Investigation of Bond Formation and Failure of Metallic Alloys

Roll bonding enables the targeted combination of different materials and thus their mechanical and thermal properties in a single material composite. However, the connection created under pressure load can tear up again due to shear stress at the end of the roll gap. Thus, industrial production requires very long process chains, which are often determined by trial and error. In order to allow for a knowledge based design of such process chains, a basic experiment, using the torsion plastometer TA STD 812, was developed at the Institute of Metal Forming to characterize bond formation and failure. In this test, the connecting partners are heated inductively and joined under combined pressure and shear stress. After bond formation, the strength can be tested at deformation temperature under a combination of tensile, compressive and shear stress. The procedure thus enables testing under near-process conditions.

For further information, please contact Tobias Teeuwen.

Friction Determination With Extended Conical Tube-Upsetting Test

The conical tube-upsetting test is used to determine the friction coefficients for different friction models. Similar to the ring compression test, which is also available at the institute, the geometric change of the specimen is measured and evaluated by means of nomograms. The conical contact surface suppresses the occurrence of static friction and thus enables more homogeneous test conditions, even under high prevailing friction. In addition, a blue line laser has been added to the test setup in order to record the contour change during forming. These measured values can be used on the one hand for the inverse determination of the prevailing friction and on the other hand for the examination of the change in friction during the compression process. The test setup allows the testing of workpiece temperatures up to approx. 1200 °C. In addition, the tools can be preheated to temperatures of up to 300 °C.

For further information, please contact Michel Henze.

Hot-Gas-Bulge Test

With the hot gas bulge test, the IBF is researching a novel test method for determining material data for virtual testing and design of hot stamping processes. A pneumatic bulge test for steel sheets has been developed for this purpose. The bulge test consists basically of two rings fixing a sheet metal. The sheet metal is then loaded with a controlled gas pressure, which causes the sheet to bulge through one of these rings. Although the controlled gas forming of sheets is challenging, flow curves in the temperature range of 600 to 950 °C can be obtained at constant strain rates up to 0.5 1/s.
In addition to the material data determination for steel, the flow curve determination for high temperature sheet forming of aluminium with the hot gas bulge test is also being investigated. The softer material behaviour of aluminium compared to steel as well as the high reflectivity of aluminium in combination with a large surface expansion lead to further challenges regarding gas control as well as optical strain measurement, which are being researched at the IBF. 

For further information, please contact Karl Tilly.

Thermal Characterization of Surface Contacts

For simulations of annealing processes of coiled metal strips, knowledge of the effective thermal conductivity in radial direction is indispensable. This is lower than the thermal conductivity in axial direction due to the high contact thermal resistance that occurs during heat transport in radial direction. The contact thermal resistance is a parallel connection of three thermal resistances of basic heat transfer mechanisms and therefore depends on various influencing factors. In cooperation with the Institutes of Industrial Furnace and Heat Engineering as well as Mineral Engineering, the IBF is developing a test rig with which the contact thermal resistance can be indirectly determined from measured temperature values up to temperatures of 1250 °C and contact pressures of 25 MPa. Other parameters that are varied within the test campaigns are the material, the surface roughness profile and the ambient atmosphere with up to 100 % hydrogen. The measured values obtained serve to validate an analytical model and can be used in the simulation of hot forming process chains.

For further information, please contact Lena Koch.