EP0198290B1 - Method of casting aluminium alloys - Google Patents

Method of casting aluminium alloys Download PDF

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Publication number
EP0198290B1
EP0198290B1 EP86104247A EP86104247A EP0198290B1 EP 0198290 B1 EP0198290 B1 EP 0198290B1 EP 86104247 A EP86104247 A EP 86104247A EP 86104247 A EP86104247 A EP 86104247A EP 0198290 B1 EP0198290 B1 EP 0198290B1
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Prior art keywords
salt mixture
wall
casting
mould
alkali
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EP86104247A
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German (de)
French (fr)
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EP0198290A2 (en
EP0198290A3 (en
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Friedrich Dr.-Ing. Behr
Heinrich Dipl.-Ing. Ballewski
Wolfgang Grossmann
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ThyssenKrupp Technologies AG
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Thyssen Industrie AG
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
    • B22D21/002—Castings of light metals
    • B22D21/007—Castings of light metals with low melting point, e.g. Al 659 degrees C, Mg 650 degrees C
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C3/00—Selection of compositions for coating the surfaces of moulds, cores, or patterns
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/20—Measures not previously mentioned for influencing the grain structure or texture; Selection of compositions therefor

Definitions

  • the invention relates to a method for casting aluminum alloys which contain more aluminum than the eutectic with the alloy partners corresponds to in order to achieve improved strength values by reducing the distances between the dendrite arm distances formed by solidification.
  • the technological properties of the aluminum alloys in particular the strength, the yield strength and the elongation, can be improved by fine-grain of the casting.
  • the strength properties of aluminum alloys are directly dependent on the number and fineness of the smallest possible dendrite arm spacing - the secondary dendrite arm spacing. According to Foundry, June 1963, pp. 78-82, the fine-grained nature of aluminum and aluminum-based alloys is improved by adding a pre-alloy to the aluminum alloy before casting, which is used as heterogeneous nuclei e.g. Contains titanium diboride.
  • the refractory material for dipping is added in the same way as the step just mentioned as germs, noble metals or reducible metal oxides.
  • the US-PS 3 157 926 works in the same way and names nickel-III-oxide, cobalt-II-oxide and -III-oxide and nickel-cobalt oxyhydrate as seeds.
  • the seeds of the above-mentioned US patents are not effective for reducing the secondary dendrite arm spacings and thus for improving the strength properties of hypoeutectic aluminum alloys and are not proposed by the US patents. No suitable seeds have yet been found for aluminum base melts, which are suitable for embedding in the mold wall to produce a fine-grained casting.
  • DE-PS 963 642 teaches to influence the surface of castings by adding additives to the molding material and alloys the surface with lead released by chemical reaction with the casting metal.
  • protective materials which reduce the molding material are added according to DE-AS 12 71 909, the melting point of which lies between the casting temperature and the firing temperature of the casting mold.
  • DE-AS 12 65 356 discloses to add a hydrogen-releasing metal hydride to the casting mold in its mold cavity.
  • the hydrogen is said to be the oxide skin of e.g. Reduce incoming iron as a casting material and thus increase the flowability.
  • the aluminum oxide of the cast skin of aluminum cannot be reduced with hydrogen.
  • the presence of hydrogen when aluminum alloys are cast is also highly undesirable because gas bubbles form.
  • German cited documents only disclose that the surface of the casting can be influenced by means of substances introduced into the mold.
  • the object is achieved in a method of the type mentioned in that the inner wall of the mold is produced with numerous roughness in the micro range, that the inner wall is provided with a thin layer of a salt mixture, the cations of the salt mixture predominantly from those of the alkali and / or alkaline earth metals and their anions consist predominantly of those of the halogens, and that the liquidus temperature of the salt mixture is set lower than the casting temperature of the aluminum alloy.
  • the transport of broken dendrite arms through the melt into central areas of the casting in addition to the growth of the fine dendritic solidification front, further improves the fine-grained interior or also the area of the inner mold wall, which is not very effective, with a sufficient number of species Germs supplied.
  • the ceramic shape the roughness and pores after drying and firing are provided with the thin salt layer, the liquidus temperature of which is lower than the casting temperature of the alloy, the salt mixture as a thin, film-like layer, which liquefies when the alloy is poured, evenly into the depressions of the alloy Spread roughness and does not prevent the cast aluminum from penetrating into the pores due to the thin layer.
  • the salts whose cations predominantly consist of alkali and / or alkaline earth and whose anions consist predominantly of halogens reliably bring about a reduction in the dendrite arm distances.
  • the invention can be advantageously configured as follows.
  • the invention recommends producing as many, but more than 10 5 roughnesses per cm 2 of the mold inner wall, the ratio of depth to diameter or depth to gap width of cracks of which is greater than 1 to 3. Roughness in the form of pores, fracture edges, cracks and crevices and, preferably, funnel-shaped depressions formed by sequences of microcrystalline fracture edges, which face the casting with the enlarging opening, are recommended.
