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.
This research investigates a novel hole-flanging process by paddle forming through the use of finite element (FE) simulations. Paddles of different shapes rotating at high speeds were used to deform clamped sheets with pre-drilled holes at their centers. The results of the simulations show that the paddle shape determines
the geometry and principal strains of the formed flanges. A convex-shaped paddle forms flanges with predominant strains in the left quadrant of the forming limit diagram (FLD). However, the convex paddle promotes unwanted bulge formation at the clamped end of the flange. A concave paddle forms flanges with no bulge but the principal strains of elements in the middle section of the flange are in the right quadrant of the FLD which indicates an increased probability for crack occurrence. An optimization of the paddle shape was conducted to prevent bulging at the clamped end while avoiding crack occurrence. The paddle shape was optimized by mapping the deformation of some elements along the flange length to a pre-defined strain path on the FLD while maintaining the bulge height within the desired geometric tolerance. The radii and lengths
of the paddle edge were varied to obtain an optimum paddle shape.
Dual-phase (DP) steels are widely used in sheet metal stamping. However, they are typically characterized by low hole expansion ratios. Since hole flanging is very often applied to sheet metal parts, solutions for improving hole flangeability are needed. In this study, high-speed punch rotation is applied in hole flanging of DP 1000 to generate frictional heat and increase formability. The flanges were formed using a punch rotating at 8000 rev/min and varying axial feeds. A maximum hole expansion ratio (HER) of 3.6 is obtained in the tests compared to ~1.58 in conventional hole flanging. The high formability is explained by the high temperature recorded in the process. The effects of temperature and strain rate on the formability of DP 1000 are examined by tensile tests conducted at different conditions. The tensile tests show an increase in formability at high temperatures. Optical microscopy at the flange edge reveals a change in the microstructure of the steel from the characteristic dual phase to a martensitic structure with elongated grains.
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