Solidification of Metals | Industries | Metallurgy

A thorough understanding of the mechanism of solidification, the rate of heat loss from the material to the mould etc. is essential to predict, how the casting will solidify and thus avoid casting defects like seams, gas porosity, hot tears etc. Since, solidification requires energy to produce a crystalline structure, some super cooling (cooling below the freezing point) is required before the liquid starts to solidify.

It is provided by the walls of the mould, which provide sites around which crystals can grow initially and subsequently, by the solidified particles and the metal itself. So the crystals start to grow from the mould walls and the process continues as more heat is lost, with crystals growing inwards until the whole of the metal has solidified.

The crystals near the mould walls are small and equiaxed (i.e. their axes randomly oriented). On further solidification, crystals grow with their axes perpendicular to the mould and these are columnar in shape (Fig. 3.34).

The reason for columnar crystal growth in direction perpendicular to mould wall will be obvious from curves in Fig. 3.35, which shows the distribution of temperature along the distance from the mould face at the time of pouring of metal and subsequently as time lapses. It will be seen that temperature gradient is decreasing, being maximum at mould face and thus short gains near face and columnar further away from face of mould.

This was the case with pure metals which have single freezing point. However, in case of alloys, these freeze over a range of temperature. When the alloy has cooled to a given temperature it starts to solidify, but remains in a mushy state, until further cooling renders it completely solid.

The temperatures at which alloy solidification starts and finishes varies with its composition, and can be determined by the liquid us and solidus lines. Since the component of the alloy having higher freezing point starts solidifying first, the frozen metal adhering to the mould walls will have a different composition from that of the original alloy.

In the case of castings, after pouring of the molten metal in the mould, the temperature falls steadily until freezing commences at a particular point. During solidification, the temperature more or less remains constant due to release of latent heat. Actually, there may be a slight increase in temperature if super cooling has occurred (Refer Fig. 3.36).

The temperature again starts falling steadily in case of pure metal. In the case of alloy, commencement of crystallisation is followed by a period of less steep temperature reduction while the metal is passing through the mushy state [Refer Fig. 3.36 (b)].

Since an alloy does not have a sharply defined freezing temperature, solidification takes place over a range of temperature. The solids separating out at different temperatures, therefore, possess different compositions.

The direction of crystal growth is thus dependent upon the composition gradient within the casting, variation of solidus temperature with composition, and the thermal gradient within the mould. The crystal growth in the case of alloys is of dendritic structure.

Experiments have been conducted to find the distribution of temperature at various distances from centre line as the casting cools. It will be noted from Fig. 3.37 (a) that in the case of a sand mould, the extraction of heat is considerably slower as compared to metal mould [Fig. 3.37 (b)].

If heat is extracted quickly then a narrow mushy zone quickly sweeps through the cooling metal [Fig. 3.37 (b)], but if heat is slowly extracted then mushy zone may extend throughout the casting [Refer dotted lines in Fig. 3.37 (a)].

It has been found that alloys having the smallest temperature difference between the start and finish of solidification (narrowest mushy zone) are easiest to feed. Ease of feeding is defined by the CFR (centre line feeding resistance) which is the ratio of time during which crystals are forming at centre line and solidification time of casting, or BD/AD. If CFR > 70%, then alloys are difficult to feed.

Heat Loss from Castings:

In order to provide a riser of adequate size, it is necessary to know the rate of heat removal through the wall of the mould. Risers act as a reservoir of liquid metal above the casting and supply liquid metal to the casting throughout its solidification period.

where k = thermal conductivity of the mould material

θ₁ = is the initial mould temperature and θ₂ is the mould/metal interface temperature (assumed constant during solidification),

t_s = time required to solidify a plate of surface area A

d = thermal diffusivity of the mould.

Q = considering the amount of heat which must be lost to produce solidification

= ρV [L + c (θ_p – θ₁)]

where, ρ = density of metal,

V = volume of metal

L = latent heat of fusion of metal

θ_p = pouring temperature

c = specific heat of metal

This time for solidification for a plate is proportional to (V/A)². It can also be shown that the time needed to solidify to a given depth is proportional to the square of the distance from the mould metal interface.

