The machining process relieves, as the practitioners say, stresses again. Indeed, the removal of material leads to a new stress equilibrium. Only the consideration of this aspect leads to useful results for the end user.
Process steps after the actual casting process also impact the properties of the microstructure and of the casting. It is possible today to couple a heat treat simulation with the consideration of diffusion and phase transfer phenomena to predict the microstructure and mechanical properties of the final product. This especially helps the steel foundries to rec- ognize potential hardness immediately, as it helps aluminum foundries to find the parameters for an optimal aging process to achieve the desired strength and elongation.
From Casting to Mold—Sand Simulation
Clearly, the focus of casting process simulation develop- ment of the last thirty years has been the casting and its production. However, many production-related questions regarding molds and cores are still answered through a much bigger experimental effort (usually through trial and error) than used in the development of the rigging of a casting. The typical approach of optimizing a core box is to provide a maximum number of vents, many of which will then be closed based on the results of the trial and error process. This uncertainty in the mold and core making is caused by the complexity of the physics involved in the filling, shooting and compressing of mold and core materials.
The coremaking process is, from a physical point of view, a multi-phase fluid problem. After opening the valve, sand is engulfed by air and thereby accelerated. Air transports the sand into the core box. At the end of the filling process, the
air needs to be separated (vented) from the sand. Recently steps have been taken into this new complex world with its demanding physics. One challenge is the proper description of those physics. A bigger challenge is the multitude of in- ternal and external boundary conditions (how many different valves and vents are there in the market?). Initial successes have motivated continuing development of simulations for the core room (Fig. 24). The final goal of the current devel- opment is the simulation of the entire process chain, with the consideration of shooting, gassing, venting, and degradation of binders during the casting process.
Time [s] (Logarithmic)
Figure 22. Microstructure prediction through heat treat- ment simulation. The coupling of diffusion calculation and TTT-diagrams provides the option to predict local micro-
structures after heat treatment, i.e. Martensite distribution.
Time
Figure 23. Stress history in a cylinder head over the entire manufacturing process. Casting Process Simulation is nowadays capable of supporting all essential process steps and thereby capable of predicting residual stresses at the
end of the machining process. This is the information a designer is looking for. 18 International Journal of Metalcasting/Spring 10
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