The Family of Aircraft Aluminums – continued
See TM Technologies
Aluminum Gas Welding System
By Kent White
This article appeared in Sport Aviation magazine, November 1999, and is a continuation of the article
The Family of Aircraft Aluminum Alloys
Note: This version from the November issue of Sport Aviation is the original UNEDITED version. Due to
major changes in staff at Sport Aviation, the published version had severe accuracy edits.
From the previous article we learned that working metals into new shapes increases their strength and
hardness, resulting in a work-strengthened material with new shape. While simple parts, like low-crowned
skins, instrument panels, tanks, and some side cowlings can be formed easily in the required temper, the
complex and deeply formed may not. These complex beauties need sustained heavy working. To remove this
work hardness from the
aluminum alloys is as
simple as elevating the metal to the proper temperature, holding for the appropriate length of time,
quenching, and voila! Soft metal (with a distinct
of cracks or melted holes) is now ready to do your bidding. What? You get melty spots, cracks, and unhappy
parts with your present method? Read on, dear craftsmen, as we introduce you to a highly reliable method
designed to give you successful results.
Metal - An opaque crystalline solid, having high thermal and electrical conductivity, and the ability to flow
Recrystallization –After all metal crystals have been dissolved by heating enough to lose its structural
strength, the metal temperature then falls, allowing the crystals to re-form.
Cooling metal at a specific rate, with a given medium.
Precipitation hardening – As the quenched alloy ages, a new material precipitates out of the metallic crystal
lattice, filling in abutting spaces, and increasing hardness.
aluminum alloys may be torch-annealed many times to achieve the necessary shape. Fairings and fillets in
particular require such heavy working and such great changes in shape that frequent annealings are quite
necessary. Both proper heat application and temperature measurement are important. For 1100 the proper
annealing temperature is 650F, for 3003, 775F, and for 5052 it is 650F.
The amount of
time at temperature
for these particular alloys must only be enough to thoroughly heat either the entire work-hardened area or
the complete part. If 3003 alloy is held too long at temperature its
becomes excessive. Our goal is only to soften the metal enough to work it, while not heating it so much that
the grain structure increases excessively, causing the panel surface to become roughened when it is bent
or folded over. This surface roughness can be a source of stress-corrosion cracking later on, so this
grain-growth is to be avoided. The
of cooling (quench) is unimportant, although it is safer to cool quickly as aluminum does not show its heat
by changing color.
Complex shapes like this tail fillet need considerable working and annealing cycles to achieve consistent
shape, strength, and fit.
Many years ago I would heat a large panel in a busy open shop setting and leave it to cool slowly in the
ambient air whilst away at lunch. Once upon my return, I was dismayed to see a hand print burned onto the
surface from an unsuspecting ``looker". Since then I have always quenched my work, whether with a
dripping rag, a convenient snow bank, or perhaps even a CO2 fire extinguisher. A cold blast of air from
the blowgun works well too, and like the CO2, it requires no extra time spent drying off the work.
When it comes to marking or measuring the right temperature for annealing, I can imagine a good winter's
night roundtable topic tossed about in front of the cheery woodstove. Some prefer to heat with the
neutral torch flame, and rub the panel with either a bar of soap or a stick of soft pine, waiting for
the material to char. At that point the temperature is somewhere around 650F. Some like to mark the
aluminum with black felt tip pens before heating, but some pens are heavier in carbon content than
others, so watch out for the more interesting results!
These gently dished wheelpants were made from 3003, H14, .050" aluminum with only minor spot-annealing.
The techno-reader might now be thinking of the digital infrared thermocouple devices now sold for analysis
work, but they do not work well at all on these reflective surfaces. Aside from the traditional and very
convenient torch-soot method, I personally tend toward using the temperature indicating crayons sold in
many welding stores. These crayons are remarkably accurate, easy to use and usually cost less than
replacing most ruined new panels.
of annealing aluminum sheet needs to be specifically addressed here, partly due to certain well-meaning
automotive influences, and partly because of the temporary wane of the aircraft metalworking tradition.
Tempilstik temperature indicator crayons make marks that melt within 1% of their rated temperatures, changing
appearance from chalky to glossy. There are over 100 temperature ratings, systematically spaced
between 100ºF -2500ºF (38ºC - 1371ºC.) Most can be certified to be lead, sulfur and halogen free.
The torch-soot process is the method whereby the workman sets the torch to either pure acetylene or an
acetylene-rich condition and coats the panel with the appropriate amount of soot. After resetting the
torch either to a neutral flame (hot) or to an oxidizing flame (really hot), he then burns off the soot,
achieving the proper heat necessary for
of the aluminum alloy, and its attendant annealing.