All about Aluminum

What is 6061 and ‘7000 Series’ Aluminum?

In the mid-90’s we saw aluminum quickly become the popular medium for building bikes due to it’s low weight.  By the end of the 90’s bike companies were plastering stickers on their frames showing the fancy ‘7000 series’, 7075, and 6061 aluminum tubing.  What’s the difference and what do the numbers mean?

Aluminum is an element that is naturally occurring in the earth and is one of the most plentiful metals found in the crust.   It is mined as a material called Bauxite Ore that contains aluminum oxide (sometimes referred to at alumina).   After Bauxite is removed from the ground, it’s crushed and sent to a refinery where the aluminum oxide is separated from other impurities.  After refinery, it is in a powder form that is then sent to the smelter where the aluminum is extracted from the alumina.  This is done by running a large amount of electrical current through molten alumina that’s been dissolved in a 1,750 degree cryolite bath. The molten aluminum can then be made into different alloys by mixing other minerals with it. 
   After the smelting process is complete, it is then cast by mixing it with other minerals to impart different qualities to the metal, creating an aluminum alloy. It’s important to note that pure aluminum is rarely ever used - it is a very soft metal that forms easily but does not have much strength.

The largest alumina mining and refinery company in the world is Alcoa, producing 14 million metric tons per year- nearly one-third of the world’s supply.  It is mined in the US, Jamaica, Brazil, Spain, and Australia   Alcoa’s Australian mining and refinery plant is the largest aluminum producing country in the world and produces 15% of the world's supply of aluminum alone.  Alcoa’s Tennesee  smelting facitlity produces enough aluminum sheet to make 100 billion beverage cans per year.

In North America, aluminum alloy compositions are registered with The Aluminum Association Inc .  There are currently more than 400 aluminum alloys registered with the Aluminum Association.   These alloys are categorized into several groups based on their main alloying element.
 The first digit (Xxxx) indicates the principal alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series, i.e., 1000 series, 2000 series, 3000 series, up to 8000 series.
The second single digit (xXxx), if different from 0, indicates a modification of the specific alloy, and the third and fourth digits (xxXX) are arbitrary numbers given to identify a specific alloy in the series. Example: In alloy 5183, the number 5 indicates that it is of the magnesium alloy series, the 1 indicates that it is the 1st modification to the original alloy 5083, and the 83 identifies it in the 5xxx series.
The only exception to this alloy numbering system is with the 1xxx series aluminum alloys (pure aluminums) in which case, the last 2 digits provide the minimum aluminum percentage above 99%, i.e.,  1350 aluminum alloy is 99.50% minimum aluminum.

1000 series aluminum alloy are nearly pure aluminum, being 99+% pure.  These are soft, weak alloys that can be welded easily but can not be heat treated.  They have good corrosion resistance and are used where high electrical conductivity is required.
2000 series aluminum alloys are high strength alloys that are considered non-weldable.  The bike industry uses 2024 alloy for components such as handlebars and seat posts because of it’s high strength and resistance to cracking.  It’s main additive is copper.
3000 series aluminum have manganese added to increase strength and response to cold work.  They are moderate strength, have good corrosion resistance, and can easily be welded.  It’s used mainly for non-structural situations such as air conditioning systems.
4000 series aluminum alloys add silicon to reduce the melting point and are used primarily for welding and brazing filler material and in castings.  They are the most sensitive to cracking of all the aluminum alloys.
5000 series alloys have magnesium added to increase strength and ability to work-harden.  They are very corrosion resistant, easily weldable, and are the strongest of the non-heat treatable alloys.  They are normally available as sheets or plates and are the most common structural alloys.  However, they are normally not available in extruded sections (tubing) because of the high cost to extrude.

The 6XXX series of alloys are the alloys most often encountered in structural work, and the most common alloy used in bicycle frames. They are relatively strong (although not as strong as the 2XXX or 7XXX series) and have good corrosion resistance.  6061 aluminum is cast with magnesium and silicon as its primary additives.  This makes 6061 aluminum alloy much easier to weld than other aluminum alloys. However, care must be taken to use the correct type and amount of filler material when welding, otherwise it can crack easily.  Even so, it is the most common heat treatable structural aluminum alloy.   It is easily extruded and cold worked so it is available in many different tube shapes and sizes.   6061 alloy is used on a vast majority of aluminum bike frames and makes very light, strong, and low-cost frames.