  • Geometrically particularly favorable roughnesses are e.g. obtained by arranging a ceramic material which tends to break the shell in particularly finely ground grains with a diameter of predominantly less than 10 ⁇ m on the inner wall of the mold. This happens e.g. by "dipping", i.e. Dipping the wax model in a slip on an aqueous or alcoholic basis, which also contains a binder, e.g. based on silicon dioxide. It is also possible to use other ceramic powders which have suitable pore sizes and / or a suitable fine grain because of the way in which they are produced.
  • the salt mixture additionally contains one or more alkali and / or alkaline earth pseudohalogen compounds or also organic salts of the alkali and / or alkaline earth metals, an improved removal of oxygen residues, in particular in the pores of the mold, can be achieved.
  • Suitable alkali or alkaline earth pseudohalogen compounds are cyanate, cyanide, rhodanite, hexa- or tetracyano compounds, amines or amides or the like, the atkaticyanides, cyanates and rhodanides chemically related compounds.
  • the removal of oxygen radicals is effective not only in casting in air, but also for casting in a vacuum at about 10 -2 Torr.
  • this additional salt in about 2-40% by weight of the total salt mixture.
  • the amount of salt added is expediently limited in such a way that the gas released during casting does not form bubbles on the surface of the casting, the released gas does not contain any molecular hydrogen and, furthermore, the salt is not stable under the pressure and temperature conditions of the preheating of the mold shell Has hydrates.
  • the inner wall and its pore entrances can be provided with different salts simultaneously and in an even, finest distribution and also apply finely ground, slurried salts that are insoluble or insufficiently soluble to the inner wall.
  • the intimate mixture of the various salts leads to rapid liquefaction.
  • the preheating of the mold which takes place before the casting, in order to improve the flow of the casting, serves at the same time to dry the applied salts. Water and / or alcohol are suitable as solvents.
  • salt mixture consisting predominantly of sodium-lithium chloride fluoride is used to coat the inner wall of the ceramic mold, the melting point of which is below 650 ° C., enables the salt mixture to be liquefied very quickly.
  • these salt mixtures there are deep-melting mixtures of reciprocal salt pairs with low hydrostability of the individual salts, especially when compared to the salts of potassium.
  • An aqueous and / or alcoholic solution of LiCl, NaF, NaCl and Na 4 Fe (CN) e is particularly suitable. No premelting and grinding of the salt mixture is necessary.
  • Sodium fluoride is water soluble. Due to ion exchange with the lithium chloride, fine-grained lithium fluoride precipitates after a few hours.
  • the solution and / or slurry of the salt mixture contains a dispersing agent, fine-grained insoluble salts which separate out from the solution after some time, such as lithium fluoride, can be kept in suspension and a uniform distribution of the salt mixture on the inner wall of the mold can be promoted.
  • Suitable dispersants are e.g. Methyl cellulose.
  • the wax grapes were given a first coating by dipping into a slurry consisting of an aqueous binder, finely ground ( ⁇ 30 11 m) zirconium silicate and silicon dioxide as filler and sanded with coarse zirconium silicate powder. After drying, a further six layers were used up by dipping, sanding and drying in the usual way, so that ceramic molds with wall thicknesses of approximately 8 mm were formed. The molds were waxed under pressure in an autoclave and then fired at approx. 800 ° C.
  • the solution was poured into the ceramic molds one after the other, immediately poured out again and filtered in order to remove any ceramic granules that had been rinsed out.
  • the ceramic molds were then heated to about 470 ° C, used in the warm state in the vacuum casting, and at about 250 ° C mold temperature with the aluminum alloy GAISi7Mg0.6 at a melt temperature of 700 ° C at 10- 2 Torr filled.
  • the aluminum melt was premelted in air, then degassed with a flushing gas mixture and then degassed in vacuo.
  • the wax models of an aircraft structural part with an average wall thickness of 5 mm and at the nodes with a wall thickness of 15 mm were assembled into wax grapes according to the method described in exemplary embodiment 1, covered with the ceramic shell, waxed under pressure in the autoclave and then at approx. Fired at 800 ° C.
  • the solution was poured into the ceramic molds one after the other, immediately poured out again and filtered in order to remove any ceramic granules that had been rinsed out.
  • the ceramic molds were then heated to approx. 470 ° C, inserted into the vacuum casting machine when hot and at approx. 250 ° C mold temperature with the aluminum alloy GAISi7MgO, 6 at a melt temperature of 700 ° C, at 10- 2 Torr filled.
  • the aluminum melt was premelted in air, then degassed with a purging gas mixture and subsequently degassed in vacuo.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Mold Materials And Core Materials (AREA)

Description

Die Erfindung betrifft ein Verfahren zum Vergießen von Aluminium-Legierungen, die mehr Aluminium enthalten, als dem Eutektikum mit den Legierungspartnern entspricht, zur Erzielung verbesserter Festigkeitswerte durch Verringerung der Abstände der sekundär durch Erstarrung gebildeten Dendritenarmabstände.The invention relates to a method for casting aluminum alloys which contain more aluminum than the eutectic with the alloy partners corresponds to in order to achieve improved strength values by reducing the distances between the dendrite arm distances formed by solidification.

