⚡ Key takeaways
- Aluminium bolts weigh 60–65 % less than steel. An M8×30 in steel is about 110 g; in 7075-T6 it is about 38 g. That is why aerospace, EV and robotics programmes specify them.
- But aluminium carries only one third to one half the strength. 6061-T6 reaches about 310 MPa and 7075-T6 about 570 MPa, against 800 MPa for class 8.8 steel. You cannot swap a steel bolt for the same size in aluminium — go up a size or re-check the joint.
- The biggest engineering risk is galling. Aluminium on aluminium, or aluminium on stainless, cold-welds at the thread under heat and pressure, seizing halfway and refusing to come apart. Lubricate, pair dissimilar materials and control torque.
- Galvanic corrosion is the outdoor killer. In contact with steel, stainless or copper in damp conditions, aluminium becomes the anode and corrodes fast. Insulate the interface or use matching materials.
- The torque coefficient is 50–100 % higher than steel. Steel runs at μ ≈ 0.15, aluminium at 0.25–0.35, so the same torque gives only about 60 % of the preload. Recalculate the tightening torque.
- Three common grades: 6061-T6 general purpose and easy to machine, 7075-T6 high strength for aerospace, 2024-T351 high strength with good fatigue. EV structures favour 7075-T6.
- When not to use aluminium: high-preload joints, frequently dismantled joints, service above 150 °C, and outdoor contact with steel plate. Titanium or steel is the safer answer there.
Contents
1. Why aluminium fasteners: weight data and applications
2. Grades compared: 6061, 7075, 2024
3. Mechanical properties against class 8.8 and 10.9
4. Torque coefficient and preload
5. Galling: the most common failure and how to prevent it
6. Galvanic corrosion: pairing aluminium with other metals
7. Surface finish: choosing an anodising type
8. Decision tree and typical applications
2. Grades compared: 6061, 7075, 2024
3. Mechanical properties against class 8.8 and 10.9
4. Torque coefficient and preload
5. Galling: the most common failure and how to prevent it
6. Galvanic corrosion: pairing aluminium with other metals
7. Surface finish: choosing an anodising type
8. Decision tree and typical applications
1. Why aluminium fasteners
Fasteners are a small share of total mass — perhaps 5–10 % of a robot joint — but in aerospace, eVTOL, humanoid robotics and EVs, where every gram is counted, aluminium fasteners earn their place.
| Material | Density (g/cm³) | M8×30 bolt | Weight saved |
|---|---|---|---|
| Steel (class 8.8) | 7.85 | ~110 g | Baseline |
| A2-70 stainless | 7.93 | ~112 g | -2 % |
| 7075-T6 aluminium | 2.81 | ~39 g | -65 % |
| Ti-6Al-4V titanium | 4.43 | ~62 g | -44 % |
Typical applications
- eVTOL and drones: skin panels, motor mounts, avionics bays — every kilogram saved buys five to ten minutes of endurance.
- Electric vehicles: battery pack covers, body panels, interior frames, matched to aluminium extrusion and die castings.
- Humanoid robots: joint housings and lightweight brackets, with steel or titanium retained inside the joint itself.
- Semiconductor equipment: non-magnetic, low-mass assemblies.
2. Grades compared
| Grade | Temper | Tensile (MPa) | Yield (MPa) | Notes | Typical use |
|---|---|---|---|---|---|
| 6061 | T6 | 310 | 275 | Machines well, welds well, moderate strength, cheap | General structure, housings, fixtures |
| 7075 | T6 | 570 | 500 | Highest strength aerospace alloy, moderate corrosion resistance | Aircraft structure, robot joints, loaded brackets |
| 2024 | T351 | 480 | 345 | Good fatigue, heat resistant | Aircraft skin, long-term vibration |
| 5052 | H32 | 230 | 195 | Excellent corrosion, not heat treatable | Housings, sheet, outdoor parts |
💡 Rule of thumb: for bolts and nuts take 7075-T6 — strong enough, and both cold headable and machinable. 6061-T6 for less demanding parts; 5052 belongs in sheet work, not fasteners.
3. Mechanical properties against steel
This is where joints go wrong: engineers substitute the same size in aluminium and the joint comes loose.
| Property | Class 8.8 steel | A2-70 stainless | 7075-T6 aluminium | Ti-6Al-4V |
|---|---|---|---|---|
| Tensile (MPa) | 800 | 700 | 570 | 900 |
| Yield (MPa) | 640 | 450 | 500 | 830 |
| Young's modulus (GPa) | 210 | 193 | 71 | 114 |
| Density (g/cm³) | 7.85 | 7.93 | 2.81 | 4.43 |
| Proof load, M8 (kN) | 29 | 20 | ~11 | ~30 |
⚠️ The critical difference: aluminium has one third the modulus of steel (71 GPa against 210 GPa). At the same preload it stretches three times as much, making the joint highly sensitive to gasket and member stiffness — and prone to "torqued but not preloaded".
