Fabian Schongen

Analysis of friction between stainless steel sheets and machine hammer peened structured tool surfaces: experimental and numerical investigation of the lubricated interaction gap

Forming of stainless steel with stringent requirements on surface integrity is currently realized using protective foils as a separating agent between tools and workpiece. The protective foils are applied with special machines and need to be removed after the forming process or at the end customer. This approach has pronounced economic and ecological disadvantages. Alternative tribological systems for a foil free forming are insufficiently researched and not yet reliably applicable in a production process. Recent developments in the field of machine hammer peening motivate the investigation of surface modifications for foil free sheet metal forming. The research question of this paper is: under which tribological boundary conditions do structured tool surfaces provide a total separation of tools and workpiece? The performed research work is based on experimental analyses investigating the friction behavior of surface structured tools. Numerical simulations of normal and sliding contact using finite element method enable the investigation of the lubricated interaction gap in order to identify significant tribological process parameters.

Time-Efficient and Precise FEM/BEM Simulation of a Cold Forging Process Verified by Tool Load Determination

Developing green processes establishes new possibilities for cold forging industry. Current technological developments require automotive parts with less mass, but higher material-efficiency. To achieve these goals, high-strength steels and complex geometries are used. The rising process forces lead to increased tool loads and subsequently elastic tool deformation resulting in early tool failure or dimensional deviations. A numerical determination of tool loads during process enables their reduction by a load-dependent design of the tool geometry. Aim of this work is a time-efficient and precise determination of tool loads considering the complete tool system using the example of a lateral extrusion process. By domain decomposition into Finite Element Method (FEM) and Boundary Element Method (BEM) domains and subsequently an integrated FEM/BEM simulation, a significant computation time reduction towards a conventional FEM model is achieved. Experiments of the examined lateral extrusion process provide data for the verification of the investigated process simulation models. In order to be able to validate the simulated elastic tool deformations, strain gauges are installed on the die insert and allow an experimental measurement of the elastic radial die strains. Additionally the simulated process force development and the final workpiece geometry of the simulation models are compared with experimental results.

Energy-Efficient Solid Forward Extrusion through Hybridization Based on Process-Integrated Resistance Heating

In this paper a hybridized solid forward extrusion process is proposed that uses a process-integrated resistance heating for the energy-efficient heating of the workpiece material in order to avoid the occurrence of chevron cracks. As for the process-integration of the resistance heating two variations are regarded: the preheating of the wrought material prior to the forming process as well as a resistance heating concurrent with the extrusion process. Based on a three-shouldered solid forward extrusion of Cf53 with emerging chevron cracks the broad temperature interval for crack elimination is derived from experiments where the wrought material is preheated in a furnace. With this derived temperature a numerical approach for the dimensioning of a resistance heating of both prior to the forming process and during extrusion is shown. The approach is based on solving the Fourier heat transfer equation using both numerical algorithms in MATLAB and finite element method (FEM) in Deform-2D. In a final step the two scenarios heating prior to and during the extrusion process are evaluated in terms of their energy-efficiency using FEM.

Riblet Application in Compressors: Towards Efficient Blade Design?

Nowadays, modern axial compressors have already reached a very high level of development. The current study is focused on the question, if the application of riblets on the surfaces of a highly efficient modern compressor blade can be a further step towards more efficient blade design. Therefore, a highly loaded compressor cascade has been designed and optimized specifically for low Reynolds number conditions, as encountered at high altitudes and under consideration of the application of riblets. The optimization was performed at a Mach number of 0.6 and a Reynolds number of 1.5×105. Two objective functions were used. The aim of the first objective function was to minimize the cascade losses at the design point and at incidence angles of +5 and −5 degrees. The intention of the second objective function was to achieve a smooth distribution of the skin friction coefficient on the suction side of the blade by influencing the blade curvature in order to apply riblets. The MISES flow solver as well as the DLR optimizer “AutoOpti” were used for the optimization process.The developed compressor cascade was investigated in the transonic cascade wind tunnel of DLR in Cologne, where the Reynolds number was varied in the range of 1.5×105 to 9.0×105. Furthermore, the measurements were carried out at a low turbulence level of 0.8 percent and at a high turbulence level of 4 percent, representative for high pressure compressor stages. The measurement program was divided into two parts. The first part consisted of the investigation of the reference cascade. In the second part of the study riblets were applied on suction and pressure side of the cascade blades; two different manufacturing techniques, a rolling and a coating technique were applied. The rolling technique provides riblets with a width of 70 μm and the coated riblets have a width of 50 μm.The wake measurements were performed using a 3-hole probe at midspan of the cascade in order to determine the resulting losses of the reference blade and the blades with applied riblets. The detailed analysis of the measurements shows, that the riblets have only a slight influence on the viscous losses in the case of the compressor application in this study. Finally, these results are discussed and assessed against the background of feasibility and effort of riblet applications within the industrial design and manufacturing process.© 2014 ASME

Investigation of Surface Crack Types under Axial Compression Loading for a Case-Hardening Steel

During cold-forming operations the formability of the materials can be reached due to the low process temperature, leading to crack initiation. For the investigated case-hardening steel 16MnCrS5 longitudinal cracks and shear cracks are identified as the main crack types. The evolution of both crack types is determined by scanning electron microscope (SEM)-investigations and a simplified finite element method (FEM)-model. The results reveal that the initiation point of shear cracks is located at the surface. In contrast thereto longitudinal cracks emanate from second phase particles in the rim zone.