Aluminum & Mixed-Material Joints in EV Drivetrains: Why Torque-Only Control Falls Short

Aluminum is everywhere in EV drivetrains: housings, motor structures, busbars. The material saves weight, dissipates heat efficiently - and creates fastening challenges that a simple torque wrench simply cannot handle. Ignoring this risks gradual clamp-force loss, elevated contact resistance, and, in the worst case, joint failure under operating load.

This article examines the physical effects that interact in aluminum and mixed-material joints - and explains why torque-angle analysis is the only reliable way to assess the true condition of a bolted connection.

The Four Critical Effects in Aluminum Bolted Joints

1. Embedment: More Settlement, Less Clamp Force

Embedment refers to the plastic deformation of the bolt and clamped parts at their contact surfaces. Asperities flatten out, threads and bearing surfaces smooth down - this is unavoidable in any bolted joint. In aluminum, however, the effect is significantly more pronounced than in steel.

The reason: aluminum has a substantially lower yield strength than steel. When designing aluminum bolted joints, the material's low strength, its tendency to flow under load, and the resulting low permissible surface pressure must all be taken into account - there is a real risk of bending and deforming components during assembly. If the permissible surface pressure is exceeded, the material yields under the bolt head or in the threaded engagement, and clamp force drops.

Embedment losses of 5-25% of preload after assembly are normal - critical joints must be re-torqued. With aluminum components, values tend to fall toward the upper end of that range.

2. Oxide Layer: The Invisible Insulation Problem

Aluminum forms a dense aluminum oxide layer within seconds of exposure to air. Due to aluminum's extremely high affinity for oxygen, this oxide layer is both dense and electrically insulating. The existing oxide layer impedes or entirely prevents electrical contact between two aluminum parts, or between aluminum and another conductive metal.

For busbars and current rails in high-voltage systems, this is a direct safety concern: if the oxide layer is not broken through, contact resistance rises - the terminal heats up, and the risk of fire increases significantly. Aluminum also tends to undergo long-term creep under sustained pressure, which can cause bolted joints to loosen over time.

This means: even a correctly torqued busbar connection can lose clamp force over its service life through relaxation and oxide regrowth - with direct consequences for electrical contact resistance.

3. Differential Thermal Expansion in Mixed-Material Joints

EV drivetrains routinely combine different materials: aluminum housings with steel bolts, copper busbars in aluminum receptacles, steel bearings in aluminum motor housings. Aluminum has a coefficient of thermal expansion of approximately 23 × 10⁻⁶ K⁻¹, while steel sits at roughly 12 × 10⁻⁶ K⁻¹ - nearly twice as high.

The differing thermal expansion of the joined materials has a detrimental effect on bolted joints, leading to preload loss and contact corrosion. With every thermal cycle - and EV drivetrains go through many each day - the aluminum component expands more than the steel bolt. As the component cools, its thickness changes thermally to a greater degree than the length of the steel bolt. This is why conventional steel bolts can cause joint failure in aluminum assemblies.

Tests with steel bolts in aluminum showed a residual preload of only 50%; in magnesium components, that figure dropped to just 10%. These numbers illustrate how dramatic clamp-force loss can be when materials are mixed.

4. Relaxation: The Creeping Loss of Clamp Force

Relaxation is the loss of preload caused by plastic deformation of the materials involved. It often goes unnoticed until a joint rattles, leaks, or fails outright. In aluminum, this process begins earlier than in steel because the material's creep tendency is higher - especially at elevated operating temperatures.

In lightweight metal components, steel bolts retain their preload far less effectively than expected - relaxation effects cause significant preload loss even after a short time. In EV drivetrains, where temperatures can range from -30 °C to above 120 °C, this effect is particularly pronounced.

Why Torque-Only Control Fails Here

Torque-controlled tightening is the most widely used fastening method - because it is technically straightforward to implement. But it has a fundamental weakness: it measures friction, not clamp force.

