Theoretical Fundamentals of Foundry

Foundry practice is a branch of science and engineering, which deals with the methods used to obtain cast half-finished parts named castings. Foundry production is a branch of machine industry. The principle of casting consists of pouring the molten metal into sand or metalmould whose cavity conforms to the shape of the required casting. The casting forms when metal cools and solidifies.

Main advantages of foundry processes are:

- possibility of production of parts from tenths of gram (zipper element) to a few hundred tons (machine tool bases, turbine parts, monuments) in mass;

- parts of intricate shapes may be produced;

- in many cases, casting process proves to be the only method to manufacture the required parts (large and heavy parts, intricate castings and the parts where the alloy used is not enable to machine tool operations, or to metal forming);

- foundry technology provides low production cost of half-finished parts.

But foundry processes are connected with procedures of metal melting, its pouring and solidification during which such foundry defects as thermal stresses, cracks, segregation, shrinkage pipes and porosity appear in castings.

The quality of castings depends on foundry properties of alloys, such as:

- fluidity;

- linear and volumetric shrinkage;

- formation of shrinkage cavity and porosity;

- crack formation, etc.

The fluidity is an ability of an alloy to fill the mould cavity and reproduce exactly its configuration. Foundrymen use fluidity tests to gain an idea of the alloy's ability to flow through long passage of definite shape and cross - section sizes (Fig. 3.1). During movement in channel the metal cools and freezes. The length of the passage l in cm filled by the metal is considered to be the index of fluidity.

Fig. 3.1. Fluidity test: 1 – liquid metal; 2 – pouring basin; 3 – half of the mould;

4 – downgate; 5 – channel (passage)

 

The fluidity depends on:

- the chemical composition of the alloy and its solidification nature;

- the temperature of the alloy;

- the temperature of the mould.

A number of defects may be developed in cast alloys. Most of the defects are caused by volumetric shrinkage or contraction during solidification. Let us consider the solidification of a casting (Fig. 3.2a).

Fig. 3.2. Formation of a pipe (a) and porosity (b) during:

1 – porosity; 2 – separated volumes of liquid metal; 3 – shrinkage cavity

 

At time 0 all metal is in liquid state. At time 1 a solid shell t₁ is formed on the surface of the casting and due to contraction of liquid metal its level drops to the line a-a. At time 2 a next solid shell t₂ is formed and level of metal further drops to the line b-b and so on.

Because of contraction of liquid metal the shrinkage cavity 3 and porosity 1, 2 are formed inside the casting (Fig. 3.2b). Because of contraction in solid state the decrease in sizes clearly exhibits.

So, shrinkage is usually understood as a percentage change in volume (volume shrinkage e _v) or in length (linear shrinkage e ₁):

;                                                  (3.1)

;                                                    (3.2)

where V₀, V₁ is initial and final volume;

1₀, l₁ is initial and final length;

A free linear shrinkage of steel is from 2 to 2.8 %,

of cast irons is 0.8...1.2 %,

of non-ferrous alloys is 1…2 %.

 

The volumetric shrinkage is three times as large as the linear shrinkage So, volumetric shrinkage amounts to 8.5 % for steel and the volume of the shrinkage cavity amounts to 6...8 % of the casting's volume.

To have sound (dense, defectless) castings foundrymen use the principle of directional solidification in process of design of foundry technology. It allows them to take out a shrinkage cavity from casting's body (Fig.3.3 a) into a riser (Fig. 3.3 b). It is necessary to avoid hot spots within casting. When metal has a low shrinkage, for instance, cast iron, the principle of simultaneous solidification is used (Fig. 3.3 c).

During cooling the part A of the casting (Fig. 3.3 d) is contracted and parts B and C move one to other. But the mould prevents the displacement and, as a result, hot or cold cracks may appear.

Cast stresses, cracks, dendritic and zone segregation develop in castings, especially, in heavy castings and those having intricate shape.

 

Fig. 3.3. Defects in castings and principles of their design: a – formation of shrinkage cavity in casting;

b-installation of a riser; c-principle of simultaneous solidification; d-formation of cracks:

1-casting; 2-crack; 3-core; 4-upper half-mould; 5-lower half-mould