Formulation of Differential Work Hardening of 6000 Series Aluminum Alloy Sheet and Application to Finite Element Analysis

The deformation behavior of 6000 series aluminum alloy sheets under biaxial tension was precisely measured for linear stress paths, using servo-controlled biaxial tensile tests with both cruciform and tubular specimens. The tubular specimens were fabricated by bending and laser-welding as-received flat sheet materials. Differential work hardening (DWH) behavior was observed; the shapes of the contours of the plastic work constructed in the principal stress space changed with an increase in plastic work. A new constitutive model that can reproduce the DWH behavior was proposed; the model is based on the Yld2000-2d yield function with the exponent and material parameters changing as functions of plastic work. Finite element analysis of hydraulic bulge forming was performed using both the proposed DWH model and the conventional isotropic hardening model based on selected yield functions. The calculated results based on the DWH model were in closest agreement with the experimental results. Thus, the new constitutive model has been verified to be effective for improving the accuracy of the FEA.