4. Torque coefficient and preload
Preload follows F = T / (K·d), where K is the torque coefficient, dominated by friction, and d is nominal diameter.
| Surface condition | Steel K | Aluminium K |
|---|---|---|
| Dry, unlubricated | 0.18–0.20 | 0.30–0.40 |
| Oil | 0.12–0.15 | 0.20–0.25 |
| MoS₂ | 0.10–0.12 | 0.15–0.18 |
The consequence: torque to the steel figure (about 25 N·m for M8×1.25) and the aluminium bolt may reach only 60–70 % of the intended preload. Worse, the aluminium thread is likely to gall at that torque.
💡 Correct practice:
1. Use torque-angle control or a value established by test — never a steel torque table.
2. Apply MoS₂ or a dedicated anti-seize to thread and bearing face.
3. On critical joints, verify preload with a load cell.
1. Use torque-angle control or a value established by test — never a steel torque table.
2. Apply MoS₂ or a dedicated anti-seize to thread and bearing face.
3. On critical joints, verify preload with a load cell.
5. Galling
Galling is one of the most notorious fastener failures: two similar metals, especially aluminium, stainless or titanium, cold-weld at micro-asperities under pressure, friction rises sharply and the thread seizes solid, so the joint has to be drilled out.
Why aluminium galls so readily
- The oxide film is thin and ruptures easily, exposing fresh metal.
- The melting point is low (660 °C), so friction heat causes local melting.
- Aluminium against aluminium diffuses and welds very readily.
Prevention
- Pair dissimilar materials: aluminium bolt with a steel or stainless nut, never aluminium on aluminium.
- Surface treatment: hard anodising at 25–50 μm isolates the metal surfaces.
- Lubrication: MoS₂ paste or nickel-based anti-seize.
- Control speed: no hammering with a pneumatic gun; slow driving reduces friction heat.
- Thread fit: avoid coarse classes below 6H; a little more clearance helps.
6. Galvanic corrosion
Aluminium sits at about -0.85 V, well below steel, stainless, copper and titanium. In the presence of an electrolyte — rain, sweat, salt spray — it becomes the anode and corrodes quickly, sometimes perforating within months.
| Couple | Risk | Treatment |
|---|---|---|
| Aluminium–aluminium | None | Direct contact |
| Aluminium–stainless | High | Insulating washer or coating required |
| Aluminium–zinc-plated steel | Moderate | Zinc acts as sacrificial anode; inspect periodically |
| Aluminium–copper or brass | Very high | Must be isolated; never direct outdoors |
| Aluminium–titanium | Moderate | Isolate in wet environments |
⚠️ A classic failure: stainless bolts driven directly into an aluminium body. Three years later the holes around the bolts are corroded and enlarged and the bolts are loose. The fix is an aluminium bolt with an aluminium nut, or a nylon or stainless insulating washer.
7. Surface finish: choosing an anodising type
| Process | Thickness (μm) | Corrosion | Appearance | Use |
|---|---|---|---|---|
| Natural anodising | 5–10 | Moderate | Clear or silver | Indoor parts |
| Sulphuric anodising (standard) | 10–20 | Good | Dyeable (black, red, blue) | Visible parts |
| Hard anodising | 25–50 | Very good | Dark grey or black | Wear resistance, anti-galling, outdoor |
| Chemical conversion coating | 1–3 | Moderate | Gold | Where electrical conductivity is needed |
💡 Note: anodising grows the coating into the thread — 25 μm of film is 12.5 μm per flank. Re-tap or chase the thread after anodising, or the nut will not go on.
8. Decision tree and typical applications
- Is it structurally loaded?
Yes: 7075-T6, check yield, go up a size if needed.
No, appearance only: 6061-T6. - Outdoor or damp?
Yes: hard anodise plus galvanic isolation.
No: natural anodising is enough. - Will it be dismantled repeatedly?
Yes: aluminium bolt with a steel nut and anti-seize.
No: aluminium on aluminium is acceptable. - Direct contact with steel plate?
Yes: insulating washer, or change to a steel bolt. - High preload required?
Yes, above 30 kN: do not use aluminium; use titanium or steel.
FAQ
Q. 6061 or 7075? 7075-T6 reaches 570 MPa, about 1.8 times 6061-T6 at 310 MPa, but costs more and machines less easily.
Q. Can an aluminium bolt use a steel nut? Yes, but insulate the interface or the aluminium will corrode preferentially.
Q. Does anodising stay conductive? Standard anodising insulates. Only chromate conversion coating conducts, so grounding points need that finish.
🛠️ Engineering support: for lightweighting programmes in robotics, eVTOL, EVs or semiconductor equipment, send us the load, temperature, environment and annual volume and our engineering team will return a written recommendation within 24 hours, covering grade, finish, torque values and whether galvanic isolation is needed.
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