Framebuilders - Aluminium for bikes...

Anyone know what kind of Aluminium is used in bike frames generally?

I'm going to build one and am thinking that using standard Alu isn't going to do the job....

Most common is 6061-T6 and requires post-weld heat-treatment. Not as common is 7005. Doesn't require heat-treatment afterwards, but is more difficult to weld.

Most common is 6061-T6 and requires post-weld heat-treatment. Not as common is 7005. Doesn't require heat-treatment afterwards, but is more difficult to weld.


Doesn't require it, no, but I'd not ride one that hasn't. Having said that, I wouldn't ride Al anyway....

There are two alloy families that are commonly used. As Danno says, there are the 6000 series - of which there are a number of alloy compositions and heattreatment regimes, and the 7000 series, just the same.

6000 series are principally alloyed with silicon and magnesium and are at best medium strength alloys. 7000 series are the high strength alloys and are a strengthened with zinc, magnesium, silicon, copper, and often flavoured with titanium and zirconium, too. Both are available in a variety of sections and specific compositions from most aluminium alloy stockholders.

Well, "heat-treatment" is a general term that encompasses many stages. Both 6000 and 7000-series typically have a 2-stage process of quenching and artificial-aging. The 6061 loses a signficant amoun to of its strength -40-60% in the HAZ. The final artificial-aging is required afterwards in order to restore it's T6 strength. Although work-hardening will build this back up over time with use. The 7005 on the other hand doesn't lose anywhere nearly as much strength, only about -10-20% and it started at a higher-strength than 6061 initially as well.

I wouldn't attempt to build an alloy bike as a first project. Do a couple of steel ones first, get down brazing and welding on steel. Then practice welding beer-cans together with TIG and once you've mastered that, alloy bikes would be next. :)

I can already weld steel. Have done pooloads of that from shipping containers to furniture, from Massive MIG to Oxy and Arc welding... Was hoping to give Aluminium a go if I was going to have to buy a new machine anyway...

Those TIG machines are damn expensive though... Just looked into them.

Yeah they can be pricey. Bikes don't need much though, a basic Miller TIG welder will do. You'll need AC and high-frequency for best results on alloy. Welding thin-wall tubing is completely different than anything else. Due to their delicate nature, attention to little details makes a big difference. Such as flushing the tubing with gas as you weld. Even pre-filling the tubing beforehand makes a difference as well. :)

Kinda hard to get double-butted 7005 alloy in just the right lengths though. I ended up trimming them externally on a lathe.

Well, "heat-treatment" is a general term that encompasses many stages. Both 6000 and 7000-series typically have a 2-stage process of quenching and artificial-aging. The 6061 loses a signficant amoun to of its strength -40-60% in the HAZ. The final artificial-aging is required afterwards in order to restore it's T6 strength. Although work-hardening will build this back up over time with use. The 7005 on the other hand doesn't lose anywhere nearly as much strength, only about -10-20% and it started at a higher-strength than 6061 initially as well.

I wouldn't attempt to build an alloy bike as a first project. Do a couple of steel ones first, get down brazing and welding on steel. Then practice welding beer-cans together with TIG and once you've mastered that, alloy bikes would be next. :)


I hope that was for The OP's benefit ;)

The degree of strength lost is based upon the as-supplied mechanical properties, depend on working and thermal treatment. 7005 does lose a lot more than you think... But it's much stronger to start with, given.

Anyway, +1 on the practise techniques :)

Such as flushing the tubing with gas as you weld.
That is what's known as "purging":rolleyes:

Even pre-filling the tubing beforehand makes a difference as well. :)
Since the most common way of "purging" is using a "Y" connector from the very tank that running through the gas line that would be recommendable. If you don't properly prepare material & fully purge interior beforehand it is a waste of setup time & preferred gas.

Back purging is always a good idea. I don't think it's required for aluminum though. Stainless is one metal where no BP is a really bad thing. But most all aluminum is welded without BP. Most aluminum frames are done in production settings and I can't see them BPing it, but maybe they do. The main issue with Aluminum is that it oxydizes instantly but only superficially, so you need an AC welder properly set up to blow the crud off the weld while you run, but unlike steel it isn't going to be burning in from the back since it oxydizes so fast you can't purge it fast enough to do any good, but once it skins over it's about as bad as it gets. It oxydizes so fast the show is over the moment you stopped shaping the joint.

But most all aluminum is welded without BP.
Idon't know what shop you work at but at the one i do we do it regularly.

you need an AC welder properly set up to blow the crud off the weld while you run
FYI, it doesn't "blow it off" it more like sucks the "oxides" up on to the tungsten.
i run both reverse & straight polarities on aluminum & most of the time when i run ac i run the sync all the way up(more on the straight polarity cycle) on syncrowave Millers.