ZT68 High-Strength Aluminum Alloy Bolts: How 600 MPa Strength Balances Lightweighting
Executive Summary
Magnesium alloy die castings are being used more and more, yet steel bolts look increasingly strained: 6056 aluminum bolts are only 420 MPa and lack strength; carbon steel bolts have enough strength but corrode the magnesium base and suffer thermal expansion mismatch. ZT68 raises the tensile strength of cold-headed high-strength aluminum alloy to over 600 MPa, while staying close in potential to magnesium alloy and matching its coefficient of thermal expansion. This article breaks down how 600 MPa and lightweighting hold true at the same time, and gives a selection comparison table.
Table of Contents
Three Hurdles Steel Bolts Face on Magnesium Alloy
On NEV e-drive housings or battery tray projects, engineers quickly run into a deadlock: the structural parts have switched to magnesium or aluminum alloy die castings, yet the fasteners are still carbon steel.
First is corrosion. The electrode potential of magnesium alloy is about −1.6 V, while carbon steel is about −0.44 V — a difference of more than 1 V. In humid or water-exposed conditions, this potential difference is enough to drive a continuously operating galvanic cell: the magnesium base, as the anode, is constantly consumed, the threaded hole corrodes away, and the joint ultimately fails.
Second is thermal expansion mismatch. The coefficient of thermal expansion of magnesium alloy is about twice that of steel. As the vehicle undergoes high-low temperature cycles, the bolt and the parent material deform differently, preload is repeatedly consumed, and the end result is loosening, abnormal noise and seal failure.
The 6056 aluminum bolts on the market solve corrosion and weight, but their tensile strength is only about 420 MPa, which is hard to meet the requirements of NEV three-electric systems for high torque, high load and long life. Strength, corrosion resistance and lightweighting have long had to be chosen two out of three.
Where 600 MPa Comes From
ZT68 belongs to the 7-series aluminum alloy family, with tensile strength steadily above 600 MPa, about 40% higher than the mainstream 420 MPa high-strength aluminum bolts on the market. This number is not achieved by simply switching grades; it is jointly determined by the material system and the heat-treatment route.
Solutionizing, quenching and aging — three steps
- Solutionizing: Fully dissolve solute atoms such as Zn, Mg and Cu into the matrix, preparing for later precipitation.
- Quenching: Rapid cooling produces a supersaturated solid solution, forcibly keeping solute atoms in the lattice.
- Artificial aging: Promotes uniform precipitation of fine, dispersed strengthening phases (such as MgZn2) that pin dislocations; this is the main source of strength.
Thread rolling after heat treatment and cold heading forming
The thread root is the fatigue weak point of a bolt. Rolling the threads after heat treatment introduces a uniform layer of residual compressive stress at the root; the tensile stress in service must first cancel out this compressive layer, so fatigue crack initiation is significantly delayed.
ZT68 Technical Specifications
| Item | Typical Value | Note |
|---|---|---|
| Material system | 7-series high-strength aluminum alloy (Zn-Mg-Cu) | Solutionized + quenched + aged |
| Tensile strength Rm | ≥ 600 MPa | Vs. mainstream aluminum bolts at 420 MPa |
| Yield strength Rp0.2 | ≥ 520 MPa | The upper preload limit is set by this |
| Elongation A | ≥ 8% | Balances strength and toughness |
| Density | about 2.8 g/cm³ | About 36% of steel |
| Weight reduction | About 60%–65% vs. same-size steel bolt | By equal volume |
| Thread engagement length | 1.3d – 1.5d | Steel bolts usually need 2d or more |
| Total friction coefficient μtot | 0.09 – 0.15 | Controlled by a three-stage lubrication system |
| Salt-spray performance | Stably passes 60 days of neutral salt spray | Anodizing + dense microstructure working together |
How Strength and Lightweighting Hold at the Same Time
Many people think strength and lightness must be opposed, but the key is specific strength — strength divided by density.
