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Metal Casting Technologies : March 2006
Models and their validity The basic problem in the process of calculating residual stresses is the determination of laws to describe the material behavior in high temperatures. The creeping phenomenon at high temperatures, as well as the con- tact between die wall and casting, are often inaccurately described. These effects are currently subject of intense research projects (G. Hartmann, 2005). Therefore, the technical calculation of residual stresses is primarily based on the cooling of the casting after ejection. Regarding the processes taking place in the closed die, it is assumed that arising stresses in the cast- ing are plastically relieved. Thus, the free shrinkage of the casting is calculated. Evaluation of residual stresses and distortion The locally arising von Mises equivalent stresses can be used for the evaluation of residual stresses. This equiva- lent stress is determined as vector product of the arising three-dimensional stresses. The development of residual stress until the removal of the ingate is initially calcu- lated based on the results of the solidification simulation and during the cooling of the casting. Depending on the proportion between cross section of the ingate and wall thickness of the casting, the removal time of the ingate has a significant influence on the development of re- sidual stress as this process leads to stress transfer in the casting (Fig. 18). Due to the occurring stress transfer, machining of the casting can also lead to distortion (Fig. 19). The finally important value is the distortion caused by the residual stress in the machined casting (Fig. 20). Simulation of stresses in dies Stresses in dies are basically caused by the following three reasons: ● The load caused by the closing mechanism of the machine, ● The load caused by the melt that is highly pressurized at least for a short moment in time, and ● The cyclically changing temperatures in the die segments that lead to cyclically changing residual stresses. These conditions cause a highly complex and unsteady overall load, whose entirety is difficult to display in simulations, but where the single elements can be mea- sured quite accurately (C. Rosbrook et al., 2000). Stress and distortion in the die frame The simulation of stress and distortion in the frame is always advisable when huge castings like engine blocks or castings with a large projected area, like structural components, are combined with tight die tolerances (due to dimensional limitations of the machine). At complex, i.e., entire models with side cores (Fig. 21), an initial stress value is assumed at the side core locks (Fig. 22). It can be assumed that there are little temperature gradients between the frame and the insert. If the die spotting was carried out in a warm environ- ment, the thermal residual stresses especially in the die inserts have nearly no influence on the stress in the frame (Fig 23). Closing force and pressure of the melt as well as a stable temperature of the frame are considered for the examination of the distortion in the frame. Figure 21 Figure 22 Figure 23 METAL Casting Technologies March 2006 ADVERTORIAL