Thixoforming : Semi-solid Metal Processing

436j 11 Thixoextrusion
Table 11.6 Pyramid diamond hardness of extruded bars for
different sample location and an impact time of 15 s.
Primary grain (HV 0.3) Eutectic (HV 0.3) Macro hardness (HV 10)
333 63 722 195 409 33
independent of the extrusion direction, the eutectic had hardness values (HV 0.3) of
722 on average, which were about double those for the primary grains, which had an
average hardness (HV 0.3) of 333 (Table 11.6). With the hardness test HV 10, an
average macro hardness of 409 for all bars could be observed.
11.5.5
Numerical Simulation of the Thixoextrusion Process
The estimation of the dwell time of the material required in the extrusion channel in
Section 11.5.3 shows the requirement for an extremely long extrusion channel or
extremely low press velocity. To determine this required press velocity that ensures
complete solidification, the process of thixoextrusion was simulated with the
simulation program Forge 2005, which uses a Lagrange approach and a rigid plastic
material law. For the implicit two-dimensional axis symmetric simulations, triangle
elements were used. The material data and boundary conditions were chosen
according to the investigations of Shimahara et al. [16]. For a complete description
of the process chain, the simulations of the process steps cooling after heating into
the semi-solid state and cooling during manipulation of the billet revealed the
temperature distribution of the billet before the forming process. The measured
temperature distribution of approximately 1270 C directly after heating was used as
initial temperatures for the first cooling simulation. Due to radiation and heat
transfer during the manipulation of 10 s, the cooling simulations showed a total
temperature decrease until inserting the billets in the container of about 20 C.
The simulation model of the bar extrusion process is shown in Figure 11.28 and is
an illustration of the real extrusion tool. It consists of the billet (diameter 76 mm,
height 100 mm), the forming die, a die holder and the container with an inner
diameter of 85 mm. The initial temperature of the billet was 1250 C. The forming die
temperature was varied from room temperature up to 600 C to investigate if this
could prevent shell formation. The extrusion channel had a diameter of 15 mm and a
length of 50 mm.
The simulated press velocities were varied from 0.5 to 10 mm s 1 to achieve
complete solidification of the material on the one hand and to simulate real
experiments on the other. The extrusion was performed in a vertical direction and
gravity force was considered. The temperature development of the extrusion material
and the forming die is shown in Figure 11.29 for a simulation of the real experiments
with a press velocity of 10 mm s 1 . After coming in contact with the forming die, the
billet cooled immediately. During upsetting the billet up to the container wall, the
material continued to cool. When the material was extruded through the extrusion