Process for the manufacture of ceramic molds to be used for the preparation of unidirectional and single crystal metal components

Abstract: In the manufacture of ceramic molds starting from aqueous suspensions, the combination of products already known in the field of ceramic molds (although not bound to the production of unidirectional and single crystal metal components), together with a particular coating sequence and with particular drying and sintering proceedings, allows one to considerably reduce the overall manufacturing time, in particular the drying time, as well as to improve the quality of the castings.

Description: BACKGROUND OF THE INVENTION The present invention refers to an improved process for the manufacture of ceramic molds to be used for the preparation of unidirectional and syngle crystal metal components and particularly for the manufacure of ceramic molds starting from aqueous suspensions. In the field of precision investment casting (lost wax process), the usual method resides in the preparation of a wax pattern of the component to be manufactured, in the investment (coating) of the pattern with a certain number of ceramic layers, as to form a "green" mold, in the removal of the wax, thus allowing the formation of a cavity having exactly the size and the shape of the component to be manufactured, in the sintering of the empty mold and finally in the filling of the mold with the desired alloy. Such a basic method is showing several critical aspects, according to the kind of the desired product. In the case, for instance, of high quality components bound to heavy uses, like air turbines showing high speed and powerful thrust, there were developed single crystal or unidirectional polycrystalline structures (namely solidified in a directional way). Such components require very precise shapes, which can be obtained only at the cost of the greatest care both in designing and in manufacturing; a big importance is bound to the ceramic molds, which have to undergo the exposure to the molten metal for a considerable period of time, while maintaining the shape within a close tolerance, without breaking and without any chemical exchange with the molten metal. That's why only selected ceramics and thoroughly planned production cycles can be employed for the manufacture of such molds, with a remarkable rise in the cost of products which are already per se expensive. Until now, there was obtained a fair saving, as to the output rate of the ceramic molds, by using alcoholic solutions which get quickly dry. However, the safety and health hazard, bound to the alcoholic suspensions, as well as the possibility of environmental damages, caused by alcohol and by the organic solvents bound to its use, are strongly pushing towards the legal suppression of the alcoholic suspensions. Consequently, for the manufacture of ceramic molds there were proposed different aqueous suspensions, which, although allowing products having a high quality, show the big drawback of requiring an excessively long drying time, up to more than 24 h after each layer and up to more than two weeks for the final drying, thus leading to low productivities and to an undesired increase of the cost. Different solutions were proposed for such a further problem, for instance solutions concerning accelerated drying methods, like the one described in the fifth participation at the XXI European Conference on the Precision Casting "Manufacture of water based molds in one day" by Challinor, Williams, Lang and McCallum, which makes use of a turbulent air flow, in order to grant a uniform treatment even in the recesses together with a high flow rate of the water under removal, in order to enhance the drying effect. Other solutions (of the problem) refer to the use of an alumina suspension, like the ones described in U.S. 4,024,300, where it is stated that the use of a mixture of two or more granulometries of fused alumina unexpectedly leads to an excellent material for the first required layers, as to have proper inner walls in the mold. Such solutions of the problem, however, are likely to be complex and expensive or alternatively only partial and are not indicated as fit for the manufacture of unidirectional and single crystal components. There are other materials, purposely planned for reducing the drying time of green ceramic molds, which are not fit, however, for the manufacture of unidirectional and single crystal components. For instance, the products commercially known as PRIMCOTE® and FASCOTE® refer to silica based binding agents and refractories, which cannot therefore be exploited in these manufactures, because silica is too reactive towards the molten alloys for unidirectional and single crystal components. Moreover, the binding agents and the refractories, when in combination, appear as a