KR100509938B1 - Method for fabricating TiAl intermetallic articles by metal injection molding - Google Patents

Abstract

The present invention is a process for forming a powder mixture by mixing a metal powder composed of titanium and aluminum with a binder; Injection molding the powder mixture to form an injection molded body; A degreasing step of performing a solvent extraction step of dipping the injection molded product in a solvent and stirring to extract the binder and a pyrolysis step of removing the binder remaining in the injection molded product by pyrolysis; Presintering the injection molded product which has undergone the degreasing step; And a step of main sintering the pre-sintered injection molded body. The method of manufacturing the titanium aluminide intermetallic article of the present invention minimizes the porosity of the article to increase the density, so that the engine structure of the supersonic aircraft and spacecraft in the aerospace field, the furnace wall material of nuclear fusion nuclear power generation, the highly efficient gas turbine structural material It can be usefully used in the manufacture of automotive engine valves and the like.

Description

 Method for fabricating titanium aluminide intermetallic article by metal injection molding {Method for fabricating TiAl intermetallic articles by metal injection molding}

The present invention relates to a method for producing an intermetallic compound article using a metal injection molding method, and more particularly, to a method for manufacturing a titanium aluminide (TiAl) intermetallic article using a metal powder composed of titanium and aluminum using a metal injection molding method. It is about.

In general, manufacturing techniques capable of precisely manufacturing a complicated and difficult-to-process article include cutting, precision casting, die casting, and powder metallurgy.

Cutting is problematic in terms of manufacturing cost as well as mass production in terms of machining. Precision casting can mold complex shaped articles, but the cost of production of small precision articles is expensive and unsuitable for mass production. The die casting method is limited to low melting point alloys such as Al, Zn and the like, and the powder metallurgy is cheaper in producing two-dimensional simple shaped articles, but complicated shapes increase in manufacturing cost due to increased post-processing.

On the other hand, unlike the existing manufacturing and processing technology, the metal injection molding method is a production technology capable of mass-producing a highly functional three-dimensional complex shape article at a relatively low cost.

Titanium aluminide (TiAl) intermetallic compound has high temperature strength, oxidation resistance and light weight, and is applied to aircraft surface, structural products, automobile engines, etc., and has high weight and cost reduction effect. However, it has been difficult to mold titanium aluminide by the metal injection molding method due to poor ductility and the like, difficult molding, high cost, and easy oxidation.

Examples of metal injection molding of TiAl or Ti ₃ Al intermetallic compounds are shown in US Pat. No. 4,707,332. They are characterized by lowering the manufacturing cost of the post-processing process by manufacturing mechanical articles using CIP (Cold Isostatic Pressing) or powder injection molding using alloy powder. However, the cited patent performs a heat treatment process up to 600 ° C to remove the binder after the powder injection, there is a disadvantage in that the injection shape is easily broken because the binder is rapidly removed in this process.

The conventional method for producing articles by powder injection molding of titanium aluminide intermetallic compounds disclosed in Korean Patent Application No. 10-1997-0074925 does not consider the process characteristics and manufacturing process variables of titanium aluminide intermetallic compounds. There is a problem that is difficult to apply to production. Therefore, in order to be able to continuously manufacture a complicated shape article by powder injection molding method using titanium aluminide intermetallic compound, it is necessary to develop a manufacturing method that considers process characteristics and manufacturing process parameters of titanium aluminide intermetallic compound in detail. There is this.

Accordingly, the present invention is to provide a method for producing a titanium aluminide intermetallic compound that can continuously produce a titanium aluminide intermetallic article having a dense structure using a metal injection molding method without large restrictions on size. .

One embodiment of the present invention to achieve the above technical problem is a process of forming a powder mixture by mixing a metal powder containing titanium and aluminum with a binder;

Injection molding the powder mixture into a mold to form an injection molded body;

A degreasing step of performing a solvent extraction step of dipping the injection molded product in a solvent and stirring to extract the binder and a pyrolysis step of removing the binder remaining in the injection molded product by pyrolysis;

Presintering the injection molded product which has undergone the degreasing step; And

A method for producing a titanium aluminide intermetallic article comprising the step of main sintering the presintered injection molded body is provided.

