Why use aluminum alloy conductors?

Gary Kardys | May 30, 2017

In order to understand where aluminum is a good choice as a conductor, we need to look at the properties of aluminum, copper, and other conductive metals and alloys. While aluminum has only 60% of the conductivity of copper, aluminum is the smarter choice for many electrical conductor applications when density and cost consideration are taken into account.

Figure 1 - Properties of several high conductivity metals used in electrical and electronic conductor applications.

The optimal conductor material depends on the specific applications such as microelectronics, PCBs, electronic connectors, electrical power cables, electrical power connectors or lugs, electric vehicles and circuit breaker electrical contacts. Copper has replaced aluminum in microelectronics conductors because of the continued trend toward miniaturization. As the conductive interconnects shrunk in size, copper on IC chips became a better choice because copper’s higher conductivity reduced resistance and joule heating. Future IC chips use photonics interconnects because even copper conductors would generate too much heat. Copper tends to be the material of choice on PCBs to reduce heat as circuits become denser. In certain conductor applications, reducing connector or interconnect size and heat generation are a factor. A large fraction (80% in some cases) of the resistance in electrical and electronic devices arises from the contact resistance and not the bulk resistance in the conductors. Contact resistance is a function of the contact material conductivity, contact hardness, contact pressure, contact sliding and environmental factors such as oxidation and corrosion. Silver and gold have excellent oxidation and corrosion resistance, which is a requirement on certain conductors for electronic and microelectronic connectors and contacts. While we do not typically use solid gold conductors, many microelectronic connector contacts are plated with gold, which maintains uniform contact resistance over time. Electrical power contacts in circuit breakers and contactors utilize silver or a silver composite, which has resistance to wear, oxidation, arcing and welding. While silver has the highest conductivity (108% IACS [International Anode Copper Standard]), silver is a poor choice for interconnects or conductive paths on circuit boards due the migration of the metal. Silver and gold are very expensive, so these materials are used sparingly and engineers are constantly looking for ways to minimize their use by increasing performance or by finding lower cost alternatives.

Aluminum is a good conductor choice in application such as aluminum conductor cables or electrical power lugs or connectors. While aluminum has approximately 60% of the conductivity of copper, we can compensate for the higher resistivity by increasing the diameter or cross section of the conductor about 30%. We can calculate how much larger a radius or diameter of aluminum is required for equivalent resistance by understanding how conductivity (σ), resistivity (ρ) and resistance (R) are related.

σ_Cu = (1 /ρ_Cu)

σ_Al = (0.6) σ_Cu = (1 /ρ_Al) = (0.6 /ρ_Cu)

ρ_Al = (0.6) ρ_Cu and ρ_Cu/ρ_Al = (0.6)

ρ_Al / ρ_Cu= 1/(0.6) = 1.6667

L is the length of the cable,
A_Cu= area of copper cable, r_Cu= radius of copper cable
A_Al= area of aluminum cable, r_Al= radius of aluminum cable

R_{Cu Cable} = ρ_CuL/A_Cu

A_Cu = π (r_Cu)²

R_{Al Cable} = ρ_AlL/A_Al=

If we want the resistance of the aluminum cable equal to the copper cable,
R_{Cu Cable} = R_{Al Cable} = ρ_AlL/A_Al= ρ_CuL/A_Cu=

ρ_Al/A_Al= ρ_Cu/A_Cu

ρ_Al/ ρ_Cu = A_Al/A_Cu= π (r_Al)²/ π (r_Cu)²

ρ_Al/ ρ_Cu = 1.6667 = (r_Al)²/ (r_Cu)²

(r_Al)² = (1.6667) (r_Cu)²

r_Al = ((1.6667) (r_Cu)²)^½

r_Al = 1.291 r_Cu

We can adjust for the lower conductivity of aluminum by increasing the radius, diameter or wire gauge size. The increase in volume will not increase the weight because aluminum is so much lower in density compared to copper. Cost is another advantage of aluminum alloys conductors over copper. The cost of aluminum is about one third the cost of copper. At the time of publication, copper is U.S. $2.57/lb and U.S. $0.88/lb. If a copper conductor has a volume of 1 cubic inch, then an aluminum conductor with equivalent resistance will have a volume of 1.67 cubic inches. The aluminum conductor will have half the weight of the copper conductor. The copper conductor would have $0.83 of material cost, while the equivalent aluminum conductor would cost $0.14.