formulation particularly fit for manufacturing quick drying coatings; the use therefore of the ones (binding agents or refractories) without the others would substantially reduce or eliminate whatsoever interesting benefit, as reported in the corresponding instructions as well as in the Foundry Trade Journal, April 21, 1989, pp 292-293. US-A-4,131,475 discloses ceramic molds obtained by using alumina of specific grain sizes in the baths and for spreading, which could produce fragile shells which crack on dewaxing. "Foundry Trade Journal" Vol. 165, No. 3434, 7 June 1991, Redhill, Surrey, GB, pages 402-408 refers to the production of shells for manufacturing equiaxed components, in that fused silica as refractory is used. There is thus still a strong need of short manufacturing cycles, which do not employ alcohol for the ceramic molds bound to the manufacture of unidirectional and single crystal metal components. DISCLOSURE According to the present invention, the Applicant did find that the combination of a fine grained binding agent, based on colloidal silica, with fused alumina, as the refractory, does work in an excellent way. According to the invention, the Applicant did find that is proper to use first the following primary bath: a binding agent, based on water, containing 25-35% by weight of colloidal silica, having a granulometry lower than 15 micrometer, preferably lower than 10 micrometer, and less than 0.6% by weight of Na₂O, with a 25°C viscosity lower than 10 mPa.s and a pH between 9.6 and 10.5; a refractory based on fused alumina, having a granulometry lower than 200 mesh; the required wetting agents and foam suppressing agents; and to use then the following secondary bath: a binding agent based on water, containing 20-30% by weight of a colloidal silica having a granulometry lower than 15 micrometer, preferably lower than 10 micrometer, and less than 0.50% by weight of Na₂O, with a 25°C viscosity lower than 10 mPa.s and a pH between 9.2 and 10.5; a refractory based on fused alumina, having a granulometry lower than 200 mesh; the required wetting agents and foam suppressing agents. The primary bath contains from 2.2 to 2.8 kg of alumina per litre of binding agent, has a pH between 9.4 and 10.6, shows a viscosity (Ford B4 reservoir or cup) between 85 and 90 s and may be exploited at a temperature from 20 to 24°C. The secondary bath contains from 1.5 to 2.2 kg of alumina per litre of binding agent, has a pH between 9.4 and 10.6, shows a viscosity (Ford B4 reservoir or cup) between 25 and 30 s and may be exploited at a temperature from 20 to 24 °C. According to the present invention, a suitable binding agent for the primary bath is the one traded as PRIMCOTE® and a suitable binding agent for the secondary bath is the one traded as FASCOTE®. An additional feature of the process according to the invention resides in the steps used during the manufacture of the ceramic molds. A wax pattern of the component part to be produced undergoes at least one immersion into the primary bath, each immersion being followed by spreading with fused alumina, having a granulometry between 0.125 and 0.250 mm, and by a drying step lasting from 1.5 to 4 h at a temperature between 18 and 22°C and with a relative humidity between 45 and 55%. The number of the immersions into the primary bath is at least three, preferably from four to seven. The mold thus obtained, is furtherly treated with a plurality of immersions into the secondary bath, each immersion being followed by spreading with fused alumina, having granulometry between 0.25 and 1.00 mm, and by a drying step lasting from 1.5 and 4 h at a temperature between 18 and 22 °C and with a relative hummidity between 45 and 55%. The number of the immersions into the secondary bath is at least three, preberably from four to seven. After the last spreading with fused alumina, the mold undergoes a last immersion into the secondary bath, followed by a drying step, lasting from 1.5 to 4 h, and by a final step (prolonged drying) lasting from 50 to 90 h, both the steps being carried out at a temperature between 18 and 22°C, with a relative humidity between 45 and 55%. The drying step after each immersion is characterized by a weight loss at least equal to 7% of the weight obtained from the respective immersion (preferably at least 9%). The green mold is then dewaxed, according to known techniques, and subsequently sintered. It can undergo a first optional treatment at a temperature between 1000 and 1100°C, according to a heating rate between 50 and 120 °C/h, followed by a cooling in the switched off oven. After such a treatment, or in its stead, the mold undergo a sintering treatment at a temperature from 1400 to 1550°C, for a time between 3 and 6 h, with a heating rate (gradient) between 100 and 200 °C/h and with at least two homogenization