Magnesium Fasteners: Applications, limits, and when to specify them
Magnesium fasteners are the lightest structural fastener material commercially available. Pure magnesium is 1.74 g/cm³ ; the common fastener alloy AZ31 sits at about 1.77 g/cm³. Either way, that's roughly a quarter the weight of steel and two-thirds the weight of aluminum. Strength-to-weight is close to titanium at roughly a third of titanium's price. If you're sourcing for an application where every gram counts, magnesium can buy back weight that nothing else will at that price point.
They're also a procurement landmine if your engineer specs them without thinking through galvanic corrosion, thread engagement, and the coating system that has to ride along with them. This post walks through the four applications where they earn their place, the corrosion math that determines whether they last, and when you should push back on the spec.
The properties that matter
| Property | Magnesium (AZ31) | Aluminum (6061-T6) | Steel (Grade 8) | Titanium (Grade 5) |
|---|---|---|---|---|
| Density (g/cm³) | 1.77 | 2.70 | 7.85 | 4.43 |
| Tensile strength (MPa) | 260 | 310 | 1,034 | 1,000 |
| Strength-to-weight (MPa/g/cm³) | 147 | 115 | 132 | 226 |
| Rough finished-fastener cost vs Grade 8 steel | ~6–8x | ~3–5x | baseline | ~20x+ |
The two numbers that matter to a buyer reading the table: density (where magnesium wins outright) and strength-to-weight (where magnesium beats aluminum, beats steel, and trails titanium by ~35%). Magnesium isn't the strongest material on the table. It's the lightest material that's still strong enough to be a real structural fastener, and the price puts it within range of mid-tier manufacturing budgets that titanium would blow apart.
The alloy designation tells you the recipe. The two common workhorses you'll see on a magnesium fastener BOM:
AZ31: 3% aluminum, 1% zinc, balance magnesium. The default. Good ductility, good machinability, lowest cost.
AZ91 : 9% aluminum, 1% zinc, balance magnesium. Higher strength, more brittle, used where the spec needs more tensile.
Specialty alloys (WE54, ZK60, Elektron 21) exist for higher-temperature or higher-strength applications and command an additional premium.
The four applications that fit
Magnesium fasteners are a special-order product at most distributors and the spec doesn't make sense for every part. The four applications where they consistently earn the cost premium:
Electric vehicle battery enclosures. Lightweight fasteners are showing up in battery-cell-to-frame attachments, module brackets, and pack cover assemblies as OEMs work the weight budget down. Industry analysts project the EV lightweight fastener market growing roughly 13.5% annually through 2030 (Stratview Research via GlobeNewsWire, March 2025. Magnesium is one of the materials in that mix wherever the part doesn't see standing water or chloride exposure and the galvanic interface can be controlled.
Drone and UAV frames. Commercial drone manufacturers have moved magnesium fasteners into airframe assembly. The weight savings affect flight time directly. Industrial UAV builders are the most active group buying mid-volume magnesium fasteners outside of aerospace prime contractors.
High-performance motorcycle and racing components. Engine cases, wheels, brackets. MotoGP teams run forged magnesium wheels because a pair weighs roughly half what aluminum die-cast rims do (Box Repsol); the bracketry and case hardware around those assemblies is where magnesium fasteners earn their keep.
Portable equipment housings and frames. Camera tripods, professional video rigs, surveying equipment, ruggedized portable electronics. Anywhere a worker carries the equipment all day, the weight savings is paid back in field productivity.
What the spec is not for: building HVAC, conveyor framing, structural steel attachments, fastener applications exposed to standing water or chlorides, anything in continuous contact with dissimilar metals without isolation.
The galvanic corrosion trap
This is where most magnesium spec decisions go wrong.
Magnesium is the most anodic structural metal in the galvanic series. It corrodes faster than any other commonly-used metal when paired in an electrolyte — water, condensation, salt spray, even high humidity. A magnesium bolt in direct contact with a stainless steel washer, exposed to outdoor humidity, can show measurable corrosion in months.
The mitigation is mechanical and chemical:
Isolation. Polymer or fiber washers between the magnesium fastener and any steel or aluminum it touches. Insulating sleeves through the bore. Standard practice in aerospace; same approach applies to industrial.
Coatings. The fastener gets coated before it ever touches the assembly. Three real coating systems:
Chromate conversion (DOW17, Type I, II, III). The original. Cheap, well-understood. Hexavalent chromium varieties are environmentally regulated in most jurisdictions now; trivalent chromate alternatives are common.
Anodizing (AMS 2476 baseline; proprietary systems include Tagnite and Keronite). Thicker oxide layer; better corrosion resistance than chromate but more brittle under shock loading.
Plasma Electrolytic Oxidation (PEO). Premium option. Forms a hard ceramic oxide layer integral to the surface (PEO process review, NIH PMC). Best corrosion resistance available; runs several times the cost of chromate.
NASA's published corrosion-protection work on AZ31B magnesium (NASA/TP-97-206239) is a useful public reference for coating choice and mixed-metal assembly behavior.
If the spec on your BOM calls for a magnesium fastener and doesn't call out the coating system and isolation hardware, the spec is incomplete. Push it back to engineering before you order.
Thread engagement is different
Magnesium is softer than steel. Bearing strength under the bolt head is lower. Three changes versus a steel install:
Increase thread engagement. The 1.5×D rule of thumb for steel-to-steel becomes at least 2×D for steel-into-magnesium — some engineering references recommend 2.2×D (Component Solutions Group).
Add a load-distribution washer. A flat washer larger than standard prevents the bolt head from gouging the magnesium surface during torque.
Control the torque. Magnesium creeps under sustained load. Use a calibrated torque wrench, never an impact driver. Re-torque after the first thermal cycle.
This isn't a steel install with a different material number. It's a different fastener system that uses a magnesium bolt as one component.
The buyer's playbook
Verify the spec is complete. Alloy designation (AZ31 vs AZ91 vs specialty), coating system named explicitly, isolation hardware called out. If any of those three are missing, the part will fail in the field and the warranty conversation is yours.
Source through a distributor with specialty experience. Magnesium isn't sitting on the average distributor's shelf. Lead times are typically 4-8 weeks for AZ31, 8-12 weeks for specialty alloys. Melfast specializes in sourcing custom and special-order fasteners for OEM applications.
Don't over-spec. If the application can use aluminum and tolerate the extra weight, aluminum is a third the cost and ships from stock. Magnesium pays back its premium only where weight is genuinely the binding constraint.
Magnesium fasteners are a real engineering tool when the application fits. They're a procurement headache when they're being spec'd by someone who saw an aerospace trend article and didn't think through the install conditions. The line between those two cases is the spec sheet.
For pricing, availability, or help sourcing magnesium fasteners for your application, contact us at [email protected].
