Thixoforming : Semi-solid Metal Processing

286j 8 Tool Technologies for Forming of Semi-solid Metals
determined by the eutectic temperature of the respective chemical composition,
being typically in the range 1200–1500 C for commercially available Si3N4 ceramics.
Owing to the better sinterability and hence reduced amount of sintering additives
required for densification, the critical temperatures for SiC are in the range
1700–1800 C [79].
The oxidation mechanisms of SiC and Si3N4 are strongly dependent on the oxygen
partial pressure. Whereas at high oxygen partial pressures passive corrosion with
formation of SiO2 occurs, at lower partial pressures gaseous SiO is formed [81, 82],
leading to linear weight loss with time by evaporation. Oxygen partial pressures in
non-encapsulated steel thixoforming tool setups vary from close to 1 bar prior to billet
insertion to estimated values of 1250 C and reacts with
iron to form iron silicides [80] accompanied by release of gaseous nitrogen. Silicon
carbide shows similar behaviour, but reaction starts already at 850 C [81, 82].
Moreover, instead of nitrogen gas, carbon is released, being readily incorporated
in the iron structure. This in consequence leads to an increase in the carbon content
of the steel alloy, which is undesired in terms of mechanical properties of the
thixoformed parts.
Hence silicon nitride was selected as the die material for the semi-solid processing
of steel. Apart from forming trials to investigate the resistance of Si3N4 dies to
thermomechanically induced stresses, the tribochemical attack on the die surface
during forming has to be investigated carefully to ensure sufficient shape accuracy
and service life.
8.6.3.2 Corrosion Experiments and Forming Series
Fully dense silicon nitride ceramics with Y2O3 and Al2O3 added as sintering additives
were applied in corrosion experiments and small-scale forming tests. A typical
microstructure in the as-received state is shown in Figure 8.40. The use of standard
grades of typical composition allows for comparison of the present results with the
findings of other workers in similar applications. Steel-grade 100Cr6 was used for
thixoforging experiments; for details on composition and thermophysical properties,
see Chapter 3.
Following the general scheme given in Figure 8.6, model tests were developed
in order to isolate specific loads from the load profile (cf. Section 8.1). Apart from
typical melt corrosion tests performed to examine the corrosion resistance against
non-ferrous and ferrous liquids at temperatures up to 1650 C, adapted corrosion
tests were used to reproduce characteristic conditions during semi-solid forming
[6]. Equipping the melt corrosion test with a linear unit allowed for cyclic
testing by repeated rapid immersion of the sample in the melt. Thereby, test