stops at intemediate temperatures, each lasting from 30 to 120 minutes. At the end of this treatment, the sintered mold is cooled in the switched off oven. DETAILED DESCRIPTION The present invention will be now described in greater detail, referring to a few experimental works carried out in order to compare the molds prepared according to the present invention with reference molds (blanks) prepared starting from a classical water based suspension. The description of such works is supplied for illustrating purposes and does not limit in any way the scope of the invention. A few binding agents were prepared according to the following Table 1: BATH Blank According to the invention Primary Secondary Colloid SiO₂ (% b.w.) 30 30 25 Granulometry (micrometer) 25 10 10 pH (20 °C) 10 10.6 10.6 Viscosity (m.Pa.s) to 20 °C 2.5 6.7 5.5 Na₂O (% b.w.) 0.30 0.48 0.41 Specific gravity (g/cm³) 1.20 1.18 1.15 The binding agents hereinabove were loaded with a fused alumina having a granulometry lower than 200 mesh, in amounts equal to 2.2 kg per litre of binding agent (as to the blank) and equal to 2.5 and 1.8 Kg per litre of binding agent respectively as to the primary bath and secondary bath according to the invention. There were then prepared a few wax patterns, subdivided into two groups, for unidirectional and single crystal components. The first group (blank) was coated with eight layers in the reference bath, whereas the second group was coated with three layers in the primary bath and with five layers in the secondary bath. Both the groups were spreaded with fused alumina, according to a a sequence comprising three fine spreadings (granulometry between 0.125 and 0.250 mm), two middle spreadings (granulometry between 0.250 and 0.50 mm) and three gross spreadings (granulometry between 0.50 and 1.00 mm). The last gross spreading was followed by an immersion into the bath. Each layer was dried under controlled conditions, with a relative humidity equal to 50% (at 21°C) and with an air change, in the drying chamber, equal to 12 change/h. The drying time, for the blank, was 24 h between layer and layer, and 15 days for the final drying, thus accounting for an overall time of 24 days, whereas the group according to the invention required 3 h between layer and layer and 3 days for the final drying, thus accounting for an overall time slightly higher than four days. A first advantage coming from the invention is residing in a 6 times shortage of the manufacturing time for the green molds. The average weight loss of the molds according to the present invention, between the second and the seventh layer, was 24 g/layer, against an average weight increase (coming from suspension accumulation and spreading material) equal to 260 g/layer, namely an average weight loss equal to 9.2%. From the other side, the average weight loss of the blank group was considerably lower, about 4%. A dry green mold according to the invention had a weight of 2.06 kg and a thickness of 4.6 mm, whereas an equivalent comparison mold had a weight of 2.56 kg and a thickness of 5.5 mm. Both the molds were steam-dewaxed in an autoclave according to a known method. Both the groups of molds were then pre-heated from room temperature to 1050°C, according to a heating rate (gradient) of 60°C/h, then cooled in the oven. Finally the molds were sintered at 1500°C for 5 h, according to a heating rate (gradient) of 180°C/h, with two homogenization periods, one hour each, at 600 and at 1200°C respectively, and at last cooled in the oven. In the group of the molds according to the invention it was not found any surface blemish nor any thermal break or distortion, whereas 10% of the comparison group was showing blemishes. The manufacture of turbine blades and of unidirectional and single crystal specimen was representing the final test. The molds were arranged in a casting oven under vacuum at room temperature, brought to 1500°C in 80 minutes then filled with molten alloy and cooled according to the standard methods for unidirectional and single crystal components. After cooling and removal of the ceramic molds, all the produced pieces were checked as to the surface blemishes, as to the shape and as to the size. The unidirectional and single crystal components obtained from the molds according to the present invention did not show any macroscopic surface blemish and had shape and size in conformity with the strict tolerances required by the single crystal components. The pieces obtained from the comparison molds were showing a high faultiness, coming from a breaking of the mold and from shape distortion. It is worth while to underline that all along the instant description the term "wax" is understood as defining whatsoever material fit for the preparation of patterns, like for instance waxes, paraffines, polystyrene and so on.

