Constitutive Modelling and Springback Prediction in Creep Age Forming of AA7055 Doubly Curved Panels

Abstract

The creep‐ageing behaviour of AA7055 has been carried out for a range of stress levels at 120° C for 20 hours, which is the typical period for a Creep Age Forming (CAF) process. There is a strong interaction between microstructure evolution, such as second‐phase precipitation, and mechanical properties in CAF, thus a set of mechanism‐based unified creep ageing constitutive equations has been formulated based on experimental observations, which models creep induced evolution of precipitates, dislocation hardening, solid solution hardening and age‐precipitation hardening under the creep ageing conditions. Multi‐step finite element simulation procedures have been developed using the commercial FE solver MSC.MARC for the study of springback and creep induced precipitation hardening in creep age forming of single and double curvature aluminium panels. The mechanism‐based unified constitutive equation set has been implemented into MSC.MARC via the user defined subroutine, CRPLAW, to describe the creep‐ageing behaviour of the metal during CAF. Not only the stress distributions along work‐piece thickness are studied, the springback, evolution of precipitation hardening and yield strength are also analyzed for both single and double curvature panels. It is found that springback varies with single and double curvature panel parts, the double curvature panel has lower springback values and more uniform springback distribution than the single curvature panel.