Es ist bekannt, daß man die technologischen Eigenschaften der Aluminium-Legierungen, insbesondere die Festigkeit, die Streckgrenze und die Dehnung durch Feinkörnigkeit des Gußstückes verbessern kann. Bakanntlich sind die Festigkeitseigenschaften bei Aluminium-Legierungen direkt abhängig von der Zahl und der Feinheit der möglichst kleinen Dendritenarmabstände-den sekundären Dendritenarmabständen. Nach Foundry, Juni 1963, S. 78-82, wird die Feinkörnigkeit von Aluminium- und Aluminium-Basis-Legierungen dadurch verbessert, daß in die Aluminium-Legierung vor dem Abguß eine Vorlegierung zugeführt wird, die als heterogene Keime z.B. Titan-Diborid enthält.It is known that the technological properties of the aluminum alloys, in particular the strength, the yield strength and the elongation, can be improved by fine-grain of the casting. The strength properties of aluminum alloys are directly dependent on the number and fineness of the smallest possible dendrite arm spacing - the secondary dendrite arm spacing. According to Foundry, June 1963, pp. 78-82, the fine-grained nature of aluminum and aluminum-based alloys is improved by adding a pre-alloy to the aluminum alloy before casting, which is used as heterogeneous nuclei e.g. Contains titanium diboride.

Aus der US-PS 3 259 948 ist es bekannt, die Feinkörnigkeit von Gußstücken aus Kobalt- oder NickelBasis-Legierungen durch Einbringen von Keimen in die innere Gießformoberfläkhe zu verbessern. Diese Keine, z.B. Kobaltaluminat und Kobaltsilikat werden nach der US-PS auf das Wachsmodell auftragen und durch das anschließende Eintauchen des Wachsmodelles in einem Schlicker aus feuerfestem Formmaterial und Ausschmelzen des Wachsmodelles zum Teil in die innere Formoberfläche eingebettet (Dippen). Nach der US-PS 3 019 497 werden, ebenfalls zum Zwecke der Kornverfeinerung, Keime mit dem Feuerfestmaterial für das Dippen gemischt und auf das Wachsmodell aufgetragen. Nach der US-PS 3 158 912 werden dem Feuerfestmaterial für das Dippen in gleicher Weise die in der eben genannten Schritt als Keime, Edelmetalle oder reduzierbare Metalloxyde beigefügt. Die US-PS 3 157 926 arbeitet in gleicher Weise und nennt Nickel-III-Oxyd, Kobalt-II-Oxyd und -III-Oxyd und Nickel-Kobaltoxy-hydrat als Keime. Die Keime der genannten US-Patentschriften sind für die Verringerung der sekundären Dendritenarmabstände und damit zur Verbesserung der Festigkeitseigenschaften von untereutektischen Aluminium-Legierungen nicht wirksam und werden von den US-Patentschriften nicht vorgeschlagen. Für Aluminium-Basisschmelzen sind noch keine entsprechenden Keime, die sich zur Einbettung in die Formwand zur Erzeugung eines feinkörnigen Gußstücks eignen, gefunden werden. Nach Foundry 1963 wird die Feinkörnigkeit von Aluminium-Legierungen durch Zugabe der Keime mit der Vorlegierung verbessert. Dies ist jedoch bezüglich der Zuverlässigkeit und des möglichst geringen sekundären Dendritenarmabstandes unbefriedigend. Die DE-PS 963 642 lehrt, die Oberfläche von Gußkörpern durch Zusätze zum Formwerkstoff zu beeinflussen und legiert die Oberfläche mit durch chemische Reaktion mit dem Gießmetall freigesetzten Blei. Zur Vermeidung der Randentkohlung werden nach der DE-AS 12 71 909 dem Formstoff reduzierende Schtuzstoffe zugesetzt, deren Schmelzpunkt zwischen der Gießtemperatur und der Brenntemperatur der Gießform liegt. Die DE-AS 12 65 356 offenbart, die Gießform in ihrem Formhohlraum mit einem Wasserstoff freisetzenden Metallhydrid zu versetzen. Der Wasserstoff soll die Oxydhaut von z.B. einlaufendem Eisen als Gießmaterial reduzieren und damit die Fließfähigkeit erhöhen. Bekanntlich ist das Aluminiumoxyd der Gußhaut von Aluminium nicht mit Wasserstoff reduzierbar. Die Gegenwart von Wasserstoff bei Abguß von Aluminium-Legierungen ist zudem in höchstem Maße unerwünscht, da Gasblasenbildung einsetzt.It is known from US Pat. No. 3,259,948 to improve the fine-grain nature of castings made of cobalt or nickel-based alloys by introducing germs into the inner mold surface. These none, e.g. According to the US PS, cobalt aluminate and cobalt silicate are applied to the wax model and, by subsequently immersing the wax model in a slip made of refractory molding material and melting the wax model, it is partially embedded in the inner mold surface (dipping). According to US Pat. No. 3,019,497, germs are also mixed with the refractory material for dipping and applied to the wax model, likewise for the purpose of grain refinement. According to US Pat. No. 3,158,912, the refractory material for dipping is added in the same way as the step just mentioned as germs, noble metals or reducible metal oxides. The US-PS 3 157 926 works in the same way and names nickel-III-oxide, cobalt-II-oxide and -III-oxide and nickel-cobalt oxyhydrate as seeds. The seeds of the above-mentioned US patents are not effective for reducing the secondary dendrite arm spacings and thus for improving the strength properties of hypoeutectic aluminum alloys and are not proposed by the US patents. No suitable seeds have yet been found for aluminum base melts, which are suitable for embedding in the mold wall to produce a fine-grained casting. According to Foundry 1963, the fine grain of aluminum alloys is improved by adding the germs with the master alloy. However, this is unsatisfactory in terms of reliability and the smallest possible secondary dendrite arm spacing. DE-PS 963 642 teaches to influence the surface of castings by adding additives to the molding material and alloys the surface with lead released by chemical reaction with the casting metal. To avoid edge decarburization, protective materials which reduce the molding material are added according to DE-AS 12 71 909, the melting point of which lies between the casting temperature and the firing temperature of the casting mold. DE-AS 12 65 356 discloses to add a hydrogen-releasing metal hydride to the casting mold in its mold cavity. The hydrogen is said to be the oxide skin of e.g. Reduce incoming iron as a casting material and thus increase the flowability. As is known, the aluminum oxide of the cast skin of aluminum cannot be reduced with hydrogen. The presence of hydrogen when aluminum alloys are cast is also highly undesirable because gas bubbles form.

