Magnesium AZ91D vs. Steel AISI 1045

Compare Magnesium AZ91D vs. Steel AISI 1045 strength, stiffness, weight and thermal properties.

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Mechanical
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Yield strength	150MPasourcetypical, 0.2% tensile yield, ambient temperature	531MPasourceproducer-stated, cold drawn
Ultimate tensile strength	230MPasourcetypical, ambient temperature	627MPasourceproducer-stated, cold drawn
Elongation at break	3%sourcetypical, ambient temperature	12%sourcein 2 in (50.8 mm)
Young's modulus (stiffness)	45GPasourcetypical	210GPasourcetypical4.67x stiffer
Density	1,810kg/m³sourcetypical4.31x lighter	7,800kg/m³sourcetypical
Strength to weight	82.9kN·m/kg22% higher	68.1kN·m/kg
Stiffness to weight	24.9MN·m/kg	26.9MN·m/kg8% higher
Poisson's ratio	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)	0.3sourcetypical
Shear modulus	16.7GPa	80.8GPa4.85x stiffer in shear
Bulk modulus	50GPa	175GPa
Speed of sound	4,986m/s	5,189m/s
Thermal
Thermal conductivity	72W/m·Ksourcetypical	40–45W/m·Ksourcetypical range, ambient
Thermal expansion	25µm/m·Ksourcetypical, temperature range not stated	12µm/m·Ksourcetypical, mean 20-300 °C
100 mm part over a 50 °C swing	125µm growth	60µm growth2.08x less
Specific heat	1,050J/kg·Ksourcetypical	460–480J/kg·Ksourcetypical range, 50/100 °C
Heats up and cools (diffusivity)	37.9mm²/s3.27x faster	11.6mm²/s
Thermal shock resistance	6,240W/m	6,269W/m
Melting point	470–595°Csourcemelting range	not sourced
Values for	ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.	AISI 1045 cold drawn bar (Niagara LaSalle) for mechanical; Ovako C45 (lists 'SAE 1045' as a similar designation) for physical constants

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 Magnesium AZ91D stronger than Steel AISI 1045?

Steel AISI 1045 is stronger: its yield strength is 531 MPa against 150 MPa for Magnesium AZ91D (3.54x).

Which is lighter, Magnesium AZ91D or Steel AISI 1045?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 7,800 kg/m³ for Steel AISI 1045.

Which is stiffer, Magnesium AZ91D or Steel AISI 1045?

Steel AISI 1045 is stiffer: Young's modulus 210 GPa against 45 GPa for Magnesium AZ91D, so the same part in Steel AISI 1045 deflects less under the same load.

Which is lighter for the same job, Magnesium AZ91D or Steel AISI 1045?

For the same stiffness or strength: Magnesium AZ91D is lighter for a stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate; Steel AISI 1045 is lighter for a stiff rod or tie (tension).

Part that must be	Magnesium AZ91D	Steel AISI 1045
Stiff rod or tie (tension)	8% heavier	lighter
Stiff beam (bending)	lighter	1.99x heavier
Stiff panel or plate	lighter	2.58x heavier
Strong rod or tie	lighter	22% heavier
Strong beam	lighter	1.86x heavier
Strong panel or plate	lighter	2.29x 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, Magnesium AZ91D or Steel AISI 1045?

Magnesium AZ91D conducts heat better: 72 W/m·K against 42.5 W/m·K for Steel AISI 1045.

Which expands less with temperature?

Steel AISI 1045 expands less: 12 µm/m·K against 25 µm/m·K for Magnesium AZ91D.

Among 72 materials

Stiffness against density

Strength against density