In aluminum specifications, a four-digit index system is used for the designation of cast and wrought aluminum alloys.
These two classes of aluminum alloys can be further subdivided into families of alloys based on chemical composition and on temper designation.
Pure aluminum is alloyed with many other metals to produce a wide range of physical and mechanical properties. The alloying elements are used as the basis to classify aluminum alloys into two categories: non-heat-treatable and heat-treatable.
Most of aluminum specifications designate aluminum alloys in the following way:
- First digit – principal alloying constituent(s),
- Second digit – variations of initial alloy,
- Third and fourth digits – individual alloy variations (number has no significance but is unique).
Wrought Aluminum Specifications. The wrought alloy group is shown as: 1xxx-pure Al (99.00% or greater); 2xxx-Al-Cu alloys; 3xxx-Al-Mn alloys; 4xxx-Al-Si alloys; 5xxx-Al-Mg alloys; 6xxx-Al-Mg-Si alloys.
Cast Aluminum Specifications. The designation system and specifications for cast aluminum alloys are similar in some respects to that of wrought alloys. The cast alloy designation system also has four digits and the first digit specifies the major alloying constituent(s). However, a decimal point is used between the third and fourth digits to make clear that these are designations used to identify alloys in the form of castings (0) or foundry ingot (1,2).
A letter before the numerical designation indicates a modification of the original alloy or an impurity limit. These serial letters are assigned in alphabetical sequence starting with A, but omitting I,O,Q and X, with X being reserved for experimental alloys. The cast alloy group is shown as: 1xx.x-Pure Al (99.00% or greater), 2xx.x Al-Cu alloys; 3xx.x Al-Si + Cu and/or Mg; 4xx.x Al-Si; 5xx.x Al-Mg; 7xx.x – Al-Zn; 8xx.x- Al-Sn; 9xx.x-Al+Other elements and 6xx.x unused series.
Aluminum Alloy Temper-Designation System. Tempers are very important for aluminum specifications due to their crucial influence to material properties. The temper designation system is based on the sequences of mechanical or thermal treatments, or both, used to produce the various tempers. The temper designation is always presented immediately following the alloy designation with a hyphen between the designation and the temper (e.g.2014-T6, 3003-H14, 1350-H19 (extra hard) etc.). The first character in the temper designation is a capital letter indicating the general class of treatment such as: F-as fabricated; O-annealed; H-strain hardened; W-solution heat treated and T-thermally treated to produce stable tempers other than F,O, or H. Note that the temper designations differ between non heat-treatable alloys and heat-treatable alloys and their meanings are given below.
Non Heat-Treatable Aluminum Alloys. The letter "H" is always followed by 2 or 3 digits. The first digit indicates the particular method used to obtain the temper as follows: H1-means strain hardened only, H2-means strain hardened, then partially annealed and H3-means strain hardened, then stabilized. The temper is indicated by the second digit as follows: 2-1/4 hard; 4-1/2 hard; 6-3/4 hard; 8-full hard and 9-extra hard which means that added digits indicate modification of standard practice. The non heat-treatable alloys are mainly found in the 1xxx, 3xxx, 4xxx, and 5xxx alloys series depending upon their major alloying elements.
Heat-Treatable Aluminum Alloys (e.g. F, O or T): The letter "T" is always followed by one or more digits. These digits indicate the method used to produce the stable tempers, as follows: T3-Solution heat treated and then cold worked, T4-Solution heat treated and then naturally aged, T5-Artificially aged only, T6-Solution heat treated and then artificially aged etc. The heat-treatable alloys are found primarily in the 2xxx, 6xxx and 7xxx alloys series.
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