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Metal Casting Technologies : March 2006
temperatures. Non-linear, elastoplastic ap- proaches are very sophisticated. It is difficult to describe phenomena like the formation of hot tears as the material laws in combination with high temperatures are not well enough known yet. Further, it is difficult to consider the con- tact conditions between die and casting, which has a substantial influence on the formation of stresses, especially in high pressure die casting. Phases of a simulation project In order to run a casting simulation, the following fundamental steps need to be carried out. 3-D-modeling Basis for the simulation is a three-dimensional geometry model of the raw casting or the machined part. The casting developers in the automotive industry focus on maintaining a centrally managed and up-to-date record of ge- ometries that exclusively consists of 3D-CAD data. This way it is nearly impossible that such model doesn't exist in the automotive industry. In other industries it might happen that a model is not existent and it needs to be developed based on drawings. The geometries of ingates and overflows, as well as of die segments including cooling/heating lines are prepared as 3D-models and need to be avail- able for the simulation, too. Enmeshment The complete 3D-model, which consists of the raw casting, ingates, overflows and/or vacuum channels, as well as die segments including cooling/heating lines need to be enmeshed for the mathematical calculation. Depending on the operation method, these meshes are exclusively automatically generated (finite vol- ume method), or automatically generated and manually reworked (finite elements method). The completion of the 3D-models and the enmeshment are known as 'preprocessing'. Predefinition of the process parameters Before starting the calculations, the required process parameters need to be entered via interactive user interfaces. These process parameters are shot curve, temperatures of melt, thermal regulation medium and die, as well as the chronological sequence of the whole casting process including the spraying of the die agent. Some simulation programs include subroutines that automatically forecast and propose appropriate process parameters. Execution of the calculations The actual calculation can be carried out on various hardware platforms. The complete calculations usually run overnight on powerful machines, but with the use of cluster comput- ers and appropriate simulation programs, the calculation can be completed within minutes (W. Schäfer et al., 2000). Evaluation of the results 'Postprocessors' prepare the results in colored graphics or movies that visualize and docu- ment the calculated operations during die filling, solidification, formation of microstruc- ture and properties, as well as the formation of residual stress and distortion. The time needed for the run of the simulation is depending on various parameters and can range between 30 minutes (for the determina- tion of the ingate position and the requirements for thermal balancing of the die) and two days (for the calculation of die filling, solidification, and formation of residual stress with more cycles for a complexly structured casting). Automated numerical optimization With the help of simulations it is possible to 'x-ray' the casting process that was elaborated by qualified personnel. The results of the simu- lations support the decision-making-process in order to implement improvements. Thus, the use of casting simulations is always dependent on the employment of qualified personnel. The labor costs in a simulation project increases in correlation with the velocity of the available hardware. This raises the question on how the simulation can be more effective in respect to the use of personnel. www.metals.rala.com.au Figure 1