ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.
Ultimaker TPU 95A, 3D-printed specimens, XY (flat); Ultimaker S5 Pro, engineering intent profile, 0.15 mm layer height, AA 0.4 print core, 100% infill, Cura 4.9, printed one-at-a-time, conditioned >=24 h at room temperature. TDS (file v5.00), April 29, 2022.
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.
Plastic TPU is 3D printed: its properties depend on build direction and print settings; these are typical values.
Questions
Is Magnesium AZ91D stronger than Plastic TPU?
Magnesium AZ91D is stronger: its yield strength is 150 MPa against tensile strength 23.7 MPa for Plastic TPU (6.33x).
Which is lighter, Magnesium AZ91D or Plastic TPU?
Plastic TPU is lighter: 1,220 kg/m³ against 1,810 kg/m³ for Magnesium AZ91D.
Which is stiffer, Magnesium AZ91D or Plastic TPU?
Magnesium AZ91D is stiffer: Young's modulus 45 GPa against 0.067 GPa for Plastic TPU, so the same part in Magnesium AZ91D deflects less under the same load.
Which is lighter for the same job, Magnesium AZ91D or Plastic TPU?
For the same stiffness or strength: Magnesium AZ91D is lighter for a stiff rod or tie (tension), stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate.
Part that must be
Magnesium AZ91D
Plastic TPU
Stiff rod or tie (tension)
lighter
453x heavier
Stiff beam (bending)
lighter
17.5x heavier
Stiff panel or plate
lighter
5.9x heavier
Strong rod or tie
lighter
4.27x heavier
Strong beam
lighter
2.31x heavier
Strong panel or plate
lighter
1.7x 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).