030

Source: Aluminum Association [12].

® The alloys listed are those that have been included in Federal Specifications QQ-A-596d, ALUMINUM ALLOYS PERMANENT AND SEMIPERMANENT MOLD CASTINGS, QQ-A-601E, ALUMINUM ALLOY SAND CASTINGS, and Military Specification MIL-A-21180c, ALUMINUM ALLOY CASTINGS, HIGH STRENGTH. Other alloys are registered with The Aluminum Association and are available. Information on these should be requested from individual foundries or ingot suppliers.

® Except for "Aluminum" and "Others," analysis normally is made for elements for which specific limits are shown. For purposes of determining conformance to these limits, an observed value or calculated value obtained from analysis is rounded off to the nearest unit in the last right-hand place of figures used in expressing the specified limit, in accordance with the following:

When the figure next beyond the last figure or place to be retained is less than 5, the figure in the last place retained should be kept unchanged. When the figure next beyond the last figure or place to be retained is greater than 5, the figure in the last place retained should be increased by 1. When the figure next beyond the last figure or place to be retained is 5 and

(1) there are no figures or only zeros, beyond this 5, if the figure in the last place to be retained is odd, it should be increased by 1; If even, it should be kept unchanged;

(2) If the 5 next beyond the figure in the last place to be retained is followed by any figures other than zero, the figure in the last place retained should be increased by 1; whether odd or even.

© If iron exceeds 0.45%, manganese content shall not be less than one-half the iron content. © Also contains 0.04-0.07% beryllium.

© Also contains 0.003-0.007% beryllium, boron 0.005% maximum. © Also contains 5.5-7.0% tin. © Also contains 0.40-1.0% silver. © Also contains 0.05 max. % tin.

® The sum of those "Others" metallic elements 0.010% or more each, expressed to the second decimal before determining the sum.

Alloys are divided into two groups based on whether their strengths can be increased by heat-treating or not. Both heat-treatable and non-heat-treatable alloys can be strengthened by strain-hardening, also called cold working. The alloys that are not heat treatable may only be strengthened by cold working. Whether or not an alloy is heat treatable depends on its alloying elements. Alloys in which the amount of alloying element in solid solution in aluminum increases with temperature are heat treatable. The 1xxx, 3xxx, 4xxx, and 5xxx series wrought alloys are not heat treatable, while the 2xxx, 6xxx, and 7xxx wrought series are, with a few exceptions. Strengthening methods are summarized in Table 9.6.

Non-heat-treatable alloys may also be heat treated, but this treatment is only used to stabilize properties so that strengths don't decrease over time (behavior called age softening) and is only required for alloys with an appreciable amount of magnesium (the 5xxx series). Heating to 225-350°F (110-180°C) causes all the softening to occur at once and thus is used as the stabilization heat treatment.

Before tempering, alloys begin in the annealed condition, the weakest but most ductile condition. Tempering, while increasing the strength, decreases ductility and therefore decreases workability. To reduce material to the annealed condition, the typical annealing treatments given in Table 9.7 can be used.

Strain-hardening is achieved by mechanical deformation of the material at ambient temperature. In the case of sheet and plate, this is done by reducing its thickness by rolling. As the material is worked, it becomes resistant to further deformation and its strength increases. The effect of this work on the yield strength of some common non-heat-treatable alloys is shown in Fig. 9.1. Two heat treatments can be applied to annealed condition heat-treatable alloys. First, the material can be solution heat treated. This allows soluble alloying elements to enter into solid solution; they are retained in a supersaturated state upon quenching, a controlled rapid cooling usually performed using air or water. Next, the material may undergo a precipitation heat treatment, also called artificial aging, by which constituents are precipitated from solid solution to increase the strength. An example of this process is the production of 6061-T6 sheet. From its initial condition, 6061-0 annealed material is heated to 990°F (530°C) as rapidly as possible (solution heat treated), then cooled as rapidly as possible (quenched), which renders the temper T4. Then the material is heated to 320°F (160°C) and

TABLE 9.6 Strengthening Methods

2xxx-Cu 6xxx-Mg, Si 7xxx-Zn

Heat Treatment

Solution heat treatment; natural aging or artificial aging

Strain Hardening (Cold Working)

-T tempers

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