Die deutschen genannten Schriften offenbaren lediglich, daß man das Gußstück mit Hilfe von in die Form eingebrachten Stoffen an seiner Oberfläche beeinflussen kann. Das Problem der Erzielung einer verbesserten Feinkörnigkeit, insbesondere der verringerten sekundären Dendritenarmabstände untereutektischer Aluminium-Legierungen wird dabei weder angesprochen, noch ist est mit den darin genannten Maßnahmen lösbar.The German cited documents only disclose that the surface of the casting can be influenced by means of substances introduced into the mold. The problem of achieving an improved fine grain, in particular the reduced secondary dendrite arm spacing of hypoeutectic aluminum alloys, is neither addressed here, nor is it solvable with the measures mentioned therein.

Es ist Aufgabe der Erfindung, die Feinkörnigkeit von Aluminium-Legierungen und insbesondere auch die Zuverlässigkeit bei deren Einstellung wesentlichen zu verbessern.It is an object of the invention to substantially improve the fine-grained nature of aluminum alloys and, in particular, the reliability when adjusting them.

Die Aufgabe wird bei einem Verfahren der eingangs genannten Gattung dadurch gelöst, daß die Innenwand der Gießform mit zahlreichen Rauhigkeiten im Mikrobereich hergestellt wird, daß die Innenwand mit einer dünnen Schicht einer Salzmischung versehen wird, wobei die Kationen der Salzmischung überwiegend aus denen der Alkali- und/oder Erdalkali-Metalle bestehen und deren Anionen überwiegend aus denen der Halogene bestehen, und daß die Liquidustemperatur der Salzmischung tiefer eingestellt wird als die Abgußtemperatur der Aluminium-Legierung.The object is achieved in a method of the type mentioned in that the inner wall of the mold is produced with numerous roughness in the micro range, that the inner wall is provided with a thin layer of a salt mixture, the cations of the salt mixture predominantly from those of the alkali and / or alkaline earth metals and their anions consist predominantly of those of the halogens, and that the liquidus temperature of the salt mixture is set lower than the casting temperature of the aluminum alloy.

Mit diesem Verfahren wurden gegenüber den bekannten Verfahren wesentlichen höhere Festigkeits-und Dehnungswerte erreicht. Dies ist nach Ansicht der Anmelder in darauf zurückzuführen, daß bei den erfindungsgemäßen Verfahren die Metallschmelze mit Hilfe der Salzmischung, die nach Abguß der Metallschemlze möglichst dünnflüssig sein sollte, auch feinste Poren und andere Rauhigkeiten der Formschalen-Innenwand auffüllt. Das Metall kühlt dort zuerst ab und erstarrt, so daß viele Kristallisationszentren in der schon auf Grund der durch die Geometrie der Formschale vorgegebenen Richtung der Wärmeabfuhr in einer kristallin günstigen Ausrichtung mit dem noch flüssigen Gußstück in Verbindung stehen und als arteigener Keim wirksam werden können. Darüber hinaus wird wohl auch durch Transport von abgebrochenen Dendritenarmen durch die Schmelze in zentrale Bereiche des Gußstückes hinein zusätzlich zu dem Heranwachsen der feinen dendritischen Erstarrungsfront die Feinkörnigkeit im Inneren noch weiter verbessert oder auch Bereich der Forminnenwand, die wenig wirksam wird, mit einer genügenden Anzahl arteigner Keime versorgt. Dadurch, daß die keramische Form mit den Rauhigkeiten und Poren nach dem Trocknen und Brennen mit der dünne Salzschicht versehen wird, deren Liquidustemperatur geringer ist als Abgußtemperatur der Legierung, kann sich die Salzmischung als dünne, filmartige Schicht, die sich bei Eingießen der Legierung verflüssigt, gleichmäßig auch in die Vertiefungen der Rauhigkeiten hinein ausbreiten und hindert wegen der dünnen Schicht das eingegossene Aluminium auch nicht am Eindringen in die Poren. Die Salze, deren Kationen überwiegend aus Alkali und/oder Erdalkali und deren Anionen überwiegend aus Halogenen bestehen, bewirken nach Versuchen der Anmelderin zuverlässig eine Verringerung der Dendritenarmabstände.With this method, significantly higher strength and elongation values were achieved compared to the known methods. In the opinion of the applicants, this can be attributed to the fact that, in the processes according to the invention, the metal melt fills even the finest pores and other roughnesses of the inner shell of the mold with the aid of the salt mixture, which should be as thin as possible after the metal melt has been poured off. The metal cools there first and solidifies, so that many crystallization centers in the direction of heat dissipation, which is already predetermined by the geometry of the molded shell, are in a crystalline favorable direction in connection with the still liquid casting and can act as a native germ. In addition, the transport of broken dendrite arms through the melt into central areas of the casting, in addition to the growth of the fine dendritic solidification front, further improves the fine-grained interior or also the area of the inner mold wall, which is not very effective, with a sufficient number of species Germs supplied. Because of the ceramic shape the roughness and pores after drying and firing are provided with the thin salt layer, the liquidus temperature of which is lower than the casting temperature of the alloy, the salt mixture as a thin, film-like layer, which liquefies when the alloy is poured, evenly into the depressions of the alloy Spread roughness and does not prevent the cast aluminum from penetrating into the pores due to the thin layer. According to the applicant's attempts, the salts whose cations predominantly consist of alkali and / or alkaline earth and whose anions consist predominantly of halogens reliably bring about a reduction in the dendrite arm distances.

