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.
Showing posts with label Investment Casting. Show all posts
Showing posts with label Investment Casting. Show all posts
Tuesday, 29 November 2011
Friday, 6 May 2011
Hard Spots in Investment Castings
Every so often I get a request for help regarding ‘hard spots’ in investment castings. Typically this defect can usually be attributed to one of three possible causes.
The first and most frequent cause is a ceramic particle; by this I mean investment powder, which has contaminated the casting. This is usually as a consequence of re-cycling scrap material. To prevent this it is very important that any scrap material is completely clean before it is re-melted. This cleaning process should involve acid dipping and also the use of magnetic pins. It is also possible to get investment powder contamination in a casting if there has been a problem with the flask burn-out and the investment has not reached its optimum strength or if there are any sharp bends or corners in the spruing of the pieces, this can lead to small pieces of investment being washed into the casting as a consequence of turbulent metal flow on casting.
The second possibility is iron contamination, which can be something as simple as a piece of stainless steel in the form of a magnetic pin or piece of burnishing shot getting mixed in with a melt, rather than issues of dust or rusty components which are usually associated with this sort of problem. The iron typically combines with the copper present in the casting to give a hard inter-metallic particle which then acts as the source of the ‘drag marks’ on polishing typical of hard spots.
The final possibility is the presence of a grain refiner in the alloy being cast which has agglomerated at the surface; rather than been distributed throughout the body of the cast piece. Typically this is associated with alloys which contain iridium as a grain refiner. The Argentium family of alloys also contain a grain refiner, boron, at a very low level of parts per million. If there are a large number of hard spots then the small amount of the grain refiner is unlikely to be the primary cause of the hard spots. To prevent this being a concern it is important that any silver casting alloy which contains a grain refiner is well stirred once it reaches the casting temperature. The induction pulse settings on the modern induction casters are very useful in ensuring that the molten metal is a homogeneous mix prior to casting.
When I receive samples of this type of complaint and examine them using a scanning electron microscope then 7 times out of 10 it is a ceramic particle I find (i.e. investment powder), 2 out of 10 it is a piece of iron or steel and the rest of the time it is an issue with grain contamination or poor mixing on melting.
The first and most frequent cause is a ceramic particle; by this I mean investment powder, which has contaminated the casting. This is usually as a consequence of re-cycling scrap material. To prevent this it is very important that any scrap material is completely clean before it is re-melted. This cleaning process should involve acid dipping and also the use of magnetic pins. It is also possible to get investment powder contamination in a casting if there has been a problem with the flask burn-out and the investment has not reached its optimum strength or if there are any sharp bends or corners in the spruing of the pieces, this can lead to small pieces of investment being washed into the casting as a consequence of turbulent metal flow on casting.
The second possibility is iron contamination, which can be something as simple as a piece of stainless steel in the form of a magnetic pin or piece of burnishing shot getting mixed in with a melt, rather than issues of dust or rusty components which are usually associated with this sort of problem. The iron typically combines with the copper present in the casting to give a hard inter-metallic particle which then acts as the source of the ‘drag marks’ on polishing typical of hard spots.
The final possibility is the presence of a grain refiner in the alloy being cast which has agglomerated at the surface; rather than been distributed throughout the body of the cast piece. Typically this is associated with alloys which contain iridium as a grain refiner. The Argentium family of alloys also contain a grain refiner, boron, at a very low level of parts per million. If there are a large number of hard spots then the small amount of the grain refiner is unlikely to be the primary cause of the hard spots. To prevent this being a concern it is important that any silver casting alloy which contains a grain refiner is well stirred once it reaches the casting temperature. The induction pulse settings on the modern induction casters are very useful in ensuring that the molten metal is a homogeneous mix prior to casting.
When I receive samples of this type of complaint and examine them using a scanning electron microscope then 7 times out of 10 it is a ceramic particle I find (i.e. investment powder), 2 out of 10 it is a piece of iron or steel and the rest of the time it is an issue with grain contamination or poor mixing on melting.
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