The weight ratio of the titanium to the aluminum in the metal powder is preferably 52:48 to 51:49, and the volume ratio of the metal powder to the binder is preferably 50:50 to 55:45. The binder preferably includes at least one selected from the group consisting of paraffin wax, polyethylene-lan-vinylacetate resin and stearic acid.

In the process of forming the injection molded body, the powder mixture is preferably heated to a temperature of 90 to 130 ° C. to injection molding, and more preferably 115 to 125 ° C.

The solvent extraction step is preferably carried out in alkane compound solvent of C5 ~ C11, more preferably carried out in alkane compound solvent of C6 ~ C8.

The pyrolysis step may include heating the injection molded product at 250 to 350 ° C. for 30 minutes to 1 hour 30 minutes in an inert gas or vacuum atmosphere; And it may include the step of further heating 30 minutes to 1 hour 30 minutes at 400 to 500 ℃.

The pre-sintering process is preferably carried out by sintering the injection molded body at a temperature of 850 to 1100 ℃ for 2 to 4 hours in a vacuum state of 10 ^-4 to 10 ^-9 torr, the injection molded body 10 More preferably, the sintering is carried out at a temperature of 950 to 1050 ° C. for 2 hours 30 minutes to 3 hours 30 minutes in a vacuum state of ⁻⁵ to 10 ⁻⁸ torr.

The main sintering process is preferably carried out by sintering the injection molded body at a heating rate of 2 to 4 ℃ / min at 1300 to 1450 ℃ for 10 to 35 hours, the injection molded body of 2.5 to 3.5 ℃ / min More preferably, it is carried out by sintering at 1330 to 1370 ° C. for 10 to 30 hours at a heating rate.

Hereinafter, the present invention will be described in more detail with reference to FIG. 1.

The process for producing a titanium aluminide intermetallic article of the present invention comprises: (1) a step of forming a powder mixture by mixing a metal powder containing titanium and aluminum with a binder; (2) molding the powder mixture into a mold to form an injection molded body; (3) a degreasing step of performing a solvent extraction step of immersing the injection molded product in a solvent and stirring to extract the binder and a pyrolysis step of removing the binder remaining in the injection molded product by pyrolysis; (4) pre-sintering the injection molded product that has undergone the degreasing step; And (5) main sintering the pre-sintered injection molded body. Hereinafter, each step will be described in detail.

The process of forming the powder mixture by mixing the metal powder containing titanium and aluminum of the present invention with a binder is prepared by the self-propagating high-temperature synthesis (SHS) reaction of the powder containing titanium and aluminum A titanium aluminide alloy powder and a binder are dry mixed at 130 to 160 ° C. for at least 1 hour, and then finely pulverized to a size of 1 to 5 mm to form a powder mixture.

At this time, the weight ratio of the titanium to the aluminum in the metal powder is preferably 52:48 to 51:49. When the weight ratio difference is more than 7% by weight, there is a problem in that it is difficult to form a complete titanium aluminide intermetallic compound because a metal component having a high weight ratio remains after preparation.

In the above process, the selection of an appropriate binder is important for easy mixing and injection molding, and to obtain a material of desired physical properties when the binder used after injection molding is completely removed. In view of this, it is preferable to use at least one selected from the group consisting of paraffin wax, polyethylene-lan-vinylacetate resin and stearic acid as a binder.

The volume ratio of the metal powder to the binder is preferably about 50:50 to 55:45. If the difference in volume ratio is more than 25% by weight, the moldability is lowered, so that it becomes difficult to maintain the shape of the intermetallic compound product in forming a complex three-dimensional shape.

