6061-T6/T651, Kaiser Aluminum typical properties (sheet, coil & plate datasheet); 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 stronger than Magnesium AZ91D?
Aluminum is stronger: its yield strength is 276 MPa against 150 MPa for Magnesium AZ91D (1.84x).
Which is lighter, Aluminum or Magnesium AZ91D?
Magnesium AZ91D is lighter: 1,810 kg/m³ against 2,700 kg/m³ for Aluminum.
Which is stiffer, Aluminum or Magnesium AZ91D?
Aluminum is stiffer: Young's modulus 68.3 GPa against 45 GPa for Magnesium AZ91D, so the same part in Aluminum deflects less under the same load.
Which is lighter for the same job, Aluminum or Magnesium AZ91D?
For the same stiffness or strength: Aluminum is lighter for a stiff rod or tie (tension), strong rod or tie; Magnesium AZ91D is lighter for a stiff beam (bending), stiff panel or plate, strong panel or plate.
Part that must be
Aluminum
Magnesium AZ91D
Stiff rod or tie (tension)
lighter
2% heavier
Stiff beam (bending)
21% heavier
lighter
Stiff panel or plate
30% heavier
lighter
Strong rod or tie
lighter
23% heavier
Strong beam
about equal
about equal
Strong panel or plate
10% heavier
lighter
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 or Magnesium AZ91D?
Aluminum conducts heat better: 167 W/m·K against 72 W/m·K for Magnesium AZ91D.
Which expands less with temperature?
Aluminum expands less: 23.6 µm/m·K against 25 µm/m·K for Magnesium AZ91D.