JPH06128604A - Production of metallic material - Google Patents

Abstract

PURPOSE:To obtain a dense, high-strength and high-corrosion resistance sintered metallic member at low cost without any plastic working or HIP. CONSTITUTION:A fine oxide grain having a lower standard free energy in the formation of the oxide than a metal powder and having a lower oxygen content than stoichiometry is uniformly dispersed in the metal powder and then sintered, and the high-strength sintered compact having a density close to its true density is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a metallic material which is a high-density and high-strength sintered member by pressureless sintering.

[0002]

2. Description of the Related Art Tungsten (W), molybdenum (M
o), rhenium (Re) and their alloys have a melting point of 2000 ° C. or higher and have been conventionally used mainly as electronic parts, electrode materials, and filament materials. In recent years, attention has been paid to their excellent high temperature strength and corrosion resistance It is expected to be used as a structural material. However, since W, Mo, and Re have extremely high melting points and poor workability, it is difficult to produce a product by a usual method such as melting + working (plastic working, mechanical working). Therefore, in general, various members are currently manufactured by the powder sintering method.

FIG. 7 shows a manufacturing process of a metal member by a general powder sintering method. First, in the first step (a), a metal powder is filled in a rubber 6 and the metal powder 1 is solidified by pressure molding. Then, in the second step (b), the pressure molded body 7 is heated and sintered in the electric furnace 8 for about 10 to 20 hours, and the sintered body 9 is obtained in the third step (c).

However, the relative density of the sintered body obtained by such a method is only about 90% at most, and a large number of pores remain inside. It is known that properties such as strength and corrosion resistance of these metal sintered bodies are largely dependent on the density, and the pores inside the sintered body significantly reduce the strength,
In many cases, the corrosive solution or gas penetrates into the internal pores, which significantly impairs the corrosion resistance. Therefore, it is essential to obtain a dense sintered body in order to bring out the original characteristics of the sintered body. In general, a dense sintered body can be obtained by sintering at a higher temperature, but if the sintering temperature is too high, there is a problem that the crystal grains become coarse and the strength decreases and becomes brittle. Therefore, it is the current situation that the densification is usually done by plastic working such as hot rolling and hot forging, but such a method can produce only a simple shape such as a bar or plate. There are also drawbacks.

Further, iron (Fe), nickel (Ni),
Even for general-purpose metal materials such as copper (Cu), the technology of manufacturing products with a near-net shape close to the actual product by powder sintering has advanced, but the above W, Mo, Re
As in the case of refractory metals such as the above, it is not possible to obtain a dense and high-strength sintered body only by sintering, and it has not been used as a structural material.

In recent years, as a method for densifying such a sintered member, high temperature and high pressure HIP treatment (Hot Isostati) after sintering is performed.
c Pressing) is reported to be effective. However, in order to densify the sintered body by the HIP process, a work called "canning", in which the sintered body is vacuum-sealed in a container made of metal or glass, is required, which causes a significant increase in manufacturing cost. Further, the production of large-sized and heavy-weight members is limited in capacity of the apparatus, and it is difficult to apply it to all sintered members.

[0007]

As described above, in order to improve the strength, corrosion resistance, etc. of the sintered metal member, it is necessary to densify the sintered metal member as close to the true density as possible during sintering. Even if it was raised, the relative density of about 90% was the limit.

The inventors have found that the cause of not densifying to near the true density during sintering is due to the oxide film on the surface of the metal powder. That is, the densification of the metal powder during sintering is due to the diffusion of metal atoms on the powder surface,
This is because the surface diffusion is hindered at the portion where the stable oxide film is present on the surface of the metal powder, and the sintering proceeds. Generally, as a method of removing the oxide film on the surface of the metal powder, a reduction treatment of the metal powder before sintering or a method of sintering the metal powder in a reducing atmosphere such as vacuum or hydrogen atmosphere is used. However, with this method, the oxide film near the surface of the sintered body can be reduced, but the oxide film inside the sintered body cannot be completely reduced, and a large amount of oxide remains inside the sintered body. It has been confirmed. Therefore, if the oxides inside the sintered body can be completely reduced and removed, a dense sintered member can be obtained at a lower sintering temperature than before. Furthermore, since the coarsening of the metal crystal grains is suppressed by the lowering of the sintering temperature during the sintering process, a fine structure can be obtained, and it is expected that the corrosion resistance is improved as well as the remarkable improvement.

An object of the present invention is to provide a method for producing a metal material which can be obtained at a low cost without subjecting a dense, high-strength, highly corrosion-resistant sintered metal member to post-processing such as plastic working or HIP processing. .

