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Metal Casting Technologies : March 2006
Basics and use Dr.-Ing. Erwin Flender, Dr.-Ing. Götz Hartmann, MAGMA GmbH, Dipl.-Ing. Jan Franke MAGMA Engineering Asia Pacific Pte Ltd Innovations and modifications in the tech- niques of high pressure die casting or tooling are forced by trends in part design, part load as well as by costs and times for development and manufacturing processes. All current trends require continuous improvement in planning of part performance and production processes. The quality of parts and the efficiency of development and manufacturing processes are primarily depending on the quality and accuracy of the planning process. Generally, there are two crucial factors that secure the reliability of planning: Experiences from past projects that can be used in future projects, and Modeling of processes based on general physical laws. As die castings and casting processes get more and more complex, it becomes increasingly difficult to use experience from past projects in future projects (M. Jolly, 2000). At the same time, due to restructuring processes in the cast- ing industry, less information is documented and stored and thus not available in the future. In high pressure die casting the term 'model- ing' means the reproduction of the casting pro- cess in simulation programs. In this method, the very detailed process flow is specified as a boundary condition in a calculation. The result is the representation of die filling, solidifica- tion, formation of microstructure and proper- ties, as well as development of residual stress and distortion in the castings. The simulation results can be displayed on screen, printed as color graphics, or represented three-dimension- ally and thus are excellent records of the antici- pated results of the die casting process. As this is the quickest and most cost-effective method to develop a high-value product, die casting modeling gets more and more important. Model specifications In the numerical simulation of casting pro- cesses, three-dimensional differential equations are used as mathematical-physical models. Mass flow, heat flow, or development of stress are coupled to the casting process and can be modeled by coupling the respective differential equations. For die filling, e.g., there are equa- tion systems, which are able to describe oc- curring phenomena like turbulent three-phase flow with possible phase transitions. However, simulations that use such detailed model speci- fications require very long computing times, even on supercomputers. But it is not always necessary to use a very detailed description for the practical use of the simulation. In order to get a basic understanding of the processes during phase transition it is also possible to use other simulation techniques. Basically, the term 'modeling' means the ide- alized replication of an object or of a process. A good model mirrors the essential characteristics of the original, but at the same time uses valid and clever simplifications. The modeling of a complex, technical operation like the high pressure die casting process means to define, to quantify, and to take into consideration the characteristic values and influential mecha- nisms of the process (P. N. Hansen et al., 2001, W. Maus et al., 2001). The simulation of die casting needs to repli- cate the following typical problems: Patterns and temperatures in the melt flow: last filled areas, venting of the die, aggrega- tion of die agents, 'dead areas' in the runner, turbulences in the melt, disintegration of the melt and merging of melt fronts, cold shuts, or weld lines. Temperatures of the die: the complete die filling (especially during thin-wall casting), cycle times, core wear, adhesive tendency, or heat loss when spraying. Solidification of the casting: the creation of shrinkage cavities and pores, hot tears, micro- structure formation, possible feeding in the final pressure phase or during local squeezing, as well as the formation of residual stress and consequently arising distortion. The integration of the simulation results into the decision making processes during casting design or in the foundry assumes that the calculations generally last no longer than one day, counting from the availability of an ac- curate 3D-CAD-model of a casting including ingates to the creation of the documentation of the calculation. With these factors in mind, there are the following models and examples to review: Models for die filling In the majority of cases the Navier-Stokes equation is used to describe pressure-driven flow. This equation needs to be solved coupled with the Fourier heat conduction equation in order to consider the heat loss of the melt during die filling. Regarding flow, basic ap- proaches for single phase and laminar flow are used. With specific extensions of the models, further phases like air and solidified melt are considered (M. Lipinski, 1996). The phenom- ena of turbulences are taken into account by using k/ε approaches (D. B. Spalding, 1983, W. Shyy, 1994, and W. J. Minkowycz, 1988). Models for solidification The Fourier heat conduction equation is used for these models. Here, phase transformation enthalpies like melt heat need to be considered according to the solidification laws (S. Neves, 2002). Effects like the different precipita- tion of solidification phases in dependency on supercooling are increasingly in use (S. Andersen et al., 1990, E. Flender, 1993). If the used modeling approaches consider values cal- culated during heat flow simulation, like local solidification time, cooling rate, or tempera- ture gradient, the formation of microstructures can be computed (E. Flender et al., 1993, Julie Huang, 1998). Models for stress calculation The formation of residual stresses in castings is very complex, especially in the area of high MODELING AND SIMULATION IN high pressure die casting METAL Casting Technologies March 2006 ADVERTORIAL