Aluminum 6061-T6 vs. Magnesium AZ91D

Compare Aluminum 6061-T6 vs. Magnesium AZ91D strength, stiffness, weight and thermal properties.

Mechanical	Aluminum 6061-T6	Magnesium AZ91D
Yield strength	276MPa11.84x stronger	150MPa2
Ultimate tensile strength	310MPa135% stronger	230MPa2
Elongation at break	17%15.67x more ductile	3%2
Young's modulus (stiffness)	68.3GPa11.52x stiffer	45GPa2
Density	2,700kg/m³1	1,810kg/m³249% lighter
Strength to weight	102kN·m/kg23% higher	82.9kN·m/kg
Stiffness to weight	25.3MN·m/kg2% higher	24.9MN·m/kg
Poisson's ratio	0.333	base material0.353
Shear modulus	25.7GPa1.54x stiffer in shear	16.7GPa
Bulk modulus	67GPa34% higher	50GPa
Speed of sound	5,030m/s	4,986m/s

Stiffness against densityHigher and further left: stiffer for its weight

Strength against densityHigher and further left: stronger for its weight

Thermal	Aluminum 6061-T6	Magnesium AZ91D
Thermal conductivity	167W/m·K12.32x more heat through a fin	72W/m·K2
Thermal expansion	23.6µm/m·K1	25µm/m·K2
100 mm part over a 50 °C swing	118µm growth6% less	125µm growth
Specific heat	896J/kg·K1	1,050J/kg·K217% higher
Heats up and cools (diffusivity)	69mm²/s1.82x faster	37.9mm²/s
Thermal shock resistance	19,159W/m3.07x more resistant	6,240W/m
Melting point	solidus-liquidus582–652°C1	melting range470–595°C2

Thermal conductivity against densityHigher and further left: spreads more heat for its weight

Thermal expansion against thermal conductivityLower and further right: warps less when heated

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Sources3 documents: Kaiser Aluminum, Magnesium Elektron, U.S. Department of Defense

Aluminum 6061-T6: 6061-T6/T651, Kaiser Aluminum typical properties; Poisson from MIL-HDBK-5J
Magnesium AZ91D: ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.

1
Alloy 6061 Sheet, Coil & Plate Technical DataKaiser Aluminumarchived copyAluminum 6061-T6 · Yield strength: typical, 0.2% offset, .500 in dia specimenAluminum 6061-T6 · Ultimate tensile strength: typicalAluminum 6061-T6 · Elongation at break: typical, in 4D, 0.500 in dia specimenAluminum 6061-T6 · Young's modulus (stiffness): typicalAluminum 6061-T6 · Density: nominalAluminum 6061-T6 · Thermal conductivity: typical, 20 °CAluminum 6061-T6 · Thermal expansion: typical, mean 20-100 °CAluminum 6061-T6 · Specific heat: typical, at 100 °CAluminum 6061-T6 · Melting point: solidus-liquidus
2
Elektron Die Casting Alloys (Datasheet 475)Magnesium Elektronarchived copyMagnesium AZ91D · Yield strength: typical, 0.2% tensile yield, ambient temperatureMagnesium AZ91D · Ultimate tensile strength: typical, ambient temperatureMagnesium AZ91D · Elongation at break: typical, ambient temperatureMagnesium AZ91D · Young's modulus (stiffness): typicalMagnesium AZ91D · Density: typicalMagnesium AZ91D · Thermal conductivity: typicalMagnesium AZ91D · Thermal expansion: typical, temperature range not statedMagnesium AZ91D · Specific heat: typicalMagnesium AZ91D · Melting point: melting range
3
MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures (2003)U.S. Department of DefenseAluminum 6061-T6 · Poisson's ratio: typical (handbook physical constant)Magnesium AZ91D · Poisson's ratio: base material: AZ91C casting (MIL-HDBK-5J handbook physical constant)

Each number points to the document that states it, with the exact basis. Values without a number are computed from the sourced ones. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Mechanical

Is Aluminum 6061-T6 stronger than Magnesium AZ91D?

Aluminum 6061-T6 is stronger: its yield strength is 276 MPa against 150 MPa for Magnesium AZ91D (1.84x).

Which is lighter, Aluminum 6061-T6 or Magnesium AZ91D?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 2,700 kg/m³ for Aluminum 6061-T6.

Which is stiffer, Aluminum 6061-T6 or Magnesium AZ91D?

Aluminum 6061-T6 is stiffer: Young's modulus 68.3 GPa against 45 GPa for Magnesium AZ91D, so the same part in Aluminum 6061-T6 deflects less under the same load.

Which is lighter for the same job, Aluminum 6061-T6 or Magnesium AZ91D?

For the same stiffness or strength: Aluminum 6061-T6 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 6061-T6	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 is more ductile, Aluminum 6061-T6 or Magnesium AZ91D?

Aluminum 6061-T6 stretches further before it breaks: 17% elongation at break against 3% for Magnesium AZ91D.

Thermal

Which conducts heat better, Aluminum 6061-T6 or Magnesium AZ91D?

Aluminum 6061-T6 conducts heat better: 167 W/m·K against 72 W/m·K for Magnesium AZ91D.

Which expands less with temperature?

Aluminum 6061-T6 expands less: 23.6 µm/m·K against 25 µm/m·K for Magnesium AZ91D.