Pores, voids and blow holes are defects in solid castings. Voids can be regarded as an agglomeration of vacancies. Pores are discontinuities or holes in solid parts.
Such defects normally form during freezing due to one or combination of both the factors:
(i) Evolution of dissolved gas at the solid-liquid interface.
(ii) Decrease in volume that occurs during freezing in most metals. Some partly covalently-bonded crystals with coordination number less than 8 expand on heating. Table 5.3 gives change in volume on melting of some metals.
Effect of Gas Evolution:
There are many metals in which gases are much more soluble in the liquid than in the solid state. The solubility of gases in the metals (liquid as well as solid) markedly depends on the applied pressure as well as on the temperature.
Sieverts law holds good for the solubility of most common gases such as oxygen, hydrogen, nitrogen, etc., which are diatomic, as-
where, Cg is the solubility of dissolved diatomic gas, p is the pressure of the gas, and k is the constant. This law appears to hold good for the metals in the solid as well as in liquid state as illustrated for hydrogen in magnesium in Fig. 5.25 by the straight line variation. The figure also illustrates that lower partial pressure of gas is needed to obtain the same solubility if the temperature of the metal is high.
ADVERTISEMENTS:
It illustrates that under the same partial pressure, solubility is higher at higher temperature. Actually in most cases, solubility increases rapidly (almost exponentially) with the rise of temperature. Fig. 5.26 illustrates this for hydrogen is copper. The significant point to be noted is that there is a rapid decrease of solubility of hydrogen as liquid copper transforms to solid copper at 1083°c.
It has been observed, in general, that the solubility of the dissolved gases decreases rapidly when most metals solidify. The gas that is rejected accumulates in front of the advancing interface, where it may reach a concentration that causes the nucleation of gas bubbles.
ADVERTISEMENTS:
The interface may not be a good heterogeneous nucleation site for the bubble formation, as the bubble and the solid interface needs creation of a solid-gas interface which has higher energy than the liquid-gas interface. (Homogeneous nucleation of gas bubbles does not occur as it needs much higher supersaturation).
Gas bubbles may adhere to the interface at specially favourable points, such as grain boundaries. The gas-bubble formation is also dependent on the local pressure in the liquid. If a liquid metal solidifies under sufficiently high pressure, such as in pressure die-casting process (liquid metal is forced under high pressure into steel-dies), the gas is made to remain dissolved in the metal (Sieverts Law), gas evolution is prevented.
Thus, gas porosity is avoided in pressure die-cast parts. But, gas-bubble nucleation is promoted by the lowering of the pressure. Such a condition can arise when the entire outer surface of the casting solidifies, leaving the liquid in the centre. As this liquid solidifies, normal shrinkage occurs. As vacuum develops, i.e. the pressure drops, gas-bubble nucleation can occur.
Once the gas bubbles have formed, the subsequent behaviour of the bubble depends on the rate of advance of the interface. The bubbles may grow, by the diffusion of gas into it from the surrounding liquid to such an extent that it floats away from the interface.
ADVERTISEMENTS:
These rapidly growing bubbles (as compared to growth of the solid) rise to the upper surface of the casting. If the top of the casting has already solidified, large sized porosity may form near the top of the casting.
These are large sized because these have grown under a lower hydrostatic head. If the top of the casting has not solidified, these gas bubbles escape to the atmosphere. The sweeping action of bubbles rising from greater depths in the mould further helps in the elimination process.
If the rate of growth of bubbles is much slower than the solid growing around them, then the bubble is trapped inside. To reduce its surface area, the cavity takes a spherical shape, called blow hole, Fig. 5.27.
If the rate of growth of bubbles is the same as the rate of advance of the solid-liquid interface (i.e. rate of solidification), then worm-holes are formed, Fig. 5.28. These holes are elongated in the heat-flow direction. A common example of worm-holes is the ice-slab formed from water having dissolved air. The whitish area of the ice-slab has worm-holes.
This fact leads to the formation of primary and secondary pipes in ingots. Hot-top pouring reduces the extent of pipes formed. Shrinkage pores can be seen in the centre of the ingot when equiaxed grains are forming and the heat is conducted outwards. When the liquid freezes, shrinkage pores are formed.
A special type of shrinkage porosity-called interdendritic porosity occurs if highly superheated liquid is poured in the ingot mould. Chill zone gets melted due to the superheated liquid. Columnar zones are formed right on the surface of ingot.
It is possible to have liquid in the interdendritic space when the columnar grains have grown quite long. The shrinkage due to continued freezing sucks this liquid into the interior of the ingot. There is seen interdendritic porosity on the surface of the ingot.
When the effect of shrinkage and the evolution of gas combine, then fine size pores are seen to form, which are irregular in shape. When the liquid is enclosed in solidified surrounding surface, called the ‘sealed-off’ part of the liquid, and if any dissolved gas is present, a bubble forms and grows, which has tendency to form a shrinkage cavity as a result of being sealed off. Such a sealed off liquid could be interdendritic liquid.
A pore may therefore be found to contain a gas even when there is not sufficient gas content to have formed gas bubbles otherwise. As all the gas diffuses to one bubble, single pore is obtained in each sealed-off region. In the absence of gas, the same amount of shrinkage would have formed as a large number of much smaller pores.
Problem 1:
Molten copper at normal pressure has 0.01 wt. % of oxygen in it. It has been found that gas porosity shall not form in the castings if oxygen content is reduced to less than 0.00001 wt. % prior to pouring. What can be done to molten copper?
Solution:
Of the various solutions, one is to do vacuum-degassing. Sieverts law can be used to find the vacuum required for this:
Another method is to treat this molten copper with Cu-15% P alloy. Phosphorus reacts with oxygen to produce P2O5, which floats on the liquid. On an average 0.1 to 0.02% P may be added to remove the oxygen.
Problem 2:
Melting point and entropy of melting of Cu are 1356 K and 9 J/mol. K respectively. Calculate the radius and Gibbs energy of the critical nucleus during solidification at 1306 K. Assume homogeneous nucleation. Molar volume of Cu is 7 x 103 mm3 and the solid-liquid interfacial energy is 0.6 J/m2.
Solution:
Problem 3:
What would be the radius of spherical surface of the critical nucleus if the nucleation occurs heterogeneously on the flat mould wall?
Solution:
As the critical nucleus is sphere, contact angle is 180,
