Magnesium vs Aluminum: Choosing the Lightest Metal for Your Part

When weight is the enemy, two metals lead the conversation: magnesium and aluminium. Both are light, both machine well, and both are everywhere in aerospace, automotive and electronics. But magnesium is the lightest structural metal there is — about a third lighter than aluminium — while aluminium is cheaper, tougher against corrosion and far easier to source. Choosing between them is a trade between ultimate lightness and practical robustness. This guide compares the two so you can pick the right light metal for your part.

The Core Difference

Magnesium is the lightest metal used for structural parts — roughly 35% less dense than aluminium — with an excellent strength-to-weight ratio, good machinability, natural vibration damping and useful EMI shielding for electronics housings. Aluminium gives up a little of that lightness in exchange for higher strength in its top alloys, much better corrosion resistance, lower cost and an enormous supply base. In short: magnesium wins on pure weight, aluminium wins on practicality.

Side by Side

When Magnesium Wins

Reach for magnesium when every gram counts and the environment is controlled: aerospace brackets and housings, automotive components, power-tool and camera bodies, laptop and electronics enclosures (where its EMI shielding and damping are bonuses). Common alloys like AZ31 (wrought) and AZ91 (die cast) cover most needs. The catches are real: magnesium corrodes readily and sits at the very anodic end of the galvanic series, so it needs protective coating and careful isolation from other metals — see galvanic corrosion.

When Aluminium Wins

Choose aluminium for the large majority of lightweight parts, where its balance of strength, corrosion resistance, cost and availability is hard to beat. It needs no coating to resist everyday corrosion, anodises to a durable finish, and its high-strength grades carry serious load. When you need maximum strength-to-weight in a robust, easy-to-source package, aluminium — especially 7075 — is usually the answer. Compare the workhorse grades in 6061 vs 7075 and the aluminium page.

Designing With Each

• Account for stiffness, not just strength. Magnesium is less stiff, so thin magnesium sections deflect more — design ribs and thickness accordingly.
• Protect magnesium from corrosion. Specify a conversion coating, anodise or paint, and isolate it from dissimilar metals and fasteners to avoid galvanic attack.
• Mind machining safety. Magnesium machines beautifully but its fine chips are flammable — a job for a shop set up for it.
• Weigh cost and supply. Aluminium is cheaper and easier to get; magnesium is justified when the weight saving truly matters.
• Match the alloy to the process. Both have wrought and casting grades — pick the alloy for your forming route and load.

The Bottom Line

Magnesium and aluminium are both answers to “make it lighter,” at different points on the curve. Magnesium is the lightest structural metal and the choice when weight, damping or EMI shielding rule and the part can be protected from corrosion. Aluminium is the practical default — stronger in its top grades, far more corrosion-resistant, cheaper and everywhere — for nearly everything else. Decide from how much the weight saving is really worth against corrosion, stiffness, cost and supply, and the right light metal is clear.

MechPart Pro machines and casts both magnesium and aluminium — with the protective finishes and dissimilar-metal isolation magnesium needs — and our engineers will help you choose the alloy and treatment for your weight, strength and environment as part of our free design-for-manufacturability review. Explore the options in our materials guide.