The following case study shows that the previously de- scribed concept of a Multi-Objective Optimization Prob- lem (MOOP) is applicable and reliable for a variety of cast- ing related issues.
Case Study Description 1— Original Casting Arrangement
The project involved a steel forging ram manufactured by a gravity sand casting process shown in Figure 3. Based on the thermal analysis of the part itself, a riser was designed and placed on top of the heaviest section. In order to enhance the feeding ability of the riser, in- sulation was applied (Figure 4). The main cylindrical padding is insulating, the melt surface is covered by an exothermic powder, and on top of that additional insu- lating powder is applied. Next, it was determined that the part be bottom-filled using a gating system com- prised of the refractory tiles. The cross-section area of the tiles is constant over the entire gating system. Last, the chills were added around the cylindrical section of the casting to establish directional solidification and to push the macrosegregation-related flaws from the sur- face to further inside the casting.
The original casting layout was simulated using the casting conditions and parameters listed in Table 1. Both filling and solidification analyses were conducted. A stress analysis was not considered in any of the presented cases.
Heat Transfer Coefficients (HTC) at the casting/mould interface, are assumed to be temperature independent.
This assumption holds only in the case of gravity sand casting. The reasoning is that in sand casting the contact between the melt and the mould is poor from the begin- ning due to the rough surface of the mould. As a result, there is a high resistance to heat removal, resulting in low interface HTCs. When the casting shrinks during solidifi- cation and solid state cooling, an air gap is formed in the casting/mould interface, inducing even more resistance to the heat removal. Nevertheless, since the heat transfer has been poor from the beginning, the decrease in HTC due to volumetric changes is not that significant to the results, and the HTC can be assumed more or less constant (low) over the entire casting process. Moreover, it is not primar- ily the interface that induces the largest resistance to heat transfer, but rather the large sand mould and its poor ther- mal properties that really govern the heat removal. Thus, the HTC between the casting and the sand mould was set to be 800 W/m2
K.
Case Study Description 2— Manually Modified Casting Arrangement
Next, the new casting arrangement with manually rede- signed gating systems and rearranged chills were devel- oped. The shape of the forging ram (Figure 5) was some- what different compared to the initial one in Figure 3, due to the fact that the second layout was manufactured for a
Figure 3. 3-D view of the cast part used in the project (Courtesy of Vitkovice Heavy Machinery, a.s.).
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Figure 4. Initial casting layout. The riser is indicated by green, chills indicated by light blue, and insulation is denoted by dark blue (Courtesy of Vitkovice Heavy Machinery, a.s.).
International Journal of Metalcasting/Fall 10
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