Validation of stress prediction during solidification of cast components

Abstract

he development of appropriate techniques for casting complex, thin walled, light weight structures and housings, to assist the aerospace sector in achieving the required reductions in component weight and concomitant increase in strength, has become a challenging area of research. Difficulties experienced during cast-processing relate to the filling of complex thin sections, controlling solidification behaviour, as well as compensating for thermal distortions. During casting the metal starts to solidify and undergoes changes in phases where different material laws are valid. In the fluid state the metal is almost stress free but as the part starts to solidify and shrink, stresses are induced in the casting due to constraints from the mould. Some of the induced stresses are relieved through creep effects corresponding to the visco-plastic behaviour of the material. Traditionally, companies have relied on the “trial-and-error” approach with the experience of certain individuals to design and compensate for distortion. These companies have used expensive and time consuming bending and straightening techniques making corrections for distortions. Many casting simulation packages have been developed which can assist with optimising the filling and solidification behaviour of the metal during casting. Modelling the thermal distortions during casting poses many difficulties due to the complexities associated with all the thermo-mechanical influences. In order to accurately simulate distortion one has to fully integrate all the processing phenomena such as fluid filling, thermal heat transfer, solidification and stress. The current research project is aimed at predicting the distortion behaviour occurring during casting with the intention of using this capability as a tool to assist with process development of complex, thin walled, lightweight components. The initial objective is to determine the reliability of a fully coupled finite element model using A356 alloy and the investment casting process. A carefully designed box shaped experimental casting was used to validate the commercial finite-element code ProCAST (casting simulation software), with respect to distortions as well as residual stresses.