The process of forming the injection molded body by injection molding the powder mixture of the present invention comprises injection molding by injecting the powder mixture obtained by mixing the metal powder composed of titanium and aluminum with a binder at a high temperature and high pressure through a metal injection molding machine. To get it.

Powder injection molding conditions are preferably injected after heating the powder mixture to a temperature of 90-130 ℃ to control the fluidity, and more preferably injection molding by heating to 115 to 125 ℃. If the preheating temperature is less than 90 ℃ fluidity is lowered, cracking occurs in the molded body after the powder injection, and if it exceeds 130 ℃, the fluidity is excessively increased and the shape of the molded body is distorted.

The injection pressure when injecting into the metal mold is preferably 400 to 500 bar, more preferably 430 to 470 bar. If the injection pressure is less than 400 bar, the pressure is low to fill the complex three-dimensional mold sufficiently, so that a good product cannot be obtained. If the injection pressure is higher than 500 bar, solidification occurs in the middle part of the mold during filling, so that it has good uniformity. Hard to get product

The degreasing process comprises a solvent extraction step of dipping and stirring the injection molded product in a solvent and performing a pyrolysis step of removing the binder remaining in the injection molded product by pyrolysis.

The solvent extraction step is a process for preventing breakage or severe warpage of the injection molded product due to rapid binder removal occurring in a subsequent pyrolysis step in a high temperature, vacuum or inert gas atmosphere. The solvent extraction step is a very important process in the preparation of the powder injection molding, and is a preliminary step to effectively remove the binder while maintaining the shape and to relieve the stress remaining in the molding at the time of injection. That is, it is necessary to first remove a binder or more through a solvent extraction step to prevent breakage or warpage of the injection molded product which may occur in the pyrolysis step.

The solvent extraction step is a process in which the injection molded product is immersed in a specific solvent and then the binder is extracted while rotating with a stirrer.

At this time, the rotation speed of the stirrer is preferably 100 to 150 rpm, the temperature is 20 to 70 ° C, and the extraction time is 8 to 12 hours. Thereafter, the injection molded product is taken out of the solvent and then sufficiently dried for 20 to 30 hours or more.

If the binder is extracted at less than 20 ° C., the binder removal efficiency is low, and in the subsequent pyrolysis step, the contamination caused by the binder is intensified and the shape of the injection molded product is destroyed due to the removal of the excess binder. On the other hand, if it exceeds 70 ° C, the binder is severely removed during the solvent extraction step, and the shape is severely warped or cracked.

Performing the solvent extraction step removes more than 90% of the binder. The solvent for extracting the binder is preferably an alkane compound of C5 to C11, more preferably an alkane compound of C6 to C8, and among these, n-heptane (CH ₃ (CH ₂ ) ₅ CH ₃ ) is most preferred.

 After the solvent extraction step, the pyrolysis step is followed to completely remove the binder by keeping the injection molded product in a tube furnace for a certain time.

Charge the injection molded product into a tube furnace and hold for 30 minutes to 1 hour and 30 minutes at 250 to 350 ° C in an inert (especially argon atmosphere) or vacuum atmosphere and then at 30 to 1 minute at 400 to 500 ° C. The remaining time 30 minutes can be further maintained to sufficiently remove the remaining binder.

If the pyrolysis temperature is less than 250 ℃ because the reaction rate is too slow, there is a problem in productivity, if it exceeds 500 ℃ there is a problem that the reaction rate is too rapid to form a crack or deform the injection molded body.

Since the titanium aluminide intermetallic compound is easy to react with oxygen and the shape of the injection molded product is severely changed according to the sintering atmosphere or conditions, presintering is necessary to densify the injection molded product. The process of presintering the injection molded product subjected to the degreasing process is a process of sintering the degreased injection molded product for a certain time at a lower temperature than that seen in a tubular furnace for densification of the intermetallic compound article structure.

