[sub6]

Okay, I always thought that 6000 series Al needs to be heat-treated after welding, whereas 7000 series stuff doesn't. And that that was the only significant difference between the two.

Well, I was looking at one of the stickers on my Brodie the other day, and noticed that it says "7000 Series Heat-Treated, Butted Tubing," or something along those lines. So, my question is, why heat-treat 7000-series stuff? Is there some advantage even though it's not "necessary"?? What gives?

 

[EBasil]

The "difference" between the 6000 and 7000 series aluminums isn't really the need for heat-treating. The "need" for heat-treating is really more prevalent in the design of steel alloys, and almost all aluminum frames are heat treated, usually to "T-6". The heat treatment just creates a unified structure and hardness in the entire frame, correcting alterations caused when you weld or braze the joints. Technically, it makes the whole thing harder than it was before you weld it, too.

The real differences between the different aluminum alloys used in bike frames are the ingredients in the alloy. Different alloys may be more ductable or easier to weld/braze or have higher tensile strength per pound, with trade-offs in fatigue tolerace or resistance to corrosion.

The "Cu92" tubes used by Giant are 2024 Series Aluminum, and get heat treated to T-6. The alloy recipe is three pinches of copper and a pile of raw aluminum.

The 6061 tubes used by damn near everyone are...6061 aluminum and get treated to T-6. This recipe is one small pinch copper, another of silicon, another of Magnesium and two of Chromium. This is the most prevalent tubing because it's easy to weld and has great performance characteristics.

The Easton 7000 series tubes (ProGram, Elite, etc...) are 7005 aluminum, treated to T-6. That recipe is a pinch of Manganese, another of Magnesium, two of Zinc and another of Chromium. This alloy has a higher tensile strength than 6061, but is harder to weld/braze and is more susceptible to corrosion.

I bet that was more than you cared about, but the momentum kept me going...

 

[sub6]

Originally posted by EBasil
I bet that was more than you cared about, but the momentum kept me going...

Actually, that was a great answer and I thank you very much for it!

What, exactly, does "T-6" entail? Is that just a temperature/time standard, or something else?

 

[EBasil]

Yes, in simplest terms, T-6 is just a standard for a set of procedures to heat,cool and age the aluminum at particular temperatures and times.

Aluminum heat treatment is basically this:

1) superheating of the alloy, often in a solution or dip-brazing tank (depending on how you're joining the tubes). This softens the metal, lets the grain structure loosen up into something ready to get finer.
2) quench cooling: into a tank of cool, recirculated water. Rapid cooling prevents all the leetle atoms that were excited and loose from reforming into random grain structure, and you get a finer, more consistent (stronger) atomic bond.
3) Aging: heating in an oven up to a particular heat, over a particular time and cooling in the same oven. This hardens the alloy and "sets" the grain structure, almost like kilning clay in a perverse sense. The hotter you age the metal, the harder it will get, to a point, and if we over-age, it starts getting weaker.

At least, that's how I read it off of the Bazooka Joe comic on heat treating.

By the way, there's no T-7 that's "one step better than T-6"... a favorite old bike shop line.

 

[EBasil]

Yes, it's a precipitation hardening process, and I apologize if you don't get the loose analogy to kilning ceramic. Of course, the mechanism is very different...I was shooting at effect.

You may not need to post from texts: I think we've covered the concept deep enough to answer the original question. I'm out of the career where I used to dip-n-bake, so if I've slightly mistated something, I'll defer to you, but remain sure I'm correct on the major points.

Keep the rubber side dirty!

 

[ohio]

All of the above is correct as far as I know, I just wanted to add that in a lot of companies do NOT heat treat 7000 series aluminum after welding. They (think they) can get away with this because 7075 has a significantly higher strength than 6061, so that even the heat-affected zones are "strong enough" post-weld. Then they don't have to deal with alignment issues that frames sometimes have after heat-treating. It's a trade off, in that you use more expensive tubes to avoid an expensive heat-treating process... and you may or may not end up with a stronger frame.

My opinion (again OPINION) is that a 6061 frame that has been heat-treated after welding is going to be stronger than a 7075 frame that has not.

 

[heypip]

Originally posted by ohio
[B
My opinion (again OPINION) is that a 6061 frame that has been heat-treated after welding is going to be stronger than a 7075 frame that has not. [/B]

7075 is unweldable. (well practicly) 7000 series frames are made from 7004, 7005, 7029 and a few more obscure alloys. in north america, almost all 7000 series bikes are 7005. easton uses this alloy too.

t6 means: solution treated and artificially aged.

solution treating is like sugar in a hot cup of tea. when the tea is hot, the sugar will be disolved into solution to a certain volume based on temperature. when the tea cools, the sugar precipitates out of solution and forms christals on the bottom of the cup.

aluminum will only disolve a micro ammount of the alloying ellements in it at room temperature, but at 1000f it will disolve quite a lot. rapid cooling from 1000f, creates a fully disolved, unstable, supersaturated solution of alloying elements in the aluminum matrix. if you look at it under a microscope at this phase, it looks just like the tea. you can't see the disolved alloying elements in the aluminum, just as you can't see the disolved sugar in the tea.

the supersaturated solution tubes are then aged either naturally, at room temperature or artificially at around 300f. the unstable supersaturated solution wants to become stable and over time, the alloying constituents precipitate at the grain boundaries, reducing slip plane motion and hardening and strengthening the metal. by reheating (artifical ageing) the metal, to about 300f, the ageing process can be stabalized in a day or so, instead of months to years for natural ageing.

with 6000 series, like 6061, after welding in the t4 condition, you must do a formal solution treatment. (1000f for an hour and quenched to room temp in water, mist or glycol very rapidly, in less than 5 seconds in some instances). the frame comes out butter soft, so it needs to be jigged to support it during solution treatment. this is expensive. the frame also needs to be alligned, within minutes after solution treatment. if you don't have your own heat tretment facility, this can be tough. the thermal shock of the water quench induces residual stress into the tubing due to their differing cooling rates. to me, the residual stress issue is the best reason not building closed triangle structures out of 6061.

alloy 7005 on the other hand can be welded in the t4 or t6 condition. because of its chemistry, it will form precipitates just from the normal cooling rate after welding. therefore it requires no formal solution treatment after welding. thus no 1000f and high speed quench. thus reduced residual stress. of course it still needs ageing (a dual step process) but its only at <300f and theres no quench afterwards.

7005 has been efectivly banned from automotive, archetecural, and aerospace applications in the states, due to its propensity towards stress corrosion cracking(scc). that's why almost all bikes in the states are made of 6061, you can't get 7005 tubes in north america. since the alloy was developed, back in the 60's, many advances in age processing have been made and there's no threat of scc in alloy 7005 anymore. the first 7005 bikes used to split down the head tubes when they were in ship containers coming over from japan. gary fisher knows a thing or 2 about that.

pip

 

[Surly]

Wow--great reply! I've hung out a bit over a bladeforums.com and have learned a fair bit about steel, but know almost nothing about aluminum. Thanks for the primer!