Solidification Time:

During the early stages of solidification, a thin solidified skin begins to form at the cool mould walls, and as time passes, the skin thickens. With flat mould walls, this thickness is proportional to the square root of time. Thus, doubling the time will make the skin √2 = 1.41 times or 41% thicker.

According to Chvorinov’s rule, the solidification time is a function of the volume of a casting and its surface area.

where, C is a constant that reflects mould material, metal properties like latent heat, and temperature.

For sphere, cube and cylinder, the solidification times are as under:

t_sphere = 0.043 C, t_cube = 0.028 C, t_cylinder = 0.035 C, i.e. cube shaped casting will solidify the fastest and the sphere- shaped casting will solidify the slowest.

Properties and Behaviour of Molten Metals during Solidification:

The quality and soundness of a casting, its physical and mechanical properties, dimensional accuracy, surface finish are not acceptable in many cases. In order to attain these properties, it is essential to have a thorough knowledge of the properties of metal in liquid state and its behaviour during solidification as most of the casting defects can then be eliminated and taken care of.

Rate of Solidification:

A riser in a casting is provided to act as reservoir of liquid metal. It compensates for the shrinkage that takes place from the pouring temperature upto solidification. The metal in riser should not solidify before the casting. For this purpose, it is essential to have an idea of the time taken by the casting to solidify.

Further, the location of riser can be judiciously selected if the time taken by a casting to solidify upto a certain distance from the mould face can be estimated. The heat rejected by a liquid metal is dissipated by the mould wall, and various layers and temperature gradients encountered in general case are shown in Fig. 3.38. In actual practice, the solidification process is quite complicated due to complex geometry, freezing of alloys, temperature dependent thermal properties.

The solidification rates for various cases are as under:

(i) In the case of a large casting in an insulating mould (sand casting),

where, V = total volume of the casting, A = cross-sectional area of the mould-metal interface (approximately surface area of casting).

In case, the metal-mould surface is convex in place of plane surface, the heat flow is more divergent and the rate of heat transfer will be more and freezing time less. Reverse is true for concave metal-mould interface.

(ii) When heat flow is controlled significantly by the thermal resistance of the mould-metal interface (as in per­manent mould casting and die casting), solidification time

where h = film heat transfer coefficient of the interface,

ρ = density of metal,

θ₁ = temperature of liquid metal, and

θ₂ = temperature of mould surface

Fig. 3.39 shows how solidification time varies with change in Area/volume ratio for various shapes.

Methods for Proper Progressive Solidification:

The sound casting with no shrinkage void, along the centre line can be obtained by ensuring, that the temperature gradient is sufficiently steep and properly directed so that liquid metal can pass through the wedge- shaped channel to compensate for shrinkage as it occurs at the centre line.

The longitudinal solidification must be progressive toward the riser. The gating and risering should be so provided that favourable temperature gradients are set up. Pouring rate and temperature must be properly selected. It is also important to use proper padding, chills, and mould materials with different thermal conductivities.

Volume Behaviour with Change in Temperature:

When a solid melts into liquid, an additional volume increase occurs due to the structural breakdown of the crystal lattice. Similarly on solidification, a similar contraction or shrinkage of the order of 1 to 6.6% (1% for cast iron with 4.3% carbon in graphitic form and 6.6% for pure aluminium) for commonly used metals occurs.

Further, the increase in volume per unit temperature is more in case of liquid than in solid (In case of solids, it is 1% for every 100°K).

To counter these two effects:

(i) A reservoir of liquid also known as feeder (which remains liquid until complete solidification) must be provided to compensate for liquid-solid contraction during solidification, and

(ii) Special measuring rules should be used to account for contraction of casting from freezing point upto room temperature.