Im einzelnen kann die Erfindung wie folgt vorteilhaft ausgestaltet sein.In particular, the invention can be advantageously configured as follows.

Man benötigt für die Erzielung eines möglichst geringen Unterkühlungsintervalles der Erstarrung beim Aluminium-Guß Kristallisationszentren mit Durchmessern in der Größenordnung von zehn bis einigen hundert Angström, so daß der Geometrie der Rauhigkeiten eine besondere Bedeutung zukommt. Dazu empfiehlt die Erfindung, möglichst viele, aber mehr als 105 Rauhigkeiten pro cm2 der Forminnenwand herzustellen, deren Verhältnis Tiefe zu Durchmesser oder Tiefe zu Spaltbreite von Rissen größer als 1 zu 3 ist. Es empfehlen sich Rauhigkeiten in Form von Poren, Bruchkanten, Rissen und Spalten sowie vorzugsweise von Folgen mikrokristalliner Bruchkanten gebildete, trichterförmige Vertiefungen, die dem Gußstück mit der sich vergrößernden Öffnung zugewandt sind.Crystallization centers with diameters of the order of ten to a few hundred angstroms are required to achieve the lowest possible cooling interval for solidification in aluminum casting, so that the geometry of the roughness is of particular importance. To this end, the invention recommends producing as many, but more than 10 5 roughnesses per cm 2 of the mold inner wall, the ratio of depth to diameter or depth to gap width of cracks of which is greater than 1 to 3. Roughness in the form of pores, fracture edges, cracks and crevices and, preferably, funnel-shaped depressions formed by sequences of microcrystalline fracture edges, which face the casting with the enlarging opening, are recommended.

Geometrisch besonders günstige Rauhigkeiten werden z.B. erhalten, indem man ein keramisches Material, welches zum Muschelbruch neigt, in besonders feingemahlenen Körnungen mit überwiegend weniger als 10 pm Durchmesser an der Formeninnenwand anordnet. Das geschieht z.B. durch "Dippen", d.h. Tauchen des Wachsmodelles in einem Schlicker auf wässriger oder alkoholischer Basis, der auch einen Binder, z.B. auf Basis von Siliciumdioxyd, enthält. Es können auch andere keramische Pulver benutzt werden, die schon auf Grund ihres Herstellungsweges geeignete Porengrößen und/oder eine geeignete Feinkörnigkeit aufweisen.Geometrically particularly favorable roughnesses are e.g. obtained by arranging a ceramic material which tends to break the shell in particularly finely ground grains with a diameter of predominantly less than 10 μm on the inner wall of the mold. This happens e.g. by "dipping", i.e. Dipping the wax model in a slip on an aqueous or alcoholic basis, which also contains a binder, e.g. based on silicon dioxide. It is also possible to use other ceramic powders which have suitable pore sizes and / or a suitable fine grain because of the way in which they are produced.

