Aluminum 6063-T6 vs. Steel AISI 4340

Compare Aluminum 6063-T6 vs. Steel AISI 4340 strength, stiffness, weight and thermal properties.

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Mechanical
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Yield strength	215MPasourcetypical, T6, 0.0625 in specimen	1,035MPasourcetypical, transverse
Ultimate tensile strength	240MPasourcetypical, T6	1,140MPasourcetypical, transverse
Elongation at break	12%sourcetypical, T6, in 4D	18%sourcetypical, transverse
Young's modulus (stiffness)	69GPasourcetypical	200GPasourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Density	2,700kg/m³sourcenominal, 20 °C	7,840kg/m³sourcetypical
Strength to weight	79.6kN·m/kg	132kN·m/kg1.66x higher
Stiffness to weight	25.6MN·m/kg	25.5MN·m/kg
Poisson's ratio	not sourced	0.32sourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Shear modulus	not sourced	75.7GPa
Bulk modulus	not sourced	185GPa
Speed of sound	5,055m/s	5,049m/s
Thermal
Thermal conductivity	201W/m·Ksourcetypical, T6, 20 °C	37.5W/m·Ksourcetypical
Thermal expansion	23.4µm/m·Ksourcetypical, mean 20-100 °C	11.3µm/m·Ksourcemean, -18 to 93 °C
100 mm part over a 50 °C swing	117µm growth	56.5µm growth2.07x less
Specific heat	901J/kg·Ksourcetypical, 20 °C (Hydro EN AW-6063)	448J/kg·Ksourcetypical
Heats up and cools (diffusivity)	82.6mm²/s7.74x faster	10.7mm²/s
Thermal shock resistance	not sourced	11,678W/m
Melting point	615–655°Csourcemelting range	not sourced
Values for	6063-T6 extruded tube, Kaiser Aluminum Tube & Pipe Alloy 6063 Technical Data (Rev. 05/06), typical properties; specific heat from Hydro EN AW-6063 extruded-products datasheet (Rev.4, 03/2023).	Carpenter (Latrobe) Lescalloy 4340 VAC-ARC datasheet; mechanical row for 1100 °F temper; physical properties grade-level

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 6063-T6 stronger than Steel AISI 4340?

Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against 215 MPa for Aluminum 6063-T6 (4.81x).

Which is lighter, Aluminum 6063-T6 or Steel AISI 4340?

Aluminum 6063-T6 is lighter: 2,700 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.

Which is stiffer, Aluminum 6063-T6 or Steel AISI 4340?

Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 69 GPa for Aluminum 6063-T6, so the same part in Steel AISI 4340 deflects less under the same load.

Which is lighter for the same job, Aluminum 6063-T6 or Steel AISI 4340?

For the same stiffness or strength: Aluminum 6063-T6 is lighter for a stiff beam (bending), stiff panel or plate, strong beam, strong panel or plate; Steel AISI 4340 is lighter for a strong rod or tie.

Part that must be	Aluminum 6063-T6	Steel AISI 4340
Stiff rod or tie (tension)	about equal	about equal
Stiff beam (bending)	lighter	1.71x heavier
Stiff panel or plate	lighter	2.04x heavier
Strong rod or tie	1.66x heavier	lighter
Strong beam	lighter	2% heavier
Strong panel or plate	lighter	32% 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 6063-T6 or Steel AISI 4340?

Aluminum 6063-T6 conducts heat better: 201 W/m·K against 37.5 W/m·K for Steel AISI 4340.

Which expands less with temperature?

Steel AISI 4340 expands less: 11.3 µm/m·K against 23.4 µm/m·K for Aluminum 6063-T6.

Among 72 materials

Stiffness against density

Strength against density