Framebuilders - 7075 Aluminum Problems?

Although I'm not a framebuilder myself, I would like to ask you people in the know something about 7075 aluminum.

7075 aluminum, in the T6 temper grade, is one of the strongest aluminum alloys and is used frequently in aerospace engineering. Supposedly, the main problem that prevents it from being a popular material for bike frames is its problem with weld cracks. Thus, 7005 is preferred. The 7075 material is otherwise used quite frequently in chain rings, which do not require welding and can be forged/CNC-machined.

But I've seen some bikes (including here in BF) that have frames made from 7075, so how are they joined? The frames don't look brazed/lugged.

7075 can't be welded reliably so bike frames made with the stuff are bonded (glued). No idea what specific frames you are referring to so can't comment about what you've seen.

7075 can't be welded reliably so bike frames made with the stuff are bonded (glued). No idea what specific frames you are referring to so can't comment about what you've seen.

I haven't heard of this before - is it epoxy?

Funny I was searching welding 7075 topics and almost all of the answers were drawn from a single source. Rather like a conspiracy where everyone questioned has the same exact answer. Key issue is whether a stronger, much more expensive alloy would be waranted. At a given weight, stiffness, and dent resistence budget, would a higher tensile alloy yield an improved frame. If not, people will just compare the charts for the two aloys and latch on to the simplest explanation, such as unweldability. 7075 is more expensive and post weld heat treating ads to the cost. Interesting that there appear to be a number of welding processes like friction stir welding that work fine with 7075, if too expensive for our use.

Too expensive and not appropriate for bicycle frames.

7075 is strong but it is also prone to cracking compared to a more ductile alloy like 6061. Back in my weight weenie days I had a set of TNT hubs (anyone remember TNT?). The hub shells were made from 7075 and they cracked at the flange spoke holes. The replacement flanges were machined from 6061, which not as strong, was at least more ductile.

I've suspected stir-welding to work as well, but the process would have to be very difficult to do on a joint rather than two flat surfaces.

Back in the old days my campy pedals with 7075 alloy cages keep cracking. I tried TIG welding the crack, which then cracked minutes after welding.

7005 is very weldable...7075 isn't....to many sevens cause the problem