[0010]

Means for Solving the Problems The present invention is to uniformly disperse oxide fine particles having a standard free energy of oxide formation lower than those of metal powders in a state where the oxygen concentration is lower than the stoichiometric composition. It is characterized by obtaining a dense and high-strength sintered body close to the true density by dispersing and then sintering.

[0011]

Therefore, since the oxide inside the sintered body is completely reduced and removed during the sintering process, a dense sintered member can be obtained at a lower sintering temperature than the conventional one. In addition, since the coarsening of the metal crystal grains during the sintering process is suppressed by lowering the sintering temperature, a fine structure can be obtained.

[0012]

EXAMPLE An example of the present invention will be described below. First,
To obtain a dense and high-strength sintered body close to the true density,
It is necessary to remove the oxide remaining in the sintered body. That is, it is necessary to completely remove the oxide film formed on the surface of the metal powder during the sintering process. On the other hand, there is a standard free energy of formation between the metal and its oxide as a function of temperature. Figure 1 shows a typical metal oxide standard free energy of formation curve. The figure shows that the metal is stable in the upper region of the straight line and the oxide is stable in the lower region. Therefore, when a metal A having a high standard free energy of oxide standard formation and a metal B having a low standard free energy of oxide standard existence are present, the oxide of the metal B is more thermodynamically stable. When heated in the state where the oxide of A and the metal B coexist, it is expected that the oxide of the metal A is reduced by the metal B and replaces the oxide of the metal A and the metal B. However, when the metal B is mixed in the metal state, it is not completely oxidized and may be dissolved or remain in the matrix metal as an impurity to deteriorate the characteristics of the sintered body. It is not always preferable to directly add the powder of the metal B to.

On the other hand, when the metal oxide is heated in a reducing atmosphere, oxygen in the oxide is partially reduced. For example, in the case of aluminum oxide, Al ₂ O ₃ to Al ₂ O _{3-x is} called. As described above, the oxygen atom is deficient due to the stoichiometric composition. It has been confirmed that such a metal oxide in a state in which oxygen atoms are deficient is thermodynamically unstable, and when heated in the atmosphere, it easily binds to oxygen in the atmosphere and returns to the stoichiometric composition. ing. Also, unlike metal powders, metal oxides are generally stable and do not react with metal powders even if they deviate from the stoichiometric composition, and when dispersed in a matrix metal, conversely suppress coarsening of crystal grains. However, it can be expected to improve the mechanical properties.

Utilizing this phenomenon, as shown in FIG. 2, the metal powder 2 is contained in the metal powder 2 whose surface is covered with the oxide film 1.
The metal oxide particles 3 having a lower standard free energy of formation of oxide than the stoichiometric composition are finely dispersed in a state where oxygen atoms are deficient, and then sintered, so that the metal oxide particles 3 have a stoichiometric composition. In the process of returning to the composition, the oxide film on the surface of the surrounding metal powder can be reduced and the sintering of the metal powder can be promoted. As a result, as shown in FIG. 3, the metal powder is easily diffused, sintered and densified, and a sintered body having a structure in which fine metal oxide particles 3 are present in the crystal grain boundaries is obtained. . Further, from the viewpoint of efficiently reducing the oxide film and improving the mechanical properties, the metal oxide particles 3
It is necessary that the standard free energy of oxide standard formation is as low as possible, and that the shape and distribution thereof are fine and uniform.

As shown in FIG. 4, by heating in a vacuum, yttrium oxide powder 3 and tungsten powder 1 in which oxygen atoms are deficient in stoichiometric composition are put in a container 5 together with a ceramic ball 4. Then, the container 5 is rotated (a). As a result, the brittle yttrium oxide powder 3 due to the free falling motion of the ceramic ball 4 in the container 5 is crushed by the impact of the collision of the shellac ball 4 and is dispersed in the metal tungsten powder 1. At this time, as described above, the yttrium oxide powder 3 is preferably as fine as possible and is preferably pulverized to a diameter of 0.1 μm or less. After that, a mixed powder of yttrium oxide 3 and metallic tungsten powder 1 is filled in a rubber container and pressure-molded at a pressure of about 2000 atm (b), and the pressure-molded body 7 is sintered in a vacuum furnace 8. (C). In this way, a dense sintered body 9 having a substantially true density is obtained (d).

