There are many electric vehicles in Norway, and each contains many electrical conductors. These electrical conductors contribute significantly to the overall weight of the vehicles.
“Historically, electrical conductors have been made of copper, for the simple reason that this metal has excellent conductivity, formability and strength,” said PhD research fellow Jørgen A. Sørhaug at the Norwegian University of Science and Technology (NTNU).
Copper is therefore very well suited as an electrical conductor, but it has one drawback: it is also quite heavy. It has what scientists call a high mass density.
“Therefore, the weight contribution from copper in an electric vehicle is significant,” said Sørhaug.
Replacing copper with aluminium
Weight affects the energy efficiency of electric vehicles, and therefore also the range. There is thus much to be gained by reducing their weight. So, what is the solution if we don’t want to use so much copper?
“Aluminium is a good alternative to copper, as it has almost the same conductivity, good formability, and good strength when alloys are added, i.e. when it is mixed with other elements. Aluminium is also much lighter than copper,” said Sørhaug.
This means that if some of the copper in electrical conductors can be replaced with aluminium, vehicles can become both lighter and more energy-efficient. This is exactly what Sørhaug and his colleagues are working on as part of his doctoral work.
They are making ‘hybrid’ electrical conductors out of a mix of copper and aluminium.
“In our project, we have produced hybrid electrical conductors made from copper and aluminium through welding, which we have then tested and studied in great detail.”
Cold welding increases conductivity
Making such high-quality conductors is challenging, but cold welding can help harness the good properties of the metals without necessarily compromising the conductivity.
During the welding process, aluminium and copper are mixed together at the atomic level on the contact surface, and usually the higher the temperature, the better they mix. However, brittle crystals, known as intermetallic phases, are then often formed from the metals, which have poorer conductivity than the pure metals. These types of crystals are thus something you want to have as little of as possible, meaning it is unfavourable to weld at high temperatures – because both the conductivity and the strength decrease.
“As a result, we investigated cold welding as a method and used the patented Hybrid Metal Extrusion & Bonding technique,” said Sørhaug.
This technique, commonly abbreviated to HYB, has been developed at NTNU. The researchers later investigated the welds using various forms of electron microscopy, including methods such as precision electron diffraction, high-resolution transmission electron microscopy, and X-ray analysis. Fortunately, you and I don’t need to understand what these advanced methods actually involve, but the results are encouraging.
“We have found that the HYB technique is better suited for joining aluminium and copper than other cold welding techniques. Thin and slow-growing intermetallic layers form at the interface between the metals. This is beneficial because it helps prevent the mechanical and electrical properties of these conductors from changing.”
Why is heat a disadvantage?
More research is needed before aluminium can replace some of the copper. Pure aluminium is mechanically weaker than copper, and that is a disadvantage.
We can increase the strength of aluminium by making alloys. Carefully measured doses of other substances are added, so-called ‘alloying elements’. In addition, the alloy is thermomechanically treated. It is rolled or otherwise shaped before being heat-treated again.
“But aluminium alloys are often sensitive to high temperatures, and their strength will generally be weakened by welding. We have therefore also investigated what causes this strength reduction at the atomic level and how we can improve the alloys to better withstand heat,” said Sørhaug
Further research on aluminium
The project that Sørhaug has been working on ends this year, but NTNU and SINTEF have received a new project to continue researching cold welding of aluminium and copper. The aim is to better control the temperature and tailor plastic deformation at the nanoscale.
This is a collaboration in which Hydro ASA, Corvus Energy AS and Professor Grong AS are also involved.
“We want to build on Sørhaug’s research to make stronger cold-welded connections between aluminium and copper,” said Randi Holmestad, professor of physics at NTNU.
Holmestad has also been one of Sørhaug’s supervisors during the doctoral work, together with senior research scientist Per Erik Vullum from SINTEF Industry.
“By microstructuring and optimizing the welding geometry, we will form a nanostructure at the interfaces that improves both strength and conductivity. This particularly applies to electrical applications, such as those found in the battery systems from Corvus Energy,” explained Holmestad.
NTNU and SINTEF are collaborating with partners from industry, which will lay the foundation for producing new, advanced multi-material components and products in Norway.
The hope is that the work might one day contribute to lighter, and thus more efficient, electric vehicles.
Sørhaug is now employed by SINTEF Manufacturing.