KR20060060190A - Mold material for titanium cast alloys and method for manufacturing the same using in-situ synthesis - Google Patents

Abstract

The present invention relates to a mold for casting titanium alloy and a method of manufacturing the same, by effectively controlling the alpha case, which is an interfacial reactant generated during precision casting of titanium alloy through the reaction generation principle of the casting material, to prevent the mechanical properties of the titanium alloy casting In addition, the present invention relates to an economical titanium alloy casting mold and a method for producing the same, which reduce environmental load and improve dimensional accuracy by eliminating post-processing.

Titanium, Precision Casting, Molds, Alpha Case, Reaction Generation

Description

Mold material for titanium cast alloys and method for manufacturing the same using in-situ synthesis}

FIG. 1 shows an alpha case with a microstructure photograph and a hardness value change at an interface of a specimen in which titanium is cast into an Al 2 O 3 mold.

FIG. 2 illustrates the result of mapping the alpha case of FIG. 1 to Electron Probe Micro-Analyzer (EPMA).

FIG. 3 illustrates a transmission electron microscopy (TEM) image in the alpha case region of FIG. 2.

Figure 4 shows the ring, dot pattern and EDS (Energy Dispersive Spectrometer) analysis obtained by removing the C2 aperture.

FIG. 5 illustrates the results of Convergent Beam Electron Diffraction (CBED) analysis on the dot pattern shown in FIG. 4B.

FIG. 6 is a schematic diagram of alpha case formation based on the analysis results of FIGS. 3 to 5.

Figure 7 shows the XRD analysis of the SKK-I template material developed in the present invention.

Figure 8 is a photograph showing a comparison of the titanium castings manufactured using a) Al ₂ O ₃ mold and b) SKK-I mold according to the present invention.

FIG. 9 is a microstructure photograph of an interface of a titanium specimen cast with the SKK-I template of FIG. 8.

The present invention relates to an economical casting mold for titanium alloys and a method of manufacturing the same, and by effectively controlling the alpha case, which is an interfacial reactant generated during the precision casting of a titanium alloy, through the principle of reaction generation of the casting material, thereby deteriorating the mechanical properties of the titanium alloy casting. The present invention relates to a titanium alloy casting mold and a method of manufacturing the same, which prevents the damage and improves the dimensional accuracy by reducing post-processing.

Generally, titanium casting is used for precision casting of titanium alloys and composite materials. Titanium is known as a universal solvent in the molten state, which causes strong reactions in the casting of titanium alloys, resulting in an interfacial reaction with the mold, which means that the oxygen decomposed in the mold is dissolved into the invasive atoms, thus reacting with an alpha case. Will form.

Since such an alpha case is an embrittlement region that adversely affects mechanical properties, it requires post-processing such as chemical milling using strong acid to remove it, thereby causing environmental problems, cost increase, and dimensional accuracy deterioration.

To date, methods developed to control interfacial reactions in the precision casting of titanium alloys and composite materials are largely based on graphite, zirconia (ZrO ₂ ), yttria (Y ₂ O ₃ ), calcia (CaO), and LaOF. There is a method of using, a method of using a binder with a controlled silica content, a method of adding zirconium (Zr) powder, and a mold casting method.

The method using graphite molds is based on the use of graphite powders proposed by Operhall [US Pat. No. 3,257,692] and inorganic powders called "stuccos". Morozov [US Pat. No. 3,389,743] using colloidal graphite and synthetic resin, and Zusman [US Pat. No. 3,485,288].

However, such a graphite mold has high energy consumption due to high purity graphite, needs to be fired for the mold only in a reducing atmosphere, has a problem in establishing a casting method and preheating with high thermal conductivity, and forms a reaction layer with carbon on the surface of the cast product. There is a technical problem.

Feagin [US Pat. Nos. 4,787,439 and 5,535,811] reported that zirconia and yttria can be used to reduce interfacial reaction layers. Lossow [US Pat. No. 4,703,806] and Horton [US Pat. No. 5,221,336] et al. And a partial coating system was developed to control the interfacial reaction.

