Magnesium Alloys The American Foundry Society Technical Dept., Schaumburg, Illiniois
light weights. Magnesium pos- sesses properties that can open the door to structural applications and has gained widespread use in automotive components. Non- automotive applications, spurred on by the computer, electron- ics and power tool industries, continue to expand. Magnesium has a density two-
C
thirds that of aluminum and slightly higher than that of fi ber-reinforced plastics while maintaining the mechanical and physical proper- ties shown in Tables 1-2. When coupled with the inherent advantages of the metalcasting process, magnesium alloys can yield cost-effective solutions to product needs by allowing for part consolidation and weight savings over other materials and manufacturing methods.
Advantages of Magnesium Because of their properties, magne-
ast magnesium alloys have gained popularity in recent years due to their ability to main- tain high strengths at
This structural automotive liftgate met the same crash requirements as a steel alternative.
saws, luggage, laptop computers and cellular phones. Automobiles and other transportation equip- ment continue to take advantage of magnesium’s low density in expanding application areas ranging from under-hood and driveline uses found in engine brackets and transfer cases to numerous interior parts, such as steering column components, pedal brackets, instrument panel supports and seating. Damping Capacity—Mag-
sium alloys can provide a casting designer with several advantages over other light- weight alloys: Weight—The lightest of all structural
metals, magnesium preserves the light weight of a design without sacrificing strength and rigidity (Fig. 1). This benefi t is important when portability is a key ele- ment of the product design, such as with chainsaws, pneumatic nailers, circular
Table 1. Typical Mechanical Properties of Magnesium at Room Temperature Property
Tensile Yield Strength (0.2% offset )
Unit AZ91 AM60 AM50 AM20 AS41 AS21 AE42
Ultimate Tensile Strength MPa 240 225 210 190 215 175 230 (250) (240) (230) (210) (240) (220) (230)
MPa 160 130 125 90 140 110 145 (160) (130) (125) (90) (140) (120) (145)
Compressive Yield Strength MPa 160 130 125 90 140 110 145 Fracture Elongation
% 3 (7) 8 (13) 10 12 (15) (20) 6 (15) 9 (13) 10 (11)
Elastic Modulus, tension GPa 45 45 45 45 45 45 45 Elastic Modulus, shear Brinell Hardness Impact Strength,
GPa 17 17 17 17 17 17 17 70 65 60 45 60 55 60 6
J Charpy un-notched test bars (9) (18)
17 18 18 (18)
4 (18) (16) 5 (12)
Note: Values in parentheses show mean property values obtained from separately diecast test bars. Table 2. Typical Physical Properties of Magnesium Property Density
Liquidus Temperature
Incipient Melting Temperature
F
Specifi c Heat of Fusion kJ/kg Specifi c Heat
5 (12)
nesium is unique among metals because of its ability to absorb
energy. Increased vibration absorption capacity provides for quieter operation of equipment when magnesium castings are used for housings and enclosures. Dimensional Stability—Annealing,
artifi cial-aging or stress-relieving treatments normally are not necessary to achieve stable final dimensions. Metallurgical changes in the structure of some metals can affect dimensions after prolonged exposure to elevated temperatures, but this is not the case with magnesium alloys. As a result, there have been few problems associated with the dimensional change of castings in assemblies. Shrinkage rates are more con- sistent and predictable in magnesium than in other nonferrous metals. Components release from the die with minimal force and distortion, hence they have minimal residual casting stress. Impact & Dent Resistance—The elastic
Unit Temp (F) AZ91 AM60 AM50 AM20 AS41 AS21 AE42 g/cu cm 68 1.81 1.8 1.77 1.75 1.77 1.76 1.79 F
788-815 788-815 788-815 788-815 788-815 788-815 1094
Linear Thermal Expansion Coeffi cient µm/m 68-212 26
26 26 26 26.1 26.1 26.1 370 370 370 370 370 370 370 Thermal Conductivity W/K*m 68 51
kJ/kg*K 68 1.02 1.02 1.02 1.02 1.02 1.02 1.02 61
65 94 68 84 84 Electrical Conductivity MS/m 68 6.6 nm 9.1 13.1 nm 10.8 11.7 2010 CASTING SOURCE DIRECTORY 1,110 1,139 1,148 1,182 1,144 1,169 1,157
energy absorption characteristics of mag- nesium result in good impact and dent resistance and energy management, which is one reason magnesium castings can be used for automotive safety-related applica- tions, such as air bag systems. Portable tools and handheld electronics also benefi t from this combination of properties, offering mechanical shock resistance. Anti-Galling—Magnesium alloys pos-
sess a low galling tendency and can be used as a bearing surface in conjunction with a shaft hardness above 400 Brinell.
Alloy Families Magnesium alloys can be divided
into two groups: sand casting alloys and diecasting alloys. Alloys also can be clas- sifi ed as general purpose, high-ductility
METAL CASTING DESIGN & PURCHASING 37
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