In the case of a metal powder having a high sintering temperature such as tungsten, yttrium oxide powder having a stoichiometric composition and tungsten powder are crushed and mixed, and then pressure-molded,
The same effect can be obtained in the manufacturing process in which the yttrium oxide powder is reduced by heating at a low temperature so that the tungsten powder does not sinter too much and then sintered again at a high temperature such that the tungsten powder is sintered again. To be

The fine yttrium oxide-dispersed tungsten produced by the above method is finely dispersed in the metallic tungsten powder during the sintering process. The yttrium oxide is in a state in which oxygen atoms are deficient due to the stoichiometric composition. Due to the reducing action of the powder, the tungsten oxide on the surface of the metal tangten powder is reduced. As a result, the sinterability of the tungsten powder is remarkably improved as compared with the case where the yttrium oxide powder is not added, and a tungsten sintered body of substantially true density can be obtained only by pressureless sintering. Further, the sintering temperature can be lowered as compared with the conventional method, and it is superposed with the pinning effect of the yttrium oxide powder arranged at the crystal grain boundaries to obtain a sintered body of fine crystal grains.

By such a method, yttrium oxide powder was added in an amount of 5 Vol. % (Example I) and 10 Vol. FIG. 5 shows the result of plotting the relative density after sintering with respect to the case of adding (%) (Example II) with respect to the sintering temperature. The figure also shows a case where yttrium oxide powder was not added (conventional method) as a comparative material.

In both cases, as shown in FIG. 5, the relative density of the sintered body tends to increase as the sintering temperature rises. However, in the conventional method in which no yttrium oxide powder is added, sintering is performed at a high temperature of 2200 ° C. Although only a relative density of 93% was obtained at most, according to the present invention, a sintered body having a relative density of more than 99% can be obtained at a sintering temperature lower than that of the conventional method by 400 ° C. Further, in the comparison between Example I and Example II, Example II in which the amount of yttrium oxide powder added was large exhibited a higher relative density even at the same sintering temperature. The amount of yttrium oxide powder added, which affects the relative density after sintering, varies depending on the particle size of tungsten powder, the degree of oxidation, and the combination of materials. The improvement effect was confirmed even with the addition of 0.5% by volume. On the contrary, adding a large amount of yttrium oxide powder surely improves the sinterability of the metal powder, but since the strength decreases, it is preferable to suppress it to 30% by volume or less.

FIG. 6 shows a tungsten sintered body added with 5% by volume of yttrium oxide powder (Example I) and a tungsten sintered body not added with yttrium oxide powder (Example).
The 4-point bending strength of room temperature of II) is shown. From the figure, the four-point bending strength of the tungsten sintered body to which the yttrium oxide powder was added according to the present invention is 50 to 60 kgf / mm ^2. And a value of 2 to 3 times that of a tungsten sintered body produced by a conventional method without adding yttrium oxide powder, which is mainly due to densification and lowering of sintering temperature and crystallized by pinning effect of yttrium oxide particles. This is due to the miniaturization of grains.

[0021]

As described above, according to the present invention, a dense, high-strength, high-corrosion-resistant refractory metal member is subjected to plastic working or HIP.
It is possible to provide a manufacturing method that can be obtained in a near-net shape state and at low cost without performing post-treatment such as treatment.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing standard free energies of oxides of typical metals.

FIG. 2 is an explanatory view showing a state in which metal oxide particles are dispersed in a metal powder covered with an oxide film.

FIG. 3 is an explanatory view showing a state where the thing of FIG. 2 is sintered.

FIG. 4 is a process chart showing an embodiment of the present invention.

FIG. 5 is a characteristic diagram showing relative density characteristics of the present invention.

FIG. 6 is a characteristic diagram showing the 4-point bending characteristic of the present invention.

FIG. 7 is a process diagram showing a conventional example

[Explanation of symbols]

 1 ... Metal powder 2 ... Oxide film 3 ... Metal oxide particles

Claims (5)

[Claims] 1. A method for producing a metal member by powder sintering, wherein a matrix metal powder serving as a matrix and a metal oxide powder having a smaller free energy for forming an oxide than the matrix metal powder are more oxygen than a stoichiometric composition. A method for producing a metal material, which comprises mixing in a low amount and then sintering. 2. The mixture of the matrix metal powder and the metal oxide powder is heated at a temperature such that the metal oxide powder has a lower oxygen content than the stoichiometric composition, and then the metal oxide powder is added again. The method for producing a metal material according to claim 1, wherein the heating is performed at a temperature at which the metal is sintered. 3. The particle size of the matrix metal powder is 10
W, Mo, Ta, Nb, Cr, Co, Re of less than μm
The method for producing a metal material according to claim 1 or 2, which is Fe, Ni, Cu or an alloy containing these as main components. 4. The particle size of the metal oxide powder is 2 μm.
The following Y, Sc, Nd, Gd, Th, Dy, Er, C
e, Lu, Ho, Al, Tm, Zr, Hf, Ca, Mg
An oxide containing as a main component.
Alternatively, the method for manufacturing the metal material according to claim 2. 5. The amount of the metal oxide powder added is 0.5 to 30.
It is volume%, The manufacturing method of the metal material of Claim 1 or Claim 2 characterized by the above-mentioned.