Thixoforming : Semi-solid Metal Processing

292j 8 Tool Technologies for Forming of Semi-solid Metals
. shrinking of the solidifying steel part on the ceramic die parts, inducing compressive
loads that cause tensile loads at distant locations in the bulk;
. mechanically induced stresses arising due to insufficient fixation of the ceramic die
parts in metallic tool frames.
It is noteworthy that in many cases the integrity of the Si3N4 dies was maintained
even after rupture if the tool frame successfully fixed the fracture surfaces. This
enabled the operator to continue forming without any decrease in work piece quality
until a predefined number of forming cycles was accomplished. In general, the
surface quality of the as-formed steel parts was excellent in the case of Si3N4
dies [35, 85]. This is attributed to the poor wetting of Si 3N 4 by liquid steel, facilitating
part ejection and inhibiting welding of steel residues on the die surface.
8.6.3.4 Applicability of Silicon Nitride Dies in Steel Thixoforming
The experimental results using bulk Si3N4 forming dies for the semi-solid processing
of steels are discussed in terms of the performance related to the short- and long-term
effects as defined in the general load profile (cf. Figure 8.1). In view of the short-term
effects comprising mechanical and thermal loads, the results show an evident
sensitivity of Si3N4 die parts regarding thermally and mechanically induced stress
states evolving during the entire course of forming, including preheating, billet
insertion, forming, solidification stage, part ejection and die opening time [85].
Failure causes may be categorized according to (i) process loads, (ii) die construction
and (iii) process parameters.
The first category comprises the required forming forces and thermal loads exerted
by the semi-solid slurry in contact with the die surface. Experimental results indicate
that thermally induced stresses are decisive in comparison with the mechanical forces
which are reduced compared with conventional forging and casting by taking advantage
of the rheological effect of thixotropy. The second category includes construction
details that are essential when using ceramic forming dies, that is, the fixation of die
parts. Clamping or joining of ceramics to metallic tool parts is difficult due to the
distinctmismatchinthermalexpansion.Thisisfurtheraggravatedbyresidualstresses
in the solder layer after cooling from high-temperature brazing, which may contribute
to the evolution of critical stress distributions inside the ceramic part when overlaid by
the above-mentioned process-induced stresses. If rotational-symmetric dies are used,
prestressing of the dies by shrink-fitting of (hard) metal armouring rings may be
beneficial. However, the compressive stresses induced prior to application may cause
critical tensile stresses in die parts protruding into the cavity. Moreover, armouring
stresses rapidly decrease with increasing die temperature and are eliminated at
shrinking temperature, reducing the efficiency of this approach.
The third category includes the effects of various process parameters on stresses
arising in the ceramic dies, namely dwell time of the shaped steel part in the die and
deviant loads, such as insufficiently preheated steel billets inserted in the cavity. The
most crucial aspect among these is the on-shrinking of steel on ceramic dies during
solidification of the formed part. The tensile stresses generated beneath the compressedpartsofthedieareofhighmagnitude,inmostcasessignificantlyexceedingthe