| Fastener Solution | Tensile Strength (MPa) | Density (g/cm³) | Specific Strength (MPa·cm³/g) |
|---|---|---|---|
| Carbon steel class 8.8 | 800 | 7.85 | about 102 |
| Carbon steel class 10.9 | 1040 | 7.85 | about 132 |
| Conventional aluminum bolt 6056 | 420 | 2.70 | about 156 |
| ZT68 high-strength aluminum alloy | ≥ 600 | about 2.80 | about 214 |
This table shows two things. First, even though 6056 is only 420 MPa, its specific strength already exceeds that of a class 10.9 steel bolt — lightweighting itself is a kind of strength. Second, after ZT68 raises strength to 600 MPa, its specific strength reaches about 214, meaning it can carry about 1.6 times the load of a class 10.9 steel bolt at the same weight. Translated to a real project: a M8×40 class 10.9 steel bolt weighs about 21 g, while the same-size ZT68 bolt weighs about 7.5 g.
Two Bonus Points Beyond Specific Strength
Close potential, suppressing galvanic corrosion at the source
The driving force of galvanic corrosion is the potential difference. The electrode potential gap between ZT68 and a magnesium alloy base is far smaller than the 1-V-order gap between carbon steel and magnesium alloy, so the driving voltage of the galvanic cell is greatly weakened. Together with the dense oxide film formed by anodizing, it can stably pass a 60-day neutral salt spray test.
Close coefficient of thermal expansion, eliminating thermal loosening
The closer the coefficient of thermal expansion between bolt and clamped part, the smaller the preload decay under temperature cycles. ZT68 uses an aluminum-based material whose coefficient of thermal expansion is highly matched to magnesium alloy; under cycling from −40 ℃ to 150 ℃, the bolt and the parent material deform nearly in sync, with very little preload fluctuation.
Four Typical Application Scenarios
| Application Position | Main Operating Challenges | Where ZT68 Helps |
|---|---|---|
| Electric drive system (motor housing, reducer flange, inverter enclosure) | Long-term 150 ℃ high temperature + high torque | Thermal expansion matching suppresses preload decay; 600 MPa supports high torque |
| Power battery system (magnesium tray, case frame, water cooling plate) | High sealing requirements + humid corrosion | Close potential removes corrosion driving force; anodizing thickens protection |
| Lightweight chassis (control arm, knuckle, subframe) | Complex alternating loads and shocks | Post-heat-treatment thread rolling creates root residual compressive stress, raising fatigue strength |
| Large die castings (instrument panel beam, body interior structure) | High-takt automated assembly | μtot stable at 0.09–0.15, low torque dispersion |
Selection Advice and Closing
When should ZT68 be considered
- The clamped part is a magnesium or aluminum alloy die casting, with humid or water-exposed working conditions;
- Service temperature has clear cycles (e.g. −40 ℃ to 150 ℃), and steel bolts show preload decay;
- The project has explicit weight-reduction targets, and fastener weight is included in the calculation;
- The assembly line uses automated tightening and requires low torque dispersion;
- Installation space is compact and cannot provide the 2d+ engagement length required by steel bolts.
When caution is needed
- Clamped part is carbon steel or cast iron: galvanic corrosion and thermal expansion mismatch risks are low, and the strength ceiling of aluminum bolts becomes the main shortboard;
- Service temperature exceeds 150 ℃ for long periods: the stability of the precipitated phases must be re-evaluated;
- Load-bearing structures with extreme load spectra: aluminum bolts have a low elastic modulus, so stiffness-sensitive conditions require dedicated verification.
The difficulty of lightweighting has never been reducing the weight of a single part, but whether the joint still holds after weight reduction. The value of ZT68 lies in putting the lightness of aluminum alloy, the compatibility with magnesium alloy, and usable strength into the same bolt, so that designers choosing lightweight structural parts no longer have to compromise on fasteners.
