Experiment (3): Effect of nucleation

Experiment (3)
Effect of Adding Nucleating Agent on
Cast Microstructure of Pure Metal Cast Ingot

Introduction

Casting is a solidification process. The molten material being poured into a mold and then allowed to freeze into the desired shape. Many of the structural features that ultimately control product properties are set during solidification.
Solidification occurs in two stages, nucleation and growth. And it is important to control both of these processes. Nucleation occurs when a stable solid particle forms from within the molten liquid. As the material changes state, its internal energy is reduced, since, at lower temperatures, the solid phase is mom stable than the liquid. At the same time interface surfaces are created between the newly solidified metal and the parent liquid. Formation of These surfaces requires a positive contribution of energy. As a result nucleation generally occurs at temperature somewhat below the equilibrium melting point (the temperature where the internal energies of the liquid and lie solid are equal).
In most practical situations, the nucleation process prefers existing surfaces, where solidification can begin without creating a full, wrap-around interface. These surfaces are generally in the form of mold or container walls, or solid impurity particles.
If we note that each nucleation event produces a crystal or grain in the final casting and remember that line-grain materials (many small grains) possess improved strength and mechanical properties, then we conclude that anything that can he done to promote nucleation will be beneficial to the final product. Thus, we may intentionally introduce impurities into the liquid metal before pouring it into the mold. These small particles of solid provide for uniform and widespread nucleation and promote a uniform, fine-grain size in the product.
In most alloys and some pure metals, new crystals can nucleate in the center of the casting and then grow to produce another region of randomly oriented, spherical-shaped crystals, the equiaxed zone.
Low pouring temperature, alloy additions, or the addition of inoculants can be used to promote the formation of this region, which is far more desirable than the columnar-shaped grains. Isotropic properties are observed in this region of the casting.

Procedures

Prepare the two dies, of round cross sectional area, where molten aluminum will be poured.
Melt a sufficient amount of aluminum, about 2 kg in our case.
Pour the molten aluminum into the first die.
Place the nucleating agent, ferro-titanium, into the furnace, about 30 gm in our case, and stir well.
Pour the molten metal with the inoculants into the second die.
Leave both dies to cool to room temperature.
Calculate the weight fraction of the ferro-titanium with respect to aluminum.
Take samples from each cast structure.
Prepare the samples for microstructure analysis.
Study the effect of adding a nucleating agent on the microstructure of aluminum.

Lab Experiments Page

Back to Main Page