Thixoforming : Semi-solid Metal Processing

346j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Table 9.4 Variables of the cooling channel process. a
No. TC ( C) TPv ( C) TGn ( C) mB (g) mR (g) mG (g) Dm (%) tP (s) Dt12 (min)
1 141 653 613 1623 347 1970 17,6 5,9 5:58
2 137 642 613 1759 382 2141 17,8 5,2 —
3 137 633 613 1496 425 1921 22,1 4,7 —
4 144 638 614 1575 360 1935 18,6 4,2 4:30
5 144 633 611 1561 402 1963 20,5 3,5 4:29
6 140 629 613 1465 467 1932 24,2 3,6 5:54
7 143 628 612 1517 487 2004 24,3 3,4 5:08
8 144 634 613 1646 371 2017 18,4 2,9 4:45
9 139 642 614 1682 331 2013 16,4 2,9 5:27
a TC, temperature of the channel; TPv, temperature of the melt before pouring; TGn, temperature of the
melt after pouring; mB, weight of the billet; m R, weight of the remaining metal on the
channel ¼ metal loss; m G, poured metal ¼ m B þ m V; Dm, percentage metal loss; t P, pouring time;
Dt 12, time between pourings.
Theincreaseinthegraindensityisaffectedbytwomechanisms:thecoolingofthemelt
due to the channel contact and the shearing of formed dendrites as a result of the flow.
The residence time of the melt on the channel is the decisive factor for the
temperature decrease of the melt, provided that the channel temperature is not
varied. This residence time is changeable on the one hand by varying the channel
length. Over the channel length the melt cools continuously. With a longer flow length
the melt becomes increasingly colder, but in addition the metal loss also increases.
Because the pouring temperature should be as low as possible (less hydrogen
absorption and lower costs for heating of the melt), pouring of 15 C supercooled
melt is useful. In this respect, the channel length is chosen appropriately. Simulations
accomplished with MAGMAsoft confirmed this result.
On the other hand, the contact time between the melt and the channel can be
influenced by the pouring rate and the channel inclination. Both affect the flow rate of
the melt and, as a consequence, the shearing of the solid phase and the residence
time. As a result of this, the flow length could be chosen to be shorter if the channel
were to be more tilted. In that case, the shearing rate would be increased.
Under the influence of shear, the transformation from dendritic to globular grains
occurs [43]. This effect was shown [41] while using the so-called cooling slope of the
new rheocasting process.
Comparing the microstructures of the remnant metal on the cooling slope from
the top of the slope with those from the bottom, a transition from dendritic to globular
microstructure takes place. Due to the fluid flow, melting of the dendrite arms and
deformation of the dendrites occur, resulting in crystal multiplication [39, 40]; the
grain density increases.
The main difference between the cooling slope and cooling channel is the
inclination (45–5 ). This forces a much higher shearing on the cooling slope. To
evaluate this effect in the cooling channel process, the remnant metal on the cooling
channel was investigated.