Substituting copper conductors for aluminum ones supports two urgent requirements in the automotive industry.
First, since aluminum is approximately two-thirds lighter than copper, aluminum wire can help reduce the overall weight of automotive wiring harnesses. Even taking into consideration the relationship of conductivity to density, an aluminum conductor with the same resistance is still around 50 percent lighter than its copper equivalent. This weight reduction can help reduce fuel consumption and cut CO2 emissions.
Lightweight construction is equally important for hybrid and electric vehicles, because it can substantially increase vehicle range in the electric driving mode. That, in turn, can help limit the size and weight of the required battery.
Second, since aluminum is much less expensive than copper, aluminum wire can help reduce overall vehicle cost. At press time, aluminum cost $0.70 per pound, while copper cost $2.31 per pound. And, because aluminum is more plentiful than copper, its price is less volatile.
During the past few years, TE Connectivity has introduced several products specifically for aluminum wire. For example, the Copalum crimp design was developed for aluminum conductors in power engineering applications, while the Amplivar family of crimped connectors was expanded to include models for stranded aluminum conductors.
Our experience with those connectors led to the development of the Litealum crimp design for terminating stranded aluminum conductors in automotive applications.
Challenges of Aluminum Conductors
While aluminum has numerous advantages as a wiring material, it also has some properties that can impede its use as a conductor.
The biggest is creep. Any terminal design for aluminum conductors must take that into account. Ideally, the design should create a cold weld between the conductors and the terminal.
Corrosion is another issue. In the presence of moisture at the immediate point of contact, the potential difference between copper (0.3 volts) and aluminum (-1.69 volts) results in the dissolution of aluminum, the baser of the two metals. Measures must be taken to prevent this unwanted effect.
Aluminum is a ductile metal with a pronounced degree of flexural sensitivity. Aluminum possesses only one-third of the mechanical strength of copper. This property must be considered to achieve the required degree of mechanical strength both in the wire itself and also in terms of the pull-out strength of connections.
Another challenge is that aluminum forms a dense, hard oxide layer. While this oxide layer protects the material from progressive corrosion, it also has the characteristics of one of the best-known insulators. Consequently, a good electrical connection requires the oxide layer to be reliably destroyed during termination.
A New Crimp
The Litealum crimp barrel was developed specifically for terminating aluminum conductors. The design and surface properties of the F-crimp barrel, and particularly the crimp termination zone, are tailored to the material properties of aluminum.
The inside of the Litealum crimp barrel has sharp-edged serrations that have the appearance of a washboard. The phrase “shark-fin shaped serration” adequately describes the contour of the ridged edge. During the crimping operation, the serrations break up the oxide layer, exposing the pure aluminum below and thus permitting electrical contact to be established by local cold welding.
The terminal’s design makes use of aluminum’s inherent ductility during crimping. Because of its low yield point, the conductor material undergoes far greater mechanical deformation during crimping than the copper sleeve. The volumetric flow caused by this deformation is displaced axially in both directions along, and into, the sharp ridges of the micro-serrations.
Because copper and aluminum alloy well together, the design of the crimp barrel and its compression ratio were optimized so that cold welding occurs between the materials. The electrical and mechanical properties of the cold weld remain stable over the life cycle of the product.
When the crimping tool is fully closed, partial cold-welded areas are formed between the sleeve and the conductor due to the elongation of the conductor in the longitudinal direction under the impact of the load. There’s a mutual penetration of the conductor materials.
In fact, cold welds account for more than 5 percent of the joint surface. As a result, electrical contact resistance levels are about the same as if the joint were fully welded. Due to this metallurgically bonded connection, the electrical durability is very high.
Mechanically speaking, the crimp connection between the aluminum and copper is actually stronger than between aluminum and aluminum. With a wire cross-section of 1.5 square millimeters, the crimped connection exhibits a pull-out strength of 80 newtons.
After crimping, the remaining residual surface pressure in the joint is approximately 180 newtons per square millimeter at only a small number of points. As a result, no conditions exist that could cause outward creep of the aluminum from the crimp barrel. Consequently, it is not the degree of residual stress in the crimp that is responsible for creating a good electrical contact, but only the partial cold welding.
A mechanical simulation between two cross-sections demonstrates that there is practically no difference between aluminum and copper conductors after crimping. To create the largest possible contacting surface to the copper for the greatest possible number of strands, the Litealum crimp barrel is rolled in as far as possible. The assessment criteria for the copper crimp do not apply to aluminum crimping. All the same, the sleeve does not come to rest against the floor.
The geometry of the Litealum crimp features a gradient at the rear end to preclude any notching effect on the aluminum conductor. The deformation and elongation of the conductor diminish continuously towards the rear end of the barrel, preventing edge formation and predetermined breaking points.
To prevent electrochemical corrosion, the insulation at the rear end of the crimp barrel is included in the crimping process. At the front end of the crimp barrel, corrosion protection is achieved by rolling in additional material (sealing ties) as well as spot deposits of sealing agent. The finished crimp is corrosion-protected. All the elements necessary for this are integrated in the crimp barrel.
Given the high-volume requirements involved in automotive manufacturing, the new terminal is designed to be applied with a fully automatic process.
Exemplary Weight Savings
As cable harnesses are already among the most complex and heaviest components in vehicles, any potential for weight saving is an attractive proposition.
To gauge just how much weight could realistically be saved, we conducted a model calculation for an average midsize car with a cable harness weighing just under 30 kilograms. For simplicity, we looked at substituting only the thickest wires (those with cross-sections exceeding 0.75 square millimeters). We excluded fine-signal conductors. We replaced the copper conductors with aluminum conductors of the next highest cross-section. In this scenario, we came up with a purely computational weight-savings of around 7 kilograms.
(It should be noted, however, that in Germany, at least, many automotive wiring components, such as battery terminals, are already made of aluminum. As a result, a more realistic weight-savings potential would be 2 to 3 kilograms.)
On Conductor Dimensioning
Because aluminum only has around 65 percent of the conductivity of copper, aluminum wires will generally need to be thicker than their copper counterparts. When exchanging copper for aluminum, a useful rule of thumb is to adopt the next interim conductor width.
However, when dimensioning cables for use in vehicles today, engineers tend to adopt wide safety tolerances, so there remains untapped potential for weight and cost savings. When designing cables and connections, engineers might be better served to account for actual load situations, instead of considering exclusively worst-case scenarios. In this way, they can avoid specifying overly thick cables.
The new Litealum crimp for fully automatic termination of aluminum stranded conductors is ready for use. In fact, one automaker is already using the technology. The permanently good electrical connection inside the crimp is due to a high degree of compression during the crimping process and the resulting partial cold weld of the parts. In addition, the remaining residual stress inside the crimp helps to prevent creep in the conductor.
Effective corrosion protection is integrated into the crimp barrel, making the connection suitable also for unsealed connectors inside the vehicle passenger compartment. Aluminum crimp connections have proven stable after temperature shock testing with 500 cycles (-40 C to 130 C) in humid conditions.
Implementing Litealum terminals will not require any fundamental changes for wire harness manufacturers, though they will need new applicators and will have to define new assessment criteria.