Lightweighting Applications of ZT68 High-Strength Aluminum Bolts: The Weight-Saving Calculus from Three-Electric Systems to Robots and Low-Altitude Economy
Executive Summary
Aluminum alloy density is only one-third that of steel, so a same-spec bolt can be about 65% lighter—but in the past aluminum bolts were too weak to carry load. The ZT68 cold-headed high-strength aluminum-alloy bolt reaches 600 MPa-class tensile strength yet matches magnesium/aluminum substrates in potential and thermal expansion, and is now batch-replacing steel bolts in NEV three-electric systems, humanoid-robot joints and eVTOL low-altitude economy. This article lays out the weight-saving calculation along with the boundaries for replacing steel and titanium.
Table of Contents
Why Lightweighting Starts with Fasteners
In the design constraints of NEVs, humanoid robots and eVTOLs (electric vertical take-off and landing aircraft), "every gram costs money" is no longer a slogan. Industry estimates are direct: every 100 kg off an NEV raises range by 6%–8%; for an EV where the battery accounts for nearly half the cost, lightweighting either lets you fit fewer cells or go tens of kilometers further on the same charge. Fasteners are the largest part family in the machine—an NEV has thousands to tens of thousands of bolts, and a humanoid robot's joints and skeleton connections also run to a thousand or more—so replacing this whole weight block yields a substantial cumulative gain.
In the past engineers assumed fasteners were steel, and lightweighting relied mainly on thinning sheet metal and switching to aluminum-alloy die castings. But once structural parts switch to aluminum/magnesium alloy while the bolts remain carbon steel, two new problems immediately appear: galvanic corrosion between dissimilar metals, and preload decay from thermal-expansion mismatch. It is against this backdrop that high-strength aluminum-alloy bolts have been pushed into load-bearing connections.
How Much Weight Does an Aluminum Bolt Save: A Material-Density Comparison
Aluminum-alloy density is about 2.7 g/cm³, less than one-third of carbon steel (about 7.85 g/cm³). Replacing an M8 external-hex flange bolt of the same spec and head type with aluminum theoretically saves about 65% weight. Below is a physical comparison of three common lightweight fastening materials:
| Material | Density g/cm³ | Young's Modulus GPa | Linear Expansion 10⁻⁶/°C | Weight Saving vs Steel |
|---|---|---|---|---|
| Carbon steel (8.8/10.9 grade) | ~7.85 | ~210 | ~12 | Baseline |
| Titanium alloy TC4 | ~4.5 | ~110 | ~9 | ~40% |
| High-strength aluminum ZT68 | ~2.7 | ~69 | ~23 (close to magnesium's 26) | ~65% |
More important is thermal-expansion matching. Aluminum's linear expansion coefficient is about 23×10⁻⁶/°C, very close to magnesium alloy (about 26×10⁻⁶/°C), while carbon steel is only about 12×10⁻⁶/°C. When electric-drive housings and battery trays repeatedly thermally cycle between -40°C and 150°C, the deformation difference between the steel bolt and magnesium substrate exceeds a factor of two, and preload is continuously consumed; switching to an aluminum bolt, the bolt and substrate "deform in sync", preload decay drops markedly, and loosening, leakage and rattling are eased at the root.
Why ZT68: 600 MPa Puts Aluminum Bolts on Load-Bearing Parts
The industry's previously mass-produced mature aluminum bolts were mainly 6056 (EN AW-AlSi1MgCuMn), with tensile strength about 400–440 MPa and yield about 350 MPa. It solved corrosion and thermal-expansion matching, but its strength was insufficient, so it could not handle NEV three-electric high-torque, high-load connections. ZT68 is YF Zhichengjia's in-house 7-series modified high-strength aluminum-alloy cold-headed bolt; using an M8×40 external-hex flange bolt as the baseline part, the measured data of 5 parallel specimens is below:
| Indicator | 6056 Mainstream Aluminum Bolt | ZT68 Cold-Headed Aluminum Bolt |
|---|---|---|
| Tensile strength | ~400–440 MPa | Measured mean ~629 MPa (nominal 600 MPa class) |
| Yield strength | ≥350 MPa | Measured mean ~510 MPa |
| Elongation after fracture | ≥6% | ~8.2% |
| Strength retention after 150°C exposure | Long-term ceiling ~150°C | Retention 95.58%–97.16% |
| Neutral salt spray (after anodizing) | Needs paired coating | Stably passes 60 days |
| Total friction coefficient μtot | — | Controllable 0.09–0.15; suits automatic tightening |
Strength comes from the combination of "solution + quench + aging" heat treatment and post-heat-treatment thread rolling: solution treatment dissolves solute atoms such as Zn, Mg and Cu into the matrix, and aging precipitates fine, dispersed MgZn₂ strengthening phases that pin dislocations; thread rolling after heat treatment presses residual compressive stress into the thread root, offsetting service tensile stress and raising fatigue strength. In the head 6° wedge test, all specimens fractured in the unengaged thread section with no head cracking, showing the cold-headed head bond is reliable.
