Thixoforming : Semi-solid Metal Processing

In the following sections, these concepts are introduced and the general load
profile as given in Section 8.1 is modified with respect to the die materials selected for
each concept. Adapted testing schemes are derived according to Table 8.10 in order to
identify the specific strengths and limits in model tests, near-application tests and
small-scale forming series. Experimental results are reflected on the predefined load
profile, forming test results and work piece quality obtained. An evaluation of the
applicability of bulk ceramic dies for the semi-solid processing of steels is given in
concluding remarks.
8.6.3
Conventionally Heated Tools
8.6 Bulk Ceramic Forming Toolsj285
8.6.3.1 Characteristic Load Profile and Die Material Selection
The selection of suitable die materials has to be carried out initially regarding the
short-term effects, namely mechanical and thermal loads during forming. When
using conventionally heated forming tools operating at 500 C, the characteristic load
profile essentially consists of severe thermal shock on the die surface in contact with
the partially liquid alloy. Hence the most critical die material property derived from
this load profile is thermal shock resistance, in conjunction with high strength levels.
Silicon carbide (SiC) and silicon nitride (Si3N4) meet those demands on die
material properties and are commonly used as structural materials in high-temperature
applications. Silicon carbide exhibits a high thermal conductivity, which is why
it is often used for heat exchangers, whereas silicon nitride shows very high thermal
shock resistance among dense ceramic materials. Silicon nitride ceramics typically
consist of two modifications: a-Si3N4, typically being present in the form of equiaxed
grains, and b-Si3N4, in the form of elongated grains of needle-like appearance, owing
to significant differences in the c-axis length of the respective unit cells. Controlling
the a/b ratio and the grain morphology allows for tailoring of the microstructure and
mechanical properties of Si3N4. The inherent high thermal conductivity of silicon
nitride further promotes thermal shock resistance. Moreover, mechanical strength
and fracture toughness are high compared with other engineering ceramics, being in
the region of 800 MPa at 1000 C and 5 MPa m 0.5 , respectively.
Hence the chemical interaction of SiC and Si3N4 with semi-solid steels under
thixoforming conditions is decisive for material selection. Both ceramics react with
oxygen in ambient air to form a dense, superficial SiO2 layer acting as a diffusion
barrier for further reaction, thereby passivating the surface. Owing to the poor
sinterability of these materials, sintering aids are required to promote liquid phase
formation during densification, with the respective elements to be taken into account
when considering oxidation and chemical interaction with steels. These sintering
additives are known to affect drastically the oxidation behaviour of the parent ceramic
by the formation of ternary or quaternary phases facilitating oxygen diffusion,
inhibiting in turn the formation of a passivating oxide layer [68]. In the case of
Si3N4, the release of gaseous nitrogen after decomposition further aggravates this
effect by causing pores in the reaction layer. Some workers have also reported an
enrichment of the sintering additive elements in the contact zone [77, 78]. Thus,
critical application temperatures for SiC and Si 3N 4 under oxidizing conditions are