Metallurgy for Cyclists III: Aluminum's Future is Bright and Shiny
by Scot Nicol
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's popularity as a frame material has increased dramatically 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 bauxite ore, named for the town where it was first discovered -- Les-Baux-de-Provence, in France. The ore contains hydrated alumina 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.
Tubing from Aluminum
The 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 route 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 the requirement for steel. And although the production of recycled aluminum takes less than 5% of that amount of energy, virgin aluminum is needed for wrought products -- those that are rolled, extruded, or drawn. Wrought aluminum is the kind you use for bikebuilding.
A number of different alloys can be produced from raw aluminum. The wrought ones used for bicycle fabrication are designated by four-digit numbers -- like the one indicating the venerable 6061 alloy. Cast aluminum alloys, on the other hand, get a three-number tag, a period, then a fourth number. You may also have seen a T4 or T6 suffix on some alloys: 7075 T6 or 2024 T4 are pretty common. This is the temper designation, which describes the particular combination of cold work, heat treatment and aging processes to which the material has been subjected, and it's pretty important.
That's because, when you weld an aluminum downtube to an aluminum headtube, the strength of the tubes will be reduced. To restore the original strength, you will need to heat treat the frame. (In case you were wondering, this goes for the material used in the Specialized M2 bikes, too. The Duralcan base alloy is good old 6061, with about 10 percent aluminum oxide by volume.)
Heat treatment involves two distinct stages, solution heat treatment and ageing. In the first process the material is heated to temperatures between 800 and 1000 °F for a number of hours, then quenched to room temperature. The second process, also known as precipitation hardening, is usually done in an oven -- bake at 250 to 350 °F for eight to 36 hours.
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 which I'm not going to discuss here. During solution heat treatment, alloying elements go into solution. During ageing, they precipitate out. The result is a significant change in the behaviour and strength of the metal.
Although there are a few aluminum alloys that can't be treated, most are better for it. 7005 alloys like Easton Varilite which don't need solution heat treatment still need to be artificially aged. And even non-heat-treatable alloys like 5086 and 5083, presently seeing some use in bicycle frames, can often be strengthened by cold work. This kind of brute force treatment -- rolling, drawing, straightening or flattening -- is also known as strain hardening or work hardening.
The more I learn about materials, the brighter the future looks for aluminum.
The first property of aluminum is the one that makes it 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. Consider that some of the new aluminum composites have strengths close to or matching that of CrMo, with one-third the density, and it's easy to see why aluminum has become such an important player.
But, as you good students know, we can't consider properties in isolation. You need to look at other factors besides with strength and density. So let's do it.
First, we'll consider the modulus. The modulus numbers for aluminum are low compared to those of steel. Hence the folk wisdom that alu frames are soft. That's a crock. How stiff a frame rides is a function of its design. Despite aluminum's low modulus, you can still build a plenty stiff bike with it.
As you'll remember from the last installment of this series, build a frame with large-diameter tubes, and its stiffness increases dramatically. Since aluminum's density is low, it allows you to build a bike with large-diameter tubes, with walls thick enough to provide good buckle-resistance, all at no weight penalty. Remember, Cannondales are made of aluminum, and nobody -- at least, not correctly -- ever called a Cannondale flexible.
The real problem 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.
Don't be too quick to dismiss the metal, though. It's true that low elongation increases the risk of a brittle frame failure, and elongations below about 9% should be scrutinized closely. But you need to consider strength, toughness and endurance limits, too.
What we find is that most kinds of aluminum don't have an endurance limit. (There are a couple of exceptions, notably the 5086 alloy.) That means that even a minuscule load, applied enough times, will eventually produce 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 being, "Put the material where you need it." This is a very simple concept, but one that people seem to easily lose track of. The steel guys figured out how to apply it a century ago. Butt the tubes!
Well-designed butts can make your frame stronger and lighter. In fact, looking at the tube sizes which have worked in steel is an excellent way to determine what properties might be required for other materials. This is what Teixeira did in designing the excellent Varilite tubes, which were among the first on the market, and which first saw action in Doug Bradbury's Manitou bikes in 1990.
Trek had made a bike with butted aluminum tubing a few years previous to that, but widespread takeup didn't happen until Klein and Cannondale, and later Specialized, got on the program.
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.
To optimize the advantages of aluminum, you have to deal with its inherent disadvantages. One way to accomplish this is to design 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 a 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, or flexing, the lower the stress. The result is less chance of fatigue. It's also good to spread the stresses out to places of lower loading. This is the idea that underlies butts, lugs and gussets. Spreading the stresses down the tube also allows you to build a bike that has combines resilience with a lively feel, rather than an ultimately rigid, dead structure.
The alu designer also needs to keep tags on 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 many of the claims made by 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 when we look at Exotics, a few lessons down the line. Meanwhile, the next installment in our Heady Metal series will cover titanium.