How to evaluate different alloy types used for bicycle frames? Only certain alloys are used for bicycle frames besides other materials like carbon fibres (at least for the mass market). I often stumble over aluminium alloy 6061 and 7005 (and I think I saw 8004 once). It doesn't seem feasible to say that one is better than the other because they have different criteria like density/weight and break resistance. Increasing those criteria towards ideal values for biking (less weight, more break resistance) seems adverse, i.e. engineers have to make a decision between the material trade-offs when selecting it and I as buyer need to recognize their intention to make the right choice. There might be criteria which have non-obvious impact like better processing of a more dense alloy allows to construct thinner frames so that maybe an alloy with less density systematically results in a more heavy bike.

I'm excluding the price from the list of criteria to make the question more answerable.

I'm happy with a reasonable simplification like x always allows to build thinner frames than y while providing the same stability and is thus always light and preferable because of a. Or a sorting of relevance of criteria and a description of trade-offs between them, e.g. it seems that certain alloys bend before they break whereas others break immediately, but then under heavier impact only. I don't even know which one I want (I assume I can't bend the frame back after a crash, so is it a question of security? Can the broken frame be welded back together? Those a just example questions…)

I'm not interested in the criteria the manufacturer uses to select the material for the frame, like weldability, price during a certain period of economic up and down, etc, unless it has impact on the quality of the bike.

I hope you don't recognize this as primarily opinion based. I'm looking for a listing of criteria with relevance to using the bike of which the frame is a part - besides a ton of other criteria which is important in engineering, but irrelevant for biking (like the melting point).

There are certain clues, like "used primarily for less expensive bicycle frames" found in a Wikipedia article on the 7005 alloy, and of course a ton of unverified information in horrifying forums.

I can add an advertisement-like reference for the use of the alloy for different bike frames.
For all I know, between those two aluminium alloys there's no rule of thumb that could say: "this one makes stronger frame" (or lighter with the same strength, whichever way you look at it).

From a mechanical point of view 7005 is stronger, but also more brittle - and that needs to be taken into account when designing tubes and the whole frame. 

So you can build relatively light and strong frames from both these alloys, without one allowing an ultimately supperior frame compared to the other (in terms of better strength, durability, lower mass etc).

The material of choice does boil down to price - in terms of what equipment a factory has at their disposal to machine the frames, since the two materials require different processes for heat treatment, aging etc, cost of material (alloy) and so on. Then it's up to the engineer to design the frame with the chosen material.

Design challenges are making frames as light with enough strength and resistance to impacts. Butted tubes, making them stronger where it's needed and lighter where the strength is not cruicial, distributing the loads along the tube length, away from critical areas is a way to do that. Exactly how much material and where has been perfected in the previous decades, didn't see much breakthrough lately, probably won't in the future as well. It's gotten as good as it gets, just copy/paste the design, and let the poorly paid Chinese people build it for you (Chinese factory equipment could dictate the alloy of choice, whether they work with 6061, or 7005).

Whichever material was chosen, the rest is up to marketing - to say that 7005 is stronger, or that 6061 is more elastic, or whatever. 

What makes the difference is the quality of frame design and how well that is built (executed). The more time and money you spend on testing (with good engineers), the lighter can a frame be, still having enough strength (and durability). Though, like I've said, I see no breakthroughs in the past decade in these terms.

As a buyer, with marketing being put in front of engineering for the past decades, it's really hard to say which is better when choosing a frame, regardless of the material. On the plus side, metal frame designs (especially steel and aluminium ones) have been tried, tested and perfected, so you can hardly go wrong, especially if choosing a renowned manufacturer. Having said that, the cheaper, "no-name" frames, while being much heavier than needed, are often even stronger and more durable, they just "weigh a ton".

For welding aluminium frame - I don't think that you can expect to weld-repair an alu frame to be durable, without aging or heat treatment afterwards, whichever alloy was chosen, or at least adding a lot of extra weight. 7005 requres no heat treatment after welding, but I don't think that the cost of a durable weld repair is as good as it is for steel. Someone with more knowledge on repairing alu frams could probably answer that better.