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Coating quality, pressure assure dimensionally stable castings; carefully applied refractory coating and the use of isostatic pressure during casting solidification combats deformation, gas pinholing and microshrinkage.

Coating Quality, Pressure Assure Dimensionally Stable Castings

Carefully applied refractory coating and the use of isostatic pressure during casting solidification combats deformation, gas pinholing and microshrinkage.

Aluminium Pechiney of France has made a strong research effort to improve the results of foam pattern casting (FPC), developing in the process a patented system to increase the advantages of this expanding segment of the metal-casting industry.

The company's initial research has focused on doing a better job of refractory coating formulation and application. Its second area of emphasis has been to perfect a method of applying pressure to the casting as it solidifies.

The refractory coating applied to foam patterns is the principal influence on the dimensional stability of the resultant casting. Well-designed and formulated coatings applied to carefully engineered patterns can eliminate casting deformation, assure the uniform inflow of molten metal and exclude blisters and inclusions. Application and formulation of refractory coatings is important since the coating is so essential to the quality of castings made by the FPC process.

A coating affects the control of gas-induced pinholing and microshrinkage, two factors that can scrap castings. Additionally, refractory coatings have the following effects: * improve pattern rigidity; * control the metal-filling rate; * minimize or eliminate the pattern's decomposition products; * control the thermal exchange between the metal and the casting material; * influence the size and presence of defects in the casting.

Making Sound Castings

The FPC process normally uses unbonded silica sand as a filler around the pattern. This results in low solidification rates and low thermal gradients compatible to those obtained with sand casting without chills.

To improve the soundness of castings, it was found that defects could be minimized by accelerating the solidification time and by applying pressure (isostatic compression) during the solidification phase.

Isostatic Compaction

Aluminium Pechiney has developed an innovative FPC system, called Castyral, which employs the use of isostatic pressure on a casting as it solidifies. Isostatic pressure is obtained by applying a gas pressure of several bars (14.4 psi per bar) on a metal casting while it is solidifying. This consists of placing the casting flask containing the casting cluster and the compacted sand surrounding it into a tightly-sealed vessel.

The part is cast at normal atmospheric pressure, but, immediately after filling, the pressure on the sand in the flask is increased gradually and held until solidification is completed. Initial testing in pilot production confirmed that several types of cast parts, i.e., heat exchangers, intake manifolds and cylinder heads, had superior mechanical properties compared to parts produced using the standard FPC cooling cycle.

Jacques Thomas Aluminium Pechiney Compiegne, France
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Author:Thomas, Jacques
Publication:Modern Casting
Date:Dec 1, 1989
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