Porduction and investigation of graded porous metals by rolling in a time-varying rolling gap
Final Report Abstract
In the present research project, the focus was on investigating the basic understanding of rolling porous metals. The investigations were motivated by the potential application of porous materials as low-noise trailing edges for airfoils. A previous project demonstrated the acoustic effectiveness, which could be enhanced by an adjusted porosity profile. A rolling process with a time-varying rolling gap was used to set a gradient in porosity (and thus permeability) in the rolling direction. Two surprising results emerged which motivated the investigations in the present project: First, it was shown that a porosity gradient in the rolling stocks thickness direction could be generated as a function of the rolling parameters (without using the time-varying rolling gap). Secondly, it was shown that the elongation of individual pores (which in some cases is not detectable) remained far behind the macroscopic elongation of the rolled material (approx. 20% at 50% thickness reduction). Firstly, considering the rolling model for porous metals by Deshpande, the working hypothesis was developed that the porosity gradient is due to a gradient in the hydrostatic stress, which in the model is responsible for a volume change. The gradient in hydrostatic stress could be adjusted by changing the compressed length. The importance of shear strains during rolling of solid material, which could also cause the observed porosity gradient, was highlighted during the review of the proposal. The experimental matrix was chosen to cover two regimes in the experiments with respect to shear strains. These regimes were defined by Seuren et al. and differ in the shape of the trend of shear strains across the rolled material thickness. Limitation in dimensions on the mill made it necessary to prove that the experiments carried out were in both regimes. This proof could be provided. The porosity curves determined experimentally by computed tomography did not coincide consistently with the shear strain curves. Thus, it could be shown that shear strains are not significantly responsible for the densification of the porous material. The previously mentioned working hypothesis, on the other hand, was confirmed by the determined porosity curves. Secondly, the working hypothesis was that the indentation of struts into pore spaces leads to the reduction of the size of pores. As a result, the pore elongation falls short of the macroscopic rolling stock elongation. The indentation of a strut or "pore roof" would preferentially occur in the longest pore direction. This working hypothesis was investigated by means of rolling experiments, highresolution computed tomography and pore-resolved simulations and could be confirmed by the experiments and the simulation.
Publications
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Investigation of the Porosity Gradient in Thickness Direction Formed by Cold Rolling in Porous Aluminum. Metals, 13(4), 681.
Tychsen, Jörn & Rösler, Joachim
