A New 5xxx Series Alloy Developed for Automotive Applications 2002-01-2128
The Aluminium-magnesium or 5XXX alloys in sheet form have been considered for some time to be the most appropriate choice for vehicle structures as they offer attractive combinations of formability, strength, weldability and corrosion resistance.
Recent applications of the Al-Mg alloys in vehicles include the use of higher magnesium containing alloys such as AA5454 and AA5083 for applications like wheels, chassis, and sub-frames, where emphasis is placed on welded material strength.
Alloy AA5083 has a high strength, but the alloy becomes susceptible to intergranular corrosion (IGC) if the material is held at elevated temperatures for prolonged times. Therefore, if the application is such that higher temperatures cannot always be avoided, an alloy like AA5454 has to be chosen. Alloys like AA5454 are less strong than AA5083, but in the daily practice they are almost not susceptible to IGC even when held at higher temperatures for longer times.
Some years ago a modified AA5×54 alloy, designated as AlMg3.5Mn or AA5018-type, was introduced for automotive structure applications, for instance for sub-frame components. This alloy is somewhat stronger than the existing AA5×54 alloys, and its susceptibility to IGC is very limited. AlMg3.5Mn has since then been produced at Corus Aluminium Rolled Products (CARP).
More widespread usage of aluminium alloys will introduce Al-Mg alloys in areas of the vehicle structure where more careful selections need to be made with regard to the surface appearance, service temperature and corrosion resistance.
Recently a research programme was carried out at CARP to further optimize the combination of strength, formability and corrosion resistance. The programme focused on developing a new alloy with a significantly higher strength and equivalent corrosion resistance compared to existing alloys. Because welding will often be used for automotive body structures, the post-welded strength and formability are seen as important issues. Therefore the development was exclusively directed to O-temper properties.
This programme has resulted in a new alloy (patent pending) that has a much higher strength than the existing AA5×54 alloys, and in fact close to that of AA5083. However its susceptibility to IGC is at least comparable to that for the alloy of the AlMg3.5Mn type.
The purpose of this paper is to present an overview of the properties of the newly developed 5xxx alloy in comparison to the existing alloys for similar applications.