Simulation of casting defects

Casting defects occur during the casting, solidification and cooling processes. They can thus be metallurgically caused or depend on the moulding material of the tooling. Since these defects such as, for example, cavities and gas porosity degrade the component’s properties, one endeavours to predict such defects or to analyse and describe the conditions under which casting defects can occur. A severe defect is component distortion which interacts with the occurring grain structure during and following solidification. Another example is the occurrence of freckles during the directional solidification of superalloys.

Simulation and minimisation of distortion

Manufacturing dimensionally accurate castings holds huge economic potential and provides the basis for a resource-efficient production. Unfortunately, it is not currently possible to satisfy this quality criterion using the established casting technology production processes by means of specific control options. The aim is to detect the relationships between the properties (Young’s modulus, shear modulus, permeability ...) and the microstructural parameters, to model the local material properties and to directly combine these with the macroscopic, thermodynamic, multiphase simulation so that it is possible to quantitatively predict distortions. These aims are achieved via the predictions of thermodynamic and thermomechanical reciprocal effects using new coupling methods, based on the microstructure, to quantitatively predict microstructural defects, distortions and residual stresses. Using the thermomechanical multi-scaled approach, qualified in this manner, and the numerical optimisation processes, the distortion of a die-casting is subsequently minimised using a multi-step approach. For this purpose, the basic methods must also be developed in order to derive the corresponding local starting, boundary and process conditions, as compensation measures, from the global assessment criteria.

Using the models to be developed, it should be possible to predetermine the casting’s precision, which is not possible, or only possible to a limited extent, using currently known models lacking locally computed and, in part, poorly known boundary conditions and properties. Corresponding to this, by combining the application with the optimised approached to be developed, the process boundary conditions; such as subsurface cooling channels and the influence of heat transfer and mould geometries, can be quantitatively predetermined for the production of precision castings.

Prediction of freckles

Freckles are microstructural defects which, depending on the process control on the specimen’s surface, occur particularly during directional solidification of superalloys. Depending on the length of the liquid-solid region, a thermosolutal convection of segregating, light alloying elements is stimulated which, on exceeding a critical Rayleigh number, leads to the fragmentation of the dendrite arms and thereby to freckle formation. It is also a characteristic that the freckles only occur on the specimen’s shadow side. An existing multilevel criterion for freckles was enhanced and successfully tested with regard to shadow effects. In the future, the physical effects of thermosolutal convection, the dissociation of dendrite arms and the formation of freckles are to be explored by employing a combination of process, component and material simulations.