On the thermal forming limit diagram (TFLD) with GTN mesoscopic damage model for AA7075 aluminum alloy: Numerical and experimental investigation

The formability of high strength AA7075 aluminum alloy under elevated temperatures is vital in guiding the fabrication of automobile structural parts. The ductile fracture forming limit of AA7075 at elevated temperatures is essentially governed by thermal damage evolution. In this paper, the hot tensile test (300 degrees C-450 degrees C, strain rate 0.1s(-1)-0.001s(-1)) for AA7075 was conducted and the corresponding flow behavior was fitted by Hensiel-Spittel (HS) constitutive equation. The GTN mesoscopic damage model was implemented to take account of the damage evolution phenomenon of AA7075 at elevated temperatures. The temperature-dependent damage void volume fractions (VVF) were identified accurately based on a novel inverse identification procedure, named the CCD parameters design-FEM inverse simulation-genetic algorithm (CCD-FEIS-GA) optimization method. Subsequently, the thermal forming limit diagram (TFLD) was calculated based on the GTN damage parameters and further validated by hot Nakajima-type bulging experiment. The corresponding damage evolution behaviors and effects of critical forming parameters were discussed in detail. The comparison of experimental and numerical results showed that the formability of AA7075 firstly increases and then decreases with the increasing forming temperature, the TFLD0 reaches its maximum value when the temperature is 400 degrees C, and the formability also increases with the increasing strain rate and the decreasing surface coefficient of friction. The proposed simulated strategy coupled with mesoscopic GTN approach conduces to the accurate prediction of TFLD and damage fracture behavior of AA7075 in the hot forming process. (C) 2019 Elsevier B.V. All rights reserved.