Claims: Process for the manufacture of ceramic molds for producing unidirectional and single crystal metal components comprising the steps of shaping a wax pattern of the components to be manufactured, coating such pattern with a plurality of layers of ceramic material by means of a sequence of immersions into aqueous suspensions comprising said ceramic material, at least a portion of said immersion being followed by a spreading with dry ceramic powder, drying the thus obtained mold and sintering it at a high temperature; said process being characterized in that: (i) a primary and a secondary bath are prepared, said primary bath comprising: (a) a water based binding agent containing 25-35% b.w. of a colloidal silica having a granulometry lower than 15 micrometers, containing less than 0.6% b.w. of Na₂O and showing a 25°C viscosity lower than 10 mPa.s and a pH between 9.6 and 10.5; (b) fused alumina having a granulometry lower than 200 mesh; and (c) the required wetting agents and foam suppressing agents; said primary bath containing from 2.2 to 2.8 kg of alumina per litre of binding agent, having a pH between 9.4 and 10.6 and a viscosity (Ford B4 reservoir) between 85 and 90 s; and said secondary bath comprising: (a') a water based binding agent containing from 20 to 30% b.w. of a colloidal silica having a granulometry lower than 15 micrometers, containing less than 0.5% b.w. of Na₂O and showing a 25°C viscosity lower than 10 mPa.s and a pH between 9.2 and 10.5; (b') fused alumina having a granulometry lower than 200 mesh; and (c') the required wetting agents and foam suppression agents; said secondary bath containing from 1.5 to 2.2 kg of alumina per litre of binding agent, having a pH between 9.4 and 10.6 and a viscosity (Ford B4 reservoir or cup) between 25 and 30 s; (ii) the pattern is coated by subjecting it to at least one immersion into the primary bath, each immersion being followed by spreading with fused alumina having a granulometry between 0.125 and 0.250 mm and followed by a drying step lasting from 1,5 to 4 h at a temperature between 18 and 22°C in an athmosphere having a relative humidity between 45 and 55%, the number of immersions into the secondary bath being at least three; (iii) the mould thus obtained is then treated with a plurality of immersions into the secondary bath, each immersion being followed by spreading with fused alumina, having granulometry between 0.25 and 1.00 mm, and by a drying step lasting from 1.5 and 4 h at a temperature between 18 and 22°C in an athmosphere having a relative humidity between 45 and 55%, the number of immersions into the secondary bath being at least three; (iv) after the last spreading with fused alumina, the mold undergoes a last immersion into the secondary bath, followed by a drying step, lasting from 1.5 to 4 h, and by a final step lasting from 50 to 90 h, both such steps being carried out at a temperature between 18 and 22°C, in an athmosphere having a relative humidity between 45 and 55%; (v) then the mold is dewaxed. Process according to claim 1 in which the primary and secondary bath are exploited at a temperature from 20 to 24°C. Process according to claim 1 in which the colloidal silica has a granulometry lower than 10 micrometers. Process according to claim 1 in which the number of immersions into the primary bath is from four to seven. Process according to claim 1 in which the number of immersions into the secondary bath is from four to seven. Process according to claim 1 in which, after the dewaxing, the mold is termally treated at a temperature between 1000 and 1100°C, according to a heating rate between 50 and 120°C/h, followed by a cooling in the switched oven. Process according to claim 1 in which after the dewaxing the mold is subjected to a sintering treatment at a temperature from 1400 to 1550°C, for a time between 3 and 6 h, with a heating rate between 100 and 200°C/h with at least two homogenization stops at intermediate temperatures, each lasting from 30 to 120 minutes. Process according to claim 1 in which after the dewaxing the mold is termally treated at a temperature between 1000 and 1100°C, according to a heating rate between 50 and 120°C/h, followed by a cooling in the switched oven; then the mold is subjected to a sintering treatment at a temperature from 1400 to 1550°C, for a time between 3 and 6 h, with a heating rate between 100 and 200°C/h with at least two homogenization stops at intermediate temperatures, each lasting from 30 to 120 minutes. Aqueous primary bath to be used for the manufacture of ceramic molds for producing unidirectional and single crystal metal components, said bath comprising: (a) a water based binding agent containing 25-35% b.w. of a colloidal silica having a granulometry lower than 15 micrometers, containing less than 0.6% b.w. of Na₂O and showing a 25°C viscosity lower than mPa.s and a pH between 9.6 and 10.5; (b) fused alumina having a granulometry lower than 200 mesh; and (c) the required wetting agents and foam suppressing agents; said primary bath containing from 2.2 to 2.8 kg of alumina per litre of binding agent, having a pH between 9.4 and 10.6 and a viscosity (Ford B4 reservoir) between 85 and 90 s. Aqueous secondary bath to be used for the manufacture of ceramic molds for producing unidirectional and single crystal metal components, said bath comprising: (a') a water based binding agent containing from 20 to 30% b.w. of a colloidal silica having a granulometry lower than 15 micrometers, containing less than 0.5% b.w. of Na₂O and showing a 25°C viscosity lower than 10 mPa.s and a pH between 9.2 and 10.5; (b') fused alumina having a granulometry lower than 200 mesh; and (c') the required wetting agents and foam suppression agents; said secondary bath containing from 1.5 to 2.2 kg of alumina per litre of binding agent, having a pH between 9.4 and 10.6 and a viscosity (Ford B4 reservoir) between 25 and 30 s.

Inventors: BARNETT STEPHEN O, POCCI DANTE, BARNETT, STEPHEN O., POCCI, DANTE
Applicants: SVILUPPO MATERIALI SPA, CENTRO SVILUPPO MATERIALI S.P.A.
Application year: 1992
Main classification: B22C1
IPC codes: B22C9, C04B35, B22C1