Thixoforming : Semi-solid Metal Processing

10.3 Heating and Forming Operationsj381
3. supplying the required melting energy to reach the needed liquid fraction;
4. homogenization step: achieving a constant surface temperature and allowing the
billet temperature to homogenize by compensation of the radiation losses only.
The power level for the first step can be chosen arbitrarily since the beginning of
the melting process is automatically detected. The power level and duration for the
melting phase can be derived from the required melting energy, which can be roughly
estimated using chemical calculations of the enthalpy or can be determined by
experiments. The power level and duration of the homogenization step can also be
determined by experiments. For the detection of the beginning of the melting
process, a threshold for the slope of the temperature has to be chosen, which triggers
a stopwatch to obtain the melting time when the slope drops below this threshold.
The threshold has to be adapted to the induction furnace setup and the chosen alloy. A
limiting factor for the achievable liquid fraction is the vertical heating setup, because
the billet develops an elephant foot for higher liquid fractions and tends to tilt with
increasing softness caused by gravity.
Experimental Results
For the experiments, commercially available rod material of X210CrW12 alloy was
used; the billets had dimensions of 34 mm diameter 50 mm and a weight of 355 g.
The induction furnace has a variable converter power of 50 kW with a maximum
frequency of 2 kHz. To reduce the heat losses, the coil was encapsulated, surrounded
by isolating material and flushed with inert gas to prevent surface oxidation. The inert
gas has also a positive effect on the heat losses, because a blank surface exhibits a
lower radiation coefficient than an oxidized surface.
Pyrometer Control
The temperature can be measured by a quotient pyrometer through a small hole
within the coil jacket. Figure 10.7 shows the results of two different experiments.
Also, the four control steps are shown for one experiment. Despite the different
temperature behaviours, all billets have the same liquid fraction at the end of the
heating. It can be concluded from the results that it is possible to heat the billets
reproducibly to the same liquid fraction for fixed parameters of the control scheme.
The proposed control scheme has been integrated successfully into the sequential
control of the automated thixoforming plant (Section 10.3.6).
Flatness-based Control
To ensure that the needed power was fed into the billet, the converter is controlled by a
PID controller. The converter is a nonlinear system, hence the PID controller has to
be tuned carefully because the coil current tends to oscillate.
Figure 10.8 shows the calculated and measured coil currents. The coil current does
not follow the calculated trajectory exactly because the PID controller is not tuned
perfectly. Figure 10.9 shows the measured temperatures in the middle of the billet, at
the surface and the desired surface temperature. There are deviations between the
desired and the measured temperatures because the needed power is not exactly