10 main reasons why alloy castings produce poress

Porosity of the alloy casting can be spread within the casting and on its surface. The surface porosity adds to surface roughness, but can also be a sing of the internal porosity. The internal porosity could weaken the casting, may cause discoloration if spread to the surface and in extreme cases could lead to a leakage.

Main causes of the porosity of alloy castings are:

1. Alloy temperature

If the alloy temperature is too high it can attack the surface of the investment similar to the case of the prolonged heating. As a consequence, a similar surface roughness can occur.

2. Air bubbles

Occasionally, small air bubbles can become attached to the pattern during or following the investing procedure. During the casting, the bubble is filled with the casting material (alloy) and is manifested as a nodule. Occurrence of the air bubbles can be prevented if a vacuum investing procedure is adopted. Some other measures can be adopted to prevent this happening, too, such as use of a mechanical mixer with vibrations or application of a wetting agent in a thin layer.

3. Residual air

These voids occur on the inner surface of the casting and are caused by entrapped air which cannot escape through the investment. This can be prevented by adequate temperature during the casting, sufficiently high pressure and correct liquid – powder ratio.

4. High heating rates

Adopting too high heating rates for the could lead to occurrence of fins (or spines) in the casting. The mechanism for this is as follows:

a) If the investment is subject to too high heating rate the outside layer becomes hot faster than the inner, and the temperature difference between the outside layer and the centre of the investment is increased.

b) Consequently, the outside layer tends to expand more than the inner parts. However, the outside layer is held back by the inner, cooler part.

c) Because of that the outside layer is subject to compressive stresses, while the inner part is subject to tensile stresses.

d) Since the investment is a brittle material, it tends to crack under tensile stresses. In this case, the most typical cracks are radial, starting from the interior of the investment and spreading outwards.

e) During casting, these cracks are filed by the casting alloy, manifesting as fins or spines.

5. Water film

Wax is a hydrophobic material (i.e. it is repellent to water). If the investment is not in close contact with the wax pattern a water film might be formed over the surface. This water might cause occurrence of small veins and ridges on the surface of the casting. Loss of contact between the investment and the wax pattern might be caused if the pattern has moved slightly, if it has been subjected to vibrations or if the painting has not been properly applied.

6. Low temperature

Too low a temperature could cause incomplete removal of the wax. Gases, formed when the hot alloy comes in contact the residues could cause porosity or voids in the casting. Too low a temperature can be caused by too short a heating time or if insufficient air is available in the furnace.

7. Liquid – powder ratio

Both too high and too low amount of liquid could lead to a rough surface of the casting.

8. Solidification defects

Solidification defects cold lead to two different manifestations of porosity; localised shrinkage porosity and Microporosity.

Localised shrinkage porosity is caused by insufficient feeding of the alloy during solidification. As the alloy solidify, it also shrinks by over 1%, and a sufficient supply of molten alloy is required during this phase to counteract reduction in the volume caused by the shrinkage. If the sprue is not properly designed and implemented then it may solidify before the feeding is complete thus preventing a continuous supply of molten alloy. This type of defect usually occurs close to the sprue-casting junction. Microporosity is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the microvoids to segregate. This in turn is caused the mould or casting temperature being too low.

9. Prolonged heating

Prolonged heating at too high temperatures could lead to a disintegration of the investment with a consequence of rough mould walls. In addition, products of the disintegration could contaminate the alloy causing surface defects.

10. Trapped gases

Many metals dissolve or occlude gases when they are molten. On solidification, these gases are forced out of the casting causing what is usually called pinhole porosity. These voids are rather small. Larger voids could be caused by the same mechanism, but could also be caused by the gases mechanically trapped within the molten alloy during the casting procedure.