I know I'm a little late to the party, but I'm a metallurgical engineer, hopefully I can help some. For what it's worth, my job is Design Support (materials and processing selection) and mechanical test (determination of mechanical properties) in an aerospace company, so this is pretty much "up my alley" so to say. I don't know if anything I'll say is relevant at all, but I appreciate it when people who know what they're talking about explain things, and I know I'll be asking questions in the future, so maybe I can help someone else out first....various questions about metallurgy and a request for someone who knows about it...
I heard 2024, 5052, 6061, and 3003 mentioned. A quick description of each in a bit, a description of strengthening mechanisms first, heat treat tempers second.
Strengthing by coldworking - if you plastically deform certain materials (deform them past them just springing back), the crystal lattice of the metal (metals have crystal-like structures) gets disrupted. If you go to further deform the metal, it's harder for atoms to move past each other because the crystal structure is messed up, so your metal is now stronger.
Strengthening by precipitation hardening - in certain metals, there are constituents that form 2nd and 3rd phases...for example, the addition of magnesium and silicon in 6061 for MgSi compounds, which are hard materials. These materials prevent the metal from deforming by making it difficult to move the atoms in the crystal lattice, the same as coldworking. How this is typically done: 1) solution heat treat the metal. This means you heat the metal up to a certain temperature below the melting point, and those 2nd and 3rd phases dissolve into the matrix. You then quench it (cool it really quickly), which keeps the alloying elements in solution, so that they can't come out. Think about dissolving salt in water...you can dissolve more salt if you heat it up...if you cool it carefully, it won't come out of solution and you now have a "super-saturated" solution of salt water. Same idea here. 2a) you can "naturally age" the metal, which is just letting it sit on the shelf for a month or two. When you do this, some of the super-saturated constituents precipitate out as those 2nd and 3rd phases. 2b) you can "artificially age" it, which is where you heat up the metal to cause the precipitation reaction to go faster. It also completes the reaction more than naturally aging, so it's usually stronger.
Cold working before aging can also improve the response of the material (in 2024 only)
Common tempers for precipitation-hardenable alloys (2024 and 6061 in our discussion):
O - annealed - dead soft
-T3 - solution heat treated, cold worked, and naturally aged. (2024 only)
-T4 - solution heat treated and naturally aged (2024 and 6061)
-T6 - solution heat treated and artificially aged (2024 and 6061)
-T7 - solution heat treated and artificially overaged (age past the point of max strength, so it's weaker, but usually better with things like corrosion resistance) (2024 and 6061)
-T8 - solution heat treated, cold worked, artificially aged (2024 only)
Common tempers for coldworked alloys (5052 and 3003 in this discussion):
Lead with -Hxx. Quick description is that, the higher the xx number, the higher amount of cold working (and typically the stronger the resulting alloy).
2024 - the "typical" high strength aluminum used in aircraft structures. If you build a GA experimental from aluminum, it's probably from 2024 doe to the excellent strength to weight ratio. Sheet is available in -O, -T3, -T4, -T6, -T7, and -T8 conditions.
Properties (assuming 0.030" thickness): Temper, Ultimate strength (where it breaks), yield strength (when it permanently deforms), and elongation (how much will it stretch before failure):
-O 14ksi 32ksi 12% (source - Aerospace Structural Metals Handbook - ASMH) ksi = thousands of pounds per square inch
-T3 64ksi 47ksi 12% (source for all of the others - MMPDS-04, formerly Mil-HDBK-5)
-T4 62ksi 40ksi 12%
-T6 62ksi 50ksi 5%
-T7 60ksi 40ksi 5%
-T8 67ksi 58ksi 5%
As you can see, artificially aging 2024 (-T6, -T7, -T8) increases the strength quite a bit, at the expense of elongation.
For what it's worth, note that the -O and -T3 conditions have similar elongations (other sources, ie wikipedia, have -O at 10-25% and -T3 at 10-15%, which is also believable). So, in the plastic forming that you'd see in the making your channels in the aileron skins should would out just as well in either -O or -T3, from an "inducing cracking" point of view. HOWEVER, -T3 is stiffer in a sense that because the yield strength is higher, it will tend to "spring back" as you form it, so forming it is more difficult. I'd prefer the round dents because sharp corners will tend to crack more than the round ones due to the high elongations required at the outside of the sharp corner.
For what it's worth, 2024 has pretty bad corrosion resistance, so it's frequently "clad" or "Alclad" - short for "aluminum clad". Pure aluminum has great corrosion resistance, so it provides an outer "layer" of better material.
6061 - the typical "general purpose" aluminum alloy. Used less in sheet form but more in bar form. Almost always used in -T6 form.
Properties:
-O 18ksi 8ksi 25% (wikipedia)
-T4 30ksi 16ksi 14% (MMPDS)
-T6 42ksi 36ksi 8% (MMPDS)
6061 has reasonable corrosion resistance, is weldable, formable, and in general a really good alloy.
5052 - a highly corrosion resistant cold working alloy, also very good at being welded.
Properties:
-O 25ksi 9.5ksi 14%
-H32 31ksi 23ksi 4%
-H34 34ksi 26ksi 3%
-H36 37ksi 29ksi 2%
-H38 39ksi 32ksi 2%
Comments on 5052 - the only temper that really works for forming is the -O temper. However, this one work hardens a LOT (and notice the dramatic reduction in elongation as you work harden). If I were picking an alloy that needed to be coldworked into a particular form, I would NOT pick 5052.
3003 - a really "cheap" and "dirty" alloy, basically just pure aluminum with a couple of different trace elements that strengthen it some over pure aluminum.
All properties from http://www.wilkinsonsteel.com, nothing in ASMH or MMPDS about it. It's not typically used as a structural alloy.
-O 16ksi 6ksi 30%
-H12 19ksi 18ksi 10%
-H14 22ksi 21ksi 8%
-H16 26ksi 25ksi 5%
-H18 29ksi 27ksi 4%
First, the H12 condition is equivalent to the H32 condition for 5052, the 3 indicates that they're more worried about corrosion on the 5052, but the 2 for the second digit means the same level of coldworking. If we look at this, the elongation numbers stay a lot higher for longer compared to 5052, but the strength numbers are lower. So, if one were not worried about strength, 3003 would be a superior choice compared to the 5052 sheet because of the elongation difference.
I don't know what's speced on the plans as I don't have them yet. My one comment about what I see in the numbers is that 6061-T4 looks like it has a nice "combination" of properties. Middle of the road strength, good elongation, good corrosion resistance, and it's actually available in sheet form from aircraft spruce.
One other alloy not mentioned was 7075. It's a heat treatable alloy that is among the highest strength aluminum alloys available.
-O - I've never seen it available for purchase
-T6 75ksi 67ksi 7%
-T73 67ksi 55ksi 8%
-T6 temper has horrible stress corrosion cracking resistance, so you want the -T73 condition, which is a special heat treat condition to improve stress corrosion cracking and general corrosion resistance.
That said, if you want high strength and 8% elongation gets you there, -T73 is pretty fantastic.
Sorry for the mini dissertation, I hope someone gets something out of it.