7000 aluminum is cast with zinc as its primary additive, making it a very high strength alloy. It's more costly to use zinc as an aluminum additive, making the 7000 series more expensive.  It also has higher levels of magnesium and copper than 6061, making it as hard as some steel alloys.  This lightness and strength come at the expense of its corrosion resistance - 7000 series alloy will corrode much easier than 6061.  With only a couple exceptions, 7000 series aluminum can not be welded.
The only 7000 series alloy that can be welded is 7005*.  This is the tubing used in many high-end aluminum frames and creates a very strong, very light frame that is more costly than any other aluminum frame.
7075 aluminum alloy is the most commonly used of the 7000 series alloys, however, it is considered non-weldable.  It is most commonly used in the bicycle industry on components.  However, it was used in frame tubing form when bonded (lugged and glued) frames were in style during the mid-90’s.

Although 7000 series aluminum is a harder alloy that makes a frame stronger, it also makes the ride quality much more rough.  6061 aluminum alloy is softer so it dents and bends easier but will give a smoother ride.
*There are other exceptions to the general rule that 2XXX and 7XXX alloys are unweldable but not in the bike industry. They are rare, expensive and not used in the bicycle industry. 

Through the years, there have been ‘boutique’ aluminum tubing used for bikes.  many people remember Easton Elite  tubing in the late 90’s as the best available.  Easton Elite tubing is made from 7005 alloy that Easton made into double and triple butted tubes in order to make extremely light and stiff frames.  No special metallurgy, but custom thickness, diameter and shapes that give the structural qualities Easton wanted.
Aluxx aluminum that Giant uses on their aluminum bikes currently is 6061 alloy that is specially formed by Giant.  Aluxx SL is a 6011 alloy that they specially form and Aluxx SLR is a butted 6011 alloy that’s specially formed.  Giant utilizes fluidforming, pressforming,  and warmforming to get the special shapes of their tubes.  

Heat Treating
The process of welding results in a loss of strength around the weld by approximately  80%. The material can be re-heat-treated to restore -T4 or -T6 temper for the whole piece. Heat treatment is done by raising the alloy temperature to about 980 degrees F and holding it there for about an hour. The purpose of this is to dissolve all the alloying elements in a solid solution in the aluminum. Then we quench the alloy in water. At this point, this is called the T4 temper. For a T6 temper, it is once again heat treated at a temperature between 325 and 400 degrees F, so that the alloying elements begin to form ordered arrays of atoms in the aluminum matrix. These arrays are called GP zones, and they strengthen the aluminum considerably. This heat treatment is called aging, resulting in material with a T6 temp.  The bike industry nearly always heat treats to a T6 rating.

Aluminum corrosion
Many of us have been told that aluminum will never rust.    While that statement may be true, but it is very misleading .  Rust is IRON oxide-steel, not aluminum.   Aluminum has excellent  corrosion resistance because it forms a natural aluminum oxide layer that protects it.  However, it will corrode when exposed to environments with high chloride levels, e.g. sweat or areas bordering the sea or oceans.  Low or high pH values (less than 4 and more than 9) lead to the oxide layer dissolving and, consequently, rapid corrosion of the aluminium.
 This is many times found along with ‘pitting’ of the aluminum - small holes or indentions where the aluminum looks as if it’s been ‘eaten away’.  Because of this, it is important that aluminum frames be finished by painting or anodizing and making sure it’s protected from sweat or humid coastlines.

Nearly 75 percent of all aluminum ever produced is still in use today.

Aluminum is Infinitely recyclable and highly durable- nearly 75 percent of all aluminum ever produced is still in use today. Aluminum is 100 percent recyclable and retains its properties indefinitely. Aluminum is one of the only materials in the consumer disposal stream that more than pays for the cost of its own collection.

references:

http://www.ehow.com/info_8078719_difference-between-6061-7075-aluminum.html

http://www.ehow.com/info_7857163_6061-vs-7005-aluminum-bikes.html

Linclon design - properties of different series aluminum alloys

Wikipedia -7075 series aluminum, 6061 series

Alotec.com

The Aluminum Association