Why Your Magnesium Part Won't Be 35% Lighter — and What It Will Be
- EMPL
- Aug 28
- 2 min read
Open any magnesium datasheet and the pitch writes itself: 1.74 g/cc against aluminium's 2.70. Thirty-five percent lighter. Sold.
Except no real structural part ever delivers it — and knowing why is the difference between a proposal that survives the design review and one that dies at the first sharp question.
The modulus tax
Magnesium's elastic modulus is 45 GPa. A380's is 71. If your part is stiffness-driven — and most housings, brackets, and covers are — the magnesium version must carry deeper ribs or thicker walls to match the aluminium part's bending stiffness. For panel bending, wall scales with the cube root of the modulus ratio: about 16% thicker. That hands back roughly a third of the theoretical saving.
The thin-wall rebate
Magnesium claws some of it back. It fills 1.0–1.5 mm walls reliably, where A380 needs 2.0–2.5 mm — so in regions where the aluminium wall was set by castability rather than load, the magnesium redesign gets to keep the saving the modulus tax took elsewhere.
A worked example
A 640 g A380 controller housing, stiffness-driven. Density alone says 413 g in AZ91D. Apply the stiffness compensation and it lands near 480 g — a 25% reduction, about 160 g out. Strength-driven parts do better: die-cast AZ91D's yield strength is comparable to A380's, so the saving runs close to the full density ratio.
Why the smaller number wins the room
Quote 35% and one modulus question unravels the proposal. Quote 25% with the compensation shown, and the sharpest engineer in the review has nothing to correct — which is precisely the position you want to present from.
Get the compensated number for your part in 30 seconds - the Mg Swap Calculator.
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