In addition to residual stresses, hot tears, crack formation and the shrinkage and warpage of the castings, dies and permanent molds are moving into focus. Due to the high costs for permanent molds and dies, maintenance and re- pair efforts are very often the deciding factor if a process is profitable. As the temperature behavior and the resulting stress development can easily be simulated, this applica- tion of simulation provides additional value and cost re- duction potential (Fig. 20).
Simulated Properties
Simulation results describing the filling, solidification, and stress behavior of castings are important sources for developing reliable processes. The foundry wants to pro- vide their customers with defined properties of a casting and the customer wants to receive reproducible proper- ties. The list of requirements for castings is therefore very long. Aside from questions in regard to dimensional tolerances, there are many mechanical properties at the center of interest. Therefore the final goal of casting pro- cess simulation is the prediction of casting properties. The base for such predictions is the quantitative infor- mation about local microstructures and potential casting defects. Due to its specific solidification behavior, cast iron was the pioneer in this area. The local microstructure
and mechanical properties, including hardness, can today be simulated for all common graphite morphologies and compositions in a quantitative manner (Fig. 21). Similar developments are meanwhile available or will be avail- able shortly for aluminum alloys.
The Process Chain
Making castings today requires more than just pouring liquid metal into a mold. Most castings receive their fi- nal properties through processes after the casting pro- cess, i.e. heat treatment or machining. Therefore it is crucial for casting process simulation to consider these processes to reliably predict casting properties when the part is delivered (Fig. 22). One example how this ex- panded process chain can be displayed is the presenta- tion of residual stresses in a casting as it is delivered to the customer (Fig. 23). Sometimes the residual stresses are completely eliminated by a stress relieving heat treatment process. However, even the removal of the gating system can lead to a stress re-distribution, which can lead to a measurable distortion of the casting. Rapid cooling or quenching, on the other hand, can again in- duce high stresses, which can be reduced or eliminated by a successive time or temperature dependent temper- ing or aging process.
Measurement Position
Figure 21. Prediction of local hardness (HB) for a gray iron engine block in comparison to measurements. Source: Ford and Eisenwerke Brühl. Micromodeling predicts local mechanical properties. Good knowledge of the metallurgical state of the melt is the basis for predicting values within the tolerance band of measurements and predicting critical areas for the machining operation. /9/
International Journal of Metalcasting/Spring 10 17
Hardness [HB]
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