Machinery's Handbook, 31st Edition
540 WROUGHT ALUMINUM ALLOYS in various combinations. The non-heat-treatable alloys are usually designated, therefore, in the 1000, 3000, 4000, or 5000 series. These alloys are work-hardenable, so further strengthening is made possible by various degrees of cold-working, denoted by the “H” series of tempers. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called stabilizing for improving property stability. 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. These elements singly or in various combinations show increasing solid solubility in aluminum with increasing temperature, so it is possible to subject them to thermal treatments that will impart pronounced strengthening. The first step, called heat treatment or solution heat treatment, is an elevated-temperature process designed to put the soluble element in solid solution. This step is followed by rapid quenching, usually in water, which momentarily “freezes” the structure and for a short time renders the alloy very workable. Some fabricators retain this more workable structure by storing the alloys at below freezing temperatures until they can be formed. At room or elevated temperatures the alloys are not stable after quenching, however, and precipitation of the constituents from the supersaturated solution begins. After a period of several days at room temperature, termed aging or room-temperature precipitation, the alloy is considerably stronger. Many alloys approach a stable condition at room temperature, but some alloys, particularly those containing magnesium and silicon or magnesium and zinc, continue to age-harden for long periods of time at room temperature. Heating for a controlled time at slightly elevated temperatures provides even further strengthening and property stabilization. This process is called artificial aging or pre cipitation hardening. By application of the proper combination of solution heat treatment, quenching, cold-working and artificial aging, the highest strengths are obtained. Clad Aluminum Alloys.— The heat-treatable alloys in which copper or zinc are major alloying constituents are less resistant to corrosive attack than the majority of non-heat- treatable alloys. To increase their corrosion resistance in sheet and plate form, these alloys are often clad with high-purity aluminum, a low magnesium-silicon alloy, or an alloy con- taining 1 percent zinc. The cladding, usually from 2 1 ⁄ 2 to 5 percent of the total thickness on each side, not only protects the composite due to its own inherently excellent corrosion resistance but also exerts a galvanic effect that further protects the core material. Special composites, such as clad non-heat-treatable alloys for extra corrosion protec tion, for brazing purposes, or for special surface finishes, may be obtained. Some alloys in wire and tubular form are clad for similar reasons, and on an experimental basis extru sions also have been clad. Aluminum Alloys, Wrought, Sheet.— Physical Properties: In sheet form, tensile strength varies from 35,000 psi (241 MPa) for soft temper to 62,000 (427 MPa) psi for heat-treated sheets, and the elongation in 2 inches ( 5.08 cm) from 12 to 18 percent. The yield strength of a heat-treated sheet is about 40,000 psi (276 MPa) minimum. Characteristics of Principal Aluminum Alloy Series Groups.— 1000 series: These alloys are characterized by high corrosion resistance, high thermal and electrical conduc tivity, low mechanical properties and good workability. Moderate increases in strength may be obtained by strain-hardening. Iron and silicon are the major impurities. 2000 series: Copper is the principal alloying element in this group. These alloys re quire solution heat treatment to obtain optimum properties; in the heat-treated condition mechanical properties are similar to, and sometimes exceed, those of mild steel. 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 (ultimate) strength is not as great. Alloys in the 2000 series have reduced corrosion resistance compared to most other aluminum alloys, and under certain
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online