EP0648560B1 - Method for the production of ceramic cores for casting - Google Patents

Description

The present invention relates to a method for manufacturing ceramic cores for foundry from a paste thermoplastic according to the preamble of claim 1, which is based on US-A-4,685,503.

The use of so-called foundry cores "ceramics" is notably known in certain applications which require obtaining a set of characteristics and strict quality criteria such as resistance to high temperatures, lack of reactivity, stability dimensional and good mechanical characteristics. Among these applications presenting such requirements, will cite in particular aeronautical applications and by example, obtaining turbine blades in the foundry for turbojets. Improvement of foundry processes, evolving from the equiax foundry to the foundry by directed or monocrystalline solidification, further increased these kernel requirements, the use and complexity are imposed by the search for high performance for parts to be obtained, as is the case for example for hollow blades with internal cooling. These fields of application are related to the precision foundry, in particular to the process known under the lost wax foundry designation. In all cases the use of the nucleus intervenes for the manufacture of hollow parts.

In the lost wax foundry method, we use a ceramic material core which is held in the mold when casting metal, the outer surface of the core forming the inner surface of an internal cavity of the product finished up this way. Precision and stability dimensional of the nucleus are therefore essential for meet the thicknesses targeted on metal parts flows.

Examples of known compositions intended for the preparation of such nuclei are given by FR-A 2,371,257 and mainly contain fused silica, flour zircon and cristobalite which is a form of silica crystallized, a silicone resin being used as binder and additional elements in small quantities such that lubricant and catalyst being added. The process of preparation is also described.

In general, the cores used to cast the parts and blades are made of ceramic with a generally porous structure: these cores are made from a mixture consisting of a refractory fraction (in the form of particles) and an organic fraction more or less complex. Another example is described by EP-A 0 328 452.
In a manner known per se, the shaping of the foundry cores, in particular from thermoplastic pastes, can be done by molding using for example a press injection. This shaping is followed by a debinding operation during which the organic fraction of the nucleus is eliminated by various known means such as sublimation or thermal degradation, depending on the materials used. A porous structure results. A heat treatment for baking the core to consolidate the porous structure is then applied to the refractory fraction. This treatment introduces a dimensional modification, in the form of a withdrawal which is often non-isotropic in the volume of the nucleus, compared to the initial form.
At this stage, it may be necessary to strengthen the core so that it is not damaged in the cycle following use. It is known in this case in particular to perform an impregnation by means of an organic resin.

• injection du modèle cire autour du noyau
• réalisation du moule carapace
• élimination du modèle cire
• divers traitements thermiques : brûlage des résidus cire, frittage du moule carapace, préchauffage, coulée de l'alliage, refroidissement de l'alliage
• élimination du noyau.

At this stage the core is ready to be used, that is to say it must withstand the following lost wax production cycle:

• injection of the wax model around the nucleus
• shell mold creation
• elimination of the wax model
• various heat treatments: burning of wax residues, sintering of the shell mold, preheating, casting of the alloy, cooling of the alloy
• removal of the nucleus.

US-A-4,685,503 describes a process for manufacturing a core foundry where after mixing a ceramic material refractory and an organic binder and molding a core to the desired shape, the core is impregnated from a aqueous solution of an alumina and phosphorus oxide before to carry out a heat treatment.

US-A-5,067,548 further teaches a shaping process of a foundry ceramic mold comprising a molding by injection of mold elements and low heating temperature before assembly of the elements and treatment high temperature thermal.

In the implementation of these known methods difficulties remain and the results obtained are not completely satisfactory. Dispersions on the geometry of the nucleus affect the finished part while a tolerance dimensional in the range of ± 0.1 mm may be required. A improved results require stability dimension of the nuclei which remains difficult to master because the structure of the material changes during the treatments successive thermal previously indicated: cooking core, heat exchanges in the shell mold of foundry.

In addition, the cores must have good strength mechanical strength sufficient to withstand mechanical and thermomechanical constraints during the stages of lost wax process: injection of the wax model around the core, thermomechanical constraints between core and shell during dewaxing, burning, sintering and during casting of the alloy around the nucleus.

The properties of the core result from cooking but according to known methods, consolidation of the structure of the refractory core fraction is accompanied by withdrawal. This phenomenon leads to difficulties in developing core shaping products and materials such as injection mold and has an impact on the quality of nuclei, the amplitude of the withdrawal anisotropies being added dimensional dispersions. The invention aims to improve the process of manufacturing ceramic cores by reducing these dimensional changes while tightening them dimensional dispersions and retaining a hold i adequate mechanics.

