Thixoforming : Semi-solid Metal Processing

10.3.4
Thixoforging of Steel
In this section, thixoforging parts show the possibility of producing near-net shaped
components, which are not realizable in the same way in drop forging processes.
They were carried out by using standard dies of hot forming tool steels X38CrMoV5-1
and X45MoCrV5-3-1. Two different geometries were chosen: a demonstrator in the
form of a wheel hub and a pump housing. As thixoforming material, X210CrW12 and
100Cr6 billets were preheated into the semi-solid interval with about 35% liquidphase
fraction and positioned upright in the lower die. The upper tool was lowered to
close the die and, subsequently, a flexible cylinder below the lower tool frame carried
out the rest of the filling operation and maintained the pressure in the cavity until the
solidification was completed. The part was ejected immediately and allowed to cool in
the appropriate atmosphere or medium.
The demonstrator (Figure 10.14a) was chosen for basic experiments to clarify the
filling behaviour of the die depending on the different diameters of the flange and
different wall thicknesses of the bush [11]. It was also investigated with the
rheoforming process (Section 10.3.7) and was the main part for simulations
(Section 10.4.2) because of it axisymmetric geometry and the thereby reduced
calculation time. Generally, the combination of a sufficient tool velocity and compression
force ensures good form filling. Especially for thin-walled components there
is a risk of early solidification with too slow forming operations. In the region of the
bush, the material can be cooled below the solidus temperature so that the flow front
breaks open and the semi-solid material of the inner part surrounds this frozen area.
This can lead to cold shuts, which are characterized by strongly weakened material
properties.
The investigation of the pump housings produced regarding the microstructure
and local inside hardness showed that no shrinkage holes and porosity can be
identified on the examined cross-sections. However, some oxidized outer shell of the
billet is found inside the part. Several microcracks exist around the oxide inclusion
Figure 10.14 Components produced by thixoforging.
Demonstrator, 100Cr6; pump housing, X210CrW12.
10.3 Heating and Forming Operationsj387