Tuesday, 29 November 2011

Porosity in Investment Castings

Porosity is the most common defect found in investment castings. Its incidence can be greatly reduced by following a few simple guidelines but first it is important to diagnose what type of porosity you are getting in your pieces.

There are two causes of porosity in a casting; shrinkage on cooling or gas entrapment.

Shrinkage Porosity shows itself as small holes on the surface of a casting. When these holes are closely inspected using a magnifying glass they are seen to be angular, with a dendritic or grainy structure inside. Shrinkage porosity is the most common type of porosity and is generally found on large surface area pieces, parts that have shanks which increase in thickness and also in parts with that contain many angles and changes of section thickness. It occurs due to the non-uniform solidification of the piece on cooling as the metal actually tries to tear itself to pieces as it solidifies. This is due to insufficient molten metal being available to feed into the solidifying casting.

Gas Porosity typically shows itself as pin holes on the surface of the casting. These holes are generally round and smooth in nature, compared to the more angular holes of shrinkage porosity. In some cases the holes are concentrated in a layer just below the cast surface; these are not visible after casting but show up after polishing. There are generally two reasons for this type of defect occurring, either reaction with the investment powder or impurities been introduced through re-cycled metal.

The main causes of Shrinkage Porosity and the corrective actions to take are:

1) The piece has been incorrectly sprued

The sprue should be attached to the heaviest section of the pattern. This is because this area will be the last part of the casting to freeze and therefore there will be plenty of molten metal available for the piece as it contracts and solidifies. It is advisable, where possible, to use a round gate rather than a square or flat one as this prevents the point of contact between the sprue and the piece been cast become a ‘choke-point’ of restricted metal flow.

2) The piece has been sprued flat

Attach the piece to the centre sprue at a 45 degree angle rather than a horizontal 90 degree plane. This will assist the metal in flowing ‘downhill’ into the piece been cast.

3) Inadequate sprueing (too few or too small

The sprue is designed to supply the molten metal into cavity left by the melting of the wax pattern. If the sprue is too small it will solidify before the actual piece being cast; similarly if you only have a single sprue feeding a complex casting then it may also solidify before the extremities of the piece. This means that the piece cannot draw any feed metal from the centre sprue and shrinkage porosity will result. The solution is simple; provide an adequate number of suitably sized sprues to feed each casting.

4) The flask temperature is too high

If the flask temperature is too high the risk of shrinkage problems is increased. In general it is good practice to keep your flask temperature as low as possible while still maintaining a complete fill of the parts been cast as this also reduces the risk of metal/mould reactions.

5. The castings are too close to either the bottom or the top of the tree

The button of the tree (I call this the top of the casting) is obviously open to air and will cool more quickly. This cooling effect could be passed to the first row of pieces been cast so a 2 cm gap from the button to the first row of castings is recommended. Similarly at the very end of the tree (what I call the bottom) as this is filled with metal first, it will solidify first so it is not recommended to sprue pieces at the very end of the tree. Again a 2 cm gap is to be recommended.

The main causes of Gas Porosity and the corrective actions to take are:

1) The metal has been overheated or the flask is to hot.

Overheating the metal increases the risk of a metal/investment powder reaction occurring. The metal and flask temperatures should be sufficient to ensure complete fill of the pieces been cast, but in general, it is not necessary when casting silver alloys to exceed 1020C (1868F) and usually it is possible to cast at lower temperatures than this. Overheating the metal or the flask can result in the calcium sulphate in the investment powder decomposing to form a sulphur gas which can be trapped in the solidifying metal causing gas porosity.

2) Inadequate wax burnout

If there is any carbon residue left from the wax burnout then this can lead to a reaction with the molten metal as it enters the mould leading to gas entrapment just below the surface skin of the casting. To prevent this ensure that the investment powder manufacturer’s recommended burnout cycle is followed, particularly with regard to the holding time at the maximum burnout temperature.

3) Inadequate air flow in burnout oven

If the button of the casting shows a yellow ring on the investment powder after casting or there is dark colour on the investment powder next to the casting then this is probably due to a sulphur gas reaction involving the investment powder. This is due to the flask been heated in an oven where there has not been sufficient air flow to carry away the gases evolved during the flask burnout cycle. Usually this is associated with the burnout oven being overloaded with flasks but airflow through the oven can be increased by drilling holes in the bottom of the burnout oven door.

4) Scrap used contaminated with investment powder

The use of any scrap material introduces the risk of impurities or contaminants been introduced into the molten metal. It is a good general guideline not to exceed 50% of scrap in any casting. Where scrap sprues and trees are been used they should be thoroughly cleaned of any residual investment powder. The presence of any impurities or investment powder can lead to a chemical reaction involving gas evolution which then becomes trapped in the cooling casting.

To summarise these points remember; clean metal feedstock, metal and flask temperatures as low as possible and use an adequate number of sprues. This should ensure that you produce investment castings of a consistently high quality.