However, even when expensive mold materials such as zirconia and yttria are used, there is a limitation that the interface reaction layer due to contamination by the binder and oxygen can not be completely removed.

Degawa [US Pat. No. 4,710,481] developed a crucible fabrication method using calcia, which is known as a stable material that is economical and does not react with titanium, but it is hygroscopic in the back-up process and dewaxing process that imparts the working strength of the mold. It is not suitable for use as a molding material because of deformation of the mold.

Brown [US Pat. No. 4,057,433] attempted to minimize the interfacial reaction with titanium molten metal as a heat sink by adding fluorides and lanthanides of groups IIIa and IIIb on the periodic table to the mold, but it was not very effective. It was.

In order to minimize the influence of silica components of common colloidal silica binders on interfacial reactions, Lim Ok-Dong [Korea Patent Publication No. 10-2001-0044137] et al. Reduced the silica content from 30% to 3 to 10%. On the other hand, the development of a binder added to 3 to 10% of the polymer, but does not completely remove the interface reaction, there is a problem that can not obtain a sufficient working strength of the mold.

Cser [US Patent Publication 2003/0011093 Al] et al. Developed a method of inhibiting oxidation of titanium molten metal during casting by adding Zr to a mold consisting of Al ₂ O ₃ , CaO and MgO. In order to fire the mold without making it difficult to give the mold proper working strength by firing at a low temperature of 850 ° C., it is necessary to go through the additional process of firing the mold in a protective gas and vacuum state to prevent oxidation of Zr. have.

On the other hand, Choudhury [US Pat. No. 6,443,212 B1] et al., In order to completely eliminate the problem of refractory crucibles and molds, a die casting method in which a titanium alloy is cast into a mold such as Ta and Nb after scull melting using water cooling. Developed.

This mold casting method is limited to a simple shape such as an exhaust valve, and the mold material is an expensive hard material, and has a short life due to thermal shock during casting.

As such, graphite, zirconia, yttria, calcia, LaOF molds, silica-adjusted binders, Zr powder addition and mold casting methods, which have been developed to date, are not suitable for economically and efficiently shaping titanium alloys.

Accordingly, the present inventors have studied to solve the problem that the interfacial reaction could not be avoided due to the strong reactivity of titanium in the molten state in the precision casting of titanium alloys and composite materials, the oxygen and metal components decomposed in the mold The products formed by infiltrating and substituting at the interface, respectively, were specifically identified and based on this, it was found that the mold itself no longer reacted with the titanium molten metal if the mold contained the reaction product in advance.

Accordingly, the present invention does not deteriorate mechanical properties due to the interfacial reaction, and omits post-treatment using a strong acid with high environmental load, thereby reducing environmental load, improving dimensional accuracy, and providing economic precision titanium casting technology. have.

In order to achieve the above object, the present invention provides a mold for casting titanium alloy containing Al ₂ O ₃ , Ti ₃ Al and TiO ₂ which is produced by reaction mixture firing of TiO ₂ powder and Al powder.

In the present invention, the amount of TiO ₂ powder and Al powder used as the raw material is preferably from 1% by weight to 99% by weight based on the size and shape of the cast product with respect to the total raw material weight.

The size of the TiO ₂ powder and Al powder is preferably used 2 to 200 ㎛. If the size of the powder is smaller than 2 ㎛ can not produce a uniform slurry, if larger than 200 ㎛ there is a problem that the reaction product is not properly synthesized.

In addition, the present invention comprises the steps of mixing the TiO ₂ powder and Al powder, slurrying the mixture to make a mold of the desired shape, shaping the obtained slurry into a mold, the mold temperature range of 850 to 1,200 ℃ It provides a method for producing a casting for titanium alloy casting comprising the step of producing a mold containing Al ₂ O ₃ , Ti ₃ Al and TiO ₂ by firing at.