Returning back to the subject of contact resistance, aluminum conductors must be tin or silver plated, or have a corrosion-inhibitor compound used at the connectors to prevent an aluminum oxide film from producing high contact resistance. Aluminum has good corrosion resistance, but the metal gains the corrosion resistance from oxide film formation. Silver and gold are noble metals and copper is almost a noble metal. Thermodynamics do not favor the formation of noble metal oxides. Thermal conductivity, creep and thermal expansion coefficient differences need to be considered when designing an aluminum conductor and conductor connectors to replace a copper conductor and connection system. Aluminum’s higher coefficient of thermal expansion compared to copper and steel must be taken into account to avoid loosening of connectors and assemblies utilizing aluminum conductors. Loosening of the electrical connector results in increased contact resistance and overheating because contact resistance is a function of the contact force.

The Aluminum Association (AA) develops and maintains standards or grades for aluminum alloys. Aluminum alloy grade have AA number designations. There are UNS numbers for these aluminum alloys as well. Wrought or thermomechanical-formed alloy are designated using a four-digit number system. Different series of aluminum alloy are based on the major alloying element used in the alloy series.

Figure 2 Aluminum alloys series designations per Aluminum Association.

A high purity grade, AA-1350 aluminum alloy conductors were an earlier mainstay, but now the AA-8000 series aluminum alloy conductors are typically the standard choice. The loosening and subsequent increase in contact resistant and electrical joint failures has been eliminated with improved designs and the use of AA 8000 series aluminum. AA 8000 series aluminum alloys have better creep resistance compared to earlier grades. CTE, the coefficient of thermal expansion, is defined as the fractional increase in length per unit rise in temperature.

Figure 3 - Creep property comparison of aluminum and coper conductor alloys. Image from Breck Booker (Southwire),"Evaluation of Aluminum Cable Presentation" for IEEE OCS, 9/14/11. Creep occurs when metals and alloys under stress relax or change dimensionally over time. Creep rate increases with temperatures, so resistance or joule heating can accelerate creep. 8000 series alloys have greater flexibility and creep resistance closer to copper, which improves connections and safety. The 8000 series use additions of silicon, iron, copper, magnesium, zinc and boron to improve properties while maintaining high conductivity. Aluminum 8000 series alloys provide good strength combined with high conductivity. 8000 series aluminum alloys tend to have has higher strength, greater ductility, more uniform chemical compositions and better thermal stability compared to 1000 series aluminum alloys. The modern aluminum alloys use heavier metal alloying elements such as 0.5% to 0.9% iron additions to improve microstructural stability and creep resistance.

The temper condition of the aluminum alloy can also impact conductivity. In fact, metallurgists use conductivity as a test for tempering or heat treating. Conductivity increases when copper precipitates out of solution during age hardening. In some cases, an over aged aluminum-copper alloy might be utilized to provide a higher conductivity component. However, the over aging will result in lower strength. While certain alloying elements can increase mechanical properties by solid solution strengthening, these alloying methods are avoided in copper and aluminum conductors due to their impact on decreasing conductivity.

Figure 4 - Resistivity density product comparison of several high conductivity metals.

Another way to look at the advantages of aluminum is the strength-to-weight and conductivity-to-weight ratios, which are higher for aluminum compared to copper. The resistivity density product factor is favorably lower for aluminum compared to copper. The low density and weight savings can be a design consideration when selecting materials or power conductors in aerospace and automotive applications.