Aluminum and Aluminum Alloys

Sommario:

In high-purity form aluminum is soft and ductile. Most commercial uses, however, require greater strength than pure aluminum affords. This is achieved in aluminum first by the addition of other elements to produce various alloys, which singly or in combination impart strength to the metal. Further strengthening is possible by means that classify the alloys roughly into two categories, non-heat-treatable and heat-treatable ...

Non-Heat-Treatable Alloys

The initial strength of alloys in this group depends upon the hardening effect of elements such as manganese, silicon, iron and magnesium, singly or in various combinations. The non-heat-treatable alloys are usually designated, therefore, in the 1xxx, 3xxx, 4xxx, or 5xxx series.

Since these alloys are work-hard-enable, further strengthening is made possible by various degrees of cold working. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated temperature treatment called stabilizing to ensure stability of properties.

Heat-Treatable Alloys

The initial strength of alloys in this group is enhanced by the addition of alloying elements such as copper, magnesium, zinc, and silicon. Since these elements in various combinations show increasing solid solubility in aluminum with increasing temperature, it is possible to subject them to thermal treatments that will impart pronounced strengthening.

These treatments include solution heat treatment, quenching and precipitation or age, hardening. By the proper combination of solution heat treatment, quenching, cold working and artificial aging, the highest strengths are obtained.

Annealing characteristics

All wrought aluminum alloys are available in annealed form. In addition, it may be desirable to anneal an alloy from any other initial temper, after working, or between successive stages of working such as in deep drawing.

Effect of Alloying Elements

1xxx series - Aluminum of 99 percent or higher purity has many applications, especially in the electrical and chemical fields. Excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties and excellent workability characterize these compositions. Moderate increases in strength may be obtained by strain-hardening. Iron and silicon are the major impurities.

2xxx series - Copper is the principal alloying element in this group often with magnesium as secondary addition. These alloys require solution heat-treatment to obtain optimum properties. In some instances artificial aging is employed to further increase the mechanical properties. This treatment materially increases yield strength, with attendant loss in elongation. Its effect on tensile strength is not so significant. The alloys in this series do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions they may be subject to intergranular corrosion.

3xxx series - Manganese is the major alloying element of alloys in this group, which are generally non-heat-treatable. Because only a limited percentage of manganese, up to about 1.5 percent, can be effectively added to aluminum, it is used as a major element in only a few instances.

4xxx series - The major alloying element of this group is silicon, which can be added in sufficient quantities (up to 12%) to cause substantial lowering of the melting point without producing brittleness in the resulting alloys. For these reasons aluminum-silicon alloys are used in welding wire and as brazing alloys where a lower melting point than that of the parent metal is required.

5xxx series - Magnesium is one of the most effective and widely used alloying elements for aluminum. When it is used as the major alloying element or with manganese, the result is a moderate to high strength non-heat-treatable alloy. Alloys in this series possess good welding characteristics and good resistance to corrosion in marine atmosphere.

6xxx series - Alloys in this group contain silicon and magnesium in approximate proportions to form magnesium silicone, thus making them heat-treatable. Though less strong than most of the 2xxx or 7xxx alloys, the magnesium-silicon alloys possess good formability and corrosion resistance, with medium strength.

7xxx series – Zinc in amounts of 1 to 8% is the major alloying element in this group, and when coupled with magnesium and copper (or without copper) results in heat-treatable alloys of very high strength. Usually other elements such as manganese and chromium are also added in small quantities. The out-standing member of this group is 7075, 7050 and 7049, which is among the highest strength alloys available and is used in air-frame structures and for highly stressed parts.

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