Thixoforming : Semi-solid Metal Processing

330j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Standardized Form Filling Experiment This form filling experiment was developed
with the aim of providing a standardized test tool which is suitable for characterizing
the qualification of a new material for the thixocasting process by measuring the form
fill and flow properties.
Moreover, the geometry was designed with the support of multiphase simulation
in cooperation with the Aachen Chemical Engineering Institute of RWTH Aachen
University. A maximum influence of the rheological qualities of the material on the
form filling characteristics can be deduced.
Fourfold Plate Sample The plate sample is part of the preferential experiments due to
the segmentation and execution as a multiple die for the characterization of different
tool materials and the statistically based data for parameter variations. In turn, the
block-shaped tools are fixed by holders without stress, which permits the use of full
ceramic inserts.
9.3.3.3 Examinations of the Wear Behaviour of the Form Tools
Due to the high process temperatures in connection with the amplitude of the
thermal shock cycles and the corrosive attack by melt, there is a good chance,
particularly in the areas near the surface of the tool, for damage during the processing
of steel. Propagation of cracks, strong abrasive wear, adhesive damage and deformation
were frequently observed in the macroscopic area.
For this reason, the choice of the characterization and assessment of different tool
materials and surface treatments, such as coating and mould release agents, were at
the forefront of the design experiments. At the same time, the dispersion-hardened,
powder metallurgical titanium–zirconium–molybdenum (TZM) alloy was tested as a
high-temperature material. A promising and comparatively high thermal shock
resistance ceramic Si3N4 was selected.
By applying suitable mould release agents, the thermal resistance can be
increased and the heat transmission into the form tool can be reduced. At the
same time, mould release agents reduce the stickiness and protect the tool surface
against mechanical destruction. No special products have been available until now.
A selected zirconium oxide mould release agent has been developed for iron and
steel in sand casting or investment casting methods. The mould release agents were
applied evenly by spraying on the form that was cleaned and newly prepared after
every shot. The tests showed that the mould release agent is taken differently and
partly sticks after forming at the component depending on the metal speed but also
the distribution of the liquid phase. Damage occurred because of the high thermal
load in the region of the oxide bag and also at the edges of the modular step units
(Figure 9.19).
Interchangeable step modules of different wall thicknesses and coatings were
placed in the tool of the T-shaped sample up to full ceramic plates. Figure 9.20 shows
coated modules after a combination of approximately 50 aluminium and five steel
design cycles. No damage can be observed on the flat areas but the edges show
detachments. During the casting and cooling of the component, the annealing
temperature of the tool steel is exceeded in the near-surface region. The hardness loss