ZT68 High-Strength Aluminium Bolts: 630 MPa, Cold Headed

1. The screw that held lightweighting back

EVs, eVTOL and humanoid robots write lightweighting into every specification, and magnesium and aluminium structures are now common. One detail is often overlooked: the lightweight structure is still held together by small screws.

Conventional carbon steel bolts are a poor match for magnesium and aluminium — galvanic corrosion, thermal expansion mismatch and insufficient strength have troubled OEMs and tier suppliers for years. Existing 6056 aluminium bolts at about 420 MPa solve the corrosion problem but cap the strength too low for the torque and load of an e-powertrain.

Drawing on more than 20 years of fastener development and manufacturing, Toprecision developed ZT68, a cold-headed high-strength modified 7000-series aluminium bolt that puts steel strength into an aluminium weight.

2. Three failures of conventional bolts in magnesium

Problem one: galvanic corrosion

Magnesium sits at about -1.6 V and carbon steel at about -0.44 V, a difference of roughly 1.16 V. In damp, wading or condensing conditions the magnesium becomes the anode and corrodes quickly: white rust, eroded threads, flaking and leaking housings.

Problem two: thermal expansion mismatch

Magnesium expands at about twice the rate of steel. Around engines and gearboxes the two move differently and preload decays continuously. A class 10.9 steel bolt can lose more than 96 % of its preload at 150 °C, ending in loosening, seal failure and noise.

Problem three: 6056 is not strong enough

6056 (AL9) at about 420 MPa tensile and 350 MPa yield cannot meet the torque, load and life requirements of an e-powertrain, which has been the technical bottleneck on magnesium lightweighting at scale.

3. ZT68 measured against 6056

By controlling the precipitation of Mg, Zn and Cu strengthening phases and applying a solution-quench-age treatment, ZT68 reaches about 630 MPa tensile and 500 MPa yield with elongation of at least 8 % (measured mean 8.21 %).

4. Four core benefits

4.1 Weight

At 2.7 g/cm³ the bolt is about one third of the steel equivalent, a saving of up to 65 %. A single gearbox saves 0.5–1 kg, which across a production run makes a real contribution to vehicle mass and range.

4.2 No galvanic couple

Matching the potential of magnesium and aluminium removes the driving force for galvanic attack at source. With anodising it passes 60 days of neutral salt spray with good intergranular corrosion resistance, suited to the humid environment of e-drives and battery packs.

4.3 Relaxation resistance

The expansion coefficient is close to magnesium, so bolt and housing move together through thermal cycles and preload decays far less than with steel. After 12 hours at 150 °C tensile remains above 600 MPa, covering motor housings and power electronics.

4.4 Assembly

The integral flange is cold headed in one piece, eliminating a separate washer and a second assembly step. A graded lubrication package holds the total friction coefficient between 0.09 and 0.15 with low torque scatter, which suits fully automated lines.

5. The manufacturing chain

ZT68 breaks the industry assumption that high-strength aluminium bolts cannot be cold headed at volume:

1. One-piece cold heading: the flange is formed in a single step with continuous grain flow and no separate washer.
2. Cleaning and pre-treatment.
3. Precision heat treatment: solution, quench and age to precipitate the strengthening phases.
4. High-precision rolling after heat treatment: the thread root develops uniform residual compressive stress that offsets service tension and raises fatigue strength.
5. Second cleaning.
6. Finish and final inspection: anodising or a lubricating coating with controlled friction.

Tool design, cold heading, rolling, machining, heat treatment, surface finishing, automated assembly and optical sorting all run in-house, supported by metallographic microscopes, universal tensile testers and friction testers.

6. Typical applications

• E-drive systems: magnesium motor housings, reducer flanges, inverter cases.
• Battery systems: magnesium trays, frame rails, cold plates.
• Lightweight chassis: magnesium-aluminium control arms, steering knuckles, subframes.
• Large magnesium die castings: instrument panel frames, body castings.
• eVTOL and low-altitude aviation: every kilogram removed adds kilometres of range.
• Humanoid robots: joint housings, where less mass means more payload and endurance.

7. Decision guide and FAQ

Decision guide

• Steel to steel: use a steel bolt.
• Magnesium or aluminium to the same, at room temperature: ZT68.
• Magnesium to steel under heavy load: steel bolt with an insulating sleeve.
• Above 150 °C: titanium or steel.

FAQ

Q. Can ZT68 replace class 10.9? No. ZT68 matches class 6.8 and is intended for magnesium and aluminium lightweight joints, not heavy steel-to-steel connections.

Q. Why not titanium? Titanium is stronger but costs five to ten times more, and where the potential already matches the parent metal ZT68 is the better value.

Q. Can it be anodised? Yes, black or clear, with 60-day salt spray performance.

Q. What does a friction coefficient of 0.09–0.15 mean? At the same torque, preload is 30–50 % higher than with zinc-plated steel (K ≈ 0.20), so the torque schedule must be recalculated.