Thixoforming : Semi-solid Metal Processing

282j 8 Tool Technologies for Forming of Semi-solid Metals
In ferrous metallurgy, ceramics are mainly applied as refractory materials in blast
furnaces, converters, tundishes and so on, providing a means to melt ores and metals
and to handle the respective liquids in steelmaking. The main function of these
refractories is to withstand the aggressive melts and slags. However, despite the
sophisticated material solutions available today, the refractory parts are consumed
during operation. Moreover, refractories contain a defined amount of residual
porosity to improve thermal shock resistance.
Dense engineering ceramics are applied in metallurgy as boron nitride (BN)-based
side dams for thin strip steel casting [67], owing to its excellent corrosion resistance
and low friction coefficient. Oxide ceramics, for example alumina (Al2O3), zirconium
oxide (ZrO2) and aluminium titanate (Al2TiO5), are used for protective tubes for
thermocouples and various items in direct contact with ferrous melts.
The demands on materials properties in these applications are lower compared
with the load profile acting on thixoforming dies. Since refractories are not part of
load-bearing structures and merely have to sustain their own weight, the required
properties are high corrosion resistance, low thermal conductivity and a specific
compressive strength. Likewise, hexagonal boron nitride, being a solid lubricant,
exhibits relatively low strength and is commonly regarded as a functional rather than
a structural material. Oxide ceramics show reasonable strength values and excellent
corrosion and wear resistance, but thermal shock resistance is poor. Hence one of the
main prerequisites for successful application of these ceramics is to establish
thermally stable operating conditions, which is detrimental to the process conditions
of sequenced metal forming techniques, that is, thixoforging and thixocasting.
Carbides and nitrides, for example, silicon carbide and silicon nitride, although
exhibiting excellent thermal shock resistance among dense ceramics and strength
levels superior to oxide ceramics, are not chemically stable in contact with iron at
elevated and high temperatures [68, 69], which is why they are not considered
appropriate for application in ferrous metallurgy.
With semi-solid processing of high-melting alloys being based on experience and
findings obtained in light metal thixoforming, the principal approach at the beginning
of interest in steel thixoforming was merely to transfer existing knowledge to
this new group of work materials. Regarding tool development, this implied corrosion
resistance being regarded as a key property for the selection of die materials.
Thus, with the target of identifying suitable tool materials for the semi-solid
processing of steel, Beyer [70] and Behrens et al. [71] independently carried out
screenings of ceramics with a view to chemical interaction with steels at high
temperatures. Thermochemical investigations revealed that a number of oxide
ceramics (Al 2O3, ZrO2, etc.) are stable in the presence of typical steel grades at
temperatures of interest, whereas most of the carbides and nitrides considered show
significant reaction with ferrous alloys, in particular silicon nitride and silicon
carbide [74]. This is confirmed by steel melt corrosion tests performed by Beyer,
who observed severe weight loss for non-oxide ceramics after testing, whereas Al2O3and
ZrO2-based ceramics showed notably higher corrosion resistance [70]. Further
investigations included the wetting of potential ceramic die materials by liquid steel.
According to the determined wetting angle, Behrens et al. [71] classified Al2O3,ZrO2,