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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. [ABSTRACT FROM AUTHOR]

The nonuniform distributions of the residual stress were simulated by a 3D finite element model to analyze the elastic-plastic dynamic ultrasonic impact treatment (UIT) process of multiple impacts on the 2024 aluminum alloy. The evolution of the stress during the impact process was discussed. The successive impacts during the UIT process improve the uniformity of the plastic deformation and decrease the maximum compressive residual stress beneath the former impact indentations. The influences of different controlled parameters, including the initial impact velocity, pin diameter, pin tip, device moving, and offset distances, on the residual stress distributions were analyzed. The influences of the controlled parameters on the residual stress distributions are apparent in the offset direction due to the different surface coverage in different directions. The influences can be used to understand the UIT process and to obtain the desired residual stress by optimizing the controlled parameters. [ABSTRACT FROM AUTHOR]

The double curvatures encountered in aerospace skin components require significant plastic strain levels to achieve final form. In order to obtain the required part geometry and final temper strength, while avoiding failure, multistage-processing is employed. For components that are more difficult to form this requires three stretching stages and two heat treatment processes. After the first forming stage the material is annealed, then formed to a second strain level, before solution heat treatment and final form. An experiment that replicates this processing route is presented. The results indicate that for all processing routes the material exhibits a significant change in texture as described by the r-values. Furthermore, the biggest strength increase is obtained when using the lowest total strain processing route. The formability of the processed material is shown to be less than the original fully annealed sheet, although there is little difference in the formability of the various processing routes. [ABSTRACT FROM AUTHOR]