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Back to Part 2
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This P51 stabilizer tip has, from left to right, four soot marks going from pure acetylene to lightly coated.
If applied to their appropriate sheet thicknesses, they would correspond to .063" and greater,
.050 - .040", .032", and .025".
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Once heated, then it's time to quench. Splash with cold water, slosh with a drippy rag, blow with air nozzle,
or blast away with a CO2 fire extinguisher.
5052 can be a bit fussy about being torch annealed because this alloy oxidizes easily at elevated
temperatures, leaving dark splotches of oxidized metal on the surface.
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Melty spot caused by overheating, and it is rough to the touch. Causes: nitpicking soot, tip too large, flame
too hot or simple inattention. Appearance is similar on all alloys. |
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Now, about those
heat-treatable materials
…
There are certain shop-worn traditions, found mostly in commercial aviation facilities, which allow for
certain ``bending of the rules" in regard to the torch annealing of T-3, T-4, T-6 etc. tempered aluminum
alloys. Please note here that the criteria for annealing heat-treatable metals of all kinds are fairly
exact, and quite scientific.
6061 is perhaps the best place to start because it is the most forgiving of transgressions. 6061 can be
partially
annealed with the torch, but to do so means that the resultant temper, when it naturally age hardens back
again, will only be very roughly approximate. Soot the area to be annealed as we have discussed, or use a
crayon to bring it up to 650F and quench with cold
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Ignoring the melty spot, and then working it with mallet and shotbag only stresses the area, and sooo…crack!
If the alloy is weldable then it is only a quick fix, and you are off and running again. If it's
not weldable, then…scrap! |
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water. Give it a bend test with your fingers in both places, the hard and the annealed. See? Now you have a
few hours to work it before it age hardens again. 2024? Don't do it! I know some old-timers do it, and
maybe you do too, but have you ever gotten one to admit that this method is absolutely foolproof? Never
a tiny crack? Neither have I, and for that reason I strongly caution against it. The Aluminum
Association is against it, and so are Reynolds and Alcoa. ``Sometimes" shouldn't indicate a sound method
for anyone. If annealing 7075, 2024, 2014, 2017 etc. is required, then do it in an oven where the
temperature is accurate, the soaking time is sufficient, and the quench is performed with the specified
rapidity.
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This Pilatus stabilizer tip was heavily crunched and then replaced, leaving it to be used as a demonstrator
for workshop annealing instruction. The torch tip indicates one of the slightly overheated
spots. These are not detrimental to integrity in this case, as they are only slightly
discolored, not melted, but smooth to the touch. Such is the nature of 5052 that while these
marks are fine if left alone, they might not stand up too well to really heavy hammering. (end
caption) |
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Both students and craftsmen alike puzzle over how some of these danged aluminums can get softened in the oven
and then harden all by themselves by natural aging one time, and yet at other times they can come out of
the oven and sit on the shelf for years and still be soft! Boy, could I have fun with that one: It's the
amount of calcium carbonate in the quenching water, you see. Softer water makes for softer aluminum. I
know, straight answers only, please!
Without delving deeply into the fascinating field of metallurgy theory and practice, let's just clear up the
basics:
Heat treatable aluminums are
hardened
to a specified strength simply by heating to the proper temperature, (820-1010F, depending on alloy),
holding for a complete thermal soaking, (minutes to hours, depending on mass and size of
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Originally this blank Cessna 195 pant would have been clad 2024, T6, .032". Its method of manufacture would
have been pressing immediately after quenching, then aging to full hardness. This replica,
however, was hand-formed of 5 separate pieces, butt-welded with the oxy-acetylene torch and then
finished to a high polish – all with no annealing! (More about this stuff later) |
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load), and then
immediately
quenching in different solutions of varying temperatures (100F max.). After quenching, the
precipitation hardening
process starts, whereby the metal ages naturally to achieve its designed strength.
When the alloys are ``as quenched" they are nearly as soft as fully annealed and may be pressed or drawn into
shape in that condition right after quenching. If time or design criteria dictate though, artificial
aging is engaged by oven heating the parts (240-380F) for from 5 to 37 hrs, which is much faster and more
specific than the days or weeks required for natural age hardening. Sub-freezing temperatures will
suspend this aging however, hence the name ``icebox" rivets.
Annealing
heat-treatable aluminum alloys is accomplished again by heating and cooling but in a different procedure.
The metal is heated (650-825F), held for a good soak, and then cooled very accurately at the rate of 50F
per hour until the temperature is 400-500F, depending on the alloy, and then the rate of cooling is
unimportant. Different methods of annealing are required for different aspects of fabrication, such as
warpage and distortion control, secondary pressforming operations, deep drawing, etc.
Stress-relief annealing, partial annealing, and full annealing employ the different temperatures and cooling
rates specific to each heat treatable alloy. Unlike the non-heat treatable aluminum alloys, these heat
treatable ones do not anneal as fully the second or third time through and so some loss of workability
must be expected.
(Note: For learning more about the aircraft family of aluminums, their applications, annealing, and the basic
forming methods of stretching, shrinking and planishing, EAA offers a 130-minute instructional video from
TM Technologies entitled, ``Shaping Aluminum Wheelpants".)
References:
Metals Handbook, 8
th
Ed., American Society for Metals
Aluminum, Fabrication and Finishing, American Society for Metals
Metallurgy, American Technical Society
Standard Handbook for Mechanical Engineers, 8
th
Ed.
Thanks to: Alcoa, Reynolds, and The Aluminum Association
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