Forging copper alloys for a high-speed future

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Copper alloys are promising for use in vehicles of the future, such as high-speed trains.Credit: lupengyu / Moment / Getty

More than 5,300 years ago, Sumerians added tin to copper to make their weapons and tools stronger. Bronze, the world’s first copper alloy, was born.

Copper alloys have come a long way since and are now a key material in many applications, from LCD televisions and smartphone touchscreens, to lithium batteries and trains. Yunbo Zhong, a professor at Shanghai University’s National Key Laboratory of Advanced Ferrous Metallurgy, is keen to improve the properties of such alloys to meet increasing demands from new infrastructure, 5G communications, and aerospace. Zhong has successfully developed advanced copper alloys with high conductivity and strength.

Copper alloys are promising for use in vehicles of the future, Zhong says. For instance, high-speed trains need robust contact wires that can withstand high tension from strong vibratory waves, overheating, and spark wear. “For a faster and more energy efficient high-speed railway, there are extremely high requirements surrounding the tensile strength and electrical conductivity of contact wires,” Zhong explains. Adding chromium and zirconium could enhance the strength and conductivity of copper alloys.

But making such copper alloys is laden with challenges. High conductivity usually comes at the expense of strength and ductility. Meanwhile, the transition metal elements, chromium, and zirconium, that comprise these alloys in the copper melt tend to react readily with oxygen and nitrogen in the atmosphere. This is problematic given that most mass production methods occur outside a vacuum. “The instability of the ingredients, coarse grain and bar length are the biggest hurdles to making these alloys,” says Zhong.

Zhong and his team have come up with a number of innovative manufacturing methods. A coupled novel electromagnetic processing technology, Continuous Extrusion Forming (CEF), and artificial ageing technology are applied to prepare Copper-Chromium-Zirconium (Cu-Cr-Zr) alloys. The resulting alloys has higher strength and conductivity than those made from traditional processes. When subjected to stress conditions of 77% deformation ratio, the alloys display a tensile strength of 663MPa and an electrical conductivity of 79.8% IACS — which is significantly higher than the latest standard high-speed rail contact wire produced in China, and will be especially suitable for use in train systems with speeds higher than 400km/h, says Zhong.

Another of his team’s technique involves applying a tailored electromagnetic field to the alloys as they are being manufactured. The process shrinks the metal’s crystal grains and makes them more even in length, improving their conductivity and tensile strength. In addition, the oxygen content of copper tube produced via this technique falls drastically to 0.0003% — useful when it comes to manufacturing high-performance electronic components, vacuum interrupters, solar panels, and other devices where a lower oxygen content corresponds to better electrical conductivity and greater reliability.

Zhong and his team are looking to increase the conductivity of Cu-Cr-Zr alloys to 90% IACS and the strength to 700MPa, which will be a boon for China’s new trains with a speed of 400-600km/h. Zhong envisages a future that the conductivity of copper wires will be elevated to 103-106% IACS, so that they can be used in microelectronics, and electric motors.

They’re an excellent material for a high-speed future, Zhong says.