Good morning … afternoon … evening (circle one), class. Today, we are going to study aluminum. What we learn today will be based on the knowledge you’ve already gained during our two previous sessions. Did you all get a chance to review the first lesson – an overview? How about the second, on steel? Good. This one on aluminum marks the halfway point of our six-part series.
Aluminum as a frame material has increased dramatically in popularity over the last decade. In the early 1980s, aluminum bikes were a novelty, only available from a small, select group of high-end manufacturers. Then, in 1982, Cannondale jumped on the scene and began to push the material downmarket. Today, almost every medium-to-large manufacturer has at least one aluminum bike.
Furthermore, there’s plenty of material for them to use – aluminum is the most plentiful metal in the earth’s crust. And except for magnesium and beryllium, it’s also the lightest structural metal. A primary source of aluminum is the ore bauxite, named for the town where it was first discovered – Les-Baux-de-Provence, in France. The ore contains hydrated alumina (Al2O3*2 H2O) with impurities of iron and titanium oxides. Sounds like one-stop shopping for the bike industry’s metal requirements, eh? It’s not really, as we have better sources of titanium and iron ore.
Making Aluminum into Tubing
The actual process that changes the aluminum we find in the earth’s crust into a tube suitable for building a bike or lawn chair is complex, ugly and energy-intensive. It’s appropriate that the most important process for getting from bauxite to aluminum is called the Bayer method, because studying it will give you a headache…. It takes about 9 kilowatts of energy to produce a pound of aluminum – far above what’s required for steel. And although the production of recycled aluminum takes less than 5 percent of that amount of energy, virgin aluminum is needed to make wrought products – those that are rolled, extruded, or drawn.
A number of different alloys are produced using raw aluminum. For bicycle fabrication, the resultant wrought aluminum products commonly use a four-number designation system. An example of this would be the venerable 6061 alloy. (See “Aluminum alloys” for other examples.) Cast aluminum alloys use a three-number tag, a period, then a fourth number. Both wrought and cast alloys use another number that comes at the end: the temper designation. No doubt you’ve seen the T4 or T6 condition listed after some of the alloys: 7075 T6 or 2024 T4, for example. It describes what cold work, heat treatment and aging processes (if any) the material has been subjected to.
The tempering has a huge effect on the mechanical properties of many alloys of aluminum (some alloys are, and some aren’t, heat treatable). When you weld a 6061 downtube to a 6061 head tube on a bicycle frame, the as-welded condition will have lower strength than before it was welded. You then need to solution heat treat, and artificially age the frame, to return it to high strength. This goes for the Duralcan material used in the Specialized M2 bikes, too, as the base alloy is 6061, with about 10 percent aluminum oxide by volume. And although 7005 alloys, like the Easton Varilite, don’t need to be heat treated after welding, they do need to be artificially aged. When you age and heat treat, you’re mucking around with solid solutions; crystalline structures; the saturation of alloying constituents; their subsequent submicroscopic precipitation; and a bunch of other very small, but very significant changes that I’m not going to discuss.
Alloys that aren’t heat-treatable are often strengthened by cold work – also known as strain hardening, or work hardening. Rather than change the structure by recrystalizing it, cold working changes the structure through brute force, such as rolling, drawing, straightening or flattening the material. Examples of this type of alloy are the 5086 and 5083 alloys that currently are seeing some use in bicycle frames.
Note that when you heat treat – which really should be called thermal treatment – there are two different steps. The first is the solution heat treatment, which is usually done between 800 and 1000 degrees Fahrenheit for a number of hours. The aluminum is then quenched – in air or water, depending on the alloy – to room temperature. After that, the aluminum must be precipitation hardened (also known as aging).
The alloying elements that went into solution during the heat treatment will precipitate out over time, increasing the strength of the aluminum. Since the alloying elements are more soluble at elevated temperatures, aging is usually done in an oven (bake at 250 to 350 degrees Fahrenheit, for eight to 36 hours), so that the process happens more quickly. This is the process you hear about called artificial aging.
The first property of aluminum that we’ll examine is the easiest to understand, and happens to be the one that makes aluminum so desirable as a frame material. It’s called density. Aluminum, as you know already, has approximately one-third the density of steel and one-half that of titanium. Since our industry is so weight-saving conscious, aluminum has become a very important player. In fact, the more I learn about materials, the brighter the future looks for aluminum.
