Warbird Information Exchange

What specific grades & thicknesses of aluminum were used on the major production aircraft (and the given sheet thickness, billets, bloom, tube, rods, etc from Alcoa or any other supplier) in the U.S. from the 1930's-1950's?

6061 is commonly mentioned as a mass-produced alloy, but what specific variations are correct per a given type? (Ex: P-38, B-36, B-47, etc)

Further, was the Alclad used in the 1930's on the Martin Clipper flying boats any more or less corrosion resistant than the Alclad on B-29's? Many of the early jet-age aircraft appeared in natural metal finishes, to what extent were they really polished versus being a different grade of exterior aluminum?

What about steel? What alloys (+ any hardening treatments, quenches, annealing, etc) were used on engine mounts, struts, tubing in fuselages (for fabric covered aircraft)?

Detailed explanations will be appreciated,
I hope this helps bring an awareness to using the appropriate material when re-skinning an aircraft - or at least understanding the strengths & limitations of materials on aircraft (especially those exposed to the elements).

Most aircraft aluminum alloys of that period were 2024-t3, post WWII it was a combination of 2024-T3 or 7075-T6

Typically, most aircraft structure for that time period was of the 24 series. The alloy numbering system has modified that to what we now know as 2024. Also, heat treat numbering has also changed significantly. O, 1/4H, 1/2H, ect. have been converted to a T series numbering system. By decoding the numbering, you can determine the heat treat process that was used. Most sheet goods, used in the construction of aircraft are T3 heat treat. Form something that cannot be formed in in the T3 condition, it will be formed in the T0 condition and then heat treated to what is now known as the T4 condition. T3 material can be annealed, worked then allowed to age back to it's treated condition, but it will not be T3, but T42, and have nearly the same strength numbers. There is lots of info published on working 2024, and one should be fairly familiar with the information and practice some before jumping in on a part for flight.

There are other alloys used in the production of parts too, but they can vary and are tailored to forgings, castings and other fabrication methods, and will be specified by the individual part number drawing.

Material thickness is going to be specified by the design of the part. I've worked parts from material as thin as .016", to blanks from 8" custom rolled plate stock. The designer is going to use whatever thickness he needs to meet the design criteria and fatigue life. Often, thou, for manufacturing convenience, they may step up a thickness to preclude having to have special mill runs of materials. I.e. if the design criteria can be met with .014" sheet goods, .016" would be used since it's a standard thickness.

Post WW2, 2024, 6061 and 7075 have all been used in aircraft construction in significant quantities. Again, the designer will specify the alloy based on the design criteria and what alloy will meet it all and at the lightest fabricated weight.

Moving on to steel. As the war broke out, 1025 was the major steel alloy in use. As 4130 became more and more available, designers moved to it, as the increase in strength for thinner shapes helped to decrease the structural weight of the aircraft. Since the late 50's, when Lockheed had a huge part in developing forgable Titanium alloys, Ti has rapidly become more prevalent in aircraft structure as well as engine parts.

If you you are looking to fabricate parts, a couple of things to note. Your best source of information is going to be the individual part drawing for what you want to fabricate. Next best thing will be the SRM, or Structural Repair Manual. The SRM will give you the material usage on the part, or will give you a material applicable for a field repair, if any. Last method is use a competent, experienced structural/design engineer to make the proper determination. When in doubt, don't guess, unless you are going to doing static only parts. From experience, most of us that work on WW2 aircraft know current substitutions or have access to the data, but once you get into aircraft of the jet age, the data is less and less. Get into something that goes transsonic and beyond, it becomes very specialized and extremely critical that you are correct.

Thank each of you for your responses! Really, really like the details

Generally speaking, is the 7075 going to retain a polished look longer than the 2024 or 6061 varieties due to it's higher zinc content? Or is the opposite true?

Really curious in a hypothetical scenario what would be the best suited aluminum for flying boats (especially those in polished aluminum).

Notice during the inter-war years, the Sikorsky amphibians, General Aviation/Fokker PJ, early PBY's, etc appear to have largely unpainted fuselages -

Assuming the 2024 series was used (with 4.3-4.5% copper content) on the Martin China Clippers, PBY, Sikorsky Baby Clippers, Boeing 314 flying boats, etc - would there be any corrosion advantage in skinning them with a different grade of aluminum (at a consistently similar/same thickness)?

Based on my own experience with old flyingboats, the Grumman G-21 series in particular, I very much doubt that any of the aircraft that you think were "unpainted" were actually unpainted. It is much more likely that they were painted silver or light gray.

In addition to being fabricated mostly of 24ST aka 2024-T3 Aluminum (bare not Alclad*) most Grumman products built for the Navy, including fighters and torpedo bombers, were for the most part also treated with Anodize. Before there were multiple forms of Anodize and it became known as Type I, the process used by Grumman was a chromic acid Anodize that produced a dull, medium gray finish on the aluminum that was much tougher than Alodine or paint, both of which scratch easily by comparison - but for the most part too they were fully painted as well. You cannot have too much protection in a salt water environment.

(*Note: for any given thickness of aluminum sheet, "bare" is stronger than Alclad because 2.5 - 5% of the thickness of any Alclad sheet is non-structural, 1100 grade, un-alloyed, pure aluminum.)

These days with the EPA restrictions on the use of anything to do with chromium because it is both toxic and carcinogenic, Type II Anodize is more common and I believe it is used on the Canadair CL-215 and CL-415 waterbombers and many non-seaplanes as well. Instead of chromic acid the Type II process, which supposedly meets the same MIL-spec standards as Type I, uses sulfuric acid. Still nasty but not as nasty as the chromic acid. It (I am told) creates a light green finish on the unpainted aluminum that appears similar to a thin, misted coat of zinc chromate (no longer readily available for the exact same reason) or zinc phosphate primer.

Furthermore, in terms of "re-skinning" a certificated aircraft (if that's where you're going with this line of inquiry) I really believe that substituting 6061 or 7075 aluminum in an aircraft "design" that was originally approved using 2024 for example would be a whopping "major alteration" that would require an encyclopedia's worth of engineering documentation and approval. Essentially every single load path would have to be re-evaluated. That's just a guess, but I have never heard of something like that being done "wholesale" across an entire aircraft or via any kind of "blanket" approval.

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