Ali specs

[white90]

An alloy is basically a metallic element to which other elements are added, to change the properties of the main element.

With aluminium alloys, there seem to be many 'holy grails'. The two main ones being that everything is automatically better if it is made of 'billet 7075-T6' or is 'aircraft grade', whatever that may mean. To start with, a billet is nothing more than the name for a lump of cast alloy, and the name billet only depends on the lump's relative dimensions. If it had a bigger cross-sectional area to length, a billet would be an ingot! This is the basic alloy casting which is then 'worked' to produce a wrought alumiium alloy. An aluminium alloy that is not worked is merley a casting.

So, the main two groups of alloys are cast aluminium alloy and wrought aluminium alloy.

The main group of alloy we are interested in on the bike are the wrought alloys.

Aluminium in it's near pure state is extremely soft and ductile with little strength, but is highly resistant to corrosion. It is very similar to copper in that it can be work hardened easily, but to bring it back to it's annealed (natural) state, it is heated and then quenched. This is the opposite of steel, that uses the same process to harden it.

On it's own, aluminium is of little use on a vehicle other than for decorative purposes.

So, we alloy it with other elements.

The main elements it is alloyed with are copper, iron manganese, magnesium, silicon and zinc. Each of these alloying elements changes the properties of the base aluminium and gives it different properties of strength, ability to easily machine, weld , cast or further improve corrosion resistance.

You will see many designations of alloy, all beginning with a number from 1 to 9. For instance, 7075 or 1060.

The first number is merely the convention for which is the main alloying element.

So, we have the following:

1 series. Aluminium of minimum 99% purity

2 series. Alloyed with Copper - Increases strength and improves machinability

3 series. Alloyed with Manganese - Increase strength and hardness

4 series. Alloyed with Silicon - Improves ductility

5 series. Alloyed with Magnesium - Improves strength and corrosion resistance

6 series. Alloyed with Magnesium and silicon - Improve sability to cast and corrosion resistance

7 series. Alloyed with zinc - Reduces ability to be cast and improves strength

8 series. Alloyed with other elements

So you can see that the 7 in 7075 means nothing more than the main alloying element is zinc.

For the one series of pure aluminium. the 3rd and 4th numbers indicate the percentage of purity of the aluminium above 99%. Therefore a 1060 aluminium will be 99.6% pure, and a 1030 aluminium 99.3% pure. This does not work with the higher numbered groups, so that although 7075 means that the main alloying element is zinc, the last two number (75) are an identifier for a specific alloy (in this case magnesium and copper).

Unfortunately when alloying it is not as simple as just putting in a bit of each constituent to try to get the properties you want from the material at the other end.

Adding the alloying elements changes the alloy properties from that of the base aluminium, but so does the heat treatment or temper to which the alloy is subjected. The heat treatment will further alter the alloy's basic properties. Unfortunately, not all grades of wrought alloy are suitable to be heat treated, so we have two further sub-divisions:

Those that can be heat treated

Those that cannot be heat treated

The alloys that cannot be further heat treated are the 1 series, 3 series and 5 series of alloys.

Those that can are the 2 series, 6 series and 7 series.

The 4 series are a grade of cast aluminium that can be further worked through other methods.

The various types of heat treatment that can be applied are given a basic designation code as follows:

O - Annealed

H - strain hardened

T - Heat treated

W - Solution heat treated

In addition, a number is added after this letter code which relates to the specific heat treatment that is applied.

So, our 7075 T6 is a zinc based aluminium alloy with additional alloying elements that is treated and hardened.

It is nigh on impossible to give a specific list of all the alloys and all their constituents and all of their properties as these can change drastically within groups and sub-groups and with different temper states. This then affects the use to which they may be put.

I have however put together this basic list of wrought alloys.

1050-O - Extrusions - Can be welded Poor machinability - Low tensile strength

1060-H18 - Chemical Equipment - Can be welded Fair machinability when strain hardened to H18 - triple the tensile strength of annealed 1050

2014-T6 - Aircraft structure - Very poor for welding - Excellent machinability - 6 times the tensile strength of annealed 1050

3003-T18 - Kitchen utensils - Very poor for welding - Excellent machinability - 3 times the tensile strength of annealed 1050

5154-H38 - Pressure vessels - Fair weldability - Excellent machinability - 5 times the tensile strength of annealed 1050

6061-T6 - Heavy applications in marine and industrial environmentsGood weldability - good machinability - 4 times the tensile strength of annealed 1050

7075-T6 - Aircraft structure - Do not weld - Excellent machinability - 8 times the tensile strength of annealed 1050

So, you can see from this simple list some of the properties. You can also see why at the beginning I said "aircraft grade whatever that means". 2000 series, 3000 series, 5000 series 6000 series and 7000 series alloys can all be used for aircraft applications in different structural areas. Also, don't go using 7075-T6 for the steering head on your next special as you cannot weld it so how are you going to attach it to the rest of the frame?

As with anything else in engineering, it is a case of choosing the right material for the right job. In the vast majority of cases, 6061-T6 will be more than man enough for most jobs on any bike, it is excellent for machining and welding, has good tensile strength and is excellent for anodizing (many of the other grades can't be anodized!).

The question of heat treatment is more complex. In general, most heat treatable alloys are solution heat treated by bringing them to temperatures of around 500 degC and then quenching them in water at room temperature. They are then tempered through precipitation heat teatment by heating to around 120 to 190 degC (dependant on material) for around 24 hours and then allowing them to cool slowly. All of this depends on what the original form of the material was, so exact figures cannot be given.

I hope this has given you a brief insight into the complexity of aluminium alloys, and if nothing else it may give you the knowledge to ask for the 'right bit of aluminium' next time you ask someone to make you a bit