This chapter contains the engineering properties and characteristics of wrought and cast magnesium alloys used in aircraft and missile applications. Magnesium is a lightweight structural metal that can be strengthened greatly by alloying, and in some cases by heat treatment or cold work or by both.
The magnesium alloys in this chapter are listed in alphanumeric sequence in each of two parts, the first one being wrought forms of magnesium and the second cast forms. These sections and the alloys covered under each are shown in Table 4.1.
The mechanical properties are given either as design values or for information purposes. The tensile strength (Ftu), tensile yield strength (Fty), elongation (e), and sometimes the compressive yield strength (Fcy) are guaranteed by procurement specifications. The properties obtained reflect the location of sample, type of test specimen and method of testing required by the product specification. The remaining design values are “derived” values; that is, sufficient tests have been made to ascertain that if a given material meets the requirements of the product specification, the material will have the compression (Fcy), shear (Fsu) and bearing (Fbru and Fbry) strengths listed.
4.1.2.1.1 Tension Testing — Room-temperature tension tests are made according to ASTM E 8. The yield strength (Fty) is obtained by the “offset method” using an offset of 0.2 percent. The speed of testing for room-temperature tests has a small effect on the strength and elongation values obtained on most magnesium alloys. The rate of stressing generally specified to the yield strength is less than 100,000 psi per minute and the rate of straining from the yield strength to fracture is less than 0.5 in./in./min. It can be expected that the speed of testing used for room-temperature tension tests will approach the maximum permitted.
Elevated-temperature tension tests are made according to ASTM E 21. The speed of testing has a considerable effect on the results obtained and no one standard rate of straining is given in ASTM E 21. The strain rates most commonly used on magnesium are 0.005 in./in./min. to the yield and 0.10 in./in./min. from yield to fracture.
4.1.2.1.2 Compression Testing — Compression test methods used for magnesium are specified in ASTM E 9. The values given for the compressive yield strength (Fcy) are taken at an offset of 0.2 percent.
4.1.2.1.3 Bearing Testing — Bearing tests of magnesium alloys are made according to ASTM E 238. The size of pin used has a significant effect on the values obtained, especially the bearing ultimate strength (Fbru). On tests made to obtain the data on magnesium alloys shown in this document, pin diameters of 0.187 and 0.250 inch were used. For pin diameters significantly larger than 0.250 inch lower values may be obtained. Bearing values in the property tables are considered to be “dry pin” values in accordance with the discussion in Section 1.4.7.1.
4.1.2.1.4 Shear Testing — The shear strength values used in this document were obtained by the “double shear” method using a pin-type specimen, the “punch shear” method and the “tension shear” method as applicable. Just as tensile ultimate strength (Ftu) values vary with location and direction of sample in relation to the method of fabrication, the shear strength (Fsu) may be expected to reflect the effect of orientation, either as a function of the sampling or the maximum stresses imposed by the method of test.
4.1.2.1.5 Stress Raisers — The effect of notches, holes, and stress raisers on the static properties of magnesium alloys, along with additional strength data, is discussed in the references cited for this section.
4.1.2.1.6 Creep — Creep data on magnesium alloys are summarized in the references cited for this section.
4.1.2.1.7 Fatigue — Room-temperature axial load fatigue data for several magnesium alloys are presented in the appropriate alloy sections.
Selected experimental data from the literature were used in determining values for physical properties. In other cases, enough information was available to calculate the constants. Estimated values of some of the remaining constants were also included. Estimated values are noted.
Corrosion protection must be considered for all magnesium applications. Protection can be provided by anodic films, chemical conversion coatings, paint systems, platings, or a combination of these methods. Proper drainage must be provided to prevent entrapment of water or other fluids. Dissimilar metal joints must be properly and completely insulated, including barrier strips and sealants.
Strain-hardened or age-hardened alloys may be annealed or overaged by prolonged exposure to elevated temperatures, with a resulting decrease in strength. Maximum recommended temperatures for prolonged service are reported, where available, for specific alloys.
Standard ASTM nomenclature is used for the alloys listed. Temper designations are given in ASTM B 296. A summary of the temper designations is given in Table 4.1.5.
| Temper | Description |
|---|---|
| F | As fabricated. Applies to products that acquire some temper from shaping processes not having special control over the amount of strain-hardening or thermal treatment. |
| O | Annealed, recrystallized (wrought products only). Applies to the softest temper of wrought products. |
| H | Strain-hardened (wrought products only), with or without supplementary thermal treatments to produce partial softening. Always followed by two or more digits: H1 = strain-hardened only; H2 = strain-hardened and then partially annealed; H3 = strain-hardened and stabilized. The digit following H1/H2/H3 (1–7, with 4 = half hard, 2 = quarter hard, 6 = three-quarter hard) indicates the final degree of strain-hardening remaining; a third digit, when used, indicates a variation of a two-digit H temper. |
| W | Solution heat-treated. An unstable temper applicable only to alloys that spontaneously age at room temperature after solution heat treatment; specific only when the period of natural aging is stated (e.g., W – ½ hr). |
| T | Thermally treated to produce stable tempers other than F, O, or H, with or without supplementary strain-hardening. Always followed by one or more digits (T1–T10, see below). |
| T1 | Cooled from an elevated-temperature shaping process (e.g., casting or extrusion) and naturally aged to a substantially stable condition. |
| T3 | Solution heat-treated and then cold worked. |
| T4 | Solution heat-treated and naturally aged to a substantially stable condition. |
| T5 | Cooled from an elevated-temperature shaping process and then artificially aged. |
| T6 | Solution heat-treated and then artificially aged. |
| T7 | Solution heat-treated and then stabilized. |
| T8 | Solution heat-treated, cold worked, and then artificially aged. |
| T9 | Solution heat-treated, artificially aged, and then cold worked. |
| T10 | Cooled from an elevated-temperature shaping process, artificially aged, and then cold worked. |
| A period of natural aging at room temperature may occur between or after the operations listed for tempers T3 through T10; additional digits may be added to designations T1 through T10 to indicate a variation in treatment that significantly alters the characteristics of the product. |
Most magnesium alloys may be welded; refer to “Comments and Properties” in individual alloy sections. Adhesive bonding and brazing may be used to join magnesium to itself or other alloys. All types of mechanical fasteners may be used to join magnesium. Refer to Section 4.1.4 when using mechanical fasteners or joining of dissimilar materials with magnesium alloys.