Moreover, for description of the various crystals and crystal aggregates (e.g. eutectic cells or grains) occuring during solidification of the melt, the concepts of macro- and micromorphology have also been introduced, which are grouped in the concept of solidification morphology. Micromorphology describes the individual crystals, whereas macromorphology involves the entire sum of crystals growing in contact with the melt. Differentiation is made between exogenic and endogenic crystals and grains with regard to time-related beginning of formation and the appertaining local position within the melt.
Endogenic crystals predominantly form at the same time throughout the entire melt and continue growth at the same or at different rates. Since they grow freely within the melt, they may change their position during solidification of the cast material, e.g. through differences in density, through convection flow, or transport through the melt in which they occur.
The phase boundry surfaces of the individual crystals or grains in contact with each other form the solidification front. In addition to the size of the flow channels, which is expressed by crystal spacing, batches, and grains, their formation shape is of particular importance for transport of molten baths for post-feeding. For that reason, the above classification of exogenic and endogenic is supplemented by information on the structure of the solidification front and on the continued solidification progress after solidification start (Fig. 1).
Exogenic smooth-wall: Exogenic compact crystals or crystal aggregates grow next to one another in any optional number and form contact with each other so that a smooth-walled solidification front is established towards the melt.
Exogenic rough-wall: Exogenic branched crystals (e.g. dendrites) grow parallel with each other and form a more or less rugged boundary surface towards the melt - the solidification front displays distinctive furrows.
Exogenic spongy : Exogenic highly branched crystals (often referred to as dendrite packages) grow into the melt. They form a kind of lattice or network with interspaces in which the residual melt is present. During the solidification sequence, residual melt channels that are originally connected with each other become more and more necked and are eventually disconnected from one another.
Endogenic mushy : Endogenic crystals or crystal aggregates, both compact and branched, are evenly distributed and grow into the melt. For the time being, there will not be a direct solidification front since the individual grains are isolated from one another by the melt. However, towards the end of solidification, these grains get in contact and constricts the residual melt to form islands.
Endogenic shell-forming: Endogenic crystals or crystal aggregates grow into the melt either in compact shape or as dendrites, but this time the predominant growth takes place from the outside so that a kind of marginal shell and from a certain time onwards also a kind of solidification front is formed. Within the shell both connected and at a later stage also isolated residual melt areas can be found.
Iin case of the exogenic types, the macrostructure may consist of stem crystals, in case of endogenic types it may consist of globulites. The solidification types described above, are merely a verbal designation of specific distinctive states in the solidification sequence so that several transitions and passage states are possible in the same casting. Naturally, this may lead to rather subjective assessments. Another difficult aspect is classification into the individual solidification types if the solidification behavior of various materials is very similar.
In this context it must be additionally taken into account that the solidification type of the casting materials is not consistent but changes in dependence of the heat dissipation conditions and the crystalization situations. Therefore, specification of the solidification type must always be connected with information on the technological conditions, since the material may undergo several different stages of crystallization in one casting. The tendency of individual materials for responding to different technological conditions differs greatly.