Modelling of microstructure formation in solidification processes

Abstract

Modelling of heat flow has become a standard practice in many solidification processes. Effort is currently being made to couple heat flow calculations to related macroscopic phenomena such as mould filling, fluid flow, macrosegregation, or thermal stresses. If these macroscopic aspects are important in predicting formation of macroscopic defects or optimising process conditions, then microstructural features such as phase appearance, morphology, grain size, spacings, or microdefects are certainly no less important in determining the ultimate mechanical properties of the solidified product. The aim of the present paper is to introduce the basic concepts of macroscopic and microscopic phenomena which enter normally into any solidification process. At the macroscopic level, i.e. at the scale of the whole process (casting, weldment, … ), the latest developments in numerical techniques which are used to solve the continuity equations are briefly presented together with the advantages and inconveniences of each technique. Emphasis is placed on modelling of the heat flow equation when a phase change is present, but modelling of fluid flow is also considered. At the microscopic level, the mechanisms of microstructure formation are outlined for both eutectic and dendritic alloys solidified with equiaxed and columnar morphologies. Besides microstructural spacings and dendrite tip or eutectic front undercoolings, solute diffusion models which describe the evolution of the mushy region are also presented. Finally, it is shown how the microscopic models of microstructure formation can be coupled to macroscopic heat flow calculations in order to predict microstructural features at the scale of the whole process. Recent micro-macroscopic simulation results concerning various processes such as casting, welding, laser remelting of material surfaces, or splat cooling, are compared with experiment.