And the degreased injection molded quartz tube (quartz tube) charged it is preferable to from 850 to 1100 ℃ 2 to 4 hours pre-sintering using a tubular furnace in a high vacuum state of 10 ^-4 to 10 ^-9 torr to 10 ^-5 It is more preferable to presinter for 2 hours 30 minutes to 3 hours 30 minutes at a vacuum state of from 10 to ⁸ torr, 950 to 1050 ° C.

When the pre-sintering process performed in a vacuum of less than 10 ^-4 torr is a problem that tends to react with oxygen in the nature of the titanium oxide is produced and, when the high vacuum of more than 10 ^-9 torr maintenance costs of the vacuum There is a problem such as a sharp rise. If the pre-sintering temperature is less than 850 ℃ fine pores (pore) is widely distributed there is a problem that the density of the product falls, there is a problem that the large pores are present inside the product above 1000 ℃.

If the presintering time is less than 2 hours, there is a problem that presintering may be insufficient, and if more than 4 hours there is a problem that the maintenance cost of the process increases compared to the effect of presintering.

The pre-sintering of the pre-sintered injection molded product is characterized in that it comprises the step of heating the injection molded product in a tubular furnace at a constant heating rate and sintering it for a predetermined temperature and time.

This sintering is preferably sintered under the α / (α + γ) transition temperature of the Ti-48Al alloy. As a result of sintering under conditions above and below the transition temperature, the density may be almost unchanged. However, sintering under the transition temperature is preferable because there is a disadvantage in deteriorating physical properties by coarsening crystal grains and pores during high temperature sintering.

The sintering temperature is preferably performed at a temperature of 1300-1450 ° C. where sufficient strength can be obtained. If the sintering temperature is less than 1300 ℃, a large amount of pores are formed inside the injection molding or the bond strength is lowered easily breakage problem occurs, while the injection molding is melted to maintain the shape at a temperature above 1450 ℃ It becomes impossible.

On the other hand, since the shape and shrinkage rate of the material change a lot depending on the temperature, heating rate and sintering time of the tubular furnace during sintering, the main sintering process is performed at 1300 to 1450 ° C for 10 to 35 hours at a heating rate of 2 to 4 ° C / min. It is preferable to carry out by sintering during the process, and more preferably by sintering for 10 to 30 hours at a heating rate of 2.5 to 3.5 ℃ / min at 1330 to 1370 ℃.

If the heating rate is less than 2 ° C / min, there is a problem of an increase in cost according to the increase in heating time, and if the heating rate exceeds 4 ° C / min, there is a problem that the density of the tissue is lowered.

If the sintering time is less than 10 hours, there is a problem in that the effect of sintering is inferior, and if it exceeds 35 hours, there exists a problem that the execution cost of a process increases compared with the effect of improving a density.

The main sintering is preferably carried out in a vacuum because the injection molded body reacts violently under low vacuum conditions, and the shape thereof is destroyed.

In addition, hot isostatic pressing may be performed to increase the density of the sintered intermetallic article.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are merely exemplary and should not be construed as limiting the present invention.

Powder Mixture Forming Process

The chemical composition of the Ti-48Al alloy used in this example is as follows.

element Al O C Ti Content (% by weight) 48.0 1.19 0.05 Bal.

As the binder to be mixed with the metal powder, paraffin wax, polyethylene-lan-vinyl acetate resin and stearic acid should be used because it is easy to mix and injection molding and to obtain a material of desired physical properties when the binder used after injection molding is completely removed. One or more mixtures were used. The TiAl powder was dry mixed with the binder at 150 ° C. for at least 1 hour and then finely ground to a size of 1 to 5 mm.

Process of forming injection molded body

The powder mixture was charged into a 27 ton metal injection molding machine and injected into a metal mold at a pressure of 450 bar and a temperature of 120 ° C.

Degreasing process

Heptane (CH ₃ (CH ₂ ) ₅ CH ₃ ) was used as a solvent in the solvent extraction step of the degreasing process. After dipping the 200-mesh sieve on which the injection molded product was placed in a solvent, the binder was extracted by rotating the stirrer at 45 ° C. for 10 hours at a speed of 100 to 150 rpm, and then the molded product taken out of the solvent was sufficiently dried for at least 24 hours. This process removed more than 90% of the binder.