These improved results are obtained by a process for manufacturing ceramic cores comprising the steps known per se of shaping and then elimination of the organic fraction of the core, characterized in that the heat treatment is limited to a minimum consolidation of the structure of the ceramic fraction of the core, providing a mechanical resistance just sufficient for handling the cores and so as to limit the shrinkage to a minimum value and is followed by a step of impregnating the porous structure of the core by means of a solution consisting of at least one simple colloid taken from the colloidal silica and alumina group and possible additions comprising several mixed soils or mixtures of soils and salts, then a step of eliminating the liquid part of the impregnation product.
The elimination of the liquid part of the impregnation product can be obtained in particular by drying.
In some cases, an additional heat treatment may be necessary after impregnation to ensure dilatometric stability of the product.
It follows from the process according to the invention that the dry residues of the impregnation products form particles which partially fill the porosity of the core and have the effect of reinforcing the mechanical resistance of the core by consolidating it and blocking at a low level the removal, without significant modification during subsequent heat treatments.

• la figure 1 représente les courbes de variation de températures lors d'essais de pièces représentatives des noyaux de fonderie obtenus par un procédé conforme à l'invention,
• la figure 2 représente une courbe de variation des retraits en pourcentages en fonction d'un cycle de température pour une pièce obtenue selon un procédé antérieur connu,
• la figure 3 représente les courbes de variation comparative des retraits en pourcentages en fonction de la température pour diverses variantes du procédé de fabrication.

Other characteristics and advantages of the invention will be better understood on reading the following description of examples of embodiments of the invention, with reference to the accompanying drawings in which:

• FIG. 1 represents the temperature variation curves during tests of parts representative of the foundry cores obtained by a process in accordance with the invention,
• FIG. 2 represents a curve of variation of the withdrawals in percentages as a function of a temperature cycle for a part obtained according to a known prior process,
• FIG. 3 represents the curves of comparative variation of the withdrawals in percentages as a function of the temperature for various variants of the manufacturing process.

Development of the core manufacturing process precision foundry ceramics was carried out at from experimental tests. Representative pieces of cores are thus produced according to known techniques injection of a thermoplastic ceramic paste. A first composition I contains a ceramic mineral filler based on fused silica mixed with zircon flour and a waxy organic binder based on synthetic wax.

A second composition II in addition to the components of the first composition described above, further comprises in the mineral filler a small fraction of silica crystallized and a mineral release agent.

The parts obtained are then unbound by heating to 200 ° C, as it is known per se.

A heat treatment is then applied to the parts. Of satisfactory results are obtained after treatment with 1100 ° C for 5 hours. Pre-sintering is thus obtained without that this results in a significant withdrawal and acceptable mechanics is obtained in such a way as to allow handling of the cores without risk of deterioration. At minus 30% of open porosity is observed. According to applications, the heat treatment temperature is between 1000 ° C and 1150 ° C and the duration between 1 and 5 hours.

In order to impregnate the pieces, several compositions were tested. Product A is an aqueous colloidal suspension of silica particles having 40% by mass of silica. After 24 hours of impregnation, approximately 90% of the open porosity is impregnated. After drying in an oven at 70 ° C for 24 hours, a gain in mass of the parts of between 8.7% and 9.5% is observed. A clear improvement in mechanical strength is noted. Drying can be done under vacuum
A second product B was tested consisting of a colloidal suspension at 10% by mass of alumina, obtained by dispersing boehmite / AlOOH powder in a solution of acetic acid at 0.7%. Impregnation of 90% of the open pores is also obtained after 24 hours. After drying and high-temperature decomposition of the boehmite into alumina, a mass gain of 3% of the parts is obtained.

A mullite formation reaction occurs as follows: 3 Al ₂ O ₃ + 2 SiO ₂ → 3 Al ₂ O _{3. 2} SiO ₂

A third product C is obtained by mixing the two previous A and B. To obtain it, colloidal silica is added to the boehmite solution in acetic acid to 0.7%. The impregnation allows in this case to fill 80 to 90% porosity opened in 24 hours and the mass gain of parts is 3 to 3.5% after heat treatment.

A fourth soil used D is obtained by mixing silica colloidal (product A above) and a nitrate solution aluminum.

