Thixoforming : Semi-solid Metal Processing

44j 3 Material Aspects of Steel Thixoforming
In Section 3.2, the requirements for successful thixoforming are summarized
from the materials and processing points of view. In Section 3.3, the alloying systems
which have been investigated in detail are introduced.
In Section 3.4, the phenomena during quenching from the partial liquid state and
their influence on the liquid state concentration and the structural morphology are
examined more closely. Furthermore, the analysis procedures for the appraisal of the
microstructural parameters and the material choice are evaluated. An estimation and
evaluation concerning the accuracy of the determination of the structural parameters
have to be conducted.
In Section 3.5, investigations of the regulation of the casting behaviour are described.
In addition to the structural parameters, the adjustment of a fluid metallic suspension
is also of importance for the process optimization of the thixoforming of steel.
In Section 3.6, the potential of adjusted heat-treatment strategies based on the
partial liquid state are examined, because completely new material properties can be
expected due to the multiphase structure and resulting different element-content
distributions in the solid and liquid phases. The liquid phase is highly enriched with
alloying elements, in which the considerable carbon enrichment plays a specific role
during the subsequent transformation behaviour. Therefore, the emphasis is on the
structure examination and the development of adjusted heat-treatment strategies
based on the partial liquid state of the steel grades X210CrW12 and 100Cr6.
The research presented in this chapter aims at a reassessment of the potential and
the suitability of the different steel grades for thixoforming.
3.2
Background
3.2.1
Material Prerequisites for the Thixoforming of Steel
The thixotropic behaviour of partial liquid metallic suspensions was analysed for the
first time in the mid-1970s and developed further during the ensuing years [1–3]. In
the examination of tin–lead alloys, which were sheared during solidification within
their solidification-interval, remarkably low viscosities were detected despite their
high solid fraction. Based on these observations, different semi-solid metal (SSM)
technologies for aluminium, magnesium, copper and steel were developed during
the following decades. A precondition for SSM technologies is a sufficiently large
solidification interval, in which a globular grain structure in the partial liquid state
can develop. It was proved to be advantageous if the primary material already exhibited
a globular grain structure. Therefore, various primary material creation routes
for the different materials were developed.
Whereas thixoforming of low-melting metals such as aluminium and magnesium
is already applied industrially, thixoforming of steels is still being developed.
Especially the high process temperatures of the steels pose challenges concerning
suitable tools, the heating technology and the process conduct [4]. Due to the