Norwegian University of Science and Technology (NTNU) developed a hybrid metal extrusion and bonding technique (HYB), which is able to cold weld aluminum and copper to create hybrid conductors to help reduce electric vehicle (EV) weight. This is because lighter weight EVs are more energy efficient, which extends the driving range of the vehicle.
Traditionally, conductors have been made of copper, which is largely due to tradition. When the International Annealed Copper Standard (IACS) was created in 1914, copper was the seen as the only material for conductivity — so it was assigned a conductivity of 100% by volume. However, aluminum also has an excellent level of conductivity and, when factoring in weight, aluminum has 200% the conductivity of copper.
“Aluminum is also much lighter than copper,” said Jørgen A. Sørhaug, who is employed at SINTEF Manufacturing. “This means that if we can replace some of the copper in conductors with aluminum, vehicles will be both lighter and more energy efficient.”
Sørhaug’s recent doctoral work at NTNU focused on developing hybrid electrical conductors by welding together copper and aluminum. Welding of generally occurs at high temperatures. When welding aluminum and copper together, the heat typically causes intermetallic phases (brittle crystals) to form, which lowers the conductivity of the metal and can make the joint prone to cracking. Therefore, it is important to reduce the number of crystals formed.
The research team turned to cold welding as a solution to this challenge by utilizing the HYB method, which was developed and patented by NTNU. The HYB process uses a rotating pin to perform continuous extrusion of an aluminum alloy filler metal (with high concentrations of silicon and magnesium) to cold weld the aluminum and copper pieces.
Following welding, the researchers used electron microscopy to carefully test and examine the welds. “We’ve observed that the HYB technique is better suited for joining aluminum and copper than other cold welding techniques,” said Sørhaug. “Thin and slow-growing intermetallic layers form at the interface between the metals. This is beneficial, because then the mechanical and electrical properties of the conductors are less prone to changing.”
More research is needed before aluminum can replace some of the copper in these kinds of conductors. Therefore, NTNU and SINTEF are now collaborating with industry partners — Hydro ASA, Corvus Energy AS, and Professor Grong AS — to further explore the cold welding of aluminum and copper. This will lay the foundation for producing new advanced multi-material components and products in Norway, which could one day contribute to lighter and more efficient electric vehicles.
“We plan to build on Sørhaug’s research to create stronger cold-welded connections between aluminum and copper,” said Randi Holmestad, a professor of physics at NTNU one of Sørhaug’s supervisors during his doctoral work. “By microstructuring and optimizing the welding geometry, we’ll create a nanostructure in the interfaces that improves both strength and conductivity. This is especially true for electrical applications like the ones found in the battery systems from Corvus Energy.”
Learn more about this research in the paper, “Al-Cu intermetallic phase growth in hybrid metal extrusion & bonding welds exposed to isothermal annealing or direct current cycling,” written by Jørgen A. Sørhaug, Aksel Elkjaer, Elisabeth Thronsen, Tina Bergh, Øystein Grong, Per Erik Vullum, and Randi Holmestad, and published in Materials & Design, Volume 240.