The pyrolysis step for removing the remaining binder is carried out by charging the injection molded product into a tube furnace, maintaining the reactor at 300 ° C. for 1 hour in an inert (particularly Ar atmosphere) or vacuum atmosphere, and then at 450 ° C. for 1 hour. Was carried out.

Pre-sintering process

The degreased molded body was charged into a quartz tube and presintered at 1000 ° C. for 3 hours using a tubular furnace at a high vacuum of 10 ⁻⁵ torr.

Figure 2 is a photograph of the microstructure of the article according to the presence or absence of presintering during sintering according to the embodiment with a scanning electron microscope (a) is a tissue photograph sintered for 3 hours at 1350 ℃ without pre-sintering, (b) Is a tissue photograph presintered at 1000 ° C. for 3 hours and then sintered at 1350 ° C. for 3 hours. As can be seen in Figure 2, the pre-sintered intermetallic compound article can be seen that the pores are reduced and a more dense structure can be obtained.

The following table is for the results of examining the density of the observed microstructures.

Skim atmosphere Pre-sintering conditions Heating rate (℃ / min)  Sintered Condition Density (%) argon 3 1350 ℃, 3 hours 91.7 argon 1000 ℃, 3 hours 3 1350 ℃, 3 hours 93.4 H ₂ 1000 ℃, 3 hours 3 1350 ℃, 3 hours 97.0

As can be seen from the above table, a high density improvement of about 2% was obtained through presintering. On the other hand, in H ₂ gas atmosphere other than Ar gas atmosphere, the density increases about 5%, but in the actual process, blocking with external air may be insufficient, in which case oxygen in the air reacts with Al and Al ₂ O ₃ and By forming the same oxide, physical properties such as ductility and fracture toughness may be greatly reduced, which may be undesirable.

Main Sintering Process

This sintered was sintered at 1350 ° C. below the α / (α + γ) transition temperature of the Ti-48Al alloy. In the embodiment of the present invention, as a result of sintering under the conditions above and below the α / (α + γ) transition temperature, the density was almost unchanged, but the disadvantage of lowering the physical properties by coarsening crystal grains and pores during high temperature sintering Since it is preferable to adopt 1350 ℃. On the other hand, since the shape and shrinkage rate of the material change a lot according to the heating rate of the tubular furnace during sintering, the heating rate was changed to 3, 6, 10 ° C / min, and the sintering holding time was 1, 3, 10, 30 hours. Differently, the microstructure change with sintering time was investigated.

The following table shows the results of examining the density of the article according to the heating rate during sintering carried out according to the present invention.

Skim atmosphere Pre-sintering conditions Heating rate (℃ / min)  Sintered Condition Density (%) argon 1000 ℃, 3 hours 3 1350 ℃, 3 hours 93.4 argon 1000 ℃, 3 hours 6 1350 ℃, 3 hours 89.7 argon 1000 ℃, 3 hours 10 1350 ℃, 3 hours 88.4

As can be seen from the table, as the heating rate is increased to 3 ℃ / min, 6 ℃ / min, 10 ℃ / min showed a tendency to decrease the density to 93.4%, 89.7%, 88.4%. The slower the heating rate during sintering, the higher the density, but considering the time taken for sintering, the heating rate should be about 3 ° C./min.

The degree of densification of the article according to the heating rate during the main sintering was observed by scanning micrographs, and the results are shown in FIG. 3. 3 (a)-(c) are microstructure photographs of articles when the heating rates are 3 ° C./min, 6 ° C./min and 10 ° C./min, respectively. As can be seen in FIG. 3, as the heating rate increases, the volume fraction of the pores formed in the grain boundary also increases and the size tends to increase.

