Thixoforming : Semi-solid Metal Processing

420j 11 Thixoextrusion
should be just above the solidus temperature, for example 1250 C for X210CrW12.
The heating core constructed for the first extrusion experiments provided a peak
temperature of approximately 1350 C in the centre of the container when the
sampling point at the die inlet was set to 1250 C. Hence heat losses of the preheated
steel billet in the container were inhibited.
11.4.2
Extrusion Performance and Bar Analysis
Laboratory-scale thixoextrusion tests yield very low values of the pressure required for
forming, being less than 22 N mm 2 during the entire experiment. This indicates
smooth and effortless forming behaviour of the semi-solid slurry and confirms the
efficiency of this tool concept in preventing heat losses of the work alloy. A
characteristic load curve of a laboratory-scale isothermal thixoextrusion test is
observed, showing a steep exponential increase in forming forces until the maximum
values are reached at the end of extrusion. Intermediate peak values observed in
laboratory tests (Figure 11.6) are ascribed to flow irregularities resulting from the
relatively hard oxide shell at the bottom of the steel billet that is cracked and deformed
during extrusion.
This is contradictory to typical load curves recorded in conventional extrusion
processes showing a yield stress at the onset of forming and nearly constant forces
during forming (Table 11.2). However, comparison of the extrusion forces applied in
the present experiments with relevant work on steel thixoextrusion by other workers
[5–8] using cold dies substantiates these findings, revealing a significant reduction
in the required process forces.
Extrusion load (kN)
25
20
15
10
5
30
28
0
0 10 20 30 40 50 60 70 80
Displacement (mm)
Figure 11.6 Typical extrusion load versus displacement curves
obtained in laboratory-scale isothermal thixoextrusion tests.