Thixoforming : Semi-solid Metal Processing

428j 11 Thixoextrusion
impacts. Extensive finite element method simulations of temperature profiles and
process-induced stresses are essential for defect-free long-term application of
ceramic dies in order to benefit from the excellent corrosion and wear resistance
of these materials.
This also includes effective control of the solidification step during thixoextrusion.
A stable and preferably flat solidification front is required in order to retain the shape
of extruded bars and minimize residual porosity by solidification at least under low
pressure. Potential solutions include providing guide rollers, as depicted in the tool
concept scheme in Figure 11.2, not available in the experiments reported here, on the
target to stabilize the as-formed bars until solidification is fully accomplished.
Alternatively, a secondary, non-isothermal die exhibiting a defined temperature
profile adjusted to the work alloy and extrusion velocity may be provided below the
isothermal forming die to control solidification. In the latter case, independently
controlled heating cores and a segmented die construction using tailored die
materials are essential.
In this respect, the tool setups and experimental results obtained hitherto merely
mark the beginning of possibilities for the isothermal semi-solid processing of highmelting
alloys. Similarly to other complex, but established, metal forming processes
relying on accurate solidification control such as continuous casting and thin strip
casting, the transfer of laboratory-scale results to a table process on an industrial scale
remains a challenging task.
11.5
Non-isothermal Thixoextrusion
11.5.1
Experimental Strategy, Tools and Process Parameters
The non-isothermal thixoextrusion experiments were carried out on the SMS Meer
open die forging press. Due to the press design, the extrusion experiments were
carried out in a vertical direction. The lowest press velocity possible was 10 mm s 1 .
For this series of experiments, the version of direct extrusion was chosen. The punch
moved in the same direction as the extruded bar. The dimensions of the container
(inner diameter 85 mm) were based on the calculations of the maximum hydrostatic
pressure. The extrusion tool was designed in a modular way (Figure 11.16a). The
forming die holder could be opened to remove the forming die, the discard and the
dummy plate afterwards. The tool was designed to extrude billets with a diameter of
76 mm and a height of 100 mm (Figure 11.16b).
The extrusion tool was actively heated to 300 C whereas the forming die was
heated to 600 C in an external furnace to prevent early solidification of the billet
material after coming in contact with the forming die. The forming die was inserted
in the container before the extrusion process. This allowed the height of the forming
die and also the geometry of the extrusion channel to be changed but prohibited
integration of an active cooling system. Additional cooling tube was not installed