DR S. & S. S. GHANDHY GOVERNMENT ENGINEERING COLLEGE , SURAT. EN. NO : 130230119021 130230119022 130230119023 130230119024 130230119025 BRANCH : MECHANICAL SEM : THIRD SUB : MSM

Solidification of Metals The solidification of metals and alloys is an important industrial process since most metals are melted and then cast into semifinished of finished shape. When molten alloys are cast, solidification starts at the wallls of the mold

1. Solidification of Polycrystalline Material Almost all engineering crystalline materials are composed of many crystals. Two steps of solidification: Nucleation: Formation of stable nuclei in the melt Growth: Crystals grow until they meet each other Liquid Nuclei Crystals that will form grains Grains Grain boundaries

Homogeneous Nucleation Homogeneous nucleation is much rarer than heterogeneous nucleation. For a microscopic nucleus of the new phase, we can write the free energy of a droplet as the sum of a bulk term that is proportional to the volume of the nucleus, and a surface term, that is proportional to its surface area

(1)Volume free energy 4/3 Π r3 G (2) Surface energy 4 Π r2 γ

Heterogeneous nucleation Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation. Heterogeneous nucleation is typically much faster than homogeneous nucleation because the nucleation barrier ΔG* is much lower at a surface. This is because the nucleation barrier comes from the positive term in the free energy ΔG, which is the surface term. For homogeneous nucleation the nucleus is approximated by a sphere and so has a free energy equal to the surface area of a sphere, 4πr2, times the surface tension σ.

Dendritic growth At slow rates of crystal growth, the interface between melt and solid remains planar, and growth occurs uniformly across the surface. At faster rates of crystal growth, instabilities are more likely to occur; this leads to dendritic growth. Solidification releases excess energy in the form of heat at the interface between solid and melt. At slow growth rates, the heat leaves the surface by diffusion. Rapid growth creates more heat, which is dissipated by convection (liquid flow) when diffusion is too slow.

Planner Growth The Temperature of the liquid metal is Greater than the freezing Temperature of the solid formed is at or below freezing Temperature. The latent heat of fusion must be removed by condition from solid – liquid interface through the solid.

Solidification time hat relates the solidification time for a simple casting to the volume and surface area of the casting.  In simple terms the rule establishes that under otherwise identical conditions, the casting with large surface area and small volume will cool more rapidly than a casting with small surface area and a large volume. The relationship can be written as t = B * ( V / A )^n

Shrinkage Defect Shrinkage defects can occur when standard feed metal is not available to compensate for shrinkage as the thick metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere therefore as the shrinkage cavity forms air compensates. There are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting

Shrinkage Defect Gas Porosity

Gas porosity To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon or carbon dioxide or under a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mold can introduce gases, so the molds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top.

Gas porosity For instance, oxygen can be removed from copper by adding phosphorus; aluminum or silicon can be added to steel to remove oxygen. A third source consists of reactions of the molten metal with grease or other residues in the mold. Hydrogen is normally produced by the reaction of the metal with humidity or residual moisture in the mold. Drying the mold can eliminate this source of hydrogen formation

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