Consider that some of the new aluminum composites have strengths close to or matching that of CrMo, with one-third the density. But, as you good students know, we need to look at many things in combination with strength and density, so let’s do it. Even though the modulus numbers for aluminum are low compared to other common framebuilding materials, you are able to build a plenty stiff bike with it, because the low density allows you to build a bike with large-diameter tubes, without a weight penalty.
As you’ll remember from the last installment of this series, build a bike with large-diameter tubes, and the stiffness increases dramatically. And since the density is low, the walls can be thick enough to provide good buckle-resistance along with the stiffness. How stiff a frame rides is a function of its design. Alans and Cannondales are both made of aluminum, but nobody – at least not correctly – ever called an Alan stiff, nor a Cannondale flexible.The first big property challenge for aluminum is elongation. How far will aluminum bend before it breaks? Not nearly as far as titanium, and usually not approaching the limits of steel, either. If you’ve learned anything from this series, though, it’s that you have to look at a combination of factors before making a judgment.
It’s true that low elongation increases the risk of a brittle frame failure, and elongations below about 9 percent should get close scrutiny. But we need to look at strength, toughness, and the endurance limit, too.
What we find is that aluminum (except for a couple of exceptions like the 5086 alloy) doesn’t have an endurance limit. That means that even a minuscule load, if applied enough times, will eventually result in a fatigue failure. Kinda scary, don’t you think? Steel and titanium are fine in this department, aluminum is not. Clearly, there are a lot of aluminum bikes out there. Are they all going to break? No, they’re not.
How do you design around this? I posed the question to “Sir” Charles Teixeira, the Easton engineer who is responsible for the Varilite tubeset (I added the “Sir” part, we’ll call him Chuck). Chuck Teixeira is a smart guy, and he knows materials. When he designs things, he pays attention to a few simple rules: One of them is to put the material where you need it. This is a very simple concept, but one that people seem to easily loose track of. The steel guys figured it out a century ago: butt the tubes.
Well-designed butts can make your frame stronger and lighter. In fact, looking at what tube sizes have worked in steel is an excellent way to determine what properties are required for other materials. This is what Teixeira did in designing the excellent Varilite tubes, which came out in 1990 and were first used for Doug Bradbury’s Manitou bikes. These were some of the first butted aluminum tubes to see wide use in the market.
Trek had been doing a bonded aluminum bike with butted tubing for a few years previous to that, but widespread use didn’t happen until the last couple of years. Klein and Cannondale got on the program a couple of years ago, and the Specialized M2 just got butted this year.
The Varilite tubes have extremely thick walls in the areas of high stress, and they taper down in the areas that handle less stress. In this way, stresses are dispersed in the tube, and the life of the structure is increased. It’s not rocket science, just good design.
Optimizing Aluminum’s Advantages
To optimize the advantages of aluminum, you have to deal with its inherent disadvantages. One of the ways to accomplish this is by designing in a large margin for error. Although there are many different situations, Teixeira said that one rule of thumb he uses is to increase the tube’s static strength by about three times that of the steel bike.
A lot of factors come into play here, so this isn’t an iron- (or aluminum-) clad rule. A basic premise is that the lower the displacement (flexing), the lower the stress, resulting in less chance for fatigue. It’s also good to spread the stresses out to places of lower loading. This is the idea behind butts, lugs and gussets. Spreading the stresses down the tube also allows you to build a bike that has more resilience and a lively feel, rather than an ultimately rigid structure.
Then there’s stress corrosion, another eyebrow raiser. If you mess up that artificial aging, then stress corrosion may come back to haunt you. As you can see, we have a very complicated puzzle in front of us.
What does the future hold? I asked Teixeira this question, and the outlook wasn’t full of fantastic new materials formerly used for Space Shuttle muffler bearings and F-16 dipsticks, as you might think. There will be advances, but the claims made by many of the slick marketers aren’t panning out. It’s still hard to beat good old 6061, when you look at the whole package. It’s the most versatile of all alloys, has excellent toughness for an aluminum, and good elongation, too. Like the point I made last time with high-zoot CrMo versus generic CrMo, we know that you can make a good bike out of either – it’s just that it takes smart design from the tube on up to build a good bike.We’ll learn more about some of the new higher-strength aluminum alloys and associated materials in the exotics part of our series, which will come at the end, after titanium and carbon fiber.
As you may have guessed, the next installment in our Heady Metal series will cover titanium.