Proper positioning of the feeder and employing means to ensure that metal solidifies last of all in the feeder, are important aspects. The latter aspect is taken care of by adopting techniques like directional solidification (to ensure that castings solidify in a directional manner, towards the feeder) which is achieved by having widest parts near the feeders, and/or using chills at points away from the feeders.

These way shrinkage cavities are confined to feeders only. Since feeders represent wastage, these should be as few and as small as possible, consistent with the requirements.

Macroscopic Effects of the Shrinkage and Its Distribution:

If a liquid metal solidifies with a narrow freezing range (skin freezing), then interface between solid and liquid phases during solidification will be nearly a smooth one. As solidification proceeds towards central portion, top molten metal feeds for the shrinkage and if the simple shape without feeder is solidified in this way, a central sinking surface is formed, which eventually leads to a deep ‘pipe’ in the centre (Refer Fig. 3.40 (a)).

If a liquid metal solidifies with a wide freezing range, then dendritic growth of solid into liquid phase can be observed. The interface between solid and liquid exists as a wide mushy or pasty zone. Thus, metal can’t flow easily and volumes are cut off.

The cut off volumes shrink further and cavities known as micro shrinkage cavities are dispersed throughout the casting, as shown in Fig. 3.40 (b). Central surface shrinking is less and no piping occurs.

Alloys solidify as primary phase plus eutectic. A skeleton of solid dendrites forms within the solidifying liquid and the liquid drains away from around them as it feeds the lower portions of the casting, resulting in sound metal near the bottom and voids around the dendrites near the top (known as inter-dendritic feeding).

To avoid these three categories of shrinkage defects, feeding problem must be overcome by suitably placing adequate size feeders at proper places depending on the geometrical shape of the casting and incorporating slight modification to wall thickness.

In any case, it may be appreciated that if volume change on solidification for any metal is low, it is easiest to cast since most problems are being experienced for this reason only. It is interesting to note that the eutectic liquid of 4.3% carbon solidifies to form the eutectic with the carbon present as graphite and then shrinkage loss is less than 1% and luckily most of the engineering applications are with iron. Graphite having lower density, the shrinkage may sometimes even be negative and thus sound iron castings are produced compared to other alloys.

Cooling of Solidified Metal:

While shrinkage effects during freezing causes problems, the problems associated during contraction after solidification also deserve due attention. In several cases, it is observed that outer skin ruptures shortly after it has formed and the liquid core spills out, leaving a ragged tear in the wall of the casting.

Actually what happens is that the outer skin solidifies first and due to contraction tends to draw away from the mould. The contraction of skin leads to increase in hydrostatic pressure of the liquid metal at core and this combined with weak skin still at high temperature and evolution of dissolved gases exerting pressure at the skin, the skin gets ruptured and liquid flows out into the gap between skin and mould wall.

An effect known as hot tearing (rupture of skin by thermal contraction) is observed, where a thin section joins thick section. The thin section solidifies first and thicker section still contains liquid metal. The thinner section than begins to contract thermally and pulls away from the large, partly molten section, and fracture occurs at the corner of the thicker section.

While mushy liquid from thicker section is unable to flow out, the oxidation of the exposed surfaces takes place rapidly. In some cases shape gets distorted and gets weakened due to above reasons. The presence of thick and thin sections in complex castings after results in differential contraction, which results in residual stresses. These residual stresses can be removed by proper annealing operation, the heating being carried out very gradually.

Evolution of Gas and Degassing:

During melting, the metal is in contact with a gaseous atmosphere (excepting under vacuum melting) and as such gases are dissolved in the liquid. Further, old machinery scrap having oxide skins of metal, contaminants, moisture etc. also produce gases, which get dissolved in melt. Air can also be entrapped by turbulence while pouring molten metal into mould, or by picking up moisture or oil from the mould walls.

Liquid metals have capability of dissolving considerable amounts of gases, whereas solids do not have (there is significant drop in solubility of gas during freezing, and as such all dissolved gases in liquid metal, get expelled in the form of bubbles during solidification producing gas cavities (blow holes) within the metal if these can’t escape. It may be noted that gas cavities are smooth and rounded but shrinkage cavity has irregular surface due to the last liquid seeping away between the dendrites.

