Increasing formability in hole-flanging through the use of punch rotation based on temperature and strain rate dependent forming limit curves

Abstract

Conventional hole-flanging by stamping is characterized by low formability. It is common knowledge that formability can be improved by forming at high temperatures. High-speed punch rotation is introduced to conventional hole-flanging to use frictional heat to improve and control formability. Thermomechanical finite element (FE) simulations of conventional hole-flanging and hole-flanging with punch rotation are used to determine the effects of punch rotation on the process temperature. Hot tensile tests were conducted to find the effects of temperature and strain rate on the forming limit of the blank. The Marciniak–Kuczynski (M–K) forming limit model is used to estimate temperature and strain-rate dependent forming limits of the material. Hole flanging experiments were conducted at different punch speeds and feeds to determine process windows that maximize formability. A maximum hole expansion ratio (HER) of 4 was obtained in hole-flanging with punch rotation compared to 1.48 in conventional hole-flanging experiments. In theory, a rise in blank temperature to 400 °C in hole-flanging with punch rotation enhances the HER by 30% based on the FE simulations. However, experiments of hole-flanging with punch rotation reveal a 170% increase in formability. The difference in formability between the experiments and FE simulations is attributed to the influence of high-speed deformation, in-plane shear and non-proportional loading paths. To control formability in hole-flanging with high-speed punch rotation, it seems sufficient to establish a closed-loop control of the process with a pre-defined temperature profile.