Because these foils are made of aluminum alloys, their compositions are exactly the same as any aluminum products made of those same alloys. Nothing is added to the metal for foil
stock. Each wrought alloy has its own four-digit number, which indicates its composition.
Mechanical properties of any given aluminum foil, therefore, are directly related and similar to those of sheet or plate made of the same alloy in comparable temper or metallurgical condition. The only differences arise from the extreme thinness of the foil gauges of the metal.
The Metallurgy of aluminum is a technical subject beyond the scope of this book. However, an understanding of the mechanical-property-related terms used for aluminum, hence for foil, requires a general familiarity with some of aluminum’s metallurgical properties and their related terminology. It is of particular importance to be familiar with how aluminum alloys are tempered and annealed, or hardened and softened.
Tempering Aluminum-As earlier implied, adding certain other elements strengthens aluminum; the specific alloys thus produced can be further strengthened by mechanical and/or thermal treatments of varying degree and combinations.
The lowest or basic
strength of aluminum and each of its alloys is determined when the metal is in the annealed or soft
condition. This is designated as the O (letter O) temper. For example, the annealed condition for alloy 1145 is written 1145-O. Annealing consists of heating the metal to the appropriate temperature for the correct period of time. The reroll stock, from which foil gauges are produced, for example, is annealed prior to the rolling operations, so that it will be of maximum workability. All alloys can be annealed.
All alloys also are strain-hardened and strengthened when cold worked, as in foil rolling. When the final product is wanted in the soft condition, it is given a final anneal.
Because aluminum alloys are strengthened beyond their basic strengths only by strain hardening, they are non-heat-treatable. All of the foil presently produced is rolled from non-heat-treatable alloys, which are given H
number temper designations for the various specific strain-hardened conditions in which they are made. For example, a common foil availability is alloy 1145-H19; the H1 means strain hardened, and the next digit indicates the degree of hardening, the 9 meaning full hard.