Thixoforming : Semi-solid Metal Processing

432j 11 Thixoextrusion
shell formation but allowed easier removal of the shell and the discard compared with
uncoated forming dies.
11.5.3
Estimation of the Material Dwell Time Required in the Extrusion Channel
As the first experiments did not show sufficient solidification of the billet material in
the extrusion channel, a first estimation of the solidification time of the material
required in the extrusion channel was performed with a one-dimensional numerical
model also allowing the determination of the influence of the forming die material
(Figure 11.21).
The simulation consists of the stationary billet material with a homogeneous
temperature distribution of 1240 C(fL 10%), a total diameter of 15 mm and a
simplified forming die with an initial temperature of 300 C. By assuming adiabatic
boundary conditions, the time required for the complete solidification of the material
is estimated by transferring the heat from the material into the forming die. The
complete solidification is reached at 1220 C. The forming die material was varied
between the hot working steel X38CrMoV5-1, the copper–beryllium alloy CuCoBe,
the nickel-base alloy TZM and an Si3N4 ceramic to determine their influence on the
solidification time. As shown in Figure 11.22, the time for complete solidification can
be assumed to be 2.5 s using a heat transfer coefficient of 15 000 W m 2 K 1 .
Simulations with higher heat transfer coefficients do not influence the total solidification
time significantly. Lower heat transfer coefficients are not suitable according
to [13]. The solidification time is almost independent of the forming die material.
used For the three metallic forming die materials, the solidification time remains
almost constant. Only the Si3N4 ceramic shows a slower cooling behaviour regarding
longer time periods.
With the simulation shown and a required solidification time of 2.5 s using a
15 mm extrusion channel, the required ratio between the length of the extrusion
channel and the bar extrusion velocity can be estimated as
lchannel
vbar
> tsolidification ¼ 2:5s ð11:1Þ
Billet
q
Point of temperature
capture
T B = 1240°C
fL = 10%
T0 = 300°C
Figure 11.21 Schematic diagram of the model for the one-dimensional cooling simulation.
Die