Dadurch, daß die Salzmischung eine oder mehrere Alkali- und/oder Erdalkali-Pseudohalogenverbindungen oder auch organische Salze der Alkali- und/oder Erdalkali-Metalle zusätzlich enthält, kann eine verbesserte Entfernung von Sauerstoffresten, insbesondere in den Poren der Form erreicht werden. Als Alkali- oder Erdalkali-Pseudohalogenverbindungen eignen sich Cyanat-, Cyanid-, Rhodanit-, Hexa- oder Tetracyano-Verbindungen, Amine oder Amide o.ä, den Atkaticyaniden, -cyanaten und -rhodaniden chemisch verwandte Verbindungen. Die Entfernung der Sauerstoffreste ist nicht nur bei Abguß an Luft wirksam, sondern auch bei Abguß im Vakuum bei ca. 10-2 Torr. Dabei ist es vorteilhaft, diese zusätzlichen Salz in etwa 2-40 Gew.% der Gesamtsalzmischung zuzugeben. Zweckmaßig wird der Salzzusatz in seiner Menge so begrenzt, daß es durch beim Abguß freigesetztes Gas nicht zur Bildung von Blasen an der Gußstückoberfläche kommt, das freigesetzte Gas keinen molekularen Wasserstoff enthält und weiterhin das Salz bei den Druck- und Temperaturbedingungen der Vorwärmung der Formschale keine stabilen Hydrate aufweist.Because the salt mixture additionally contains one or more alkali and / or alkaline earth pseudohalogen compounds or also organic salts of the alkali and / or alkaline earth metals, an improved removal of oxygen residues, in particular in the pores of the mold, can be achieved. Suitable alkali or alkaline earth pseudohalogen compounds are cyanate, cyanide, rhodanite, hexa- or tetracyano compounds, amines or amides or the like, the atkaticyanides, cyanates and rhodanides chemically related compounds. The removal of oxygen radicals is effective not only in casting in air, but also for casting in a vacuum at about 10 -2 Torr. It is advantageous to add this additional salt in about 2-40% by weight of the total salt mixture. The amount of salt added is expediently limited in such a way that the gas released during casting does not form bubbles on the surface of the casting, the released gas does not contain any molecular hydrogen and, furthermore, the salt is not stable under the pressure and temperature conditions of the preheating of the mold shell Has hydrates.

Dadurch, daß die Salzmischung in Form einer Lösung und/oder feindisperser Aufschlämmung durch Ein- und Ausgießen in die gebrachte Form auf deren Innenwand mit anschließender Trocknung aufgebracht wird, kann man die Innenwand und deren Poreneingänge mit verschiedenen Salzen gleichzeitig und in gleichmäßiger, feinster Verteilung versehen und außerdem feinstgemahlene, aufgeschlämmte Salze, die nicht löslich oder zu wenig löslich sind, auf die Innenwand aufbringen. Bei der Abgußtemperatur führt die innige Mischung der verschiedenen Salze zu einer schnellen Verflüssigung. Die Vorwärmung der Form, die vor dem Abguß erfolgt, um das Auslaufen des Gußstückes zu verbessern, dient dabei gleichzeitig zum Trocknen der aufgebrachten Salze. Als Lösungsmittel eignen sich Wasser und/oder Alkohol.The fact that the salt mixture in the form of a solution and / or finely dispersed slurry is applied to the inner wall by subsequent pouring and pouring into the molded form with subsequent drying, the inner wall and its pore entrances can be provided with different salts simultaneously and in an even, finest distribution and also apply finely ground, slurried salts that are insoluble or insufficiently soluble to the inner wall. At the pouring temperature, the intimate mixture of the various salts leads to rapid liquefaction. The preheating of the mold, which takes place before the casting, in order to improve the flow of the casting, serves at the same time to dry the applied salts. Water and / or alcohol are suitable as solvents.

Dadurch, daß eine überwiegend aus Natrium-Lithium-Chlorid-Fluorid bestehend Salzmischung zur Beschichtung der Innenwand der keramischen Form verwendet wird, deren Schmelzpunkt unterhalb von 650°C liegt, kann eine sehr schnelle Verflüssigung des Salzgemisches erreicht werden. Bei diesen Salzgemischen existieren tiefschmelzende Mischungen reziproker Salzpaare mit geringer Hydrostabilität der Einzelnsalze, insbesondere verglichen mit den Salzen des Kaliums.The fact that a salt mixture consisting predominantly of sodium-lithium chloride fluoride is used to coat the inner wall of the ceramic mold, the melting point of which is below 650 ° C., enables the salt mixture to be liquefied very quickly. In these salt mixtures there are deep-melting mixtures of reciprocal salt pairs with low hydrostability of the individual salts, especially when compared to the salts of potassium.

Besonders geeignet ist eine wässrige und/oder alkoholische Lösung aus LiCI, NaF, NaCI und Na4Fe(CN)e. Dabei ist kein Vorschmelzen und Mahlen der Salzmischung notwendig. Natriumfluorid ist wasserlöslich. Durch Ionenaustausch mit dem Lithium-Chlorid fällt nach einigen Stunden feinkörniges Lithiumfluorid aus.An aqueous and / or alcoholic solution of LiCl, NaF, NaCl and Na 4 Fe (CN) e is particularly suitable. No premelting and grinding of the salt mixture is necessary. Sodium fluoride is water soluble. Due to ion exchange with the lithium chloride, fine-grained lithium fluoride precipitates after a few hours.

Dadurch, daß die Lösung und/oder Aufschlämmung des Salzgemisches ein Dispergiermittel enthält, können feinkörnige unlösliche Salze, die sich nach einiger Zeit aus der Lösung ausscheiden wie das Lithiumfluorid, in der Schwebe gehalten werden und eine gleichmäßige Verteilung der Salzmischung auf der Forminnenwand gefördert werden. Als Dispergiermittel eignet sich z.B. Methyl-Zellulose.Because the solution and / or slurry of the salt mixture contains a dispersing agent, fine-grained insoluble salts which separate out from the solution after some time, such as lithium fluoride, can be kept in suspension and a uniform distribution of the salt mixture on the inner wall of the mold can be promoted. Suitable dispersants are e.g. Methyl cellulose.

