Melting

Melting is performed in a furnace. Virgin material, external scrap, internal scrap, and alloying elements are used to charge the furnace. Virgin material refers to commercially pure forms of the primary metal used to form a particular alloy. Alloying elements are either pure forms of an alloying element, like electrolytic nickel, or alloys of limited composition, such as ferroalloys or master alloys. External scrap is material from other forming processes such as punching, forging, or machining. Internal scrap consists of gates, risers, defective castings, and other extraneous metal oddments produced within the facility.

The process includes melting the charge, refining the melt, adjusting the melt chemistry and tapping into a transport vessel. Refining is done to remove deleterious gases and elements from the molten metal to avoid casting defects. Material is added during the melting process to bring the final chemistry within a specific range specified by industry and/or internal standards. Certain fluxes may be used to separate the metal from slag and/or dross and degassers are used to remove dissolved gas from metals that readily dissolve certain gasses. During the tap, final chemistry adjustments are made.

Pouring

In a foundry, molten metal is poured into molds. Pouring can be accomplished with gravity, or it may be assisted with a vacuum or pressurized gas. Many modern foundries use robots or automatic pouring machines for pouring molten metal. Traditionally, molds were poured by hand using ladles.

Pouring of the Metal: When manufacturing by metal casting, pouring refers to the process by which the molten metal is delivered into the mold. It involves it's flow through the gating system and into the main cavity (casting itself).

Goal: Metal must flow into all regions of the mold, particularly the casting's main cavity, before solidifying.

Factors of Pouring:

Pouring Temperature:

Pouring temperature refers to the initial temperature of the molten metal used for the casting as it is poured into the mold. This temperature will obviously be higher than the solidification temperature of the metal. The difference between the solidification temperature and the pouring temperature of the metal is called the superheat.

Figure:8

Pouring Rate:

Volumetric rate in which the liquid metal is introduced into the mold. Pouring rate needs to be carefully controlled during the metal casting operation, since it has certain effects on the manufacture of the part. If the pouring rate it too fast then turbulence can result. If it is too slow the metal may begin to solidify before filling the mold.

Cooling and Solidification

Mechanism of Solidification

Pure metals solidifies at a constant temp. equal to its freezing point, which same as its melting point. The change form liquid to solid does not occur all at once. The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal. Nuclei of solid phase, generally a few hundred atom in size, start appearing at a temperature below the freezing temperature.

A nuclei, more than a certain critical size grows, and causes solidification.

Mechanism of Solidification

By adding, certain foreign materials (nucleating agents) the undercooling temp. is reduced which causes enhanced nucleation. In case of pure metals fine equi-axed grains are formed near the wall of the mold and columnar grain growth takes place upto the centre of the ingot. In typical alloy, the columnar grains do not extend upto the center of casting but are interrupted by an inner zone of equiaxed graines. By adding typical nucleating agents like sodium, magnesium

or bismuth the inner zone of equiaxed grained can be extended in whole casting.