In this sintering, the sintering holding time also greatly influences the density of the intermetallic article. Therefore, the present inventors investigated the density of the intermetallic compound article while varying the sintering holding time during sintering while keeping the other conditions the same, and the results are shown in Table 3 below.

Skim atmosphere Pre-sintering conditions Heating rate (℃ / min)  Sintered Condition Density (%) argon 1000 ℃, 3 hours 3 1350 ° C, 1 hour 91.7 argon 1000 ℃, 3 hours 3 1350 ℃, 3 hours 93.4 argon 1000 ℃, 3 hours 3 1350 ℃, 10 hours 96.0 argon 1000 ℃, 3 hours 3 1350 ℃, 30 hours 97.8

As can be seen from the above table, the sintering holding time is increased from 1 hour to 10 hours in the density of time, but in the case of 30 hours thereafter, the increase in the density was not so large.

The method for producing a titanium aluminide intermetallic article of the present invention can continuously produce a titanium aluminide intermetallic article by minimizing the pores of the intermetallic article and increasing the density through optimization of the manufacturing process and process parameters. have. Titanium aluminide intermetallic compounds prepared using the present invention are useful for the production of engine structures for supersonic aircraft and spacecraft in aerospace applications, furnace wall materials for nuclear fusion nuclear power generation, high efficiency gas turbine structural materials, automotive engine valves, and the like. Can be used.

1 is a flow chart of a manufacturing process according to an embodiment of the present invention.

Figure 2 is a photograph of the microstructure of the article with or without presintering during sintering according to one embodiment of the present invention observed with a scanning electron microscope.

3 is a photograph observing the degree of densification of the article according to the heating rate with a scanning electron microscope.

Claims (13)

Mixing a metal powder containing titanium and aluminum with a binder to form a powder mixture; Injection molding the powder mixture into a mold to form an injection molded body; A degreasing step of performing a solvent extraction step of dipping the injection molded product in a solvent and stirring to extract the binder and a pyrolysis step of removing the binder remaining in the injection molded product by pyrolysis; Pre-sintering the injection molded product subjected to the degreasing step at a temperature of 850 to 1100 ° C .; And It includes a step of main sintering the pre-sintered injection molded body at a temperature of 1300 to 1450 ℃, The process of forming the injection molding is injection molding by heating the powder mixture to a temperature of 115 to 125 ℃, The solvent extraction step is a method for producing a titanium aluminide intermetallic article, characterized in that carried out in a C6 ~ C8 alkane compound solvent. The method of claim 1, wherein the weight ratio of the titanium to the aluminum in the metal powder is 52:48 to 51:49. The method of claim 1, wherein the volume ratio of the metal powder to the binder is 50:50 to 55:45. The method of claim 1, wherein the binder comprises at least one selected from the group consisting of paraffin wax, polyethylene-lan-vinylacetate resin, and stearic acid. delete delete delete delete The method of claim 1, wherein the pyrolysis step comprises: heating the injection molded product at 250 to 350 ° C. for 30 minutes to 1 hour 30 minutes in an inert gas or vacuum atmosphere; And further heating at 400 to 500 ° C. for 30 minutes to 1 hour and 30 minutes. The method of claim 1, wherein the presintering is performed by sintering the injection molded body in a vacuum of 10 ⁻⁴ to 10 ⁻⁹ torr for 2 to 4 hours. The method of claim 10, wherein the pre-sintering process is carried out by sintering the injection molded body at a temperature of 950 to 1050 ℃ for 2 hours 30 minutes to 3 hours 30 minutes in a vacuum state of 10 ^-5 to 10 ^-8 torr The manufacturing method characterized by the above-mentioned. The method of claim 1, wherein the main sintering step is performed by sintering the injection molded product at a heating rate of 2 to 4 ° C./min for 10 to 35 hours. The method of claim 12, wherein the main sintering process is performed by sintering the injection molded product at a heating rate of 2.5 to 3.5 ° C./min at 1330 to 1370 ° C. for 10 to 30 hours.