rous area (also predicted by the simulation) was detected and measured. Besides the macro-shrinkage, both V- and A-type macrosegregation bands are spotted in the casting. A general cause of the macrosegregation is relative move- ment or flow of segregated liquid and solid during solidi- fication. The most common form of solid movement is the settling or floating up of small solid pieces formed early in the solidification process.
These solid pieces may be dendrite fragments that separat- ed from an existing solid structure or equiaxed grains that nucleated in the bulk liquid. They settle or float, depending on their density relative to the liquid. The solid pieces gen- erally have a composition different from the nominal alloy composition, and their movement to different parts of the casting thus induces macrosegregation.31- 33
Simulation Results— Manually Refined Layout
After the initial assessment of various stages of the manu- facturing process, potential drawbacks and defects were recognized. This was then verified through casting trials by creating a solid ground for subsequent improvements and optimization.
Since centerline porosity depends primarily on thermal gradi- ents and the cooling rate, the next step was to induce steeper thermal gradients in the section surrounded by the chills and
to establish a directional solidification towards the heaviest top section where the riser is placed to keep the feeding path open as long as it is necessary. This was pursued by the rear- rangement of the chills and by adding a chill plate underneath the casting bottom. Due to the chill plate, the bottom filling was no longer feasible. Therefore, the gating system was somewhat redesigned, which after the first filling analysis was strongly recommended anyhow. The reason for adding the vertical extensions of the two horizontal runners is to reduce the kinetic energy of the melt. This is a very easy way to slow down the melt front. However, in many foundries worldwide it has been overlooked and not applied.
Figure 11 shows early stages of the filling process. One can argue that the new gating system really improved the filling pattern. The two vertical extension channels signifi- cantly slowed down the melt (approx. 4m/s in the runners compared to almost 10m/s in the original layout) and thus it propagates uniformly towards the thin gates. When the melt reaches the cavity it does not form fountains but creates a relatively small splash during impingement of the streams. A solution to this problem might be an application of tan- gentially oriented thin gates, which then help to avoid any impingement of propagating melt fronts.
However, since the shape of the downsprue remained un- touched, melt aspiration is still seen at that area. This issue should really be addressed by the manufacturing foundry to eliminate oxidation of the melt in the downsprue area.
Figure 9. Prediction of the centerline macro/micro shrinkage and its experimental validation obtained from the foundry.
International Journal of Metalcasting/Fall 10
Figure 10. Results from the casting trial - presence of porous areas and bands of macrosegregation, (Courtesy of Vitkovice Heavy Machinery, a.s.).
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