Lightweighting New Energy Vehicles: How Aluminum-Alloy Bolts Achieve Structural Weight Reduction
Executive Summary
Weight reduction is a key way to extend EV range, and connecting parts are often an overlooked opportunity. Aluminum-alloy bolts, at roughly one third the density of steel, can replace steel bolts in powertrain and housing joints while preserving joint strength. Combining real aluminum grades such as AW-6056 and ZT68 with manufacturing process, this article explains why aluminum bolts allow short engagement, why they avoid galvanic corrosion with magnesium/aluminum parts, and the boundaries engineers must respect.
Table of Contents
- Why Lightweighting Starts with Connecting Parts
- The Aluminum-Bolt Material System: AW-6056 and ZT68
- Short Engagement Length: The Mechanical Logic of 1.3d to 1.5d
- Galvanic Corrosion: The Natural Match Between Aluminum Bolts and Light Alloys
- Manufacturing: Cold Heading, Solution Treatment, Aging and Thread Rolling
- Selection and Engineering Recommendations
Every certain percentage of weight saved in a new energy vehicle improves range and energy consumption. Body structural parts already make heavy use of aluminum and magnesium die castings, but if the mating connecting parts still use steel bolts, weight is added and galvanic-corrosion risk is introduced. With a density about one third that of steel, aluminum-alloy bolts reduce weight while performing the connection function, becoming an important choice for powertrain and light-alloy housing joints.
Why Lightweighting Starts with Connecting Parts
A vehicle carries thousands of bolts. If the medium-and-high-strength bolts at critical joints were all changed from steel to aluminum, the accumulated weight reduction would be considerable. More importantly, aluminum bolts are closer in electrochemical potential to aluminum and magnesium clamped parts, which avoids galvanic corrosion between steel bolts and light alloys at the source; this is especially critical in hot, humid and salt-spray environments. Of course, aluminum bolts are weaker than same-size steel bolts, so they are not a universal replacement. Instead they are precisely substituted at joints where strength allows and the weight-saving benefit is large.
The Aluminum-Bolt Material System: AW-6056 and ZT68
Engineering-grade high-strength aluminum-alloy bolts are mainly heat-treatable Al-Si-Mg-Cu alloys. According to the Volkswagen material standard VW 60469, AW-6056 (EN AW-AlSiMgCuMn, material code AL9) is a typical grade, with about 0.7%–1.2% silicon, 0.65%–0.85% copper, 0.6%–1.0% magnesium and 0.55%–0.65% manganese. In M6–M12 sizes, 6056 aluminum bolts offer tensile strength around 420 MPa, yield strength around 350–600 MPa and elongation after fracture of at least 7%. For higher strength demands, there is the ZT68 high-strength aluminum bolt, reaching about 630 MPa tensile strength, about 500 MPa yield strength, elongation of at least 6%, hardness around 187–204 HV, and strength retention above 95% after thermal exposure.
| Comparison | Steel bolt (10.9/12.9) | AW-6056 aluminum bolt | ZT68 high-strength aluminum bolt |
|---|---|---|---|
| Relative density | ~7.85 g/cm³ | ~2.7 g/cm³ (1/3 of steel) | ~2.7 g/cm³ |
| Tensile strength | 1000 MPa class and above | ~420 MPa | ~630 MPa |
| Yield strength | 900 MPa class and above | ~350–600 MPa | ~500 MPa |
| Typical engagement length | ~2d–3d | ~1.3d–1.5d | ~1.3d–1.5d |
| Galvanic corrosion with Mg/Al parts | Clear potential difference | Similar potential, basically avoided | Similar potential, basically avoided |
Short Engagement Length: The Mechanical Logic of 1.3d to 1.5d
When aluminum bolts are used in aluminum or magnesium components, the engagement length usually only needs 1.3d–1.5d (d is nominal thread diameter), whereas steel bolts in soft materials often need 2d–3d. This is because the aluminum bolt and the clamped material are similar, so their thread strengths match and the problem of "the bolt is fine but the base thread strips first" does not occur. Short engagement means the boss can be smaller and thinner, further saving weight and space. It should be emphasized that the tightening torque–strain curve of aluminum bolts is relatively flat in the yield region, so torque-angle control is more suitable; tightening directly by steel-bolt feel is unreliable.
