Thixoforming : Semi-solid Metal Processing

spinel MgAl2O4 and SiAlONs as suitable for steel thixoforming. Several Si3N4-based
ceramics, aluminium titanate (Al2TiO5) and numerous mixed oxides are categorized
as suitable to a limited extent; SiC, TiN and cordierite are considered unsuitable.
Since the first forming trials using ceramic dies and die segments revealed thermal
shock acting on the die surface to be critical, corrosion resistance was replaced by
thermal shock resistance as a selection criterion for the semi-solid processing of highmeltingalloys.Hencesiliconcarbideandsiliconnitridecamebackintofocusduetotheir
excellent strength, thermal shock resistance and fracture toughness, despite their lower
ranking in corrosion resistance and wetting angle screenings. Consequently, in recent
work on semi-solid processing of steel, silicon nitride was selected exclusively as tool
material [7, 32, 70–73]. However, only a few data are available on the actual performance
of Si3N4 dies. Beyer [70] observed rupture of externally prestressed, axial-symmetric
Si3N4 liners after only a few forming cycles. Failure was attributed to tensile stresses
generated by the armouring on the one hand and insufficient thermal shock resistance
and fracture toughness on the other. Analysis of die surfaces after application yielded
evidence of corrosive attack, with deterioration of the surface quality [74].
Behrens and co-workers [7, 75] used Si3N4 plunger tops for steel thixoforming and
regularly observed rupture of the ceramic die parts due to shrinking of the steel part on
the ceramic. Neither analysis of the stresses arising in the ceramic die part nor analysis
of the die surface after testing was reported. Other workers, although reporting the
application ofSi3N4 die parts, indicated that stress resistance was unsatisfactory in steel
thixoforming experiments, without giving any details on tool performance [73, 76].
8.6.2
Ceramic Die Concepts for Steel Thixoforming
8.6 Bulk Ceramic Forming Toolsj283
In a first approach to the selection of ceramic die materials for the semi-solid
processing of metals, the tribochemical attack on forming dies is the most critical
load, since the service life of metal forming dies is typically limited by the maximum
tolerated shape inaccuracy. However, with the envisaged thixoforming of highmelting
alloys using ceramic dies, the thermal process loads proved to be decisive
for material selection. As discussed in Section 8.1, the upper thermal process
boundary is given by the work alloy, whereas the die preheating temperature
determining the lower thermal process boundary may be varied. Any change in
thermal process conditions unequivocally affects the entire process layout, owing to
the pronounced temperature dependence of the liquid phase content and thus the
rheological properties of the semi-solid steels. Furthermore, the microstructure of
the work pieces and the resulting mechanical properties are affected. A schematic
diagram of the interconnection of thermal boundary conditions, process parameters
and work piece quality is depicted in Figure 8.39.
With reference to the above-mentioned process analysis, bulk ceramic tools for
the semi-solid processing of steels were applied following two different die concepts:
(i) conventionally heated tools at working temperatures of 1000 C. The characteristics of the different approaches are given
in Table 8.10.