During solidification of a pure molten metal, the grains in the casting near the mould wall are
Answers
Solidification starts at the surface of the mold material and molten metal outer boundary, due to high rate of cooling randomly oriented fine grains are produced. In EDM, the metal removal takes place due to melting and evaporation of the work piece material.
During solidification of a pure molten metal, the grains in the casting near the mould wall are fine and equiaxed or randomly oriented.
EXPLANATION:
When a pure molten metal is under the mechanism of 'solidification', there will be heat loss from material to the 'mould'. This is to avoid gas porosity, seams etc. As solidification need energy to produce the crystal, cooling before the 'freezing point' is essential for the liquid to solidify.
The walls of the mould will provide the cooling, with which the crystals can grow by the solid particles. As more amount of heat is lost and the process continues, the crystals or the grains grow inward whole the metal is solidified. These grains near the wall are fine and equiaxed or randomly oriented.