The equilibrium crystallization temperature or theoretical crystallization temperature of aluminum is 660℃. However, in the actual crystallization process, the actual crystallization temperature is always lower than the theoretical crystallization temperature. This phenomenon is called undercooling phenomenon, and the temperature difference between them is called undercooling degree. The undercooling degree is closely related to the cooling rate. The faster the cooling rate is, the lower the actual crystallization temperature will be and the greater the undercooling degree will be. On the contrary, the slower the cooling rate is, the smaller the undercooling degree will be, and the actual crystallization temperature will be closer to the theoretical crystallization temperature.
In brief, undercooling is the difference between the theoretical and actual phase transition temperature of the liquid aluminum.
During the crystallization and solidification of aluminum liquid, there are two forms of undercooling.
One form is to mainly affect the aluminum casting surface, called thermal undercooling, which is mainly the influence of the external temperature on the shallow aluminum liquid, so that it can quickly form small and uniform equiaxed crystals. Al-Ti-B (aluminum titanium boron) grain refiner is especially suitable for this thermal undercooling, and can exert the best refining effect in this environment. But if the thermal undercooling is much too excessive, a layer of coarse acicular grain structure is usually formed on the inner side of the surface very fine layer at the same time.
The other form is to mainly affect the aluminum casting interior, called constitutional undercooling. Mainly during the solidification process of aluminum alloy, although the actual temperature distribution is certain, the solute distribution in the liquid phase changes, which changes the theoretical solidification temperature of the liquid phase. At this time, the undercooling is determined by the two factors, changes of composition and actual temperature distribution, which forms the constitutional undercooling. Al-Ti-C (aluminum Titanium Carbon) grain refiner particularly prefers this constitutional undercooling, and can exert the best refining effect in this environment.
Because Al-Ti-C and Al-Ti-B mainly take effect in the different undercooling environment, the aluminum grains refined by Al-Ti-C tend to have a more uniform particle size distribution from the inside to the outside in the casting. While, the aluminum grains refined by Al-Ti-B tend to have smaller grain sizes on the surface and outside of the casting body, and the deeper the inside of the casting body, the larger the grain size.
In summary, it is different in the actual refinement effect between Al-Ti-C and Al-Ti-B due to the different preferred undercooling environment. Correspondingly, the mechanical properties and plasticity toughness of aluminum castings refined by Al-Ti-C will be better and more stable.
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