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Metal Casting Technologies : March 2006
Stress and distortion in the die inserts For the evaluation of stress in the inserts, the thermal residual stress is a considerable part of the overall load. The thermal shocks in each cycle caused by the melt and later by the die agent in correlation with the die tempering generate inhomogeneous and cyclically unsteady temperature distributions with partially very high gradients. This results in thermal residual stresses, where a likewise cyclically changing distortion is espe- cially important for the lifetime of the die insert. These calculations need to be carried out with non-linear, elastoplastic model approaches, too. Temperatures in a die casting tool can differ signifi- cantly. High pressure die casting dies in Aluminum applications can reach temperatures of more than 450°C near the impressions just after the filling. Temperatures close to the cooling channels are much lower (Fig. 24). Due to the temperature gradients appearing in all high pressure dies, residual stresses arise especially in the die insert, where the tool steel load can reach the yield point (Fig. 25). Compressive and tensile stresses successively occur due to strong temperature changes that are also caused by spraying of die agents. This leads to the de- velopment of cracks, especially in slim cores, where big changes in temperature happen due to intense heating up and cooling (spraying). Integrating the casting simulation into development and manufacturing A lot of information is generated during casting simula- tion, a lot of which would not be available when using conventional processes. To what extent this informa- tion can actually lead to a measurable improvement of casting design or processes is depending on the level of integration of the simulation into development and manufacturing processes (H.-G. Haldenwanger, 2000, G. Hartmann et al, 2000, E. Beutner et al., 2001, W. Sequeira et al., 2002). In this respect, we can differenti- ate between firstly technical integration with focus on the communication of CAE-tools via interfaces, and secondly structural integration with focus on informa- tion management during planning and performance of operating procedures. Technical integration of the casting simulation The information generated by the casting simulation is of significant importance to the constructing engineer. The local properties of the casting, like microstructure distribution, mechanical properties, or casting errors can be calculated and displayed with the help of casting simulation. Whereas the calculated distribution of cast- ing errors is only a qualitative approximation of the lo- cal damage of the casting, FE-calculations can consider calculated, local, mechanical properties (T. McMillin et al., 2002). There is the fundamental problem that different simulations, like the casting simulation and FE-analysis or crash simulation, are carried out with different calcu- lation meshes. One mesh of a die-dashboard for casting simulation consists of two million elements whereas the mesh of the very same part for crash simulation consists of only some hundred nodes. Thus, interfaces need to be used that transfer the calculated and interpolated value fields from one calculation mesh to the other. Figure 24 Figure 25 Figure 26 www.metals.rala.com.au