Thixoforming : Semi-solid Metal Processing

Figure 10.12 Segregation in components with different geometries.
flange ensures that the flow front surface area remains nearly constant with
increasing diameter so that the material flow and the die filling are more homogeneous,
as shown in Figure 10.11a. According to this behaviour, complete filling is
possible without any inclusions.
The comparison of different relations between wall thickness and flow length in
the upper cup shows basically tendencies to higher segregation in the case of longer
flow lengths (Figure 10.12). The metallography shows a homogeneous contribution
of the solid and liquid phases at the bottom of the cup. In the course of material flow
for the thinner cup, the liquid-phase fraction increases until the top of the cup is
reached. In the case of a greater wall thickness, the gradient between the distribution
of solid and liquid phases is much lower. Two-phase simulations (see Chapter 6),
which started with a contribution of 50% solid-phase fraction, confirm these results.
The analyses of different velocities show generally the tendency for higher segregation
with increasing strain rates. This segregation mechanism is based on the better
fluidity of the liquid-phase fraction whereas the solid-phase particles could interfere
with each other and slow down in their movement. In conclusion, an adequate
forming strategy for aluminium alloys has to consider a short forming procedure
regarding the temperature development and also adapted geometries to ensure good
form filling of the dies without causing high chemical gradients by segregation of the
liquid and solid material during the forming operation.
10.3.3
Thixo Lateral Extrusion of Steel
10.3 Heating and Forming Operationsj385
Within the SFB289, first investigations of steel were realized with the thixo lateral
extrusion process. This section shows the general feasibility of producing first