The various attributes which affects and impacts the process of solidification in castings are range of freezing, thermal gradient and rate of cooling. The phenomenon of solidification takes an important role in various fields since it causes great impact in time taken for production, quality of product cast and the quantity of material used for the production.
Since all the above attributes are important factors in any process of production the solidification phenomenon takes a vital role in any field. Let us discuss in detail about the factor freezing range, which takes a vital role in the casting solidification process.
Generally the melting point associated with alloys to be caste is not fixed. The process of solidification of these alloys takes place at different ranges in temperature. The two important terms used in this aspect are liquidus and solidus.
The liquidius refers to the temperature higher than which the ally to be cast remains in full liquid state. The solidus refers to the temperature below which the alloy to be cast remains in full solid state. The difference between these two temperatures is coined as freezing range. For instance if Tl is used to refer the liquidius and Ts refer to the solidus then the difference between these two namely referred as freezing range denoted by FR is given as
The zones that could be referred in general during the above solidification process are there in number. They are :
- Fully Solid
- Fully Liquid
- Mushy Zone
As the name implies fully solid and fully liquid refers to zone in which the alloy is completely solid and completely liquid respectively. The third zone called as mushy zone is one in which dendrites gets formed which are nothing but tree like structure with branches having metal in liquid form. It has both forms of solid and liquid in it and hence the name mushy zone.
The structure of casting particularly the macrostructures gets affected by various factors and one of the important factors among this is freezing range. This is because the freezing range varies for instance pure metals have lesser freezing range and therefore the process of solidifications is done one layer after another which contributes columnar grains for macrostructure.
Some examples in which range of freezing is shorter are steels and casting and so on. In contrast the metals with longer range of freezing the process of solidification gets started out at number of points, which contributes to equi-azed grains for macrostructure. Some examples in which range of freezing is longer are aluminum, sand casting and so on.
The process of solidification can be modeled by considering and treating both liquid and solid as a fluid whose attributes gets changed over the freezing range. The use of proper alloys plays an essential part in phenomenon of solidification since uniform freezing of the casting is determined by this aspect.
Also the shrinkage defect type depends a lot on freezing range. Thus proper care and analysis must be made to have freezing range maintained properly to reduce shrinkage defects and improve final quality of product.