Foundry Metallurgy

• Casting processes

a)      Sand casting:

Moist, bonded sand is packed around a wood or metal pattern; the pattern is then removed and molten metal is poured into the cavity in the sand formed by removing the pattern. When the metal gets solidifies, the sand mold is broken and the casting is removed.

b)     Shell molding:

Sand coated with a thermosetting plastic resin is dropped onto a heated metal pattern (which cures resin), shell halves are stripped off and assembled. When poured, metal solidifies; shell is broken away to take out the finished casting.

c)      Gravity die (permanent mold) casting:

Mold cavities are machined into metal die blocks designed for respective use. Molten metal is gravity fed into the mold cavity. The mold consists of two or more parts and is hinged and clamped for easy removal of the casting when it has solidified.

d)     Pressure die casting:

Molten metal is poured into closed steel (split but clamped) die under pressure varying 100 to 1700 kg/cm2. When the metal solidifies, the die is opened and casting is ejected.

e)      Plaster mold casting

Slurry of special gypsum plaster, water and other ingredients is poured over pattern and allowed to set, pattern is removed and the mold is baked. When poured metal cools, mold is broken for removal of casting.

f)       Investment casting:

Refractory slurry is cast around a pattern formed from wax, plastic or frozen mercury; when slurry hardens, pattern is melted out and mold is baked. When poured metal solidifies, mold is broken away to take out the casting.

g)      Centrifugal casting:

Sand, metal or graphite mold is rotated in horizontal or vertical plane; molten metal introduced into the revolving mold is thrown to mold wall where it is held by centrifugal force until solidified.

h)     Superheating:

A metal to be cast is heated somewhat above its melting point to ensure that it possesses sufficient fluidity to completely fill the mold before it starts solidifying.

            The amount by which the metal is heated above its melting point / temperature is known as its superheat.

i)        Fluxing:

Addition of solid, liquid or gaseous material to molten metal to facilitates the flow of metal in the mold etc. or the production of sound casting is known as fluxing.

            A flux when added to molten metal:

1)      Controls dissolved gases within the melt.

2)      Controls the viscosity of oxide or dross present on the surface of molten metal

3)      Provide a cover on the molten metal to prevent its interaction with the atmosphere.

4)      Clean the molten metal

5)      Refines the metal.

• Fluxes may be classified as :

1)      Covering fluxes: covering fluxes reduce the oxidation and prevent the contamination of the melt by atmospheric gases. Commonly, mixture of chlorides serves as sufficient covers or blankets at a reasonable cost.

2)      Cleaning fluxes: are added in order to rid the molten metal from non-metallic impurities. A cleaning flux produces excellent metal from contaminated, impure metal. Fluoride is added in order to clean aluminium.

3)      Drossing-off fluxes:  reacts with caked, semisolid dross which forms on the melt (e.g. aluminium) surface and releases the entrapped metal particles, reducing the semi sticky, and semisolid dross to a powder which is then skimmed off. Thus drossing flux is added to avoid excessive metal loss.

4)      Degassing fluxes: helps removing the gases (e.g. hydrogen, oxygen, nitrogen) from the molten metal and thus helps producing sound casting free from porosity.

5)      Grain-refining fluxes: refines molten metal. For example, silicon is added to grain refinement in aluminium by controlling the nucleation process.

• Control of gas unsoundness in castings:

a)      Gas absorption:

1)      Furnace charge i.e. wet metal charge fluxes etc.

2)      Furnace refractories if they are not properly preheated.

3)      Furnace atmosphere which consist of water vapors, CO, CO2, SO2 and normal atmospheric gases such as oxygen and nitrogen.

b)     Preventing gas unsoundness:

1)      Melt the metal fast and handle it less agitation.

2)      Avoid overheating of molten metal.

3)      Cool the molten metal slowly until complete solidification has taken place.

4)      Make use of degassing.