New 7000 series aluminum alloy 7K55 is launched

January 13, 2016

TOKYO, January 13, 2016 — Kobe Steel, Ltd. has developed a 7000 series aluminum alloy called 7K55 that offers both outstanding stress corrosion cracking resistance and high yield strength in the 400 N/mm2 class. 7K55 has already been adopted for use as an extrusion for bumper beams by a Japanese automaker. Kobe Steel aims to expand the application of this new aluminum alloy in the future.

In the automotive sector, the need for lighter car bodies is growing due to stronger environmental regulations in recent years. Aluminum extrusions for bumper beams and the body framework are an effective way to lighten the weight of cars. As a result, demand for aluminum extrusions is expanding.

Bumper beams absorb the energy of a collision and serve to protect passengers from the impact.

The 7000 series aluminum alloys, which contain zinc and magnesium to increase strength, make thinner designs possible, compared with 6000 series aluminum alloys, which are the predominant material currently used in aluminum bumper beams. The 7K55 aluminum alloy, when used in bumper beams, provides a 30-percent reduction in weight, compared with 6000 series aluminum alloys.

However, a general characteristic of 7000 series aluminum alloys is that as their strength increases, their stress corrosion cracking resistance deteriorates. It has been problematic to achieve both high strength and stress corrosion cracking resistance.

Through its involvement in the fields of motorcycles and rolling stock, Kobe Steel has cultivated its development capabilities and production technologies for 7000 series alloys for over half a century. Kobe Steel optimized the chemical composition and heat treatment conditions, and through appropriate control of the structure, Kobe Steel succeeded in developing 7K55 alloy with high yield strength in the 400 N/mm2 class that is about 30 percent stronger, while maintaining stress corrosion cracking resistance, compared with Kobe Steel’s conventional 7000 series alloys.

As environmental regulations for cars become stricter, the need for lighter cars is anticipated to grow. Based on its outstanding alloy development capabilities and design technologies, Kobe Steel can offer proposals for automotive lightweighting, contributing to improved fuel efficiency and lower CO2 emissions.

Technical Terms

Bumper Beam
A bumper beam is a reinforcement mounted inside a bumper to reduce damage to the car body in the event of a collision.
Stress Corrosion Cracking
Stress corrosion cracking is the cracking that occurs when metal is subject to tensile stress in a corrosive atmosphere. When stress corrosion cracking occurs, metals are less able of absorb energy. This becomes a problem from the standpoint of collision safety.

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