THE Materials Oracle
Join Date: Feb 2006
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The xxxx series system for classifying aluminium alloys is assigned by virtue of major and minor alloying addition elements, and subsequent suffixed on their mechanical state and degree of heat treatment.
As a general rule, you will only come across three series in bicycle frame and component manufacture: 2xxx, 6xxx and 7xxx series.
The first digit refers to their major alloying addition - 2xxx series are aluminium-copper alloys, 6xxx are aluminium-magnesium-silicon alloys and 7xxx are aluminium-zinc-copper-magnesium alloys.
The reason you are unlikely to see the 1, 3, 4 and 5xxx series is because those alloys can only be strengthened by cold work and the heat of welding anneals them, removing any strength in the welded are and heat-affected zone. The 2, 6 and 7xxx series are all precipitation hardening alloys, which can be strengthened either by cold work, by heat-treatment, or a combination of both. Following welding such frames it is recommended to heat-treat them in their entirety to restore strength to the fusion zones. 8xxx and 9xxx series are experimental and aluminium-lithium alloy grades which are usually severely out of the reach of most bicycle frame and component manufacturer's pockets.
6xxx series alloys are intermediate strength aluminium alloys. They are readily forged, machined, welded and are easy to solution and age. They are weaker and less tough than either 2xxx or 7xxx series by and large. You will find them usually, but not always, in lower cost aluminium alloy frames. Likely numbers you will encounter are - 6061, 6063 and 6082. The last three digits are assigned chronologically on their development but also reference their specific compositions with regards to alloying additions.
2xxx series alloys are generally not used for frames. They are readily welded and heat-treated, but are more suitable to manufacture of components such as bars, seatposts and stems - larger section:diameter ratio components with less severe butting, as these alloys' work-hardening rate is the highest of any aluminium alloys. Numbers you are likely to encounter are 2011, 2014 and 2024.
7xxx series alloys are, with the exception of aluminium-lithium alloys, the strongest, most expensive, toughest and least readily welded. There are only two 7xxx series aluminium alloys cleared for fusion welding in USAF specifications. The reason is the low boiling point of zinc, the major alloying addition, which tends to evaporate from the weldpool and fill the resultant fillet with voids. Those alloys are 7005 and 7020. 7075 alloy, which is usually the grade used for very high strength aluminium alloy bicycle components, is presently fusion-unweldable, so you will only see as forged parts. Friction stir-welding is the only jointing process that will satisfactorily joint most 7xxx series alloys and is totally impractical for bicycle frames.