Aluminum Alloys Article
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The Family of Aircraft
Aluminum Alloys
from Sport Aviation, September 1999
by Kent White
Aircraft... the word suggests various images of brightly painted fabric, varnished wood, black tubing, or a fuselage twinkling across a grass strip. But say aluminum aircraft and rows of rivets come to mind, and a smooth skin beckoning to be stroked. For the builder, however, those simple words may invoke images of drilling holes and pounding rivets, or long hours spent forming that cowling to get the perfect fit. Regarded today as simultaneously cheap and expensive, it has been 150 years since the metal was regarded as precious enough for Napoleon to have a tea service made of it, knowing full
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well that platinum was cheaper. Aluminum for aircraft use is light in weight, sometimes high in strength, and offers both good capabilities and severe limitations for forming, straightening, and joining.
What are the aircraft aluminums? How are they different from each other? What are their limitations and capabilities? In its pure elemental form, aluminum is a very usable metal. Let's take a look at how its application changes as it is alloyed with other metals.
The aluminum alloy designation system was changed in October 1953 from the old two digit system to the present day four digit system. The old two digit number will be included here in parentheses next to the current four digit for reference. To begin, let's divide the family into two categories: the non-heat treatable and the heat treatable alloys. Some will be weldable, some will not.
Note: Be mindful that the rule of workability dictates that as strength (hardness and temper) increases, workability decreases.
Non Heat-Treatable Alloys
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These alloys can only be strengthened by strain or work hardening applied to the sheet. Either rolling or pulling the sheet includes the necessary hardness. This wrought hardness is indicated by an H, marked on the sheet, or found in the accompanying paperwork. Remember, you can only harden it by Hammering (or by otherwise working it in some fashion). Although there are numerical designations for hardness, H14, H18, H32, H34, etc., most of us metal folk refer to the hardness as quarter-hard, half-hard, three-quarters, and full, and know that friendly suppliers will follow suit. S-O means full soft, or no temper at all. 1100-(2S) is referred to as an alloy, although technically it is the element itself in its pure (99% or greater) form. 1100 is quite simply the most shapeable alloy. It works readily into very complex parts, and its low strength (5KSI-22KSI yield) and excellent weldability make it ideal for non-structural parts like tubing, tanks, gear-leg fairings, strut fairings, cowl bumps, scoops, and welding rod.
