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Heat Treatable Alloys
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This P-51 wing tip is made from 5052, and the gas weld is plainly visible. |
The temper is indicated by a T designation marked on the panel (think hardening by Thermal) and is also
indicated in the paperwork (the 'certs" required for commercial and general aviation). Fully annealed or
having no temper is designated as 0. Other common tempers are indicated in increasing order as T3, T4
and T6. While this means that the ultimate yield strength is only achieved by heat treatment, or that a
predictable strength is achieved by such tempering, the material also work hardens. As a result it may
require annealing or several annealings in order to form a complex part. The finished part is then heat
treated to bring it up to strength. Simple parts, however, may be formed in the full-hard or T-6
condition.
Duralumin, the first heat treatable aluminum, was developed by a German engineer in Durn, Germany. It was
designated as 17S (2017) and was specifically developed for dirigible frames.
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The 2024 skin is rapidly stretched in the air-shaping hammer with the use of a rubber die. The panel is
roughed into shape in this manner, saving time and effort over the use of the mallet and bag. |
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Later known as Dural, 17S eventually grew into the Duralumin family, comprising aluminum alloys having a
minimum content of 4% copper. The family included 2017 (17S), 2024 (24S), 2014 (14S), 2018 (18S) and
2025
(25
5).
2024-(245) contains 4.5% copper, 0.6% manganese, and 1.5% magnesium. It is a ternary alloy having a yield
strength from 11 KSI to 66K51, depending on temper. 2024 has long been used for stringers, longerons,
spars, bulkheads, carry-throughs, stressed skins, tubing and trusses. Non-structural parts made from 2024
such as fairings, cowlings, wheel pants and wing tips are to be found on some military and light planes
having high production runs. In these cases, either economics or extreme duty warrants such application,
and the tempers will be from T3 to T6. In the T3 or T4 temper, 2024 has limited formability and very
limited reparability. At the maximum yield strength (T6 temper), workability is minimal and so is mostly
limited to the radius bent panels found on wing leading edges or fuselages, or the low crowns for some
skins. It has extremely limited reparability in the T~ condition and also has limited corrosion
resistance, unless ALCLAD.
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When roughed to shape and checked to either station buck or original panel for loose fit, the panel is then
deemed suitable for smoothing and working to a close fit. While the identifying marks may survive
the hammer punishment, they don't survive a solvent wash, so there is no way to accurately
identify the alloy of an unmarked sheet unless you have access to a very expensive tester. |
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2024 is not considered weldable. Even though a pretty weld can be made, the weld will have no useful
strength. ALCLAD sheet are commonly spot-welded, but it is the cladding that is actually being welded.
It gradually work hardens in the 0 condition, perhaps a bit faster than 3003. Annealing temperatures
from 650
0
F to 775
0
F are required, depending on the degree of softness required, and whether work hardening or precipitation
hardening is being removed. 2024 is not recommended for torch annealing, as it has a soaking time at
temperature necessary for recrystallization, which varies from two to three hours, with a cooling rate
at 50
0
F per hour down to 500
0
F the balance being non-critical.
6061 -(61 S) has a nominal composition of 1.0% magnesium, 0.6% silicon, and 0.25% each of chromium and
copper. This quaternary alloy, having a yield strength of from 8KSI to 37K51, is similar in application
to
5052.
It has limited structural use and finds application in wing ribs, ducting, tubing, tanks, machined fittings,
fairings, framework and baffles. 6061 work hardens like 3003 and anneals similarly to 2024. It does have
some workability in the T3 and T4 tempers, though. A partial anneal, recovered over time with natural
aging, is possible with the torch, and may be followed by air quenching.
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