Fördernd für die gleichmäßige Verteilung des Salzgemisches bei der Trocknung wirkt auch, daß der Lösung und/oder der Aufschlämmung des Salzgemisches ein die Benetzung der Innenwand der keramjschen Form verbesserndes Hilfsmittel wie ein Tensid zugegeben wird.The fact that the solution and / or the slurry of the salt mixture is admixed with an auxiliary agent which improves the wetting of the inner wall of the ceramic mold, such as a surfactant, also promotes the uniform distribution of the salt mixture during drying.

Im folgenden wird die Erfindung anhand von Ausführungsbeispielen erläutert.The invention is explained below using exemplary embodiments.

Die Untersuchungen ergaben, daß die senkundären Dendritenarmabstände etwa 40-50 um betrugen, während bei nach dem Stand der Technik mit einer Titandiborid-Vorlegierung versetztem Material solche von ca. 80-90 um unter gleichen Gießbedingungen gefunden wurden.The investigations showed that the secondary dendrite arm spacings were approximately 40-50 μm, while in the case of material mixed with a titanium diboride master alloy according to the prior art, those of approximately 80-90 μm were found under the same casting conditions.

Ausführungsbeispiel 1Embodiment 1

Es wurden 20 Wachstrauben hergestellt, die aus je acht Zugproben-Modellen mit 8 mm Durchmesser zusammengesetzt waren, wobei die Zugproben um eine Einguß kreisförmig angeordnet und mit je einem ringförmigen Ober- und Unteranschnitt versehen waren.20 wax grapes were produced, each composed of eight tensile specimen models with a diameter of 8 mm, the tensile specimens being arranged in a circle around a sprue and each having an annular top and bottom cut.

Die Wachstrauben wurden mit einem ersten.. Überzug durch Tauchen in eine Aufschlämmung, bestehend aus einem wässrigen Binder, feingemahlenem (<30 11m) Zirkonsilikat und Siliziumdioxyd als Füller, versehen und mit grobem Zirkonsilikatpulver besandet. Nach der Trocknung wurden weitere sechs Schichten durch Tauchen, Besanden und Trocknen in üblicher Weise aufgebraucht, so daß keramische Formen mit Wandstärken von ca. 8 mm entstanden. Die Formen wurden unter Druck im Autoklaven entwachst und anschließend bei ca. 800°C gebrannt.The wax grapes were given a first coating by dipping into a slurry consisting of an aqueous binder, finely ground (<30 11 m) zirconium silicate and silicon dioxide as filler and sanded with coarse zirconium silicate powder. After drying, a further six layers were used up by dipping, sanding and drying in the usual way, so that ceramic molds with wall thicknesses of approximately 8 mm were formed. The molds were waxed under pressure in an autoclave and then fired at approx. 800 ° C.

Es wurde weiterhin eine wässrige Lösung aus 20 g LiCI, 20 g NaF, 5 g Na4Fe(CN)6, 40 g NaCI, 1 g Methylzellulose und 0,1 g Tensid pro Liter angesetzt.An aqueous solution of 20 g LiCl, 20 g NaF, 5 g Na 4 Fe (CN) 6 , 40 g NaCl, 1 g methyl cellulose and 0.1 g surfactant per liter was also prepared.

Die Lösung wurde nacheinander in die keramischen Formen gefüllt, sofort wieder ausgegossen und filtriert, um ggf. ausgespülte Keramikkörner zu entfernen.The solution was poured into the ceramic molds one after the other, immediately poured out again and filtered in order to remove any ceramic granules that had been rinsed out.

Die keramischen Formen wurden dann auf ca. 470°C erhitzt, im warmen Zustand in die Vakuumgießanlage eingesetzt und bei ca. 250°C Formtemperatur mit der Aluminium-Legierung GAISi7Mg0.6 bei einer Temperatur der Schmelze von 700°C, bei 10-2 Torr gefüllt.The ceramic molds were then heated to about 470 ° C, used in the warm state in the vacuum casting, and at about 250 ° C mold temperature with the aluminum alloy GAISi7Mg0.6 at a melt temperature of 700 ° C at 10- 2 Torr filled.

Die Aluminium-Schmelze wurde an Luft vorgeschmolzen, dann durch ein Spülgasgemisch entgast und im Vakuum nachentgast.The aluminum melt was premelted in air, then degassed with a flushing gas mixture and then degassed in vacuo.

Nach der üblichen Wärmebehandlung, die Lösungsglühen und Aushärten umfaßte, wurden folgende Festigkeitswerte bei geringer Streuung erhalten.

Figure imgb0001
Ausführungsbeispiel 2After the usual heat treatment, which included solution annealing and hardening, the following strength values were obtained with little scatter.
Figure imgb0001
 Embodiment 2

Es wurden die Wachsmodelle eines Flugzeugstrukturteiles mit einer mittleren Wandstärke von 5 mm und an den Knotenpunkten mit einer Wandstärke von 15 mm nach der in Ausführungsbeispiel 1 beschriebenen Methode zu Wachstrauben zusammengestellt, mit der keramischen Schale überzogen, im Autoklaven unter Druck entwachst und anschließend bei ca. 800°C gebrannt.The wax models of an aircraft structural part with an average wall thickness of 5 mm and at the nodes with a wall thickness of 15 mm were assembled into wax grapes according to the method described in exemplary embodiment 1, covered with the ceramic shell, waxed under pressure in the autoclave and then at approx. Fired at 800 ° C.