Lightweighting Gains in Three Major Scenarios
- NEV three-electric systems: magnesium-alloy motor housings, reducer flanges, electronic-control housings, battery trays and water-cooling plates. Here weight saving converts directly into range, while also solving steel-magnesium galvanic corrosion and thermal-expansion mismatch; paired with AZ91 magnesium alloy, ZT68 has almost no corrosion driving force, and after anodizing passes 60 days of neutral salt spray.
- Humanoid robots: joint modules, skeleton and housing connections are extremely weight-sensitive—every kilogram off self-weight reduces joint-motor output burden and standby power. Aluminum bolts are low-density and non-magnetic, friendlier to magnetic encoders and sensors inside joints, suiting dense connections in highly integrated cavities.
- Low-altitude economy (eVTOL/drones): every kilogram off an aircraft's empty weight directly benefits range and payload. After continuous 150°C thermal exposure, ZT68 strength stays stable above 600 MPa with no creep relaxation, and can withstand temperature cycling and continuous vibration near the power bay.
The Boundaries for Replacing Steel and Titanium
Aluminum bolts are not a universal substitute. Titanium alloy saves about 40% weight, has higher specific strength and better heat resistance, but material and processing cost is high; aluminum bolts save more weight (~65%) at far lower cost than titanium, suiting high-volume, cost-sensitive vehicle and robot scenarios. The selection boundary is clear:
| Dimension | Keep Steel Bolt | Switch to ZT68 Aluminum | Choose Titanium Alloy |
|---|---|---|---|
| Connected part | Steel/cast-iron thick wall | Mg/Al-alloy die-cast housing | High-temp / aerospace extreme service |
| Core demand | Lowest cost, highest strength | 65% weight saving + anti-galvanic corrosion | 40% weight saving + heat/corrosion resistance |
| Long-term temperature | High | Stable ~150°C | Higher |
| Cost tier | Lowest | Medium (amortized by batch cold heading) | Highest |
Selection Recommendations
- If the connected part is a magnesium/aluminum-alloy die casting and is sensitive to galvanic corrosion and preload decay → prefer ZT68 aluminum bolts, with engagement length designed at 1.3–1.5d.
- If there are many connection points and per-piece weight saving accumulates (battery trays, electric-drive housings, joint skeletons) → the weight gain scales with quantity, and is most worth switching.
- If temperature long-term exceeds 150°C, or fracture toughness is demanded at an extreme level → re-evaluate titanium alloy or superalloy.
- Whichever you choose, mark strength grade, friction-coefficient window and salt-spray requirement on the drawing, and require batch tensile, wedge and salt-spray reports from the supplier.
Conclusion
The lightweighting value of high-strength aluminum-alloy bolts is not just the single material item of "density two-thirds lower". It simultaneously solves three old problems that steel bolts brought to magnesium/aluminum lightweight structures: galvanic corrosion, thermal-expansion mismatch and preload decay. With 600 MPa-class cold-heading mass-production strength, ZT68 has moved aluminum bolts from "soft small connecting parts" onto load-bearing connections in three-electric systems, robot joints and low-altitude aircraft.
YF Zhichengjia, founded in Shenzhen in 2005 with cold heading as its core process, passed IATF 16949 certification in 2013 and entered the NEV supply chain in 2017; it can provide selection support for ZT68 high-strength aluminum bolts from sample development and joint testing to batch delivery. For more connection technical material, see the technical documentation library, or contact our technical engineers for a one-on-one weight-saving and connection evaluation.
