Frame building basics #2.1 Aluminium (Aluminum)

Aluminium rose into the world arena after they developed a way to separate it from the ore using electricity .

There are numerous alloys used in frames and related parts and each have specific characteristics that suit specific needs. I will list below a few common alloys used in parts and what their properties are:

• 6061/6063 – a very very common alloy which is used to make bicycle frames and parts. It is very weldable and very workable. Has very good corrosion resistance and can be artificially aged and precipitation hardened. Can be extruded very well. Main alloying element is Silicon.
• 6105/6106 – a very common alloy used for rims. Has similar properties to 6061(has higher tensile strength), has a little bit more silicon in it, can be aged and precipitation hardened. Can be extruded very well. Slightly inferior workability compared to 6061. Main alloying element is Silicon.
• 7005 – used in bicycle frame prodction. Very strong (high tensile strength), also expensive and moderatly easy to weld. Inferior manufacturability compared to 6000 series alloys. Does not need to be precipitation hardened after welding. Average corrosion resistance. Main alloying element is Zinc and Magnesium.
• 7075 – used in parts production, very strong(comparable to steels), very expensive and hard material to machine. Used in places where weight is important, but loss in strength is not allowable. Should be artificially aged and precipitation hardened after welding. Main alloying element is Zinc and Magnesium.

This covers some of the aluminium alloys used in todays bicycles. 7005 series aluminium frames are a lot less frequent, since most companies have migrated to 6061-6063 aluminium. Those two are also most commonly found at different metal tubing resellers /warehouses.

Since 6061-6063 are the most common alloys out there, I am going to assume that you are interested in using them to make a custom frame. I would recommend using 6061-T6 aluminium (slightly higher tensile strength compared to 6063-T6). It is available in many different tube sizes/shapes, is easy to manipulate even with basic tools and can be polished very well.

But before embarking on a frame building adventure (which involved 6061-T6 aluminium), there are three very important questions to answer:

• Do I have somebody nearby who can Tig weld 6000 series aluminium really well?
• Do I have access to tubes that have similar sidewall thicknesses ?
• Do I have access to somebody who specializes in heat-treating /aging /precipitation hardening aluminium alloys?

- Question nr 1 is relatively easy to answer and people can usually recommend a good welder nearby. An average bicycle frame takes around 1-1.5 hours to weld up (personal experience).

 - Question nr 2 is also relatively easy – having tubes with similar sidewall thicknesses helps the welder. Having a tube which is 2mm and other which is 1.5mm does not seem like a big issue, but there is a thickness difference of 33%. In amperage that means 85-90 ampers for the 2mm tube, but 65-70 ampers for the 1.5mm tube. Easier to burn through tubes and also extends the HAZ (heat affected zone) outwards more than needed.

 - Question 3 is the biggest decider in my opinion. If you have somebody nearby who can help you and can do the whole process for 50-100 usd, I'd say go for it. The reason why I had to drop aluminium as a frame material was the high cost (was quoted around 500 usd) of post-welding processing.