For product D, as for product C, the mixtures are produced so as to obtain, after drying, a mixture of alumina and silica in the stoichiometric proportion of the mullite. the soil obtained is loaded with 8% Al ₂ O ₃ and 3.1% SiO ₂ . The very low viscosity solution allows in this case a pore impregnation close to 100% After heat treatment of the parts, carried out at 1150 ° C. for 1 hour, a mass gain of 2.6% is observed.
Depending on the applications, before casting the metal, the core can be subjected to preheating, in particular at a temperature between 1000 ° C and 1100 ° C and for a period between 1 and 4 hours.
The tests carried out corresponding to the implementation of the process for manufacturing ceramic cores according to the invention made it possible to record significant and advantageous results.

Measurements by dilatometry using an absolute dilatometer make it possible in particular to note the shrinkage of the parts as a function of the temperature corresponding to the dimensional variations of the parts which represent an important criterion of quality in the use of cores in precision foundry.
Thus, parts according to the second composition II previously described were subjected either to a thermal cycle of temperature rise to 1500 ° C. which corresponds to the temperature reached by the cores during the casting of superalloys, as represented by curve 1 in FIG. 1, or in a thermal cycle comprising an intermediate level of 5 hours at 1200 ° C., as represented by curve 2 in FIG. 1 which represents the temperature in degrees on the ordinate and the time in minutes on the abscissa.

• la courbe 4, sans infiltration
• la courbe 5 avec une infiltration par le produit A
• la courbe 6 avec une infiltration par le produit B
• la courbe 7 avec une infiltration par le produit D.

The corresponding variation in shrinkage represented as a percentage on the ordinate is shown by curve 3 in FIG. 2 for a piece of composition II obtained according to a known prior process and subjected to the cycle of curve 1 in FIG. 1.
The curves in FIG. 3 show the variations in comparative withdrawals according to the impregnations carried out for pieces of composition II:

• curve 4, without infiltration
• curve 5 with infiltration by product A
• curve 6 with infiltration by product B
• curve 7 with infiltration by product D.

The tests carried out and the results observed show that the impregnation operation of the pieces of compositions used for the manufacture of ceramic cores for precision foundry using colloidal oxides of the type precursors of silica, alumina or mullite provides a shrinkage measured on parts 2 to 7 times lower at the result of a heat treatment at 1500 ° C than the result obtained for parts not impregnated according to the process prior. The impregnated support sees its mechanical behavior in cold bending increased from 50 to 70%, depending on the impregnation used.

In addition, the method according to the invention avoids a excessive fragility of the nuclei. Impregnation after sintering using an organic resin of the "glue" type previously applied and which causes disadvantages of deformation of the cores during their use can thus be avoided. Satisfactory mechanical properties of resistance of the cores is obtained by the process in accordance with the invention, in particular with regard to impact resistance thermal and mechanical resistance to heat, in particular flexion which is increased from 170% to 230%, depending on the impregnator used.

Claims (10)

1. A process for producing ceramic cores for precision casting comprising a shaping step known per se, such as by hot injection of a thermoplastic paste consisting of a refractory ceramic fraction and an organic fraction into a metal mould followed by a binder-removing step in which the organic core fraction is removed in conditions known per se such as by sublimation or heat degradation, characterised in that the subsequent heat treatment is limited to a very reduced consolidation of the structure of the refractory ceramic fraction of the core providing just sufficient mechanical strength for the core to withstand handling and so as to reduce shrinkage to a minimum and is followed by a step of impregnating the porous structure of the core with a solution consisting of at least one single oxide colloid from the group consisting of colloidal silica and colloidal alumina and of possible additions comprising a number of mixed earths or mixtures of earths and salts, then by a step of removing the liquid part from the impregnating product.
2. A process according to claim 1 wherein the binder-removing step is separate from the consolidating heat treatment.
3. A process according to claim 1 wherein the binder-removing step or elimination of the organic core fraction is performed during the heat treatment step.
4. A process according to any of claims 1 to 3 wherein the liquid part is removed from the impregnating product by drying.
5. A process according to claim 4 wherein the drying is performed in vacuo.
6. A process according to claim 4 wherein the drying is performed by baking at 70°C for 24 hours.
7. A process according to any of claims 1 to 6 wherein the heat treatment is performed at a temperature between 1000°C and 1150°C for a period between 1 and 5 hours.
8. A process according to any of claims 1 to 7 wherein the impregnating solution used is colloidal boehmite and the impregnation time is 24 hours.
9. A process according to any of claims 1 to 8 wherein before the casting of the metal the core is preheated, the said heat treatment producing a reaction between the impregnation residues and the refractory ceramic core part so as to strengthen the core and ensure good mechanical behaviour at the high casting temperatures.
10. A process according to claim 9 wherein the preheating is performed at a temperature between 1000°C and 1100°C for a period between 1 and 4 hours.

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