Figure 6(c) indicates the melt falling down from the two top runners on top of the melt front progressing from the bottom of the mould cavity. This is again a feature that should be kept at a minimum during filling for the same reason as the fountains, i.e. oxidation. Moreover, an oxide layer is already present on top of the melt front coming from the bottom, which likely gets torn apart by the melt streams, and estab- lishes surface turbulence and disintegration of the melt front. The thermal analysis during solidification evaluates the effect of the top riser and the cooling effect of the chills placed around the cylindrical section of the cast part.
In Figure 7 one can see that during the solidification pro- cess, particularly at 42% solidified, there are indications of isolated liquid pools in the lower section of the casting. This fact raises a probability of porosity formation due to a lack of liquid feeding. The chills cool the cylindrical section too rapidly, compared to the very bottom area in which the cooling rate is lower due to the enlarged cross-section, thus creating a hot spot. A potential remedy might be either to increase the thickness of the chills towards the bottom area or to redesign the gating system so that there is a chance to add a chill plate underneath the casting bottom. This would significantly promote cooling of the bottom of the casting and directional solidification towards the riser. This will be addressed in the following section.
A direct consequence of having both isolated liquid pools (Figure 7) and very flat temperature gradients (Figure 8)
in the casting domain is the presence of a shrink (porous area). Areas solidifying early always “suck out” the liq- uid melt from areas solidifying last to compensate for the volumetric changes evoked by the solidification process. As long as there is an open and active feeding path to these areas, no problem occurs. However, when the liquid melt supply is cut off and drained, areas solidifying last will be short of melt, so when the time comes for them to solidify, no compensation for the volumetric shrink- age will be available, giving rise to porosity. This issue is addressed by means of the Niyama criterion function in Figure 9.
Figure 9 shows the numerically predicted presence of cen- terline porosity in the lower areas together with results obtained from the casting trials (by the radiographic tech- nique). It is seen that the porous areas occured in the casting where the isolated pools of liquid were once present. Look- ing at the dimensions of the defect area, one can see a very good agreement between the two types of results (numeri- cal- red [right] vs. experimental- black [left]). The close cor- relation also justifies the use of the Niyama threshold value of 0.45 for this particular steel alloy. However, it should be emphasised that the geometrical extension of the shrinkage obtained numerically, to some extent is approximate due to its dependency on the mesh quality.
Figure 10 shows the results obtained from the casting tri- als. The cast part was cut into several sections, and the po-
Figure 7. Fraction liquid criterion function indicating an isolated liquid pool in the bottom section of the cast part at 42% solidified.
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Figure 8. Gradient criterion function depicting a very shallow gradient in various areas of the casting.
International Journal of Metalcasting/Fall 10
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