Thixoforming : Semi-solid Metal Processing

430j 11 Thixoextrusion
Figure 11.17 Variation of process parameters for experiments without cooling tube.
(Figure 11.18). In contrast to conventional extrusion processes, where the required
force decreases during the process due to the reducing friction length
(Figure 11.18) [12], a continuous rise of the required force was detected in thixoextrusion.
As was to be expected, an increase in the press velocity resulted in lower required
forces, because there was less time for cooling and solidification.
The friction and the deformation force increased during the process due to the
solidification of the material and reached the maximum at the end. The average
extrusion force required in the thixoextrusion experiments is very low compared with
the conventional extrusion process using the same forming dies and billet dimensions.
A rough estimate can be calculated using the elementary theory, that is, the
Siebel equation [1]. At a solid-state extrusion temperature of about 1145 C, the flow
stress would be about 70 N mm 2 and the force would reach an order of magnitude of
Extrusion load (kN)
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Force
Conventional direct extrusion
Start of process
Displacement
End of process
v 10mm s -1
0 20 40 60 80 100
Displacement (mm)
v
20mm s -1
v 50mm s -1
Figure 11.18 Extrusion load–displacement diagram for
conventional and semi-solid direct extrusion processes for
experiments carried out with a liquid fraction of 40%.