Thixoforming : Semi-solid Metal Processing

10.3
Heating and Forming Operations
The following sections present selected results and examples concerning semi-solid
forging using an inductive heating unit and a hydraulic forging press. As already
describedin Chapter 1,an essential part of any thixoforming unit isthe heating system
providing the semi-solid material. General questions concerning the requirements
arediscussedinChapter1andspecificsolutionsforinductiveheatingofsteelbilletsfor
thixocastingaredescribedinChapter9,wherethebilletisheatedinasleeveandejected
by the shot piston. This solution cannot be transferred to thixoforging. The billet
has to be placed in the lower half of the die without the sleeve. One possible solution is
the heating of freestanding billets. For this purpose, specific process control features
for steel billets have been developed and tested on experimental pilot equipment.
The further processing of the semi-solid material is investigated with regard to
different materials and process alternatives such as thixoforging, thixo lateral
extrusion and thixojoining. After first investigations, it became apparent that
especially the processing of steel requires high precision regarding the reproducibility
so that an automated thixoforming unit was developed.
Amongst already mentioned advantages of the thixo-route, further saving potential
regarding process time and energy consumption can be achieved by rheoforging.
Two different steel alloys were investigated concerning the applicability in the semisolid
state by forming experiments that use the same tools as in thixoforging.
10.3.1
Induction Process
10.3 Heating and Forming Operationsj373
In the following, inductive heating, which is applied for the thixoforging operation,
and important fundamentals are described. As already shown, the induction furnace
is an essential part of a production unit for thixoforging parts (Figure 10.5), which
mainly consists of a resonant circuit and a converter. The resonant circuit consists of a
capacity connected to an induction coil via a transformer. Energy is fed into the circuit
by the converter, the frequency of which is automatically adapted in order to match
the characteristic frequency of the resonant circuit. The heating power is brought into
the billet by eddy currents, which are mainly induced near the billet surface by the
induction coil s alternating magnetic field. The manipulated variable of the process is
the electrical power of the converter, which means that the coil current peak value is
varied since the converter voltage is constant. In the experimental facility used, the
billets are heated in the upright position to ensure simplified handling and less heat
transfer into the billet carrier.
Disturbances of the inductive heating process are caused by the temperature of the
environment, the cooling water temperature and the changing surface properties of
the billet. The losses during heating consist of convective and radiation losses. These
losses lead to a large difference between the core and the edge temperature if a
conventionally designed coil is used. Also, losses caused by heat conduction at the
bottom surface occur.