Thixoforming : Semi-solid Metal Processing

homogeneous microstructure and no difference concerning the microstructure was
observed between samples from the edge and centre.
Using a simple modulating valve, the cooling air flow could be altered. With
different lever positions, varying cooling times were achieved. Retrospectively the
heat transfer was determined from measured cooling curves. Eight experiments were
performed. In experiment number 8 the highest cooling rate was achieved by water
cooling. From the middle between the edge and centre of the cast billets, samples
were taken and image analysed, as for the preliminary experiments with MICRESS
simulation.
The results of the image analyses (with the software package IBAS) are given in
Table 9.8. The change in the microstructure with different cooling rates is illustrated
in Figure 9.40. Furthermore, this figure shows the measured frequency distributions
to illustrate the effect of the cooling rates on the microstructures. A transition from a
globular to a dendritic grain shape becomes apparent. Moreover, the microstructure
formed is much finer with higher cooling rates (see Figure 9.40, where the measured
grain amount is plotted against the cooling rate). Because a mixed microstructure
forms during the transition a from globular to a dendritic shape, no clear borderline
could be set; the change from globular to dendritic microstructure is fluent.
To estimate nevertheless the highest possible cooling rate to achieve a useful
globular microstructure, the measured and simulated shape factors were plotted
against the cooling rate (Figure 9.40). For an easier depiction, the reciprocal of the
shape factor from Equation 9.1 was used. As expected, the shape factor decreases with
an increase in cooling rate. The reading points could be approximated by a linear
Table 9.8 Results of the validation experiments for the MICRESS simulations.
Parameter
Experiment No.
1 2 3 4 5 6 7 8
9.4 Rheoroutej353
Average starting temperature a ( C) 607.0 603.5 606.0 608.5 608.0 607.3 607.4 606.7
End temperature a ( C) 585.0 585.0 585.0 585.0 585.0 585.0 585.0 585.0
Cooling duration (s) 335.5 245.5 143.0 147.5 138.5 100.0 83.0 48.0
Cooling duration extrapolated
(613–585 C) (s)
427 372 191 176 169 126 104 62
Cooling rate (K s 1 ) 0.066 0.075 0.147 0.159 0.166 0.223 0.270 0.452
Grain density d (mm 1 ) 72 88 99 104 90 120 120 170
Grain density b,c (mm 1 ) 57 67 76 73 59 82 85 86
Omitted portion c
0.09 0.12 0.12 0.22 0.22 0.25 0.24 0.43
Solid fraction d (%) 54.79 53.94 53.45 54.98 54.62 52.62 50.89 49.63
Averaged shape factor d
0.79 0.76 0.78 0.71 0.69 0.72 0.72 0.63
Average grain diameter d (mm) 81.37 75.52 69.45 68.03 69.57 62.89 61.87 50.93
a Linear cooling condition.
b Without edge-truncated grains.
c Only grains with area 1500 mm 2 .
d Determined using image analysis software; because of ripening effects during quenching, the
measurements may differ from the real microstructure at the quenching temperature, but this effect
is negligible.