5585

Source: Aluminum Association [12].

(©Values represent properties obtained from separately cast test bars and are derived from ASTM B-108, Standard Specification for Aluminum-Alloy Permanent Mold Castings; Federal Specification QQ-A-596d, Aluminum Alloy Permanent and Semi-Permanent Mold Castings; and Military Specification MIL-A-21180c, Aluminum Alloy Castings, High Strength. Unless otherwise specified, the average tensile strength, average yield strength and average elongation values of specimens cut from castings shall be not less than 75 percent of the tensile and yield strength values and not less than 25 percent of the elongation values given above. The customer should keep in mind that (1) some foundries may offer additional tempers for the above alloys, and (2) foundries are constantly improving casting techniques and, as a result, some may offer minimum properties in excess of the above.

© F indicates "as cast" condition; refer to AA-CS-M11 for recommended times and temperatures of heat treatment for other tempers to achieve properties specified.

© Hardness values are given for information only; not required for acceptance.

Source: Aluminum Association [12].

(©Values represent properties obtained from separately cast test bars and are derived from ASTM B-108, Standard Specification for Aluminum-Alloy Permanent Mold Castings; Federal Specification QQ-A-596d, Aluminum Alloy Permanent and Semi-Permanent Mold Castings; and Military Specification MIL-A-21180c, Aluminum Alloy Castings, High Strength. Unless otherwise specified, the average tensile strength, average yield strength and average elongation values of specimens cut from castings shall be not less than 75 percent of the tensile and yield strength values and not less than 25 percent of the elongation values given above. The customer should keep in mind that (1) some foundries may offer additional tempers for the above alloys, and (2) foundries are constantly improving casting techniques and, as a result, some may offer minimum properties in excess of the above.

© F indicates "as cast" condition; refer to AA-CS-M11 for recommended times and temperatures of heat treatment for other tempers to achieve properties specified.

© Hardness values are given for information only; not required for acceptance.

the specimen, rather than the actual cross-sectional area under stress. The actual area is less than the original area since necking occurs after yielding; thus the engineering stress is slightly less than the actual stress.

When strengths are not available, relationships between the unknown strength and known properties may be used. The tensile ultimate strength (Ftu) is almost always known and the tensile yield strength (F^) is usually known, so other properties are related to these:

Fcy = 0.9Fty (for cold-worked tempers)

Fcy = Fty (for heat-treatable alloys and annealed tempers)

These relationships are approximate but usually accurate enough for design purposes.

Tensile ultimate strengths vary widely among common alloys and tempers, from a minimum of 8 ksi (55 MPa) for 1060-0 and 1350-0 to a maximum of 84 ksi (580 MPa) for 7178-T62. For some tempers (usually the annealed temper) of certain alloys, strengths are also limited to a maximum value to ensure workability without cracking.

The strength of aluminum alloys is a function of temperature. Most alloys have a plateau of strength between roughly -150°F(-100°C) and 200°F (100°C), with higher strengths below this range and lower strengths above it. Ultimate strength increases 30-50% below this range, while the yield strength increase at low temperatures is not so dramatic, being on the order of 10%. Both ultimate and yield strengths drop rapidly above 200°F, dropping to nearly zero at 750°F (400°C). Some alloys (such as 2219) retain useful (albeit lower) strengths as high as 600°F (300°C). Figure 9.2 shows the effect of temperature on strength for various alloys.

Heating tempered alloys also has an effect on strength. Heating for a long enough period of time reduces the condition of the material to the annealed state, which is the weakest temper for the material. The higher the temperature, the briefer the period of time required to produce annealing. The length of time of high-temperature exposure causing no more than a 5% reduction in strength is given in Table 9.17 for 6061-T6. Since welding introduces heat to the parts being welded, welding reduces their strength. This effect is discussed in Section 9.4.1, and minimum reduced strengths for various alloys are given there.

Under a constant stress, the deformation of an aluminum part may increase over time, a behavior known as creep. Creep effects increase as the temperature increases. At room temperature, very little creep occurs unless stresses are near the tensile strength. Creep is usually not significant unless stresses are sustained at temperatures over about 200°F (95°C).

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