Creep age forming is a novel deformation process which applies creep/stress relaxation deformation on the age hardenable alloys for manufacturing large aluminium panels with complex curvature for modern vehicle and aerospace structures. One of the main issues that influence the accuracy of the forming shape in creep age forming is springback. Despite tremendous effort to predict the shape and springback of formed parts, springback has been analysed in terms of the elastic unloading of residual stress from stress relaxation. Anelastic creep strain recovery, however, may also substantially influence the springback and final shape of formed parts.
The present work was focused on evaluating the extent to which anelasticity can affect springback in the creep age forming process for the aluminium alloy AA7050-TAF. The mechanical properties of this material were characterized first with respect to age hardening and anelastic/inelastic creep deformation behaviour. Transmission electron microscopy (TEM) was used to examine microstructural variations of the material under the pure aged and creep aged conditions.
The Uniaxial Creep-Aging and Recovery Test (CART) and the Creep-Aging and Back Stress Test (CABT) were developed and performed to investigate the anelasticity and internal stresses induced during the forming process. The three-point bending Stress Relaxation and Back Stress Test (SRBT) was performed to examine the relationship between the thickness of the material and the amount of anelastic creep strain recovery after creep age forming.
A constitutive model was then developed to represent and predict the response of the material in creep age forming. The highlight of this research is that anelastic creep strain recovery has been proposed to influence the springback after the forming process; the constitutive equations allow the estimation of anelastic strain and other state variables from the creep aging histories of the material.