In addition, the present invention comprises the steps of mixing the TiO ₂ powder and Al powder, baking the mixture at a temperature range of 850 to 1,200 ℃ to obtain a product containing Al ₂ O ₃ , Ti ₃ Al and TiO ₂ , the product It provides a method for producing a mold for casting titanium alloy comprising the step of producing a mold of the desired shape using.

The firing step is preferably made in a temperature range of 850 to 1,200 ℃. When the firing temperature is 850 ° C. or less, reaction generation is not properly performed, and sufficient strength cannot be given to the mold, and when the temperature is 1200 ° C. or more, cracks are generated in the mold.

As such, the titanium alloy casting mold of the present invention

(a) TiO ₂ and Al powder may be mixed and slurryed into a desired form, and then calcined at a temperature range of 850 to 1,200 ° C.,

In another way,

(b) TiO 2 and Al powder may be mixed and calcined at a temperature range of 850 to 1,200 ° C., and then the mold may be prepared using the product.

That is, it may be used as a mold immediately after firing, or a new mold may be prepared by firing and then using the product.

Hereinafter, with reference to the accompanying drawings described in more detail with respect to the titanium alloy casting mold according to the present invention.

First, a process of identifying the interfacial reaction between a general Al ₂ O ₃ mold and titanium is shown in FIGS. 1 to 6, and the mold of the present invention developed using the characteristics of the interfacial reaction identified as described above may be 7 to 9 are shown in comparison.

As shown in FIG. 1, there is an alpha case composed of an interfacial reaction and a hardness increase layer which are visually observed between the Al ₂ O ₃ template and titanium.

Until now, this alpha case is an embrittlement area by invasive elements such as carbon, nitrogen, and oxygen decomposed in a mold, especially oxygen, and the environmental problem and cost increase by post-processing such as chemical milling using strong acid to remove it. It has been known that causing a decrease in dimensional accuracy is an obstacle to precision casting of titanium alloys.

Until now, as part of the thermodynamic approach to suppress the reaction with oxygen decomposed in the mold, mold materials such as Al ₂ O ₃ , CaO, ZrO ₂ and Y ₂ O ₃ , which have lower standard free energy than TiO ₂ , have been used. Nevertheless, the interfacial reaction could not be completely eliminated.

In FIG. 2, the result of mapping the alpha case shown in FIG. 1 to an Electron Probe Micro-Analyzer (EPMA) using characteristic X-rays is shown.

It can be seen that oxygen, which is known as a major interfacial reaction layer generating material in titanium castings cast with Al ₂ O ₃ molds, is generally detected in trace amounts at the casting interface.

On the other hand, when aluminum is uniformly distributed in a layer of about 30 μm in pure titanium having no aluminum content, it can be seen that the aluminum of the mold has a great influence on the interfacial reaction.

That is, it was confirmed that not only oxygen decomposed in the mold but also aluminum, which is a metal component, influences the interfacial reaction.

In order to confirm the phase of the detected aluminum, a TEM (Transmission Electron Microscopy) analysis was performed to show an image of the interface reaction region in FIG. 3.

Figure 4a is a ring and dot pattern obtained by removing the C2 aperture to determine what phase is on the image shown in Figure 3, through the ring pattern it can be seen that it is a fine TiO ₂ present in amorphous.

FIG. 4B is a dot pattern for contrast represented by arrows in FIG. 3, and FIG. 4C shows a pattern index of FIG. 4B.

In the image of FIG. 3, the light and dark portions indicated by the arrows show the HCP crystal structure of (2ii0) as a result of diffraction pattern analysis.

In addition, FIG. 4D is an EDS point analysis result of the arrow contrast of FIG. 3, and as a result of the EDS point analysis, it may be confirmed that the aluminum phase is likely to be Ti ₃ Al.

For clearer phase analysis, the results are shown in FIG. 5 using a convergent beam electron diffraction (CBED) method in which the unit volume of the crystal lattice is easily identified using a focused beam without using a parallel beam.

Referring to FIG. 5, it can be seen that the portion indicated by the arrow in FIG. 3 is 61.08 Å ³ , and coincides with a theoretical unit volume of 62.10 Å ³ of Ti ₃ Al with an error within 2%.

As described above, the interfacial reaction generated during the precision casting of the titanium alloy proved to form not only the invasive reaction product TiO ₂ by the oxygen decomposed in the mold but also the substitutional reaction product Ti ₃ Al by the metal component decomposed in the mold. It became.

6 shows a production schematic of such an interfacial reactant.

The titanium alloy casting mold according to the present invention can be manufactured by the following method according to the size and shape of the cast product, the present invention is not limited thereto.

1 to 99% by weight of Al powder was added to the TiO ₂ powder, and then, using a colloidal silica binder, the slurry was subjected to a viscosity using Zahn cup # 4 of ASTM D4212 to 30 to 50 seconds depending on the shape and size of the casting. To prepare.

The wax pattern is coated with a mold material to an appropriate thickness, followed by dewaxing and firing at 850 to 1,200 ° C. to complete the mold of the present invention.

Figure 7 shows the X-ray diffraction (XRD) analysis results of the SKK-I (Sungkyunkwan-I) template prepared by the above-described method, the reaction synthesis (in-situ) between the TiO ₂ and Al powder of the template material synthesis), it can be seen that penetration type TiO ₂ and substituted Ti ₃ Al and Al ₂ O _{3 which} are identified as interfacial reaction products are formed simultaneously.

FIG. 8 is a photograph showing comparisons of specimens precisely cast titanium using the Al ₂ O ₃ mold and the SKK-I mold of the present invention under the same conditions.

As can be observed visually in Figure 8, the casting using the Al ₂ O ₃ mold of a) shows a reaction layer to be removed by chemical milling, while the specimen obtained by the mold of the present invention of b) is only a mold It can be seen that there is no interfacial reaction to the extent that it maintains the metallic luster peculiar to titanium in the removed state.

FIG. 9 shows a microstructure photograph of an interface region of a specimen prepared with the SKK-I template shown in FIG. 8. It can be seen that the internal and external structures are the same without any interface reaction.

As described above, the titanium alloy casting mold according to the present invention is a stable mold that no longer reacts with titanium molten metal because the mold itself has previously formed an interfacial reaction product by reaction synthesis. There is no deterioration in characteristics, and post-processing such as chemical milling can be omitted, thereby reducing environmental load, improving dimensional accuracy, and reducing mold processing cost.

Therefore, the application area can be infinitely extended to the integrated parts of complex shape, size and thinness, and it is widely applied from the titanium alloy, which was applied only to the aerospace and military industries, to the automobile, sports leisure, and medical engineering fields of composite materials. can do.

Claims (5)

A mold for casting titanium alloy containing Al ₂ O ₃ , Ti ₃ Al and TiO ₂ , which are produced by mixing and firing TiO ₂ powder and Al powder. The mold for casting titanium alloy according to claim 1, wherein the amount of TiO ₂ powder and Al powder is used to produce Al from 1% by weight to 99% by weight depending on the size and shape of the mold with respect to the total weight of the raw material. According to claim 1, wherein the size of the TiO ₂ powder and Al powder is a titanium alloy casting mold, characterized in that 2 to 200 ㎛. Mixing the TiO ₂ powder and an Al powder, in order to make the desired shape of the mold and fired in the step, a temperature range of the mold 850 to 1,200 ℃ to the step of slurrying the mixture, shaping the resulting slurry into a mold Al ₂ A method for producing a titanium alloy casting mold comprising the step of preparing a mold containing O ₃ , Ti ₃ Al and TiO ₂ . Mixing TiO ₂ powder and Al powder, calcining the mixture at a temperature range of 850 to 1,200 ° C. to obtain a product containing Al ₂ O ₃ , Ti ₃ Al and TiO ₂ , using a desired form Method for producing a titanium alloy casting mold comprising the step of producing a mold.

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