Solidification — Crystallization and microstructures

When it comes to metal production, casting operations involve considerable quantities: every year, around one billion tonnes of metal are solidified worldwide, primarily ferrous alloys (steels and cast irons), followed by aluminium alloys.

• The products and semi-finished products thus obtained by solidification must meet quality requirements: absence of major internal defects such as shrinkage and porosity, absence of strong chemical heterogeneities at product scale (major segregations), for some castings: absence of poorly distributed microporosities, for many semi-finished products: surface appearance, etc. Shrinkage, a typical example of a defect inherited from solidification, is strongly influenced by the nature of the process involving solidification and by the operating conditions chosen for the same process. Shrinkage, a typical example of a product-scale defect inherited from solidification, is strongly influenced by the nature of the solidification process and by the operating conditions chosen for the same process; the same applies to cricks, to grain latexture (columnar or equiaxial), to grain size, to microporosities, to major segregations, and so on. The last part of this article: Macrostructures and product quality is therefore devoted to the presentation of aspects of cast products and phenomena linked to solidification, for which consideration of scale effects is essential.

• First and foremost, raw metal products, whether heat-treated or even processed, are manufactured for their final properties: suitability for chemical homogenization, deformability (stamping, etc.), mechanical properties, corrosion resistance, etc. These properties most often depend on the small-scale solidification structure, that of ladendrite. These properties most often depend on the small-scale solidification structure, that of ladendrite (between 10 µm and 1 mm); this is true not only for raw cast products, but also for products that have undergone heat treatment or wrought. To determine the influence of the solidification stage on the properties of metal products, it is necessary to establish correlations with the microstructural characteristics of the cast metal, namely: the size of the casting grain, the fineness of the dendritic branches, the nature of the first solid formed (ferrite or austenite for steels), the nature and intensity of chemical segregations, the presence of microcracks, microblowholes or inclusions. The development and maturation of dendritic structures, on the one hand, and the development and homogenization of microsegregations with their attendant minor phases, on the other, are the focus of the second section of this article. In this section, the development and control of...