Hotforming of AlZnMgCu-7xxx sheets for automotive applications has been studied extensively over the past few years. By replacing presshardening UHSS with high strength aluminium alloys, the part weight can be reduced up to 40 %.
Due to its availability and high strength, AA 7075 sheet is the most prominent material studied. However, Cu-content imposes several difficulties upon the hotforming process. Due to the low melting S-phase, overheating has to be avoided during solution heat treatment of the blanks. Transfer of the hot blank to the cold die and quenching needs to be very fast, in order to avoid pre-precipitation of the solute elements, which would impair the strength levels after ageing and reduce ductility. Last but not least, fusion welding of Cu-containing 7xxx materials is very tricky, due to solidification shrinkage.
Medium strength, weldable Cu-free 7xxx aluminium grades have been used for very long time in road construction and machining applications. Welding can be done by conventional techniques, and with a proper post-welding heat treatment, these materials restore their strength level and show an excellent corrosion resistance. Due to the absence of Cu, they allow a wide temperature window for solution heat treatment and having Zr as grain controlling element, they preserve high strength levels even with moderate quenching rates. All these advantages of Cu-free 7xxx against Cu-containing 7075 might facilitate their integration into the body-in-white.
The only drawback of Cu-free 7xxx alloys with higher Zn and Mg contents is their stress corrosion cracking (SCC) susceptibility. Several strategies to enhance SCC resistance of a high strength 7021 sheet material developed for automotive applications will be presented.