⚡ Key Takeaways
- Four products: self-tapping/self-forming screws, threaded inserts, galvanic-corrosion-resistant aluminium bolts, and sealing plugs.
- Self-forming screws: a light-alloy die casting needs only a plain hole; the screw forms its own thread and the interference fit gives excellent anti-loosening.
- Threaded inserts: carbon or stainless steel, raising a tapped hole to class 8 and above, even class 12.
- Galvanic corrosion: high-strength aluminium bolts with coated heads eliminate the problem when fastening magnesium die castings.
- Sealing plugs: pull-in or press-in types are almost 100% leak-tight with near-zero failure rates, to IP68.
- The principle: a small potential difference between aluminium and magnesium (chemical separation) plus a coated head (physical separation) — two lines of defence.
📑 Contents
1. Why light-alloy die castings are hard to fasten
2. Self-tapping, self-forming screws
3. Threaded inserts
4. Preventing galvanic corrosion
5. Sealing plugs
1. Why light-alloy die castings are hard to fasten
2. Self-tapping, self-forming screws
3. Threaded inserts
4. Preventing galvanic corrosion
5. Sealing plugs
1. Why light-alloy die castings are hard to fasten
EV gearbox housings, motor housings and battery trays use large amounts of aluminium and magnesium die casting:
- Aluminium ADC12: a directly tapped thread strips after 3–5 assemblies;
- Magnesium AZ91D: with an electrode potential of −1.6 V it is far more active than carbon steel, so galvanic corrosion is severe;
- Weight: aluminium at 2.7 and magnesium at 1.8 g/cm³ are 60–75% lighter than steel.
2. Self-tapping, self-forming screws
From the industry handbook: to raise assembly efficiency and reliability, a light-alloy die casting needs only a plain hole — the self-forming screw cuts its own thread as it goes in. Efficiency improves and the interference fit delivers excellent anti-loosening performance.
Compared with the traditional route:
| Approach | Operations | Efficiency | Anti-loosening |
|---|---|---|---|
| Die-cast, tap, then machine screw | Drill, tap, clear chips, drive screw | Low | Moderate |
| Self-forming screw | Plain hole, drive screw | High | Excellent, thanks to interference |
3. Threaded inserts
For locations that need high strength and repeated assembly, such as gearbox housings:
- Material: carbon or stainless steel;
- Reinforce tapped holes in aluminium, magnesium and other light-alloy castings and extrusions;
- Thread strength reaches class 8 and above, and even class 12 and above.
4. Preventing galvanic corrosion
The magnesium problem: magnesium sits at −1.6 V and carbon steel at −0.44 V — a 1.16 V difference. In a damp environment the magnesium becomes the anode and corrodes rapidly.
The solution is two lines of defence:
- Material matching: high-strength aluminium bolts (ZT68 and similar) sit close to magnesium in the galvanic series;
- Physical separation: an anti-galvanic coating on the bolt head isolates the joint.
5. Sealing plugs
| Type | Sealing | Best for |
|---|---|---|
| Conventional plug | Moderate | Locations that must be opened again |
| Pull-in / press-in | Close to 100%, near-zero failure, IP68 | Process holes that must be reliable and are never reopened |
FAQ
Q: How does this differ from an ordinary self-tapping screw? A self-forming screw displaces material instead of cutting it, so there are no chips and the fit is tighter.
Q: Why not use steel bolts in magnesium? Galvanic corrosion — the magnesium acts as the anode and is consumed.
Q: What does IP68 mean for a sealing plug? Protection against short immersion, so oil cannot weep from a gearbox housing.
🛠️ Engineering support: Toprecision supplies self-forming screws, threaded inserts, aluminium bolts and sealing plugs for light-alloy die castings as one package.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote