EP1611977B1 - Ceramic investment material for the production of moulds for precission castings of titanium, zirconium or alloys thereof - Google Patents

Description

The present invention relates to a ceramic investment material consisting of an oxide ceramic mixture and a binder system for the production of precision casting molds for castings of titanium, zirconium or their alloys.

Titanium and zirconium precision castings and castings from titanium-zirconium alloys and alloy types used in medical technology are characterized by their mechanical diversity, biocompatibility and extremely different quality and price levels.

In a study on the tolerability of titanium casting alloys taking into account "alternative methods of diagnostics", final report on the research project, published by the Institute of German Dentists in Cologne, the different alloys are tested and evaluated for their suitability for use in the medical field. Here, titanium and titanium alloys are classified as particularly biocompatible. However, their processing into filigree and individually shaped precision castings for medical technology and other industrial sectors such as jewelery, microtechnology, space technology, etc. is still poor. This is mainly due to the non-mature casting technique and not yet controllable higher casting temperatures for titanium and titanium alloys. Since zirconium has an even higher melting point than titanium, zirconium and zirconium casting alloys have not been used in medical technology at all for this reason, although they are particularly biocompatible.

The problem of casting titanium, zirconium or their alloys is mainly due to chemical reactions between the liquid materials and unstable molds during the casting process and the resulting unsafe technical conditions for the production of cast objects. Unstable molds lead to a casting error, the so-called "alpha-case". This casting defect is caused by excessive oxygen uptake of the alloy during casting (slag / metal reaction). So far, there is no safe technique by which this casting error would be avoided.

• Die vergossenen Teile aus den Titan- oder Zirkonium Legierungen härten insbesondere in Randzonen auf. Dies führt zu Versprödungserscheinungen der filigranen Gußteile. Erkennbar ist dies an einer starken Zunahme der Härte im Randbereich.
• Durch die Sauerstoffaufnahme, die bis zu unstöchiometrischen Titanoxiden (TiO_x) führen kann und zur unerwünschten Auflegierung führt, läßt die Biokompatibilität der gegossenen Teile stark nach. Insbesondere besteht die Gefahr der Bildung von löslichen niederwertigen Titan-Verbindungen durch Korrosionsreaktionen im menschlichen Körper.

The casting defect has serious consequences:

• The cast parts made of titanium or zirconium alloys harden especially in edge zones. This leads to embrittlement phenomena of filigree castings. This is recognizable by a strong increase in hardness in the edge area.
• Due to the uptake of oxygen, which can lead to unstoichiometric titanium oxides (TiO _x ) and leads to unwanted alloying, the biocompatibility of the cast parts decreases considerably. In particular, there is a risk of the formation of soluble low-valent titanium compounds by corrosion reactions in the human body.

This explains the large discrepancy of measured data on the corrosion resistance and biocompatibility of titanium, zirconium and their alloys between solid material on the one hand and parts obtained by casting technology on the other hand. The use of highly corrosion resistant zirconium, zirconium alloys or titanium-zirconium alloys in medical engineering as cast alloy is has not yet been described because no permanent molds are known for their processing.

However, the high biocompatibility of such castings would make the applicability of these in medical technology desirable. In particular, this could provide protection against allergies and long-term health consequences. In addition, the need for particularly biocompatible implant parts to cost-effective conditions is becoming increasingly important.

The European Patent Application EP 0 916 430 A discloses a method for controlling and adjusting the expansion of ceramic investment materials, wherein a non-quartz magnesium oxide (MgO) -containing oxide ceramic mixture, a magnesia (MgO) and monoammonium phosphate (NH ₄ H ₂ PO ₄ ) -based binder, and a make coatable investment material at least an organic acid is supplied. In this case, the addition of different organic acids such as oxalic acid dihydrate, citric acid or tartaric acid leads to different setting expansions.

Accordingly, the invention has for its object to provide a ceramic investment material for the production of molds, which allows the processing of titanium, zirconium or their alloys to cast-defect-free castings with high biocompatibility.

This object is achieved by a ceramic embedding compound, consisting of an oxide ceramic mixture and a binder system, for the production of precision casting molds for castings of titanium, zirconium or other alloys, characterized in that the oxide ceramic mixture consists essentially of calcium aluminate and magnesium oxide and optionally zirconium ( IV) oxide and is bound with a binder system consisting of a mixing liquid and a ceramic particle mixture, wherein the mixing liquid is an aqueous solution of trivalent metal oxo salts of at least one element of the group consisting of scandium, yttrium or lanthanum and the ceramic particle mixture is a calcium-aluminum Has mixed oxide, which releases OH ^- - ions in aqueous solution, wherein the binder system when firing the mold completely in under the Casting conditions of titanium, zirconium or their alloys decomposes stable metal oxides.

Surprisingly, it has been found that the alpha-case casting defect and the associated loss of quality is caused by the oxygen uptake of the cast metal from the mold. Titanium and zirconium crystallize in the hexagonal packing (hdP). Although in the hdP structure the titanium / zirconium atoms are densely packed (space filling approx. 72%), there are only a few, but large gaps in the titanium / zirconium lattice (so-called interstitial sites). In this crystal structure, small non-metal atoms such as atomic oxygen, carbon, nitrogen or hydrogen can be stored on these interstices. Titanium has the ability to store very large amounts of such atoms (intercalated crystals). So you can solve up to about 10% oxygen in alpha-titanium. It forms an oxygen-titanium / zirconium alloy. Normally, pure titanium or zirconium has an oxygen content of approx. 0.2 to 0.5 wt .-% on. With increasing oxygen content, the hardness, the 0.2% proof stress and the tensile strength values increase, but the elongation at break and the fracture constriction decrease drastically. The usual hardness is the Vickerhärte HVO, 5 (microhardness) with the hardness in the edge region of a Gußobjektes can be measured. With increasing oxygen content, brittle titanium-oxygen alloys. This oxygen uptake causes the alpha-case casting error. It has been found that with oxygen contents up to about 1% by weight, such a casting defect is avoided and the cast articles obtained have suitable hardness dimensions and biocompatibility.

At higher oxygen contents, the titanium-oxygen alloys behave like an oxide-ceramic part: during bending, a part breaks brittle without noticeable deformation. A high oxygen content in the surface area of the titanium or zirconium part leads to a bursting of the surface upon deformation. At very high oxygen contents, a gray layer of an unstoichiometric metal oxide is formed, in the case of titanium a titanium (I) oxide which forms an oxide which can not be unambiguously assigned to a molecular formula, a so-called non-daltonide compound. This "TiO _x " compound on the surface of a cast object has a specular gloss, so it can be considered that there is a metallic surface. However, this is a fallacy, because the biocompatibility of such parts loaded with casting defects decreases drastically. It forms corrosion products with low-valent titanium compounds (radicals), which lead to an impairment of tissue compatibility. In cell cultures, the DNA activity decreases drastically and the cell morphology is damaged.

In addition, liquid titanium and zirconium during casting lead to slagging reactions between the investment materials and the liquid cast metal at the moment of casting. In the context of this slagging reaction, the oxygen uptake of the cast metal occurs. It has been found that for avoiding these alpha-case cause the proportion of unstable oxides such as silica-modifications or quartz, cristobalite, amorphous silica-units or alkali silicates ( "water glass" binder) and bonded diphosphorus pentoxide (P ₂ O ₅₎ is very small must be in the sum below 0.5 wt .-%. If these compounds are present in higher concentrations in the molds, they react with the aggressive titanium or zirconium melts at the moment of the casting process by absorbing oxygen into the cast metal and taking up unwanted alloy constituents such. As silicon, phosphor, etc .. This leads to the so-called alpha-case, the irreversible embrittlement of the casting.

$$\begin{array}{l}{{\mathrm{\mu }}^{\mathrm{o}}}_{1800}\left(\mathrm{MgO}\right)=-731\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{o}}}_{1800}\left({\mathrm{ZrO}}_2\right)=-665\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{o}}}_{1800}\\ \left({\mathrm{Al}}_2{\mathrm{O}}_3\right)=-650\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right)\mathrm{.}\end{array}$$

In the context of this invention, it has surprisingly been found that the use of thermodynamically very stable oxides such as yttrium (III) oxide (Y ₂ O ₃ ), lanthanum (La) oxide (La ₂ O ₃ ), zirconium (IV) oxide (ZrO ₂ ), magnesium oxide (MgO), calcium oxide (CaO) and also limited alumina (Al ₂ O ₃ ) in the investment materials for the production of Präzisionsgußformen the avoidance of excessive oxygen uptake of the cast metals and avoiding the alpha-case Gußfehlers allows. In the order given, the thermochemical stability of yttrium (III) oxide to alumina decreases over the aggressive titanium or zirconium melts. In the thermochemical sense, this means an increase in the standard chemical potential based on one mole of oxygen in the positive direction of the components at a casting temperature of 1800 K. The following values result: $$\begin{array}{l}{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\left(\mathrm{CaO}\right)=-790\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\left({\mathrm{<figure-callout\; id="2"\; label="Y"\; filenames="imgf0007.png"\; state="\{\{state\}\}">Y</figure-callout>}}_2{\mathrm{<figure-callout\; id="3"\; label="O"\; filenames="imgf0007.png"\; state="\{\{state\}\}">O</figure-callout>}}_3\right)=-764\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\left({\mathrm{<figure-callout\; id="2"\; label="La"\; filenames="imgf0007.png"\; state="\{\{state\}\}">La</figure-callout>}}_2{\mathrm{<figure-callout\; id="3"\; label="O"\; filenames="imgf0007.png"\; state="\{\{state\}\}">O</figure-callout>}}_3\right)\\ =-743\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);\end{array}$$

$$\begin{array}{l}{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\left(\mathrm{MgO}\right)=-731\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\left({\mathrm{<figure-callout\; id="2"\; label="ZrO"\; filenames="imgf0007.png"\; state="\{\{state\}\}">ZrO</figure-callout>}}_2\right)=-665\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right);{{\mathrm{\mu }}^{\mathrm{O}}}_{1800}\\ \left({\mathrm{<figure-callout\; id="2"\; label="al"\; filenames="imgf0007.png"\; state="\{\{state\}\}">al</figure-callout>}}_2{\mathrm{<figure-callout\; id="3"\; label="O"\; filenames="imgf0007.png"\; state="\{\{state\}\}">O</figure-callout>}}_3\right)=-650\left(\mathrm{kJ}/\mathrm{mol}-\mathrm{O}\right)\mathrm{,}\end{array}$$

In addition, it has been found that the activity of said oxides in bonded forms such. As aluminates can be significantly reduced.

Both the binder for the production of the green mold and the ceramic components from which the mold is constructed may contain only thermo-chemically resistant oxides. The choice of oxides for the construction of the mold and the choice of binder depends on the caused by the amount of the material to be cast temperature stress of the mold and on the way the production of the mold. In the production of the mold is limited in the choice of oxides in an oxide mixture, as the mold must have a sufficient mechanical resistance. A mold must always be made of multiple oxide constituents by sintering processes. It has been found that in cases of high temperature load of the molds, such. B. in casting titanium amounts of more than about 100 g, the proportion of highly stable oxides such as Y ₂ O ₃ , La ₂ O ₃ or ZrO ₂ in the sum must be above 30%. In addition to the two mentioned oxides of group IIIb of the periodic table Y ₂ O ₃ and La ₂ O ₃ , scandium (III) oxide (Sc ₂ O ₃ ) is also a suitable stable oxide, which, however, is not used because of its high price.

The balance between molten metal and the oxide system of the investment at the moment of potting is due to the slagging reaction $$\begin{array}{c}{\mathrm{me}}_2{\mathrm{<figure-callout\; id="3"\; label="O"\; filenames="imgf0007.png"\; state="\{\{state\}\}">O</figure-callout>}}_3\left(\mathrm{firmly}\right)+\mathrm{Ti}.\mathrm{Zr}\left(\mathrm{Liquid}\right)\Rightarrow 2\mathrm{me}\left(\mathrm{solved\; in\; fl}\mathrm{,}\mathrm{Ti}/\mathrm{Zr}\right)+3\mathrm{O}\left(\mathrm{solved\; in\; fl}\mathrm{,}\mathrm{Ti}/\mathrm{Zr}\right).\end{array}$$

(Me = La, Y also Sc)
with the equilibrium constant K _a , which results from the standard chemical potentials μ ° ₁₈₀₀ (for example, see above for 1800 K) for this slagging reaction $${\mathrm{K}}_{\mathrm{a}}=\frac{\mathrm{a}{\left(\mathrm{me}\right)}^2\cdot \mathrm{a}{\left(\mathrm{O}\right)}^3}{\mathrm{a}\left(\mathrm{Ti}\right)}$$

With a (Me) for the activities of La / Y in the metal phase, a (O) is the oxygen activity in the metal phase and a (Ti) is the Ti activity in the metal phase, a (Me ₂ O ₃ ) = 1 Presence of the pure component. In the oxidic compounds (eg in aluminates), a (Me ₂ O ₃ ) is always << 1, so that the oxygen uptake can be reduced as a result.

K _a is in the temperature range between 1600 to 1700 ° C due to thermodynamic calculations between 10 ^-3 and 10 ^-5 . It would become one Set equilibrium in which the oxygen activity would be far below that at which alpha-case formation occurred.

The same applies to the stability of ZrO ₂ over liquid Ti. However, here the equilibrium constants lie at values of K _a ≈ 0.1 (1600 ° C.), so that ZrO _{2 is} more unstable than Y ₂ O ₃ or La ₂ O ₃ , By setting the Zr activity, by lowering the ZrO ₂ activity (for example, formation of mixed oxides Y ₂ O ₃ -ZrO ₂ ) and by a sufficiently high cooling rate of the mold after the casting process, the alpha-case formation can be undermined. The obtained surface hardness of the castings formed by such mold is well below the embrittlement limit of HV0.5 = 400.

The investment materials of the invention consist of a binder system and a suitable oxide ceramic mixture.

The binder system

A ceramic mixture of the components MgO-CaO-Al ₂ O ₃ (as calcium aluminate) -ZrO ₂ is mixed with an aqueous lanthanum or yttrium salt solution (or a mixture of the two salts) (embedding pulp). A wax part (later cavity for the Ti / Zr casting process) is encased with this potting slurry and cures to a so-called "green mold" within a few hours. The setting reaction is based on a slow reaction of alkaline components and the La ³⁺ - or Y ³⁺ -ions to the corresponding precipitated hydroxides Me (OH) ₃ with Me = La, Y: $${\mathrm{me}}^{3+}\cdot \mathrm{aq}+3{\mathrm{OH}}^{-}\cdot \mathrm{aq}\to \mathrm{me}{\left(\mathrm{<figure-callout\; id="3"\; label="OH"\; filenames="imgf0007.png"\; state="\{\{state\}\}">OH</figure-callout>}\right)}_3\left(\mathrm{firmly}\right).\mathrm{me}=\mathrm{Y}.\mathrm{La}$$

The crystallized hydroxides connect the ceramic particles and thus bind the "green mold". It turned out that the calcium aluminate component plays a crucial role as alkaliserender addition (OH ^- release in water). The calcium aluminate usually consists of the components CaO + Al ₂ O ₃ with w (CaO) = 20 to 40% and w (Al ₂ O ₃ ) = 80 to 60%. In the case of technical products, minor constituents amount to less than 0.8% (Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ ). The calcium aluminate exists from the mineralogical phases CaO.Al ₂ O ₃ + CaO .2Al ₂ O ₃ as primary phases and with 12CaO. 7Al ₂ O ₃ + α-Al ₂ O ₃ as secondary phases. The particle size distribution can be characterized by sieve analysis: 5% mass fractions are above a particle size> 90 μm, ie 95% of all calcium aluminate particles have particle sizes below 90 μm. In contact with water, alkaline components are released slowly, so that the pH increases to pH≈12 in the minute range. This leads to the above-mentioned precipitation of the sparingly soluble Me (OH) ₃ hydroxides, which produce the green strength.
The MgO component contributes to the precipitation reaction, so that this plays an essential role in the setting reaction of the system CaO + Al ₂ O ₃ . For the production of Me (OH) ₃ precipitation only good water-soluble oxo salts of lanthanum and / or yttrium or mixtures of these come into question, which form without residue La ₂ O ₃ or Y ₂ O ₃ in the firing process of the "green mold". For example, as mixing liquid, these are aqueous solutions of nitrates: (Y, La) (NO ₃ ) ₃ .aq or / and acetates: (Y, La) (OOCCH ₃ ) ₃ .aq or / and formates: (Y, La) ( OOCH) ₃ · aq, etc. These compounds decompose completely and without residue below 700 ° C. to form the corresponding oxides La ₂ O ₃ and Y ₂ O ₃ (thermal decomposition). These combine with the usual oxide components in the fired mold. In the range of higher temperatures between 700 and 1000 ° C, this leads to sintering processes (formation of mixed crystal systems (Y, La) ₂ O ₃ -Al ₂ O ₃ or ZrO ₂ -Y ₂ O ₃ , the strength of the fired mold below 1000 ° reinforce C, such as 3Y ₂ O ₃ · 5Al ₂ O ₃ or La ₂ O ₃ · Al ₂ O ₃ + La ₂ O ₃ · 11Al ₂ O _3). The use concentration of the salts in the aqueous solution is usually between 5 to 30% (depending on the solubility limits).

The mold for the production of the precision casting is made by burning the wax or plastic part (Ausmelzverfahren) that the object to be produced, images. As usual, the green mold is heated to high temperatures (up to approx. 1000 ° C) under the ingress of atmospheric oxygen. The formed cavity which precisely images the object to be cast is filled with liquid Ti or Zr alloys by applying a light pressure. In this case, the exact geometry of the object to be cast (taking into account the shrinkage dimensions) must be reproduced become. An additional requirement for the production of a mold may be that it must have a correspondingly large thermal expansion, so that so that the shrinkage of the casting object is compensated. This additional requirement is in the field of medical implant technology for the production of high-precision castings. The casting surface must be visually matt glossy. The roughness depth and roughness width must be less than 20 microns, so that a regrinding of the surface can be saved. The molds can be preheated to different temperatures (usually in the temperature range 400 to 800 ° C).

Oxidic ceramic mixture

The oxide ceramic mixture for building the mold can be summarized as follows: Oxide constituent Mass fraction (%) Grain fraction (μm) Calcium aluminates 8 to 30 5 to 150 magnesia 92 to 70 5 to 200 Unstabilized zirconium (IV) oxide 0 to 10 10 to 100

In sum, w (CaO-Al ₂ O ₃ ) + w (MgO) + w (ZrO ₂ ) must always be 100% (without the binder content).

In the following examples are given, which, however, have only descriptive character and to which the invention can not be limited.

• w(Fe) = 0,2 bis 0,25 %, w(C) = 0,07 bis 0,09 %, w(H) = 0,003 bis 0,005 %; w(O) = 0,12 bis 0,08 %; w(H) = 0,01 bis 0,012 %, Rest Ti
• 0,2 % Dehngrenze je nach Abkühlungsbedingungen und
• Konzentrationsschwankungen der Spuren Fe, C, N, 0: R_ρ0,2 = 300 bis 350 MPa Vickershärte, je nach Abkühlungsbedingungen und
• Konzentrationsschwankungen der Spuren Fe, C, N, O : HV 0,5 = 180 bis 220

The commonly used titanium casting material (TiF35 / starting material) of the following purity (w: mass percentage, mean values) is used:

• w (Fe) = 0.2 to 0.25%, w (C) = 0.07 to 0.09%, w (H) = 0.003 to 0.005%; w (O) = 0.12 to 0.08%; w (H) = 0.01 to 0.012%, balance Ti
• 0.2% proof stress depending on the cooling conditions and
• Concentration _{fluctuations of} the traces Fe, C, N, 0: R _ρ0,2 = 300 to 350 MPa Vickers hardness, depending on the cooling conditions and
• Concentration fluctuations of the tracks Fe, C, N, O: HV 0.5 = 180 to 220

1. 1. der Zusammensetzung der Gußform und
2. 2. der Vorwärmtemperatur der Gußform

ermittelt.The melter, an electric arc furnace, was operated under pure argon (99.9995%). The titanium mass used was between 10 and 50 g. Under these conditions, various molds were tested and the hardening by oxygen in the edge region of a cast object as a function of

1. 1. the composition of the mold and
2. 2. the preheating temperature of the mold

determined.

The hardness was determined as microhardness HV 0.5 in the edge region as a function of the distance. Based on this HV determination method, it is possible to obtain reliable information about the average penetration depth of the oxygen into the Ti / Zr matrix (hardening zones) and to assess whether a brittle alpha-case layer has formed. When assessing the HV changes, it must be taken into account that the base material used is subject to fluctuations which are in the nature of metallurgical technology and can not be avoided. So it depends primarily on the hardness change compared to the base material. Excessive oxygen uptake leads to alpha-case formation and is associated with an abnormally high increase in HV 0.5 (greater than 500).

Another criterion is the measurement of the thermal expansion (dilatometer results) in the conversion of the so-called "green" to the fired mold. The mold should expand slightly in the firing process (or at least show no shrinkage), hence the geometry of the casting object of the geometry of the wax part (= predetermined geometry) possible deviates slightly. An extension of the ceramic casting mold of about 1% would be advantageous, so that the decrease in the thermal expansion of the Ti / Zr casting on cooling to room temperature can be compensated by the increase in the thermal expansion of the mold. This then leads to extremely precise casting objects. The dilatometer measurements thus contain statements about the precision of the cast object.

In addition, it has been found in the study of this invention that by the addition of zirconium carbide (ZrC), higher thermal expansion can be observed by high-temperature chemical reactions. The oxidation of ZrC to ZrO ₂ is associated with a volume increase of over 40%, so that with a relatively small amount of ZrC (maximum 5 wt .-% in investment), a thermal expansion of the investment of up to 4% can be achieved. It has been found that ZrC oxidation is catalyzed by small additions of lithium compounds into the investment materials. Lithium compounds which form lithium oxide (Li ₂ O) or lithium fluoride (LiF) on heating the set investment material to the casting mold are suitable for this purpose. The proportion of Li ₂ O or LiF in the embedding mass according to the invention is between 0.01 to 0.8% Li ₂ O or LiF. As a result, titanium or zirconium alloy parts can be produced with extremely high accuracy of fit. The increased thermal expansion improves the precision of precision casting of titanium or zirconium alloys as many blends known in the art have too little thermal expansion.

With regard to the setting behavior of the ceramic mixtures containing lanthanum or yttrium salts (MgO-CaO-Al ₂ O ₃ -ZrO _{2 system} ), the mixing liquids were fundamentally not changed even when using lithium salts. The mixing liquid (based on water or water-alcohol mixtures) may contain, in dissolved form, the lithium salt which forms Li ₂ O later on heating the mold in a thermal dissociation process. Alternatively, the lithium salt may also be incorporated in finely divided form into the ceramic mixture for the production of the casting mold (investment material). This is, for example, with lithium fluoride (LiF), which is heavy in water is soluble, possible. The formed by thermal dissociation reactions Li ₂ O and ZrO ₂ formed by oxidation are thermochemically stable so that no alpha-case-forming reactions take place between these materials in the mold and the liquid titanium or zirconium alloys. This is an important prerequisite for the inventive use of ZrC and lithium compounds.

Zirconium carbide (ZrC):

ZrC is added to the investment materials described above (up to w (ZrC) = 5%). The ZrC reacts under the effect of air and at high temperatures (from 500 ° C) under increasing volume to zirconium (monoxide) (monoclinic zirconium dioxide):

ZrC + 2O ₂ → ZrO ₂ + CO ₂

• Das ZrC enthält geringe Mengen an Hafnium. Der Hf-Gehalt liegt unter 3 %, da beide Elemente nur schwer trennbar sind (Seltener Erden-Effekt).
• Die Reinheit des handelsüblichen Zirkoniumcarbides ZrC liegt bei mindestens 97 %.
• Die Korngröße liegt unterhalb von 25 µm. Je feiner ZrC ist, desto reaktiver ist die Verbindung.
• ZrC ist ein sehr reaktionsträger Stoff. Auf der Kornoberfläche bildet sich ZrO₂, das wegen der Volumenzunahme auf der Oberfläche eine geschlossene Deckschicht bildet und diese somit passiviert. Die Oxidationsgeschwindigkeit geht gegen Null.

The increase in volume due to the oxidation of the ZrC is approx. 43%, so that even smaller proportions in the investment materials generate correspondingly high thermal expansions.
To characterize the ZrC's:

• The ZrC contains small amounts of hafnium. The Hf content is below 3%, as both elements are difficult to separate (rare earth effect).
• The purity of the commercial zirconium carbide ZrC is at least 97%.
• The grain size is below 25 microns. The finer ZrC, the more reactive the compound.
• ZrC is a very inert substance. ZrO ₂ is formed on the grain surface, forming a closed surface layer on the surface due to the increase in volume and thus passivating it. The oxidation rate goes to zero.

It has been found that the ongoing diffusion processes for the oxidation of ZrC can be greatly accelerated by traces of lithium compounds (formation of defects and disruption of the ZrO _{2 formed} , enhancement of the diffusion processes). The catalytic ZrC oxidation by Li compounds in said investments is essential to the invention. The lithium compounds may be mixed as dopants in the investment or alternatively, if they are soluble in water or alcohol, dissolved in the mixing liquid.
The ZrO ₂ formed from ZrO and the Li ₂ O formed from the lithium compounds and also LiF are practically not attacked by liquid Ti or Zr alloys because of their high thermochemical stabilities. The concentration of Li ₂ O or LiF in the fired ceramic mold must be limited (maximum 2% Li ₂ O or LiF), otherwise the ceramic mold would sinter too much at high temperatures. The sintering would cause a shrinkage effect and the ceramic mold would solidify too much (problem of de-bonding).

Lithium compounds:

• Lithiumlactat (C₃H₅O₃Li), wasserfrei oder hydratisiert
• Lithiumcitrat (C₆H₅O₇Li₃), wasserfrei oder hydratisiert (z.B. Tetrahydrat)
• Lithiumbenzoat (C₇H₅O₂Li), wasserfrei oder hydratisiert
• Lithiumacetat (C₂H₃O₂Li), wasserfei oder hydratisert (z.B. Dihydrat)
• Lithiumnitrat (LiNO₃), wasserfei oder hydratisiert (z.b. Trihydrat)
• Lithiumchlorid (LiCl), wasserfei oder hydratisiert (mehrere Hydrate)
• LiCl wandelt sich bei höheren Temperaturen in Gegenwart von Wasserdampf in den Einbettmassen zu Li₂O um, so dass hierdurch die ZrC-Oxidation katalysiert wird:
  
          2 LiCl + 2 H₂O → Li₂O + 2 HCl

It has been found that all lithium compounds which thermally dissociate at high temperatures and form lithium oxide (Li ₂ O) are suitable. These are for example:

• Lithium lactate (C ₃ H ₅ O ₃ Li), anhydrous or hydrated
• Lithium citrate (C ₆ H ₅ O ₇ Li ₃ ), anhydrous or hydrated (eg tetrahydrate)
• Lithium benzoate (C ₇ H ₅ O ₂ Li), anhydrous or hydrated
• Lithium acetate (C ₂ H ₃ O ₂ Li), anhydrous or hydrated (eg dihydrate)
• Lithium nitrate (LiNO ₃ ), anhydrous or hydrated (eg trihydrate)
• Lithium chloride (LiCl), water-free or hydrated (several hydrates)
• LiCl converts to Li ₂ O at higher temperatures in the presence of water vapor in the embedding masses, thereby catalyzing the ZrC oxidation:
  
  2 LiCl + 2 H ₂ O → Li ₂ O + 2 HCl

The formed HCl is released and volatilizes.

Lithium fluoride (LiF), anhydrous, is sparingly soluble in water and can only be processed directly in the investment as a ceramic component.
The LiF attaches the ZrO ₂ to the ZrC particles and also catalyzes (like the Li ₂ O) the ZrC oxidation.
The applied lithium concentrations, as Li ₂ O or LiF, are in the concentration range of 2% to 0.01%. Low amounts of lithium are sufficient because they only cause the catalytic oxidation of the ZrC.

Addition of an organic carboxylic acid

The carboxylic acids (chemically R-COOH, R an organic residue molecule) are contained in dissolved form in the mixing liquids. They have the function, when mixing the basic constituents in the investment materials (basic MgO and calcium aluminate), to neutralize them and, first of all, to drastically lower the pH so that a hydroxide precipitation of Me (OH) ₃ , Me = Y, La, is delayed becomes. Thus, the solidification time of the investments is delayed in the range of several minutes, resulting in a longer processing time.
The polybasic carboxylic acids used, such as, for example, tartaric acid, citric acid, ascorbic acid or maleic acid also have complexing properties, so that the hydroxide precipitation is also additionally delayed by this. At the same time, the hydrolysis tendency of the lanthanum or Yttrium salts, which can lead to premature precipitation of La, Y-hydroxides in the mixing liquid prevented. A major condition for the choice of acid is that it will volatilize from thermal decomposition of the ceramic mold. Therefore, only the carboxylic acids come into question here. They escape without residue through oxidation and thermal decomposition during the firing process into carbon dioxide and water vapor from the porous ceramic casting molds (temperature range 100 to 300 ° C). In addition to the above-mentioned polybasic carboxylic acids R- (COOH) _x with x = 2.3, etc.), simple monocarboxylic acids x = 1 such as formic acid or acetic acid are also used. The choice depends on solubility parameters La ³⁺ / Y ³⁺ - carboxylic acid.

Solvent for the mixing liquid

The base solvent used is water. However, the water has small negative effects with respect to the processing time of the embedding pulp and the shrinkage of the green mold during the setting process. Too much water activity means a quick setting and an increased volume change due to shrinkage (in extreme cases up to - 0.7%). Shrinking and processing time depend not only on the water content, but also on the influence of the carboxylic acid and the nature of the lanthanum (III) and yttrium salt. These influences have been qualitatively researched and the basis of each developed recipe.
It is possible to reduce the water activity by adding short-chain alcohols and ketones (thus extending the processing time and substantially reducing the shrinkage). These organic solvents must be capable of being mixed with water in any ratio: methanol, ethanol,
1-propanol, 2-propanol and acetone (= dimethyl ketone). The content of these solvents added to the water is within limits of 0 to 50 vol .-%. The contents of the short-chain alcohols and ketones depend on the Solubilities of the lanthanum (III) or yttrium (III) salts in the respective solvent mixture of water short-chain alcohols / ketones. It has been found that the solubilities of the lanthanum (III) or yttrium (III) salts in these mixtures are good and are generally at least 10%. The lithium compounds (except for LiF) and the carboxylic acids are generally sufficiently soluble in these solvent mixtures. Homogeneous amish liquids of the type: water-organic solvent-organic carboxylic acid-lithium compound-Me ³⁺ salt (Me = La, Y) can thus be prepared. <b><u> Overview of the concentration ratios "(spans") </ u></b> The following table gives concentration limits for the different ingredients in mixing liquid and in the investment: Water content of the mixing liquid Alcohol / ketone content of the mixing fluid R (COOH) _x content, in mixing liquid Me salt content in mixing fluid (Me = La, Y) from La (III) or Y (III) salt (binder system) Me ₂ O ₃ content in investment, (Me = La, Y) from Me ³⁺ salts of the binder system Li ₂ O or LiF content in ceramic embedment of Li compounds (after thermal treatment) ZrC content in investment material particle sizes <25 μm Lower concentration limit 50% by volume 0% by volume 0% by weight 2.0% by weight 0.2% by weight 0.01% by weight 0% by weight Upper concentration limit 100% by volume 50% by volume 4.0% by weight 25% by weight 2.5% by weight 2.0% by weight 5.0% by weight

Application example 1 Composition of the ceramic mixture for the casting mold:

• 1.1.20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 1.2. 80% MgO (grain fractions 20 μm to 100 μm)

Composition of the mixing liquid for the setting reaction:

• 1.3 10% aqueous lanthanum (III) acetate solution, La (Ac) ₃ · aq with a La ₂ O ₃ content of w (La ₂ O ₃ ) = 35.9%. 21 ml per 100 g of ceramic powder mixture (see 1.1 and 1.2)

The lanthanum (III) acetate La (Ac) ₃ .aq is unstoichiometric and has a La ₂ O ₃ content of w (La ₂ O ₃ ) = 35.9%. The content of water and of acetic acid is not given in the technical quality used with clear chemical formulas. By specifying w (La ₂ O ₃ ) = 35.9%, the constituent La ₂ O ₃ , which is important for the ceramics, and which is formed by the thermal decomposition of the acetate, has been completely described. The content of acetic acid or water is of minor importance, since these components completely volatilize in subsequent decomposition reactions. The same applies to all other yttrium (III) or lanthanum (III) salts. If an exact chemical formula is given [eg. B. La (CH ₃ OOH) ₃ . 1.5 H ₂ O], where the exact La ₂ O ₃ content with w (La ₂ O ₃ ) = 47% is clearly indicated by the stoichiometric calculation.

Setting reaction to the green mold after 5 h.
Ball hardness of the set green mold (Brinell hardness), HB = 38

Results for example 1:

• From these components, the embedding pulp is stirred (1 min) and then a wax object (= later cavity for about 30 g of Ti) sheathed. It forms within 5 h at room temperature by a curing reaction from the green mold. This green mold is heated at about 300 ° C / h to 850 ° C and fired at 850 ° C for 1 h. It forms the burned (suitable for casting) mold.
• Three molds are being prepared, preheated at 400 ° C, 600 ° C and 800 ° C, to explore the influence of preheating temperature. In all cases, the surface of the casting object was very smooth. The mean roughness was less than 8 μm.
• In the FIG. 1 For these preheating temperatures, the hardness curve is shown in the edge area. HV 0.5 = 340 is in the edge region far below an embrittlement phenomenon by oxygen. The slight increase in hardness in the edge region at 20 μm from HV 0.5 ≈200 to HV 0.5 ≈ 340 (O contents below 1%) should be regarded as minimal.
• Surprisingly, the increase in hardness of the preheating temperature is practically not dependent. This is primarily due to the high casting temperature for Ti / Zr alloys of about 1700 ° C, which determines the temperature load on the mold.

The thermal expansion behavior of the mold, is in FIG. 2 shown. The expansion is almost linear from 200 ° C (heating rate 500 ° C / h) and runs to 1.1% at 950 ° C. When sintering at 950 ° C is a small Increase in volume within ½ h (sintering reaction, which increases the strength). The expansion behavior is - as from the FIG. 2 Recognizable - when cooled almost exactly reversible (no jumps etc.), so that sudden thermal stresses are avoided. This expansion behavior of the mold allows the casting of extremely precise castings, since the shrinkage of the Ti / Zr alloys is compensated in this way.

The setting expansion of the "green mold" was 0% (= no setting expansion), i. There was no shrinkage either.

Application Example 2 Composition of the ceramic mixture for the casting mold:

• 1.1. 20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 1.2. 80% MgO (grain fractions 20 μm to 100 μm)

Composition of the mixing liquid for the setting reaction

• 1.3. 10% aqueous lanthanum (III) nitrate solution, La (NO ₃ ) ₃ .6H ₂ O, 22 ml per 100 g ceramic powder mixture

Setting reaction to the green mold after 5 h.
Ball hardness of the set green mold (Brinell hardness), HB = 30

Results for example 2: For the titanium casting:

HV 0.5 = 340 in the 25 μm distance in the edge regions (ie similar to Example 1). No alpha case on the surface. Average surface roughness below 6 μm, optically clear surface.
Thermal expansion = 0.4% at 950 ° C, setting expansion = 0%.

Application example 3 Composition of the ceramic mixture for the casting mold:

• 1.1. 20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 3.2 80% MgO (grain fractions 20 μm to 100 μm)

Composition of the mixing liquid for the setting reaction

• 1.2. 10% aqueous lanthanum (III) acetate, La (Ac) ₃ · aq, (with a La ₂ O ₃ content of w (La ₂ O ₃ ) = 35.9%) in admixture with a 20% La (NO ₃ ) ₃ · 6H ₂ O,
  22 ml per 100 g of ceramic powder mixture (3.1 and 3.2)

Setting reaction to the green mold after 5 h.
Ball hardness of the set green mold (Brinell hardness), HB = 35

Results for example 3: For the titanium casting:

1. a) Thermische Expansion = 0,58 % bei 950 °C bei 22 ml Mischflüssigkeit auf 100 g Keramikpulver,
2. b) Thermische Expansion = 0,75 % bei 950 °C bei 18 ml Mischflüssigkeit auf 100 g Keramikpulver.

HV 0.5 = 340 in the 25 μm distance in the edge regions (ie similar to Example 1). No alpha case on the surface. Average surface roughness below 6 μm, optically clear surface.

1. a) Thermal expansion = 0.58% at 950 ° C with 22 ml of mixed liquid per 100 g ceramic powder,
2. b) Thermal expansion = 0.75% at 950 ° C with 18 ml of mixed liquid per 100 g ceramic powder.

The measured dilatometer function for case b) is in FIG. 3 shown. First, a slight shrinkage of -0.2% (200 ° C) by the release of free water and water from hydrates can be seen. Subsequently, in the temperature range, the salts are decomposed to La ₂ O ₃ . The expansion behavior is (advantageously, since the construction of thermal stresses is thereby avoided) almost linear.
In both cases a) and b) the setting expansion is 0%.

The use of a nitrate in conjunction with an acetate (or other organic oxo-salt component) has the advantage of greatly reducing nitrogen oxides NO _x in the nitrate decomposition reaction in the furnace during the firing process (environmental aspect). NO _x is reduced by the acetate component to N ₂ .

Application Example 4 Composition of the ceramic mixture for the casting mold:

• 4.1. 20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 4.2. 80% MgO (grain fractions 20 μm to 100 μm)

Composition of the mixing liquid for the setting reaction

• 4.3. 20 % wässrige Yttrium(III)-nitrat, Y(NO₃)₃·6H₂O,

22 ml auf 100 g der Keramikpulvermischung (4.1. und 4.2.)

• 4.3. 20% aqueous yttrium (III) nitrate, Y (NO ₃ ) ₃ .6H ₂ O,

22 ml per 100 g of ceramic powder mixture (4.1 and 4.2.)

Setting reaction to green mold after 4 h, setting expansion at 0%. Ball hardness of the set green mold (Brinell hardness), HB = 30

For the titanium casting:

HV 0.5 = 330 in the 25 μm spacing in the edge areas. No alpha case on the surface. Average surface roughness below 6 μm, optically clear surface.

Thermal expansion = -0.3% at 950 ° C. The ceramics sinter in the temperature range above 800 ° C to solid shards, resulting in a firmer form. This can be seen by the negative thermal expansion, which contains indications of a structural change of the ceramic matrix.

Application Example 5 Composition of the ceramic mixture for the casting mold:

• 5.1. 20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 5.2. 70% MgO (grain fractions 20 μm to 100 μm)
• 5.3. 10% ZrO ₂ , unstabilized, grain fraction 100 μm

Composition of the mixing liquid for the setting reaction

• 5.4. 20 % wässrige Yttrium(III)-nitrat, Y(NO₃)₃·6H₂O,

22 ml auf 100 g der Keramikpulvermischung (5.1., 5.2. und 5.3.)

• 5.4. 20% aqueous yttrium (III) nitrate, Y (NO ₃ ) ₃ .6H ₂ O,

22 ml per 100 g of the ceramic powder mixture (5.1., 5.2 and 5.3.)

Setting reaction to green mold after 4 h, setting expansion 0%. Ball hardness of the set green mold (Brinell hardness), HB = 30

For the titanium casting:

HV 0.5 = 340 in the 25 μm spacing in the edge areas. No alpha case on the surface. Average surface roughness below 8 μm, optically clear surface.
Thermal expansion = -0.32% at 950 ° C. The ceramics sinter in the temperature range above 800 ° C to solid shards, resulting in a firmer form. This can be seen by the negative thermal expansion, which contains indications of a structural change of the ceramic matrix.

Application Example 6 Composition of the ceramic mixture for the casting mold:

• 6.1. 20% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 6.2. 80% MgO (grain fractions 20 μm to 100 μm)

Composition of the mixing liquid for the setting reaction

• 6.3. 10 % wässrige Yttrium(III)-acetat-Lösung, Y(CH₃COO)₃·aq,

22 ml auf 100 g der Keramikpulvermischung (6.1., 6.2.)

• 6.3. 10% aqueous yttrium (III) acetate solution, Y (CH ₃ COO) ₃ .aq,

22 ml per 100 g of ceramic powder mixture (6.1., 6.2.)

The compound Y (CH ₃ COO) ₃ .aq used contains yttrium (III) oxide with a content of w (Y ₂ O ₃ ) = 36%.
Setting reaction to green mold after 4 h, setting expansion 0%.
Ball hardness of the set green mold (Brinell hardness), HB = 30

For the titanium casting:

HV 0.5 = 335 in the 25 μm spacing in the edge regions. No alpha case on the surface. Average surface roughness below 8 μm, optically clear surface.
Thermal expansion = + 0.30% at 950 ° C. The ceramics sinter in the temperature range above 800 ° C to solid shards, resulting in a firmer form.

Application example 7 Composition of the ceramic mixture for the casting mold:

• 7.1. 19.6% calcium aluminate
  (68% Al ₂ O ₃ + 31% CaO, balance = traces / impurities Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O +
  Li ₂ O + SiO ₂ )
• 7.2. 78.14% MgO (grain fractions 20 μm to 100 μm)
• 7.3. 0.26% lithium acetate dihydrate
• 7.4. 2.0% zirconium carbide (ZrC), particle size <25 μm

Composition of the mixing liquid for the setting reaction:

• 7.5. 5% lanthanum (III) acetate solution, La (Ac) ₃ · 1.5 H ₂ O in ethanol-water solvent. The lanthanum (III) acetate has a La ₂ O ₃ content of w (La ₂ O ₃ ) = 47%.
• 7.6. Solvent: 43% by volume ethanol with water
• 7.7. Maleic acid content: 2.3% in solvent 7.5.

The lithium acetate dihydrate can also be dissolved in the mixing liquid. The concentrations are chosen so that the same concentrations of lithium are present in the potting sludge. The setting and expansion behavior does not change.
21 ml of mixing liquid were applied to 100 g of the ceramic powder mixture (see composition of the mixture 7.5 to 7.7.)

Setting reaction to the green mold after 5 h.

Ball hardness of the set green mold (Brinell hardness), HB = 37

Results for example 7:

From these components, the embedding pulp is stirred (1 min) and then a wax object (= later cavity for about 30 g of Ti) sheathed. The setting reaction begins after 6 min. It forms within 5 h at room temperature by a curing reaction from the green mold. This green mold is heated to 850 ° C at about 500 ° C / h and fired at 850 ° C for 1 h. It forms the burned (suitable for casting) mold. The preheating temperature of the mold for Ti casting was 600 ° C.

The average roughness of the titanium casting was less than 5 μm. There was no alpha-case layer on the surface. The surface was metallic bright. She shimmered pale yellowish because of the tarnish. This is visually a very positive sign for the suppressed no reaction metal / ceramic investment. HV 0.5 for titanium is in the edge area at HV 0.5 = 330.

The same positive casting result comes with the highly biocompatible titanium-zirconium-manganese
Ti57-Zr37-Mn6 (concentration of the alloy in atomic%) achieved. This alloy has a measured very low liquid temperature (virtually identical to the solidus temperature) of only 1480 ° C, so that the metal-ceramic reactions are strongly suppressed by the lower casting temperature.

The thermal expansion behavior of the casting mold was surprising, since this mixing liquid contains a greatly reduced lanthanum (III) acetate content compared to Application Example 1 (see in this respect, comparatively FIG. 2 ). In the FIG. 4 expansion behavior has been shown. The expansion is initially from 200 ° C to 600 ° C at a Heating rate of 500 ° C / h almost linear. Then, a strong expansion by the oxidation of the ZrC takes place, so that the expansion increases dramatically. It runs up to 1.4% at 850 ° C. In comparison, in Application Example 1, the expansion at 850 ° C is 0.85%, although here the lanthanum (III) oxide concentration in the ceramic mixture is much higher. When sintering at 850 ° C, a small volume increase within ½ h is observed (strength-increasing sintering reaction). The expansion behavior is - as from the FIG. 4 Recognizable - when cooled down, almost linear (no jumps, etc.), so that sudden thermal stresses are avoided. This expansion behavior of the mold allows the casting of extremely precise castings, since the shrinkage of the Ti / Zr alloys is compensated in this way.

The setting expansion of the "green mold" was 0% (= no setting expansion), i. there was no shrinkage. The shrinkage was also 0% due to the use of the ethanol-water solution. Instead of water-ethanol, the organic component can also consist of 1-propnaol, 2-propanol, methanol or acetone. The thermal expansion behavior is not affected by this. The (weak) shrinkage of the embedding pulp during the setting process and the setting time are positively influenced. The choice of solvent depends on the solubilities of the respective components.

Application Example 8 Composition of the ceramic mixture for the casting mold:

• 8.1. 19.0% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces / impurities Fe ₂ O ₃
  + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 8.2. 77.0% MgO (grain fractions 20 μm to 100 μm)
• 8.3. 1% lithium lactate (anhydrous)
• 8.4. 3.0% zirconium carbide (ZrC), particle size <25 μm

Composition of the mixing liquid for the setting reaction:

• 8.5. 10% lanthanum (III) acetate solution, La (Ac) ₃ .aq in water with a La ₂ O ₃ content of
  w (La ₂ O ₃ ) = 35.9%.
• 8.6. 0.8% acetic acid solution (solution 8.5.)

The acetic acid solution dissolves the basic lanthanum (III) acetate to a clear solution.
The lithium lactate can also be dissolved in the mixing liquid. The concentrations are chosen so that the same concentrations of lithium are present in the potting sludge. The setting and expansion behavior does not change.
23 ml of mixing liquid was applied to 100 g of the ceramic powder mixture (see composition of the mixture 8.1 to 8.4.) Setting reaction to green mold after 3 h.
Ball hardness of the set green mold (Brinell hardness), HB = 39

Results for example 8:

The casting results were identical to those of Example 7.

The extraordinary large thermal expansion is brought about by the component ZrC (with 3% by weight) and the addition of lithium lactate (with 1% by weight). These additives allow the expansion behavior of the Adjust the investment material according to the invention. The thermal expansion is +2.2% / 850 ° C. In the FIG. 5 expansion behavior has been shown. The increased thermal expansion is necessary to compensate for high shrinkage in certain Ti / Zr alloy parts.

Application Example 9 Composition of the ceramic mixture for the casting mold:

• 9.1. 19.4% calcium aluminate (68% Al ₂ O ₃ + 31% CaO, balance = traces Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ )
• 9.2. 78.6% MgO (particle fractions 20 μm to 100 μm)
• 9.3. 2.0% zirconium carbide (ZrC), particle size <25 μm

Composition of the mixing liquid for the setting reaction

• 9.4. 10% aqueous yttrium (III) acetate solution with Y (CH ₃ COO) ₃ · aq, yttrium (III) oxide content w (Y ₂ O ₃ ) = 36%.
• 9.5. 0.3% lithium lactate in the aqueous solution 9.4. disbanded

22 ml of mixing liquid (9.4. And 9.5.) Were applied to 100 g of the ceramic powder mixture (9.1 to 9.3.).

Setting reaction to the green mold after 5 h.
Ball hardness of the set green mold (Brinell hardness), HB = 38

Results for example 9:

The casting results were identical to those of Example 7.

The thermal expansion is caused by the component ZrC (with 2% by weight) and the addition of lithium lactate (in the aqueous mixing liquid in 0.3% concentration). These additives can be used to adjust the expansion behavior of the embedding compound according to the invention. The thermal expansion is +0.85% / 850 ° C. In the FIG. 6 expansion behavior has been shown. It lies around rd. + 0.50% over thermal expansion without ZrC [see application example 6)]. Investments with Y ₂ O ₃ are more stable (thermochemical stability) than with La ₂ O ₃ or ZrO ₂ , so that higher-melting Ti / Zr alloys are used.

In the FIG. 7 exemplified is the interaction of ZrC and various lithium salts in the investment materials based on calcium aluminate / MgO with lanthanum (III) acetate as a binder (mixing liquid). Visible is the increase of the thermal expansion in the molds through the interaction ZrC / lithium salt. Thus, any Ti or Zr alloys can be produced by this precision casting technique

Further results:

As the proportion of calcium aluminate decreases (68% Al ₂ O ₃ + 31% CaO, balance = traces of Fe ₂ O ₃ + MgO + Na ₂ O + K ₂ O + Li ₂ O + SiO ₂ ), the embrittlement tendency in the titanium casting increases , so that in a 10% calcium aluminate content, the HV hardness in the 25 .mu.m edge region of the Ti cast part increases on average to HV 0.5 = 380. But that's not the point of alpha-case formation. The surfaces of the castings were without cracks and in the bending tests (60 ° angle) no brittle fractures were detectable.

Analogous results were obtained with other yttrium (III) oxo salts [yttrium (III) nitrate, yttrium (III) acetate, etc.] in the mixing liquids, with similar compositions of the ceramic mixture [calcium aluminate-MgO-ZrO _{2 system} ]. , obtained for the casting technique.

Brief Description:

FIG. 1

Hardness curve in the edge region for application example 1 with the binder system lanthanum (III) acetate / calcium aluminate

FIG. 2

Thermal expansion behavior of a mold for application example 1 with the binder system lanthanum (III) acetate / calcium aluminate

FIG. 3

Thermal expansion behavior of a casting mold for application example 3 with the binder system lanthanum (III) acetate / lanthanum (III) nitrate / calcium aluminate

FIG. 4

Thermal expansion behavior of a casting mold for application example 7 with the binder system lanthanum (III) acetate // calcium aluminate with the additives lithium acetate, zirconium carbide (2% by weight ZrC), maleic acid and the solvent mixture ethanol-water

FIG. 5

Thermal expansion behavior of a casting mold for application example 8 with the binder system lanthanum (III) acetate // calcium aluminate with the additives lithium acetate, zirconium carbide (3% by weight ZrC) and acetic acid aqueous solution

FIG. 6

Thermal expansion behavior of a casting mold for application example 9 with the binder system yttrium (III) acetate // calcium aluminate with the additives lithium acetate, zirconium carbide (2% by weight ZrC), aqueous solution of Y (Ac) ₃ · aq

FIG. 7

Thermal expansion at 800 ° C as a function of the zirconium carbide concentration (ZrC) in the investment material calcium aluminate / MgO (in the mass ratio 1: 4) using various lithium salts, mixing liquid 10% aqueous La (Ac) ₃ · aq, w ( La ₂ O ₃ ) = 35.9% by weight, 23 ml per 100 g investment

Claims (16)

1. Ceramic investment material consisting of an oxide-ceramic mixture and a binder system, for the production of moulds for precision castings of titanium, zirconium and alloys thereof, characterized in that the oxide-ceramic mixture substantially consists of calcium aluminate and magnesium oxide and optionally includes zirconium (IV) oxide and is bound with a binder system consisting of a mixing liquid and a mixture of ceramic particles, wherein the mixing liquid is an aqueous solution of trivalent metal oxosalts of at least one element of the group consisting of scandium, yttrium or lanthane, and the mixture of ceramic particles includes a calcium-aluminum mixed oxide which releases OH- ions in an aqueous solution, wherein the binder system during burning the mould is completely decomposed into metal oxides which are stable under the casting conditions of titanium, zirconium or alloys thereof.
2. Ceramic investment material according to claim 1, characterized in that the oxide-ceramic mixture solely includes metal oxides which are thermochemically stable under the casting conditions of titanium, zirconium or alloys thereof.
3. Ceramic investment material according to one or more of the preceding claims, characterized in that the metal oxides of the oxide-ceramic mixture are oxides of the group consisting of Y₂O₃, La₂O₃, ZrO₂, MgO, CaO and Al₂O₃ or mixtures thereof.
4. Ceramic investment material according to one or more of the preceding claims, characterized in that the binder system consists of compounds which after burning the mould are present as oxides of the group consisting of Y₂O₃, La₂O₃, ZrO₂, MgO, CaO and Al₂O₃.
5. Ceramic investment material according to one or more of the preceding claims, characterized in that the investment material in the burnt condition produces moulds which guarantee a surface hardness of the parts cast of titanium, zirconium or alloys thereof below the embrittlement limit, especially that the surface hardness for TiF 35 as alloy HV is 0.5 < 400.
6. Ceramic investment material according to one or more of the preceding claims, characterized in that the investment material includes zirconium carbide.
7. Ceramic investment material according to claim 6, characterized in that the investment materials include up to 5 % by weight of zirconium carbide.
8. Ceramic investment material according to claim 6 or 7, characterized in that the investment material includes a lithium compound.
9. Ceramic investment material according to claim 8, characterized in that the investment material includes Li₂O and/or LiF as a lithium compound at a concentration between 0.01 to 2.0 % by weight.
10. Ceramic investment material according to claim 8, characterized in that the investment material includes a lithium compound which is thermally dissociated at high temperatures and forms lithium oxide (Li₂O).
11. Ceramic investment material according to claim 10, characterized in that the investment material includes as a lithium compound at least one compound of the group consisting of lithium citrate, lithium benzoate, lithium acetate, lithium nitrate, lithium chloride, lithium lactate or lithium fluoride.
12. Ceramic investment material according to one of the preceding claims, characterized in that as metal oxosalts in the binder system nitrates, nitrites, acetylacetonates, acetates, formiates or other well water-soluble carboxylates are used, wherein well water-soluble means a solubility > 3 % by weight.
13. Ceramic investment material according to one of the preceding claims, characterized in that the calcium-aluminum mixed oxide in the binder system includes 20 to 40 % by weight of CaO and 60 to 80 % by weight of Al₂O₃.
14. Ceramic investment material according to one of the preceding claims, characterized in that <5 % by weight of the employed oxides are present in the binder system at a grain size >90 µm.
15. Ceramic investment material according to one of the preceding claims, characterized in that the oxide-ceramic mixture includes 8 to 30 % by weight of calcium aluminates at a grain size distribution of 5 to 150 µm, 70 to 92 % by weight of magnesium oxide at a grain size distribution of 5 to 200 µm, and 0 to 10 % by weight of zirconium (IV) oxide at a grain size distribution of 10 to 100 µm.
16. Ceramic investment material according to one of the preceding claims, characterized in that the calcium aluminates employed in the oxide-ceramic mixture include 20 to 40 % by weight of CaO and 60 to 80 % by weight of Al₂O₃.

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