Galvanic Corrosion: The Natural Match Between Aluminum Bolts and Light Alloys
When a carbon-steel bolt contacts magnesium alloy AZ91D, the potential difference between magnesium and iron is large, and corrosion pits and spalling readily appear at the magnesium hole edge under hot, humid conditions. An aluminum-alloy bolt, by contrast, is close in potential to the magnesium part, with almost no obvious corrosion around the hole; with aluminum parts it is at the same potential and galvanic corrosion basically does not occur. This is the core value of aluminum bolts on NEV light-alloy housings: they reduce weight while eliminating the corrosion hazard of multi-material joints.
Manufacturing: Cold Heading, Solution Treatment, Aging and Thread Rolling
The typical process route for high-strength aluminum bolts is cold heading → solution treatment → age hardening → post-heat-treatment thread rolling → cleaning and sorting. Solution treatment heats the bolt to about 480–530 °C so that Mg, Si and Cu dissolve into the matrix to form a supersaturated solid solution; low-temperature aging at about 150–200 °C for several hours precipitates fine, dispersed strengthening phases. Aluminum bolts emphasize "post-heat-treatment thread rolling": this improves mechanical properties, introduces residual compressive stress in thread roots, reduces notch effect and raises fatigue strength, while lowering intergranular-corrosion sensitivity and thread-damage risk.
Selection and Engineering Recommendations
- Prefer aluminum bolts where strength allows: housings, covers and brackets with medium absolute strength requirements and large weight-saving benefit.
- Choose material by load: AW-6056 for general housings; consider ZT68 where higher strength and load are required.
- Design engagement at 1.3d–1.5d, shrinking bosses for further weight saving.
- Use torque-angle tightening, recalculating the torque window from the aluminum bolt's yield strength.
- Pair with magnesium/aluminum parts to exploit potential matching and avoid galvanic corrosion.
The value of aluminum-alloy bolts is not merely "light" but "matching of the material system": it places the fastener and the light-alloy housing at the same potential and the same strength order, solving weight reduction and corrosion protection at the same time.
Conclusion
From an engineering standpoint, the shift from steel to aluminum bolts is not a simple material swap but a rebalance of the whole joint. Because the modulus of elasticity of aluminum is roughly a third that of steel, an aluminum bolt of the same nominal diameter stretches more under the same preload, which raises sensitivity to anything that reduces effective clamping—such as gasket compression, thermal cycling scatter, and settling under dynamic load. This is why engagement length is deliberately pushed to 1.3d–1.5d rather than the 1.0d typical of steel. Designers should also re-examine friction coefficient: anodized or plated aluminum surfaces behave differently from zinc-plated steel in the torque–preload relationship, so the tightening torque must be re-calibrated rather than copied from a steel BOM. In volume production, batch consistency of the heat-treat state (solution treatment plus artificial aging) becomes the controlling factor for strength; a single under-aged lot can drop yield by more than twenty percent. Pairing material selection with process control, testing, and traceability is what turns aluminum lightweighting from a laboratory idea into a reproducible production reality.
In the lightweighting of new energy vehicles, aluminum-alloy bolts are a typical "small part, big benefit" solution: at about one third the density of steel, AW-6056 and ZT68 offer a strength range from 420 MPa to 630 MPa, 1.3d–1.5d short engagement makes bosses compact, and potential matching with aluminum/magnesium parts naturally avoids galvanic corrosion. Engineering performance is guaranteed through the process window of cold heading, solution aging and post-heat-treatment thread rolling. YF Zhichengjia possesses cold-heading and heat-treat capability for high-strength aluminum-alloy bolts and can provide samples, testing and selection support in AW-6056, ZT68 and related materials. See the technical document library or contact us.
