Authors: Ahmed Rassili, Marc Robelet, Dirk Fischer

717

Authors: Alexander Schönbohm, Rainer Gasper, Dirk Abel

Abstract: The aim of the paper is to demonstrate a control scheme by which it is possible to
reproducibly reheat steel billets into the semi-solid state. Usually a heating program is used to reheat
the billet into the semi-solid state. Our experiments showed that this control scheme leads to varying
semi-solid fractions from one experiment to the next. To gain information about the billet’s state its
temperature is often used since there is a known relationship between the temperature and the liquid
fraction. Direct measurement of the temperature via thermocouples is not feasible in a production
environment, therefore a radiation pyrometer has been used as a contact-less measurement device.
The accuracy of the pyrometer depends heavily on the exact knowledge of the radiation coefficient,
which can vary from billet to billet due to different surface properties and which is subject to change
during the heating process. These uncertainties prohibit the implementation of a closed-loop control
scheme since the exact temperature cannot be measured with the required accuracy. In order to be
independent of the measurement errors the proposed control scheme only relies on the slope of the
temperature. By detecting the distinct change of slope which occurs when the solidus temperature is
crossed, the beginning of the melting process can be determined. The energy fed to the billet from
this point onward determines the resulting liquid fraction. By detecting the entry into the solidusliquidus
interval and then feeding the same amount of energy to each billet, it is guaranteed that the
billet reaches the desired liquid fraction even by uncertain absolute value of the temperature and by
small variations of the alloy composition. For the experiments the steel alloy X210 has been used
and measurement data demonstrate the feasibility of the proposed control scheme.

734

Authors: Alexander Schönbohm, Rainer Gasper, Dirk Abel

Abstract: An important step in the processing of semi-solid metals is the inductive re-heating of the
feedstock material. The heating should lead to an uniform billet temperature in order to obtain good
forming results. The billet is supposed to be heated to the target temperature as fast as possible and at
the same time it must be guaranteed, that the outer area of the billet does not melt prematurely.
Conventionally the open-loop trajectories consist of simple power over time diagrams and are
generated by extensive experiments. By using an open-loop control scheme it is possible to chose a
desired trajectory for the middle axis temperature of the billet which respects the given constraint on
the heating process. By taking advantage of the flatness property of the system, an open loop
trajectory for the coil current can be calculated which ensures the desired behavior of the axis
temperature. The shape of the trajectory is determined by the shape of the desired trajectory and the
temperature dependent material properties, which have to be known with the needed accuracy. The
losses of the converter and induction coil are estimated online so that the induced power is known.
The trajectory ensures that the billet is heated to a temperature just below the solidus temperature
without overheating of the billet’s surface and with a very homogeneous temperature distribution.
The Experiments have been conducted using A356 aluminum alloy.

766

Abstract: The heating of basic material is considered the first step of the plastic hot forming. Most
of the „prescriptions” concerning this process are based on practical data and basically they can be
limited to the checking of the surface temperature of the billet. The model described in this paper
makes it possible to determine the temperature distribution developing inside the billet while the
system of the conditions of warm-up can be planned and optimized as a function of the physical
parameters of the heated material.

55

Authors: Jing Yuan Li, Guang Tu Yang, Chun Kun Lin, Fan Wang Meng

Abstract: In the present work, a finite difference model (FDM) is built to predict the transient temperature field of the aluminum billet in the gradient cooling process. The billet is divided in 5 segments, each of which is corresponding with one water cooling ring, and all of which are water-cooled for different times respectively in order to obtain the target temperature gradient. On the following air cooling stage, the temperature of the outer layers, especially the surface, rises first, then falls slowly. With the rising of the surface temperature, the temperature deviation from the center to the surface is narrowed intensively. In comparison with the commercial finite element method (FEM) software, Deform 3D, the FDM model gets nearly the same prediction results as FEM simulation does.

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