Es wurde weiterhin eine wässrige Lösung aus 20 g LiCI, 20 g NaF, 5 g Na4Fe(CN)6' 40 g NaCl, 1 g Methylzellulose und 0,1 g Tensid pro Liter angesetzt.It was further an aqueous solution of 20 g LiCl, 20 g NaF, 5 g Na 4 Fe (CN) 6 '40 g NaCl, 1 g recognized methyl cellulose and 0.1 g of surfactant per liter.

Die Lösung wurde nacheinander in die keramischen Formen gefüllt, sofort wieder ausgegossen und filtriert, um ggf. ausgespülte Keramikkörner zu entfernen.The solution was poured into the ceramic molds one after the other, immediately poured out again and filtered in order to remove any ceramic granules that had been rinsed out.

Die keramischen Formen wurden dann auf ca. 470°C erhitzt, im warmen Zustand in die Vakuumgießanlage eingesetzt und bei ca. 250°C Formtemperatur mit der Aluminium-Legierung GAISi7MgO,6 bei einer Temperatur der Schmelze von 700°C, bei 10-2 Torr gefüllt.The ceramic molds were then heated to approx. 470 ° C, inserted into the vacuum casting machine when hot and at approx. 250 ° C mold temperature with the aluminum alloy GAISi7MgO, 6 at a melt temperature of 700 ° C, at 10- 2 Torr filled.

Die Aluminiumschmelze wurde an Luft vorgeschmolzen, dann durch ein Spülgasgemisch entgast und im Vakuum nachentgast.The aluminum melt was premelted in air, then degassed with a purging gas mixture and subsequently degassed in vacuo.

Nach der Wärmebehandlung, die Lösungsglühen und Aushärten umfaßte, wurden folgende Festigkeitswerte bei geringer Streuung an Flachproben aus dem Finegußteil erhalten:

Figure imgb0002
After the heat treatment, which included solution annealing and hardening, the following strength values were obtained with little scatter on flat samples from the fine casting:
Figure imgb0002

Beim Gießen derselben Schmelze in eine bekannte Form ohne die erfindungsgemäßen Maßnahmen ergaben sich folgende Werte

Figure imgb0003
When the same melt was poured into a known mold without the measures according to the invention, the following values resulted
Figure imgb0003

Die technologischen Werte Zugfestigkeit, Streckgrenze und Bruchdehnung wurden durch den Einsatz der Erfindung somit wesentlich verbessert.The technological values of tensile strength, yield strength and elongation at break were thus significantly improved by using the invention.

Claims (9)

1. Method for casting aluminum alloys which contain more aluminium than corresponds to the eutectic mixture with the alloy partners to achieve improved strength values by reducing the spacings of the secondary dendrite arm spacings formed by solidification, characterized in that the inner wall of the casting mould is produced with large numbers of roughnesses in the micro-range, in that the inner wall is provided with a thin layer of a salt mixture, the cations of the salt mixture consisting predominantly of those of the alkali and/or alkaline-earth metals and its anions consisting prominently of those of the halogens, and in that the liquidus temperature of the salt mixture is set lower than the casting temperature of the aluminium alloy.
2. Method according to Claim 1, characterized in that the inner wall of the casting mould has more than 105 roughnesses per cm2 of which the ratio of depth to diameter or depth to crack width is more than 1 to 3.
3. Method according to Claim 2, characterized in that the refractory material forming the inner wall of the casting mould is produced from very fine-grained oxide powders, obtained by grinding, by immersing a wax model in a slurry of the oxide powder, a filler and a binder, drying and firing with the formation of an oxide-ceramic bond by the binder.
4. Method according to one of the above Claims, characterized in that the salt mixture used also contains one or more alkali and/or alkaline-earth pseudo-halogen compounds in the form of cyanate, cyanide, thiocyanate, hexa- and/or tetra-cyano compounds, amines or amides and/or similar compounds related chemically to the alkali cyanides, cyanates and thiocyanates and/or organic salts and/or metal- organic compounds of the alkali and/or alkaline-earth metals.
5. Method according to one of the preceding Claims, characterized in that a salt mixture, consisting of predominantly of sodium-lithium-chloride-fluoride, is used for coating the inner wall of the ceramic mould, the melting point of which lies below 650°C.
6. Method according to Claim 5, characterized in that an aqueous and/or alcoholic solution of LiCI, NaF, NaCl and Na4Fe(CN)6 is used.
7. Method according to one of the preceding Claims, characterized in that the salt mixture in the form of the solution and/or of a finely-dispersed slurry is applied to the inner wall of the fired mould by pouring it into and out of the said fired mould followed by drying.
8. Method according to Claim 7, characterized in that the solution and/or slurry of the salt mixture is held in a dispersing agent, for example methyl cellulose.
9. Method according to one of the above Claims, characterized in that an auxiliary agent which improves the wetting of the inner wall of the ceramic mould, for example a surfactant, is added to the solution and/or slurry of the salt mixture.
EP86104247A 1985-04-03 1986-03-27 Method of casting aluminium alloys Expired - Lifetime EP0198290B1 (en)

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