Thixoforming : Semi-solid Metal Processing

8.6 Bulk Ceramic Forming Toolsj293
material s strength even for high-strength ceramics such as Si3N4. Consequently, the
dwell time of the steel part in the die has to be exactly adjusted to the minimum
solidification time required for ejection without any deterioration in shape.
Concerning the long-term performance of Si3N4 steel thixoforming dies, the
tribochemical wear is of crucial importance for the determination of service life.
Experimental results show substantial agreement of the corrosive effects observed in
model tests, both in air and an argon atmosphere, and thixoforging trials. This
indicates that the oxygen partial pressure in the die during thixoforming varies
strongly, since oxidation of Si3N4 is observed in addition to metallic phases containing
high amounts of iron and silicon, suggesting the formation of Fe(Si) solid
solutions and iron silicides.
After thixoforming experiments, the reaction products are found on the die surface
in the form of a discontinuous tribochemical layer, which develops according to the
following sequence:
1. superficial heating of the Si3N4 ceramic to temperatures >1000 C, accompanied
by softening of the glassy grain boundary phase;
2. relative enrichment of alloying elements (Mn, Cr, V) on the steel side due to
process-induced segregation;
3. out-squeezing of the ceramic glassy phase by compression and shear forces;
4. redox reactions between glassy phase, Si3N4 and steel/steel melt;
5. formation of a tribochemical layer as a result of the above-mentioned redox
reactions;
6. delamination, fragmentation and shearing of this layer;
7. iteration of the sequence with the subsequent forming cycle.
This sequence and the corresponding effects are confirmed by observations of
other workers on Si3N4 forming dies for the semi-solid processing of steel [70]. With
regard to tool performance, the behaviour of the resulting tribochemical layer on the
die surface is decisive. The physico-chemical properties of this layer can be controlled
only by the amount and composition of ceramic sintering additives, since other
parameters of influence cannot be altered, such as the chemical composition of the
steel grade and forming temperatures. The use of alternative sintering aids for Si3N4
ceramics on the target for promoting the formation of refractory reaction layers to
inhibit diffusion from constituents to the contact zone in both the ceramic and the
steel seems to be an appropriate measure to improve the corrosion resistance of Si3N4
dies. However, apart from chemical interaction with iron as the main constituent of
steel, the alloying elements have to be taken into account. Previous work on the
corrosion resistance of various ceramics with metals (Al, Cu, steel) aimed at
thixoforming showed that the reaction between sintering additive elements and
steel alloying elements might be decisive for corrosive attack rather than reaction
with iron [86]. Thus, with the target of tailoring the grain boundary phase of Si3N4
ceramics for application as forming dies in steel thixoforming, the entire chemical
composition of the respective alloy has to be considered. Changing the work alloy may
therefore lead to significantly reduced service life of the dies. The development of
tailored ceramic materials has not been reported hitherto, presumably owing to the