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Metal Casting Technologies : March 2010
46 www.metals.rala.com.au BacktoBASICS Example 2: Steel investment casting A second example illustrates the application of flow modeling to the investment casting process. The casting in this case is a steel Load Plate. The 8 mm investment shell was preheated to a temperature of 7600C, with a pouring time of 15 seconds. Flow modeling showed that at the outer rib of the casting, approximately 25% up from the bottom, a condition occurred in which the metal cooled to the point that it could no longer flow and thus a "miss-run" defect resulted in which a portion of this feature was not filled. Subsequent simulations were run in which the pouring time was gradually reduced, and the preheat temperature of the shell was gradually increased. Eventually it was found that with a shell preheat temperature of 9700C and a pouring time of 10 seconds; the potential for the miss-run defect was eliminated as shown in Figure 8. Examination of actual production castings verified this result. Example 3: Ductile iron casting A third example illustrates the utility of resolving casting quality production problems with flow simulation. This example concerns a ductile iron (spheroidal graphite) casting which is a base for medical equipment. In the subject casting, a defect in the form of a V-shaped notch was repeatedly forming in the end wall of the casting opposite the inlet gates. Flow simulation of the casting with the production gating design showed that this defect was a "cold-fold" defect in which the metal at the front of two opposing flows reached the solidification temperature of the metal; these flows were then not able to completely fill the casting feature and thus the defect formed as shown in Figure 9. Redesign of the gating system to resolve this defect was accomplished by simulation of various changes to the rigging system, including a vent attached to the wall of the casting at the location of the defect. Flow simulation showed that this redesign eliminated the effect of two cold flows of metal meeting at the casting wall; metal temperature was more or less uniform in this area with the redesigned gating, and the defect was eliminated as shown in Figure 10. Conclusion Casting simulation is now becoming an essential design tool both for initial design of the casting process and for assistance in elimination of defects which are occurring in castings that are currently in production. Flow simulation is an integral part of the casting design and simulation process, and when used properly can result in significant reduction in cost as well as improvement in casting quality. ■ Fig 7. Prediction of porosity with process and design modifications shows no likelihood of internal shrinkage Fig 9. Original gating design and flow simulation of ductile iron base casting showing formation of cold fold defec Fig 8. Prediction of miss-run (left) and complete fill (right) through adjustment of pouring time and shell preheat temperature Fig 10. Modified gating design and flow simulation of ductile iron base casting showing elimination of defect
Media Kit 2010