Info

Number

Alloying Element

Resistance

Strength

Heat Treatment

1xxx

None

Excellent

Fair

Non-heat-treatable

2xxx

Copper

Fair

Excellent

Heat treatable

3xxx

Manganese

Good

Fair

Non-heat-treatable

4xxx

Silicon

Non-heat-treatable

5xxx

Magnesium

Good

Good

Non-heat-treatable

6xxx

Magnesium and silicon

Good

Good

Heat-treatable

7xxx

Zinc

Fair

Excellent

Heat-treatable

For example, 6463 is a modification of 6063 with slightly more restrictive limits on certain alloying elements such as iron, manganese, and chromium to obtain better finishing characteristics. The primary alloying elements and the properties of the resulting alloys are listed below and summarized in Table 9.1:

1xxx This series is for commercially pure aluminum, defined in the industry as being at least 99% aluminum. Alloy numbers are assigned within the 1xxx series for variations in purity and which elements comprise the impurities, the main ones being iron and silicon. The primary uses for alloys of this series are electrical conductors and chemical storage or processing because the best properties of the alloys of this series are electrical conductivity and corrosion resistance. The last two digits of the alloy number denote the two digits to the right of the decimal point of the percentage of the material that is aluminum. For example, 1060 denotes an alloy that is 99.60% aluminum. The strength of pure aluminum is relatively low. 2xxx The primary alloying element for this group is copper, which produces high strength but reduced corrosion resistance. These alloys were among the first aluminum alloys developed and were originally called duralumin. Alloy 2024 is perhaps the best known and most widely used alloy in aircraft. The original aluminum-copper alloys were not very weldable, but designers have overcome this obstacle in subsequently developed alloys of this series. 3xxx Manganese is the main alloying element for the 3xxx series, increasing the strength of unalloyed aluminum by about 20%. The corrosion resistance and workability of alloys in this group, which primarily consists of alloys 3003, 3004, and 3105, are good. The 3xxx series alloys are well suited to architectural products such as rain-carrying goods and roofing and siding. 4xxx Silicon is added to alloys of the 4xxx series to reduce the melting point for welding and brazing applications. Silicon also provides good flow characteristics, which in the case of forgings provide more complete filling of complex die shapes. Alloy 4043 is commonly used for weld filler wire. 5xxx The 5xxx series alloys contain magnesium, resulting in high strength and corrosion resistance. Alloys of this group are used in ship hulls and other marine applications, weld wire, and welded storage vessels. The strength of alloys in this series is directly proportional to the magnesium content, which ranges up to about 6%. 6xxx Alloys in this group contain magnesium and silicon in proportions that form magnesium silicide (Mg2Si). These alloys have a good balance of corrosion resistance and strength. Alloy 6061 is one of the most popular of all aluminum alloys and has a yield strength comparable to mild carbon steel. The 6xxx series alloys are also very readily extruded, so they comprise the majority of extrusions produced and are used extensively in building, construction, and other structural applications. 7xxx The primary alloying element of this series is zinc. The 7xxx series includes two types of alloys—the aluminum-zinc-magnesium alloys (such as 7005) and the aluminum-zinc-magnesium-copper alloys (such as 7075 and 7178). The alloys of this series include some of the strongest aluminum alloys such as 7178, which has a minimum tensile ultimate strength of 84 ksi (580 MPa), and are used in aircraft frames and structural components. The corrosion resistance of those 7xxx series alloys alloyed with copper is less, however, than the 1xxx, 3xxx, 5xxx, or 6xxx series. The 7xxx alloys without copper are corrosion-resistant, and some (such as 7008 and 7072) are used as cladding to cathodically protect less corrosion-resistant aluminum alloys. 8xxx The 8xxx series is reserved for alloying elements other than those used for series 2xxx through 7xxx. Iron and nickel are used to increase strength without significant loss in electrical conductivity, such as in conductor alloys like 8017. 9xxx This series is not currently used.

Experimental alloys are designated in accordance with the above system, but with the prefix X until they are no longer experimental. Producers may also offer proprietary alloys to which they assign their own designation numbers or brand names.

The chemical composition limits in percent by weight for common wrought alloys are given in Table 9.2.

National variations of these alloys may be registered by other countries under this system. Such variations are assigned a capital letter following the numerical designation (e.g., 6005A, a variation on 6005 used in Europe). The chemical composition limits for national variations are similar to the Aluminum Association limits, but vary slightly. Some standards-writing organizations of other countries have their own designation systems that differ from the Aluminum Association system. A comparison of some alloy designations is given in Table 9.3.

The 2xxx and 7xxx series are sometimes referred to as aircraft alloys, but they are also used in other applications, including fasteners used in buildings. The 1xxx, 3xxx, and 6xxx series alloys are sometimes referred to as "soft," while the 2xxx, 5xxx, and 7xxx series alloys are called "hard." This description refers to the ease of extruding the alloys—hard alloys are more difficult to extrude, requiring higher capacity presses and are thus more expensive.

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