Aluminum 7075-T6 vs. Magnesium AZ91D

Compare Aluminum 7075-T6 vs. Magnesium AZ91D strength, stiffness, weight and thermal properties.

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Mechanical
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Yield strength	503MPasourcetypical, 0.2% offset, .500 in dia specimen	150MPasourcetypical, 0.2% tensile yield, ambient temperature
Ultimate tensile strength	572MPasourcetypical	230MPasourcetypical, ambient temperature
Elongation at break	11%sourcetypical, in 4D	3%sourcetypical, ambient temperature
Young's modulus (stiffness)	71GPasourcetypical1.58x stiffer	45GPasourcetypical
Density	2,800kg/m³sourcenominal	1,810kg/m³sourcetypical
Strength to weight	180kN·m/kg2.17x higher	82.9kN·m/kg
Stiffness to weight	25.4MN·m/kg2% higher	24.9MN·m/kg
Poisson's ratio	0.33sourcetypical (handbook physical constant)	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)
Shear modulus	26.7GPa1.6x stiffer in shear	16.7GPa
Bulk modulus	69.6GPa	50GPa
Speed of sound	5,036m/s	4,986m/s
Thermal
Thermal conductivity	130W/m·Ksourcetypical, 20 °C, T651	72W/m·Ksourcetypical
Thermal expansion	23.4µm/m·Ksourcetypical, mean 20-100 °C	25µm/m·Ksourcetypical, temperature range not stated
100 mm part over a 50 °C swing	117µm growth7% less	125µm growth
Specific heat	960J/kg·Ksourcetypical, at 100 °C	1,050J/kg·Ksourcetypical
Heats up and cools (diffusivity)	48.4mm²/s28% faster	37.9mm²/s
Thermal shock resistance	26,370W/m4.23x more resistant	6,240W/m
Melting point	532–635°Csourcemelting range	470–595°Csourcemelting range
Values for	7075-T6/T651, Kaiser Aluminum rod & bar (mechanical) and sheet/coil/plate (physical) typical datasheets; Poisson from MIL-HDBK-5J	ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.

Every value links to the document that states it. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Is Aluminum 7075-T6 stronger than Magnesium AZ91D?

Aluminum 7075-T6 is stronger: its yield strength is 503 MPa against 150 MPa for Magnesium AZ91D (3.35x).

Which is lighter, Aluminum 7075-T6 or Magnesium AZ91D?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 2,800 kg/m³ for Aluminum 7075-T6.

Which is stiffer, Aluminum 7075-T6 or Magnesium AZ91D?

Aluminum 7075-T6 is stiffer: Young's modulus 71 GPa against 45 GPa for Magnesium AZ91D, so the same part in Aluminum 7075-T6 deflects less under the same load.

Which is lighter for the same job, Aluminum 7075-T6 or Magnesium AZ91D?

For the same stiffness or strength: Aluminum 7075-T6 is lighter for a stiff rod or tie (tension), strong rod or tie, strong beam, strong panel or plate; Magnesium AZ91D is lighter for a stiff beam (bending), stiff panel or plate.

Part that must be	Aluminum 7075-T6	Magnesium AZ91D
Stiff rod or tie (tension)	lighter	2% heavier
Stiff beam (bending)	23% heavier	lighter
Stiff panel or plate	33% heavier	lighter
Strong rod or tie	lighter	2.17x heavier
Strong beam	lighter	45% heavier
Strong panel or plate	lighter	18% heavier

For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).

Which conducts heat better, Aluminum 7075-T6 or Magnesium AZ91D?

Aluminum 7075-T6 conducts heat better: 130 W/m·K against 72 W/m·K for Magnesium AZ91D.

Which expands less with temperature?

Aluminum 7075-T6 expands less: 23.4 µm/m·K against 25 µm/m·K for Magnesium AZ91D.

Among 72 materials

Stiffness against density

Strength against density