In the production of sheet metal components, manufacturing defects often arise in the reshaped parts, such as creases, cracks and necking, and these mean a higher level of rejects. Integrating thin-film sensor systems makes it possible to regulate process control and thus even out fluctuations and minimize rejects. At the Fraunhofer IST a new kind of thin-film sensor system is being developed which stands in direct contact with the workpiece to be formed. Sheet infeed behavior is precisely determined by measurement of pressure and temperature distributions in the drawing process. Additional heat exposures are required for the drawing processes in the case of light metals such as magnesium and aluminum. The development of thin-film sensor systems for local force and temperature measurement in 2D and 3D geometries, in combination with tribological resistance is a special challenge here.
The aim of work at the Fraunhofer IST was to build a thin film sensor system whose purpose was to detect temperature and pressure distributions on a strip drawing tool with a curved surface. The coating system consists of a piezoresistive amorphous hydrocarbon layer (DiaForce®, d ~ 6 µm), a lithographically structured metal layer (chromium, d ~ 250 nm) and an insulating and wear-protective layer (SiCON®, d ~ 3 µm). To increase the sensitivity of the pressure sensors and to enable integration of temperature sensors, the coating system is given an additional insolating coating (SiCON®, d ~ 1.5 µm) in the region of the conductors and contacts. The force and temperature sensor structures are distributed via locally offset measuring points over the curved face of the tool. The temperature sensors are meander structures made of chromium which are located between the force-sensing structures. The contact areas of the individual sensor structures are located on the edge region of the tool.
At the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz the sensor module was fitted into a strip drawing machine in which load cases during the deep drawing of metal strip were investigated. The functional capability of the multifunctional thin-film system was tested with aluminum strip (AA6016) as an example. It was preheated to a temperature of 200 °C and shaped over the curved face of the deep-drawing tool. To keep frictional losses between the sheet metal and the sensorized coating system on the drawing tool as low as possible a special lubricating oil was used. As a result of the contact between the hot sheet metal and temperature sensors T1 to T4 there is initially a rise in temperature. During the bending process the aluminum sheet moves over the individual sensor structures and thereby causes a fall in the sensor resistance of the force-measuring sensors F1 to F8. The bending process finishes with an abrupt unloading of the sensor structures which causes local resistance minima at each individual force sensor structure.
In the further course of the project the plan is to transfer the multifunctional coating system to complex-shaped tools in deep-drawing machinesin order to measure loads and temperatures with spatial resolution during the sheet infeed movement.
The results we have described were obtained within the SensoFut project (Sensorized Future – Sensing of temperature and pressure in harsh environments), on which the Fraunhofer IST worked together with the Fraunhofer Institute for Machine Tools and Forming Technology IWU and Sirris, the Belgian research association. SensoFut is funded in the 13th Cornet Call (Collective Research Networking) by the Federal Ministry of Economics and Technology (BMWI) and the German Federation of Industrial Research Associations (AiF).
This article is part of the annual report 2014.