Determination of Boundary Conditions
For the determination of the process boundary conditions, a multi-stage concept was developed at the Institute of Metal Forming. This concept ensures the distinct determination of the single parameters. The corresponding boundary conditions such as emissivity factor, heat transfer coefficient and friction coefficient are determined by using an inverse FEA calculation. The procedure is as follows:
Determination of the Emissivity Factor ε
- Cooling of a cylindrical specimen with air in combination with temperature measurement
- Comparison of the specimen’s temperature with FEA calculation
Determination of the Heat Transfer Coefficient α
- Cooling of a specimen at defined contact pressure combined with temperature measurement
- Comparison of the specimen’s temperature with FEA calculation
Determination of the Friction Coefficient μ
- Compression of a conic upsetting specimen or alternatively a ring shaped upsetting specimen
- Comparison of the specimen’s diameter with FEA calculation or nomogram
Emissivity Measurement
With increasing temperature, the influence of thermal radiation becomes significantly greater compared to the heat transfer to the environment. For this reason, precise knowledge of the radiation coefficient is often necessary for forming processes at high temperatures. The emissivity can be determined in an experiment dominated by radiation. For this purpose, a cylindrical sample is mounted on a thin ceramic sleeve to ensure minimal solid contact. The sample can then be inductively heated to the desired temperature. After reaching the target temperature, the sample is cooled in air, and the temperature of the sample is measured at several points. Based on the temperature change over time, the emissivity of the sample can finally be determined using inverse modeling techniques. The emissivity is particularly dependent on the surface properties of the sample.
Heat Transfer Coefficient Measurement
Thermodynamic boundary conditions, such as heat transfer, are critical variables that should be determined as precisely as possible, especially when it comes to hot bulk metal forming. The Institute of Metal Forming therefore offers the possibility to determine heat transfer coefficients experimentally for corresponding material pairings. The tools can be heated to temperatures of up to 300 °C and the samples to temperatures of up to 1200 °C using an external furnace. In dependence of surface properties and normal pressure, corresponding heat transfer coefficients can be determined on the basis of the measured temperature change of the sample by means of inverse modeling.
Friction Measurement
For the determination of friction coefficients in the field of bulk metal forming, both the classic ring compression test and the specially developed conical tube upsetting test, are available at the Institute of Metal Forming. Tools that can be heated up to 300 °C and specimens that can be heated up to 1200 °C in an external furnace make it possible to reproduce typical forming conditions of hot bulk metal forming. Strain rates of 0.001 - 10 s-1 can be realized. Precisely reproducible quantities of lubricant are applied to the punches by means of a lubrication spraying system. The tests are evaluated by means of inverse modeling or inverse determined nomograms.