Thixoforming : Semi-solid Metal Processing

Figure 8.51 SEM images of alumina die surfaces after steel
thixoextrusion experiments. (a) Die inlet; (b) Die land.
8.6 Bulk Ceramic Forming Toolsj299
ceramic material. Adhesion of steel on the die surfaces was observed sporadically and
only locally, owing to the poor wetting of alumina by steel. This advantageous
behaviour promotes the aforementioned high surface quality of formed parts.
Microscopic analysis of the dies carried out using SEM and EDS substantiates
these findings. Alumina die surfaces applied in steel thixoextrusion experiments, as
depicted exemplarily in Figure 8.51, appear to be unaffected by tribochemical attack
of the semi-solid steel slurry. This is demonstrated by the machining grooves
originating from green machining of the die still being visible after application,
thereby acting as indicators of tribological attack (Figure 8.51a).
Although the accumulated contact time of alumina dies with semi-solid steel was
short in small-scale forming series performed using self-heating ceramic dies, the
findings are consistent with investigations on the chemical interaction of alumina
ceramics with steel alloys in the temperature range of interest for steel thixoforming
[5]. Thus, the potential of oxide ceramics in semi-solid metal forming with regard
to improved tool life is clearly discernible.
8.6.4.4 Applicability of Self-heating Ceramic Dies for Steel Thixoforming
The die concept of self-heating ceramic dies for the semi-solid processing of steel has
been realized and tested by the construction of prototypes for thixocasting, thixoforging
and thixoextrusion. The intended beneficial effects on the flow behaviour of the
semi-solid slurries, reduction of process forces and geometric complexity of work
pieces have been accomplished in small-scale forming series.
Regarding the thermomechanically induced stresses, denoted short-term effects
according to the general load profile depicted in Figure 8.1, self-heating alumina dies
showed sufficient resistance in near-isothermal steel thixoextrusion experiments. A
detailed analysis of the temperature-dependent mechanical strength of the alumina
dies in steel thixoextrusion is given elsewhere [2]. Regardless of the excellent wear and
corrosion resistance of thixoextrusion dies observed herein, a prediction on longterm
performance of such tools is not feasible on the basis of current experiments,
since contact times with the semi-solid slurry were too short.