If gas entrapment is large resulting in holes visible to the naked eyes, it is referred to as blow-hole. If the gas holes are smaller than the grains and are dispersed, it is referred to as gas porosity or micro porosity. If concentration of dissolved gases has reached saturation limit, as with the last volume of liquid to solidify, it results in fine gas porosity. It is interesting to note that sometimes gas evolution may counteract and compensate shrinkage, thereby eliminating piping effect and the gas holes can be welded together during subsequent rolling.

It may also be noted that hydrogen gas is usually expelled during solidification but oxygen or nitrogen may be separate as oxides and nitrides and thus remain within the metals as micro constituents. Steam reacts with molten metal to form metallic oxide and atomic hydrogen which may be dissolved or may form compound. Further, equilibrium between gas and metal is attained for a given pressure and temperature; as is obvious from the following equation. (Sievert’s law or square root law)

Concentration of dissolved gas:

where k = equilibrium constant related to temperature.

In order to avoid nuisance of gas evolution in the manufacture of castings, some kind of degassing procedure needs to be followed before solidifying of liquid metal. One method is vacuum degassing in which the liquid metal is held under a vacuum just before casting and the gases come out of solution because the gas dissolved is proportional to the square root of its partial pressure above the melt.

However, this method is costly and may also remove alloying ingredients having moderate vapour pressures. In another method, another gas having much lower solubility than the one to be removed, is bubbled through the liquid melt. As a result contaminated gas mixture bubbles escape and oil interface is formed at which the concentration of the dissolved gas is extremely low so that no evolution takes place during solidification.

Hydrogen can be eliminated by melting the metal under oxidising conditions. Oxygen can be eliminated by adding deoxidants (like Mn or Si for steel) which form liquid products at the bath temperature and which coalesce and separate easily, usually by rising to the surface.

It becomes essential to take all precautions and care to control gas pick-up during melting (using clean and dry charge and melting quickly under suitable atmosphere) and before solidification. A layer of flux can be floated on the melt to avoid contact with atmosphere. Pouring should be slow and gradual and mould surfaces clean.

Development of Grain Structure:

How the grain structure develops during the process of solidification is the most important point as it directly influences the mechanical properties. A casting for engineering applications must be produced with an appropriate grain structure that is free from any heterogeneities, inclusions or structural defects.

When a pure liquid metal is poured in a mould, the temperature of liquid adjacent to mould wall quickly falls below freezing point and many solid nuclei form and each grow rapidly to form a grain boundary on meeting its neighbour. Due to large number of nuclei, equiaxed fine grained structure (chill crystals) is formed quickly near the mould wall.

The innermost chill crystals then grow into the liquid as it cools to its freezing point, giving rise to long columnar crystals. Columnar growth continues until opposite sets of columnar crystals meet, if there is no nucleation in the centre of the liquid. However, if nucleation occurs in the centre also, then coarse equiaxed crystals will be formed at the centre meeting columnar crystals originated from chill crystals near the mould wall.

It may be noted that columnar growth occurs with pure metals. In the case of alloys having wider freezing range, the columnar growth will be less as chances of nucleation occur in the centre. Any non-metallic inclusions are pushed along by the advancing front of columnar growth and form a pronounced line of weakness when they eventually meet.

Such cases, in which solid grains develop by degree of super cooling only and there is no other internal or external source of stable solid nuclei, are called homogeneous nucleation. If suitable stable nuclei like seed crystals or tiny particles of other alloy are present to initiate grain growth, it is known as heterogeneous nucleation. In the case of single phase alloy or pure metal, one crystal grows from each nucleating particle to give a fine equiaxed crystal structure.

In the case of alloys it is desirable that same composition be maintained throughout the mass of casting to have the required physical and mechanical properties uniformly. However, segregation (variation of alloy composition) occurs due to several different mechanisms operating during solidification of casting. Gravity segregation occurs due to density differences. Micro segregation occurs across individual grains within a casting.