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.
-Kent White
 

Annealing

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 non-heat treatable aluminum alloys is as

Key Words Metal - An opaque crystalline solid, having high thermal and electrical conductivity, and the ability to flow before fracture. 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. Quenching – 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.

simple as elevating the metal to the proper temperature, holding for the appropriate length of time, quenching, and voila! Soft metal (with a distinct absence 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.

The non-heat treatable 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.

Complex shapes like this tail fillet need considerable working and annealing cycles to achieve consistent shape, strength, and fit.

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 grain-growth 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 rate of cooling (quench) is unimportant, although it is safer to cool quickly as aluminum does not show its heat by changing color.

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.

These gently dished wheelpants were made from 3003, H14, .050" aluminum with only minor spot-annealing.

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!

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.

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 method 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.

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 recrystallization of the aluminum alloy, and its attendant annealing.

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