a massive melt aspiration in the downsprue section due to the accelerating melt and the constant cross-section. Using this type of a gating system, it was impossible to control the melt velocity whatsoever, which then led to a formation of fountains and subsequent melt oxidation in the mould cavity. As to the solidification results, it was shown that due to improper cooling there was a tenden- cy for cutting off liquid areas from the feeding path. A consequence was a formation of shrinkage and centerline porosity in the bottom cylindrical area of the part. These results were verified by casting trials performed by the foundry. Therefore it was decided to redesign the gating system from the bottom-fill to the side- fill type, rearrange the chills, and implement a new chill plate underneath the casting to promote cooling of that area. It showed that the filling pattern improved significantly, avoiding problems with fountains in the mould cavity. The surface turbu- lence is still present, but as long as the cross-section area of the filling channels remains constant and the velocity is uncontrolled, no further improvement is feasible. The new chill arrangement caused significant improvements in the solidification pattern. More pronounced cooling established steeper temperature gradients and induced directional solidification in the problematic bottom area, totally eliminating the shrinkage-related defects.
The manually-optimized solution served as a reference case for the geometry optimization of the riser and chills. In the multi-objective optimization case, the riser and chills dimensions have been applied as design variables together with the ranges of variation. In addition, opti- mization objectives together with potential constraints have been introduced into the optimizer together with the number of initial designs and generations in which the optimal solutions should be found. Three distinct solu- tions were selected from the design space. The first one represented a modest approach (relatively large riser), the middle one was still a safe solution but the riser was much smaller, and the third solution represented the very risky solution with the highest casting yield. It turned out that all three designs yielded different solidification patterns compared to the initial design. This was attributed to the change in the riser dimensions and the rearranged chills. No residual liquid pools were spotted; however there were some indications of potential problems in the third solution. Concerning the macro and micro-shrinkage in the optimized designs, the only problematic areas were in the riser head and bottom pins. The casting body seemed free of porosity. Eventually it was decided together with the foundry not to consider the last solution for produc- tion due to a high risk of failure in production. In other words, when taking the human factor into consideration, the risk of porosity extending from the riser to the casting body is too high. Last, findings regarding the casting yield showed that when correctly utilizing multi-objective op- timization, it is feasible to substantially increase the cast- ing yield and thus reduce production costs.
International Journal of Metalcasting/Fall 10
Acknowledgements
The authors would like to thank the Vitkovice Heavy Ma- chinery a.s. for identifying the case study and providing us with the casting data necessary to perform the relevant simu- lations and the photographs of the metallographic analysis for this case study.
REFERENCES
1. MAGMAsoft simulation software, MAGMA GmbH, Aachen, Germany
2. Svensson, I.L., Seifeddine, S., Hattel, J.H., Thorborg, J., Kotas, P., “On Modelling of Microstructure Formation, Local Mechanical Properties and Stress- Strain Development in Aluminium Castings”, Proc. Int. Conf. On Modelling of Casting, Welding and Advanced Solidification Processes, MCWASP XII, Vancouver BC, Canada, pp. 129-136 (2009).
3. Seifeddine, S., Wéssen, M., Svensson, I.L., “Use of Simulation to Predict Microstructure and Mechanical Properties in an As-Cast Aluminium Cylinder Head: Comparison with Experiments”, Metallurgical Science & Technology, vol. 24, no. 2, pp. 27-33 (2007).
4. Herman, A., Marousek, L., “The Verification of Computer Simulation for High Pressure Casting Technology”, Technológia 2003. Bratislava: Faculty of Mechanical Engineering of the Slovak Technical University, p. 56 (2003).
5. Sturm, J., Hepp, E., Enger-Walter, A., “Integration of Casting Simulation into Crash Simulation” 20th
6. Kotas, P., “Numerical Optimization of Die Filling and of High-Pressure Die Cast Deformation”, Master Thesis, Technical University of Denmark, Lyngby, Denmark (2007).
7. Hattel, J.H., Fundamentals of Numerical Modelling of Casting Processes, 1st Forlag (2005).
ed., Kgs. Lyngby: Polyteknisk
8. Egner-Walter, A., “Prediction of Distortion in Thin- Walled Die Castings”, Casting Plant and Technology International, vol. 23, no. 1, pp. 24-33 (2007).
9. Monroe, C., Beckermann, C., “Deformation during Casting of Steel: Model and Material Properties”, in Proceedings of the 61st Technical and Operating Conference, SFSA, Chicago, IL (2007).
10. Poloni, C., Pediroda, V., GA Coupled with Computationally Expensive Simulations: Tools to Improve Efficiency, Chapter Genetic Algorithms and Evolution Strategies in Engineering and Computer Science, pp. 267-288. John Wiley and Sons (1997).
11. Fonseca, C., Fleming, P., “Genetic Algorithms for Multiobjective Optimization: Formulation, Discussion and Generalization”, In Genetic Algorithms, Proceedings of the 5th International Conference, USA, pp. 416-423 (1993).
75 CAD-
FEM Users` Meeting 2002, International Congress on FEM Technology, Lake Constance, Germany (2002).
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