Thixoforming : Semi-solid Metal Processing

396j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
evolution, being dominated by nucleation, forced convection and decelerated cooling,
is characteristic for all rheo processes. The rheoforming process has already been
applied successfully to serial production using light metal alloys. However, the rheo
process in principle has greater potential concerning energy consumption and cost
reduction compared with the thixo route due to the shorter process chain. To exploit
this potential, several new variations of the process are also under development for
light metal alloys [48–52]. For the application of steel alloys, the requirements for the
process chain and the metallurgical development of the utilized materials were
basically analysed. Analogous to the thixoforming process, the duration of the
process steps has to be kept as short as possible due to temperature losses and
variations in the chemical composition because of melting losses and slagging.
The study considered two different steel grades, X210CrW12 and 100Cr6. The steel
melt is produced in a tilting induction furnace with a maximum power of 45 kWand a
frequency of 2 kHz. To avoid oxidation, the furnace and the slope are rinsed with
argon. The melt is just slightly overheated so that the temperature drops below the
liquidus temperature during flowing on the slope. This induces homogeneously
distributed nucleation in the storage cup. Figure 10.22 gives an overview of the
schematic sequence and the positions of temperature measurement by thermocouples
and a pyrometer during the rheocasting operation for steels.
The supercooled melt is collected in a manually manipulated and argon pre-rinsed
cup at the end of the slope. It is insulated and preheated (400 C) to avoid rapid
cooling of the semi-solid billet. The volume of the cup suits the size of a single part. To
guarantee a constant volume content and to remove slag or other impurities on the
surface, the excess slurry is cut off with a special mechanism directly after casting
(Figures 10.23 and 10.24). Then, the cup is immediately closed with an insulation
plate to avoid oxidation during a given holding time.
By inverting the holding cup, the billet is ejected and placed in the open lower die
for subsequent forging (Figure 10.25). The dies made of working steel
X38CrMoV5-1 are preheated to 300 C and rinsed with argon shortly before the
forging operation to avoid fast cooling and oxidation of the billet. After the insertion
of the billet (step 1), the upper die closes and the lower plunger (step 2) carries out
the main forming operation. During the complete forming operation, the upper die
is locked with a holding force of 480 kN. The peak force of the plunger reaches
190 kN (equal to 160 MPa) at the final position and it is kept during the solidification
Figure 10.23 Scheme of filling and cutting sequence.