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Granular media-based tube press hardening

Subject Area Primary Shaping and Reshaping Technology, Additive Manufacturing
Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Term from 2014 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 266685077
 
Final Report Year 2019

Final Report Abstract

The complementary fundamental studies aimed at deriving strategies for using granular media as forming media to achieve an alternative profile press hardening process using hydroforming principles. The studies showed this process is possible but the product complexity is limited due to the granule properties in force transmission compared to liquid and gaseous media. The parameter space in principle available for gaining improvements without imposing limitations on the forming dies contained the material of the granular particles, the frictional properties of the particles, the arrangement of the particles and the shape of the punch. The material of the particles had to be chosen to meet the additional criteria that the particles have to withstand the high pressures and elevated temperatures of the forming process. The process studies have identified zirconia beads as the particle material suitable for the hot forming process. Further improvements thus have to modify the particles frictional properties, which are accessible through surface coatings and additives. The studies have shown that among the tested additives only solid lubricants can lower the internal friction of the zirconia beads. Solid lubricants like HeBoFill that are stable at the elevated temperatures of the forming process are thus introduced to improve the forming performance. However, frictional losses of forces persist, which impose limitations on the distance from the punch to the forming dies. The arrangement of the particles plays a decisive role in redistributing the forces in the tube and the homogeneity of pressure. It could easily be shown in horizontal two-dimensional experiments that forces are better confined and transported in crystalline arrangements. Experiments with bulk granular media have shown that the formation of crystalline arrangements require tailoring the particle interactions. Still, gravitational forces impose collapse of the crystalline arrangement at a certain threshold, limiting the accessibility of tailored packing structures in the forming process. To achieve a better force transmission without causing a thinning on the profile product, however, the tube needs to be pushed in the forming area by an active axial feed system. This was realized by a ball screw drive axis. The punch shape could be shown to be an easily accessible approach to affect the force and pressure distribution within the granular medium, and by this the plastic deformation of the medium. The fundamental studies thus narrowed the parameter space down to solid lubricants and the punch shape, and indicated beneficial forming behavior if the punch is close to the dies. These findings guided new developments like the passive tube hydroforming, where punch pressure is initiated on the tube while the medium provides a structural support during the forming process.

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