Thixoforming : Semi-solid Metal Processing

Figure 6.24 Influence of particle diameter on the storage modulus
G 0 . Material: Sn–15%Pb.
behaviour with increasing resting time. Furthermore, it can be seen that the steadystate
value of the storage modulus increases with increasing particle diameter,
whereas the evolution of the loss angle is independent of the particle diameter.
If the storage modulus is normalized regarding its equilibrium value and plotted in
a half-logarithmic diagram versus time, two agglomeration processes can be observed
(Figure 6.24). In the first few seconds, a fast process independent of particle
diameter with an absolute slope of a0 ¼ 0.001 s 1 occurs. This is followed by a
diameter-dependent slower process with higher absolute slopes between 0.0014 and
0.0044 s 1 (a 1 a 3). During the fast process, bonds between particles are formed and
the slower process is characterized by hardening of the initially generated bonds.
6.1.6.2 Capillary Rheometer
6.1 Empirical Analysis of the Flow Behaviourj189
Experimental Results and Discussion (Sn–15%Pb alloy) The experiments were carried
out under isothermal conditions and were designed to verify rheological
modelling in terms of closely process-related conditions. At the beginning of each
experiment, the alloy, supplied in cylindrical billets of 75 mm diameter and 160 mm
length, was heated to the desired experimental temperature using a convection
furnace. While heating the billet, the core temperature was recorded using a type K
thermocouple. The heating rate was derived from the time–temperature curve as
shown in Figure 6.25. The first sets of experiments, for which the Sn–15%Pb alloy
was used, were carried out at a solid fraction fS ¼ 0.6. The flow velocity of the slurry u
was varied from 2 to 7 m s 1 , which corresponds to a variance in the true shear rate
between 1800 and 9200 s 1 (Figure 6.26).
Experimental Results and Discussion (Aluminium–Silicon alloy A356) Rheological
experiments on semi-solid aluminium alloys are characterized by extremely high tool
temperatures. At the beginning of each experiment, the alloy, supplied again in