Failure with Strain Localization of Aluminum Alloy 7075 Sheets at Elevated Temperature and its Application to Two-Step Hybrid Forming

Abstract

In this work, the onset of failure induced by severe strain at elevated temperature was numerically estimated with cross-formed empirical hardening law describing material softening. The hardening law can replicate the rate-sensitive behavior of aluminum alloy 7075 sheets (thickness of 2.0 mm) with initial hardening and progressive material deterioration caused by dynamic recrystallization, dynamic recovery, and micro-void development. The characterized material was applied to the two-step hybrid forming process consisting of a drawing at 400 °C followed by a pneumatic forming at 470 °C to produce a shock absorber housing with an extremely complex shape. The user-defined subroutine codes, VUMAT (ABAQUS/Explicit) and UMAT (ABAQUS/Standard), were sequentially utilized for the drawing and the pneumatic forming, respectively. The identified hardening parameters based on uniaxial tensile tests were validated by simulating the two-step hybrid forming process and compared with the conventional Voce type law (converging function) and the combined Swift-Voce type law (ever-increasing function) since they play a key role in accurately predicting the onset of failure induced by severe strain localization. Finally, simulation results are reasonably well matched with experiments in terms of the moment of failure occurrence, failure location, final blank shape, and thickness distribution.