The Effect of Pre-Strain and Inter-Stage Annealing on the Formability of a 2024 Aluminium Alloy

Stretch forming of both fuselage and nacelle skins often requires multi-stage processing to achieve the final form. Finding the optimum pre-strain level before inter-stage annealing is critical to avoid failure in subsequent processes and minimise the number of processing steps. This paper examines the effect of three pre-strain levels of 4%, 8% and 12%, followed by a recovery anneal process. Both yield behaviour and material formability are analysed. The forming limit diagrams indicate an immediate drop in formability in the pre-strained and annealed samples when compared with the O condition material. The aforementioned formability drop quickly saturates for the higher pre-strain magnitudes. This saturation is also evident in the uniaxial data. The underlying material texture, as described by the r-values, remains unchanged during processing, while the yield strengths are shown to increase in an almost isotropic fashion. Overall the paper indicates that to improve the formability in subsequent processing steps, the pre-strain levels should be maximized before the recovery anneal and second forming stage.