For hang-on-parts like hoods, bonnets, and doors, aluminium sheets are a natural choice due to their weight advantage over the traditionally used low carbon steels and the limited need for joining operations with other materials for these parts. In order to meet customer demands for applications with highest requirements of formability and typical requirements on in-service strength, Hydro has developed 6/30+ alloys, which are 6xxx series alloys that show elongation to fracture values of 30% or more in all directions in the T4 condition.
Dedicated 6xxx series alloys, initially based on AA6016, had been developed to provide a favourable property mix of excellent surface quality after forming, compatibility with the OEM process (i.e. strength increase in the paint-bake cycle for dent resistance, compatibility with joining operations like adhesive bonding) and in-service corrosion resistance. At present, aluminium sheets are significantly gaining market share in this segment.
However, the formability of these aluminium sheets is lower than that of plain low carbon steels. The forming constraints of today’s 6xxx alloys are exemplified by the common use of 5xxx series alloys for non-visible (inner) parts with a high demand on formability, like door inner panels, in combination with a 6xxx alloy for the visible (outer) part despite the obvious advantages of a uni-alloy concept. Consequently, a major direction of development is towards 6xxx alloys with an improved formability and a similar hardening in the paint-bake cycle.
For comparison, typical 6xxx automotive sheet alloys show elongation to fracture values of 24% and forming optimised variants are usually limited to 26-27% (see Figure 1). That these properties also lead to an improved formability is evidenced (see Figure 2) in a comparison of the forming limit curves (FLC’s) of a typical AA6016 sheet alloy and a 6/30+ alloy, as developed by Hydro.
Figure 1: Comparison of elongation values (T4 condition) of a typical AA6016 automotive sheet alloy and AA6016-6/3
Figure 2: Comparison of FLC’s of a typical AA6016 automotive sheet alloy and AA6016-6/30+ (T4 condition).
The 6/30+ alloy has a chemical composition within AA6016, which is a well established alloy type for hang-on parts, and combines outstanding forming properties in the T4 condition with a typical bake hardening response, resulting in in-service strengths that are similar to standard AA6016 alloys.
