We know that casting basically consists of pouring molten metal in the mould or cavity and waiting for it to become solidified. It is therefore necessary for the engineers that the solidification process and the changes going on at the microstructure level are well understood. This article deals with the phase change from liquid to solid of the molten metal when it is poured into the mould.
Solidification and Crystallization
We all understand what is broadly meant by the term solidification – namely the change of phase from liquid to solid – as we perceive at the macro level. At the micro level, there are changes going on in the material as the disordered structure of the liquid gets transformed into an orderly arrangement of crystals.
There are two terms associated with this transformation, glass transition and liquid transition. Glass transition refers to that particular phase transition when the starting point is not a liquid, (neither a solid obviously) but when the disordered structure is frozen as it is. Crystallization refers to the phase change to solid state starting either from liquid state or glass state and the kinematics of both are something different. Since our main focus of discussion is about casting, which is about conversion from liquid to solid phase, we will not go deeper into the details of glass transition.
Effects of Grain Size
Grain size refers to the crystals which are formed during the solidification process. Their size depends, apart from other factors, on the rate of cooling of the liquid metal. The slower the metal is cooled, the larger grain size is formed and vice versa. Since we are interested in having improved mechanical properties during manufacturing such as toughness, strength, and so on, it would be interesting to note that superior mechanical properties are associated with finer grain sizes.
Again in the casting process there may not be enough scope for suppression or inducement of a phase change in the molten metal because of the simplicity of the process. In other industrial processes this phenomenon is normally used to produce ultra-fine grain sizes using various techniques for rapid solidification.
Another term which needs to be cleared before studying the changes at microstructure level inside the mould is super-cooling. Basically super-cooling refers to the process of cooling a liquid much below its freezing point which induces certain unique features at the micro level.
In the next article we will learn about the exact kinematics of solidification that occur inside a casting mould as the hot molten metal is poured inside it.
Raghavan, V. (1989) Material Science and Engineering. New Delhi: Prentice Hall India