Materials
Misconceptions about the properties between Al and Cu can arise due to different grades of metals used in different electrical applications. The Cu used in wire and electrical equipment is nominally pure. Pure Al, however, is often not strong enough for electrical applications. Also, keep in mind that different alloys have changed over time and on account of evolving applications.
Different Al alloy properties will also change, depending on processing. For example, Al 6101 is stronger than Al 1350. Yet heat-treating Al6101 hardens it and improves its strength. Different grades of metals, such as Al 6101 and Al 1350, will vary in comparisons with Cu. During the design process, it is therefore essential to have the material properties for the specific material used.
Properties
Weight, electrical capacity, and cost are major considerations when selecting Al or Cu for an electrical application. Yet others can loom just as large. For example, resistance in electrical connectors can increase if a material’s strength and expansion are not considered. As a connection undergoes thermal cycles, the expansion can increase the clamping force that can deform contact points and promote creep in materials. This will be a larger concern with Al because its coefficient of thermal expansion, depending on the alloy, is about 42% greater than Cu, but Al may be able to dissipate the heat faster.
Taking advantage of the lower modulus of elasticity since the 1990s, extruded Al bus bars have increased the surface area, helping to keep temperatures low. It is important when designing in either material that connections are robust to prevent poor connectivity over time, owing to deformation from thermal expansion, as well as creep.
A common misconception is that Al is soft and must use compression connectors. However, with some design and plating changes, mechanical pressure connectors and compression connectors are no longer required. In some cases, alloys or processing are used to make aluminum almost on par with Cu. Al can require coating to reduce oxidation in general as this can affect the connection – even an Al to Al connection. In addition, coating and plating often include tin or silver. These materials reduce corrosion in both Al and Cu, as they are prone to oxidation when exposed to the atmosphere.
Corrosion is also a problem when there are two dissimilar metals in one system. Al will electrochemically react with the Cu if moisture is introduced (moisture that would act as an electrolyte). Al to Cu cable lugs are connectors that have been friction welded and capsulated to prevent corrosion from damaging an Al-to-Cu connection. Proper connections are important, as corrosive wear is also a concern. Al and Cu are compatible metals, so contact can create bonding that can promote wear. While corrosive wear is more of a problem for parts that move, a technician may need to take more time in the field if wires are stuck to the bus bar.
Weight and Electrical Capacity
Arguably, the main material property in deciding between using Al or Cu in an electrical application is its capacity. Cu offers a better electrical capacity per volume. However, Al has better capacity per weight. According to Uwe Schenk, global segment manager at Helukabel, “As a raw material, Al is approximately 70% lighter than Cu. For wires, Al can be up to 60% lighter than comparable current carrying copper wires.”
The weight is not a direct relation as more Al is necessary to match the capacity of Cu. Al carries about half the capacity of Cu (56% in Al6101). The difference in the weight to electrical capacity ratio means generally, one pound of Al has the electrical conductivity equal to 1.85 lb. of Cu. For example, a Cu busbar could weight around 550 lb., whereas the same busbar in Al would be about 300 lb. Reducing weight may help shipping or even labor cost.
