strongest metal.
Ageing is usually conducted to the time and temperature that gives maximum strength. That's why T6 is so common. By overageing, heat treating for a time past the peak, strength is reduced 10 to 15% but crack resistance is improved. This is a T7 heat treatment. If you can sacrifice a little strength, you can get some resistance to cracking. It's important to note that you get the highest strength when you heat treat at low temperatures for long times, but this is not economical. So, you want to heat treat at the lowest temperature that is fast enough to give you the strength you need.
I hope this gives you a little understanding of the metallurgy of aluminum alloys. At the very least, you should be able to look at an alloy designation and tell what is in it, how it was heat treated, and why heat treating is important. At best, youÇll be able to impress your friends with some trivia. If you want more, you're lucky because there's a lot left. Keep an eye out for the next in this series. We'll talk about how strong steel and aluminum are and how they break, and we'll unlock the secrets of the common aluminum alloys: 6061, 7005, 7050, and 7075...and don't forget the vaunted and taunted 4130. If you have any questions about metals or want to know more, either myself or one of my colleagues can hook you up. Call us: gt0318d@acme.gatech.edu