Thixoforming : Semi-solid Metal Processing

Figure 11.19 Shell formed while extruding in a special case with reduced liquid fraction.
5800 kN, thus being significantly above the thixoextrusion force. However, the
maximum extrusion pressure required at the end of the described thixoextrusion
experiments is between 254 and 325 N mm 2 . This is up to 10 times higher than for
the isothermal thixoextrusion experiments, which indicates that a considerable
fraction of the force is required to compress the solidifying shell of the billet.
For the chosen process parameters, the material does not solidify in the extrusion
channel and exits the forming die still semi-solid, forming clews with constant
diameter. Hence no significant influence of the forming die geometry is observed. To
achieve solidification in the extrusion channel, the liquid fraction of the billet is
reduced from 40 to 10% while keeping the other process parameters constant. For
these conditions, another defect – massive shell formation – occurs. The reduced
billet temperature results in an immediate shell formation when the billet material
comes in contact with the extrusion channel wall. In one special case a massive
formed shell is shown in Figure 11.19.
This shell reduced the diameter of the extrusion channel and the following
material was extruded through the reduced diameter channel, which allowed the
production of a very thin bar with a total length of 450 mm and a diameter of 2 mm on
average (Figure 11.20). PVD coatings on the extrusion channel did not influence the
Figure 11.20 Extruded thin bar in a special case with reduced liquid fraction.
11.5 Non-isothermal Thixoextrusionj431