Approximately 90% of the applied tightening torque is consumed by friction in the thread and under the bolt head - only around 10% is converted into axial preload. The friction coefficient varies considerably with material, surface finish, coating, and lubricant - under uncontrolled conditions, resulting preload scatter of 30 to 50% is entirely possible.

With aluminum joints, additional sources of uncertainty come into play:

• Oxide layer on the bearing surface alters the friction coefficient in unpredictable ways
• Different surface coatings (e.g., cathodic dip coatings) act as electrical insulators and affect friction behavior
• Creep of the aluminum under the bolt head means the tightening torque may have been correct - yet clamp force has already dropped significantly hours or days later

Torque-controlled tightening produces preload scatter of ±50% - only angle-controlled or yield-point-controlled tightening reduces this. A torque target value based on steel-on-steel friction coefficients is simply not meaningful for aluminum mixed-material joints.

Torque-Angle Analysis: Making Joint Condition Visible

Torque-angle analysis combines both measured variables into a complete picture of the tightening process. A newer method for assessing joint integrity is the determination of the torque-angle "fingerprint" of a bolted connection. This approach provides a straightforward, practical, and highly informative technique for evaluating the actual preload applied to a joint during the tightening process.

A typical torque-angle tightening curve begins with a non-linear zone in which the components align. This is followed by the linear, elastic region, where preload is built up while the parts are drawn together and the joint stabilizes.

This curve shape is particularly informative for aluminum joints:

• Shallow slope in the linear region -> embedment or creep of the aluminum under the bolt head
• Early inflection point -> yield strength of the aluminum component is reached before the target clamp force is achieved
• Irregularities in the tightening trace -> oxide layer fractures, friction coefficient changes abruptly
• Deviation between repeat measurements -> relaxation or embedment has already occurred

From the point at which true angle-controlled tightening begins, the process is friction-independent. Total preload scatter is therefore lower than with purely torque-controlled tightening.

QUANTEC MCS®: The Compact Bolt Lab for Real-World Use

The QUANTEC MCS® analysis tool from GWK makes exactly this kind of analysis possible in the field - no fixed reference point, no elaborate fixtures required. The fixed-point-free angle measurement captures the rotation angle directly at the tool, independent of any external reference. This is critical when joints in confined installation spaces need to be analyzed - which is the norm in EV drivetrains.

Key technical features of the QUANTEC MCS® for aluminum applications at a glance:

Practical Use Cases in EV Drivetrains

Housing fasteners (die-cast aluminum): Here, the yield strength of the housing material is often the limiting factor. The torque-angle curve shows whether tightening remains within the elastic range of the component or whether creep deformation is already occurring - an effect that is invisible with torque-only control.

E-motor fasteners (steel bolts in aluminum stator): Thermal cycles between cold start and full load generate recurring clamp-force fluctuations. Analyzing the tightening curve across multiple cycles reveals whether the joint remains stable or whether re-torquing is required.

Busbar connections (copper/aluminum): As the insulating interlayer grows, more power is dissipated as heat, which in turn increases thermal output. Torque-angle analysis detects whether the oxide layer was broken through during tightening - visible as a characteristic inflection in the curve - and whether the required contact force was actually achieved.

Process Reliability in Series Production

Analysis with the QUANTEC MCS® is not just a development tool. It provides the data foundation for designing the series production process:

For teams that want to use the QUANTEC MCS® on a project basis first, GWK offers the GWK ToolRent® rental system - calibrated instruments available on demand, by the week, month, or year, with worldwide shipping.

Conclusion

Aluminum bolted joints in EV drivetrains are not a standard case. Embedment, oxide layers, low yield strength, differential thermal expansion, and relaxation all interact - and none of these effects can be captured with a torque wrench alone.

Torque-angle analysis makes joint condition visible: it shows whether clamp force was actually achieved, whether the component remains within its elastic range, and whether the joint stays stable across thermal cycles. The QUANTEC MCS® with fixed-point-free angle measurement brings this analytical capability directly into development and quality assurance - compact, precise, and without any fixed reference point.

Accuracy by GWK.