solidification, phase relation and strengthening part 1

Solidification

Solidification may be defined as a phase change process where a liquid turns into a solid when its temperature is lowered less than its freezing point. Almost all metals and alloys, at one point in their processing, are a liquid, when then solidifies as it cools below the freezing temperature. In pure metals, the grain size and shape may be controlled by solidification.

Nucleation and Grain Growth

Solidification process occurs 2 steps- i.e. 1. nucleation and 2. growth. Nucleation phenomenon occurs when a small piece of solid forms from the liquid. Growth of crystals in solid occurs as atoms from the liquid get attached to the tiny nanocrystals solid until no liquid remains in it,

We predict that a material solidifies when the liquid cools below the freezing temperature. As the temperature falls below this temperature, the value of energy present with the crystalline structure of solid is increasingly less than the amount of energy of the liquid, this makes the solid more and more stable.

Homogeneous nucleation

As the liquid cools further below the equilibrium freezing temp, there is a greater probability that atoms will cluster together to form an embryo larger than the critical radius. This type of nucleation occurs when the undercooling is large enough to permit the embryo to exceed the critical size .On increasing undercooling, the critical radius needed for the nucleation decreases.

Heterogeneous nucleation

Nucleation process on preexisting surfaces boundary is known as heterogeneous nucleation. Such nucleation process is dependent on the contact angle (ɵ) for the nucleating phase and the surface on which nucleation occurs. The same type of phenomenon occurs in solid-state transformations

Crystal growth/ dendrite formation

Once solid nuclei have formed, growth occurs as atoms are attached to the solid surface. In pure metals, growth depends on how heat is removed from the solid-liquid system. Two types of heat must be removed – the specific heat of the liquid and the latent heat of fusion. The latent heat of fusion, which represents the energy that is evolved as the unevenly arranged liquid structure changes to a more stable crystal structure latent heat, must be removed from the solid-liquid mixture before solidification process is completed.

Small protuberance may be formed on the liquid surface. This uneven extension moves outward continuously under suitable nucleation. The mechanism known as planar growth occurs by the movement of a solid-liquid interface into the liquid. However, when nucleation is poor, the liquid may under-cool to a temp below the freezing temp. Under these conditions, a small solid protuberance called a dendrite, which forms at the interface grows. As the solid dendrite grows, the latent heat is conducted into the under-cooled liquid, raising the temp of the liquid towards the freezing temperature. Secondary dendrite arms and tertiary dendrite arms also form on the primary stalks to speed up the evolution of the latent heat

Cooling curve

A typical cooling curve is actually a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid a.

Solidification time

The rate at which the solid grows depends on the cooling rate or rate of heat extraction. A fast cooling produces rapid solidification or short solidification times. The time required for a simple casting to solidify can be calculated using Chernin's rule.

Ts = B (V/A)²

Where, Ts is the solidification time,

V is the casting volume,

A is called

the surface area of the casting in contact with the mold,

&finally B is the mold constant

Melt conditions

The melt condition relates to superheating and the influence of temperature. some of the casting processes are

1. a) Green Sand molding
2. b) The permanent mold process
3. c) Die casting
4. d) Investment casting

Solidification Defects

A well-designed casting, a corrected prepared mold and properly melted metal always results in a defect free casting structure. Further, if proper control is not exercised in the foundry development sometimes it is too expensive - a variety of unwanted and expensive defects may result in a casting structure. These defects produced may be the result of following causes:

 Improper pattern design.

 Improper mold and core pattern.

 Improper melting practice.

 Improper pouring practice.

 Molding and core making materials.

 Improper gating system.

 Improper metal composition.

It creates a deficiency or imperfection. They are

 Surface Defects

1.Blow :It is relatively large cavity produced by gasses which displace molten metal form.

2. Scar Due to improper permeability or venting. A scare is a shallow blow.

3.Scab: gas coming out while pouring damages mold.

4. Drop: This defect is caused by the break-away of a part of mold sand as a result of weak packing of the mold, low strength of the sand used in molding, malfunctioning of a molding machine.
5. Buckle A buckle is a long, fairly shallow, broad, vee type depression that is seen on the surface of flat castings structure. It continues in a fairly straight line across the entire flat surface.
6. Penetration: when the sand packing is inadequate, some projection is seen in the pattern.

 Internal Defects

1. Blowholes: Blow holes, gas holes or gas cavities are circularly rounded cavities having a clean surface. They appear on the casting surface or either in the body of a casting.
2. Pin Holes: Pin holes are small smooth gas holes either at the surface(may be a circular arc or semicircular) or just below the surface. When pin holes are present, they occur in large numbers and are almost uniformly dispersed over the whole surface.

 Visible Defects

1. Wash: A cut or wash is a low projection developed on the drag face of a casting surface that extends along the surface, decreasing in height as it extends from one side of the casting to the other end.
2. Rat Tail: A rat tail is a long, shallow, angular depression developed on the surface of a flat structure and resembles a buckle, the difference is that it is not shaped like a broad vee depression.
3. Hot Tear: Hot tears are hot cracks which appear in the form of unevenly spread crevices with a dark oxidized fractured surface.
4. Shrinkage: A shrinkage is a cavity which is a depression or an internal void(null space)in a casting that results from the volume contraction.
5. Swell: A swell is a slight, smooth bulge mostly found on vertical surfaces of casts. this is due to results from liquid metal pressure.
6. Shift : Mold shift refers to a defect caused by a sidewise movement of the mold cope with respect to the draMisrun or cold sheet or short run : It is a defect due to incomplete cavity filling. such phenomenon can occur as a result of inadequate metal supply, too low mold or melt temperature, improperly designed gate or thickness ratio to the length of the casting is too small