JNTU B.Tech III MANUFACTURING PROCESS AND MACHINE TOOLS Previous Question Paper (March - 2021 - R16)

Directional solidification is a controlled casting process where the metal is allowed to solidify progressively from the extremities of the mold cavity toward the riser. By ensuring that the molten metal in the riser remains liquid longer than the metal in the casting, the riser acts as a reservoir to feed the casting during its contraction phase.

Its influence on casting quality is significant:

• Elimination of shrinkage defects: It prevents internal voids and surface depressions by providing a continuous supply of molten metal to compensate for volume contraction.
• Improved soundness: It ensures a dense, uniform grain structure, reducing porosity and improving the mechanical integrity of the component.
• Controlled cooling: It minimizes thermal stresses and potential cracking by managing the temperature gradients within the mold.

These casting methods utilize centrifugal force to distribute molten metal within a mold, but they differ in their rotational axis and mold configuration:

• True Centrifugal Casting: The mold rotates about its own central axis at high speeds. The molten metal is poured into the rotating mold, and centrifugal force holds the metal against the inner walls, creating hollow cylindrical shapes without the need for a core. The wall thickness is determined by the amount of metal poured.
• Semi-Centrifugal Casting: Similar to true centrifugal casting, the mold rotates about its central axis, but it is used for solid parts like wheels or pulleys. The mold is completely filled with metal, and rotation helps ensure that the metal reaches the outer extremities of the mold cavity, resulting in high density in the outer sections.
• Centrifuge Casting: In this method, multiple mold cavities are arranged symmetrically around a central vertical sprue. The entire assembly rotates about the central axis. The centrifugal force acts to push the molten metal from the central sprue into the individual mold cavities, which is particularly effective for small, complex, or intricate castings that are difficult to fill by gravity alone.