JP2803455B2 - Manufacturing method of high density powder sintered titanium alloy - Google Patents

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 high-density powder sintered titanium alloy having excellent mechanical properties.

[0002]

2. Description of the Related Art Titanium alloys are lightweight, have high strength, and are excellent in corrosion resistance, so that they are expected to be widely applied to industrial parts and consumer parts. In particular, it is being studied to apply such properties to automotive parts. However, in the method of manufacturing from ingot material by machining or cold working, although excellent in mechanical properties can be obtained, titanium is a difficult-to-work material, so there is a drawback that working is difficult and cost is high. In order to avoid such a drawback, application of powder metallurgy technology in which a titanium alloy raw powder is mixed, formed into a predetermined shape, and then fired in a vacuum to obtain a sintered body has been attempted. However, when the powder metallurgy technique is used, there is a problem that the mechanical properties of the product, particularly ductility and fatigue strength, are lower than that of the ingot material, because pores remain in the product.

On the other hand, in the case of a titanium alloy ingot, β
A method of improving the fatigue strength by forming a structure composed of a fine α-phase by water cooling from a region (Japanese Patent Publication No. 3-10)
No. 91) has been proposed, and it is conceivable to apply this concept to a titanium sintered body to improve fatigue strength.

In fact, as a technique for improving the fatigue strength of a sintered titanium alloy produced by the elemental powder mixing method, a method of performing HIP after water quenching from the β region has been proposed (Japanese Patent Publication No. 1-29864). Gazette). Further, a technique has been proposed in which a sintered titanium alloy is quenched from the β region and then annealed in the α + β region to improve the fatigue strength (US Pat.
No. 36,234). These are all intended to increase the fatigue crack initiation resistance due to the refinement of the α crystal.

However, as described above, it is necessary to perform water quenching or oil quenching from the β region as described above, so that a brittle reaction layer is formed on the surface layer of the sintered product. Cracks are easily generated. Therefore, in order to secure a desired fatigue strength, it is necessary to remove this reaction layer by machining or the like.

That is, these methods eliminate the advantage of obtaining a near net-shaped sintered product in powder metallurgy. Not only that, heat treatment, HI
It is necessary to add extra steps such as P and surface layer removal,
Not practical. Further, the powder sintered titanium alloy thus obtained has insufficient ductility. Thus, a technique for obtaining a high-density powder sintered titanium alloy excellent in ductility and fatigue strength at low cost and with high productivity has not yet been established.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is capable of producing a high-density powder sintered titanium alloy excellent in ductility and fatigue strength at low cost and with high productivity. An object of the present invention is to provide a method for manufacturing an alloy.

[0008]

According to the present invention, in order to solve the above-mentioned problems, a powder having an alloy composition to be obtained is prepared, the powder is formed into a compact, and then the compact is sintered. To form a β single phase in the α + β alloy,
When the β transformation temperature of this alloy is expressed as Tβ, Tβ +
The temperature range from 50 ° C or higher to Tβ-100 ° C or lower is 1
Provided is a method for producing a high-density powdered sintered titanium alloy, characterized by cooling at a rate of -10 ° C / sec.

The inventors of the present invention have conducted various studies on a method for obtaining a high-density powder sintered titanium alloy excellent in ductility and fatigue strength at low cost and with high productivity. As a result, the cooling rate after sintering is within a certain range. It was found that it was only necessary to control the temperature and to obtain a moderately fine structure.

[0010] The ductility and fatigue strength of a titanium alloy depend on the size and form of the α crystal, and slip tends to occur at the acicular α in coarse grains and the grain boundary α at the grain boundary, and the ductility and fatigue strength are high. Decrease. In normal vacuum sintering, the sintered body is furnace-cooled after sintering, but in this process, the α-crystals of the sintered body are enlarged and fatigue cracks are easily generated, so that the fatigue strength is reduced. On the other hand, as described above, the α phase can be refined by water quenching from the β region, but in this case, a reaction layer is formed on the surface, so that it is necessary to remove it in order to increase the fatigue strength. There is. In addition, the titanium alloy thus obtained has insufficient ductility.

In consideration of the above, the inventors of the present invention have found that, after sintering, the vicinity of the β transformation point is cooled at a certain rate faster than furnace cooling and slower than water quenching, so that a moderate They have found that a high-density powder sintered titanium alloy having a balance between ductility and fatigue strength can be obtained, and have completed the present invention. Hereinafter, the present invention will be described in detail.

In the present invention, a powder having an alloy composition to be obtained is prepared, and this powder is formed into a compact.
In this case, the composition of the powder is adjusted to the composition of the α + β type alloy. For molding, any method usually used in the field of powder metallurgy may be used. For example, a molded article can be formed by pressure molding using a mold.

Next, the compact is sintered, and the sintering is performed by heating the compact to the β region.
At this time, as described above, the cooling rate after sintering determines the form and size of the precipitated α-crystal, so that the cooling rate is regulated within a certain range to control the form and size of the α-crystal. In this case, if the cooling rate is less than 1 ° C./sec, the α crystal becomes coarse,
Fatigue strength decreases. On the other hand, when the temperature exceeds 10 ° C./sec, the structure becomes very fine and the strength is increased, but the ductility is reduced. Therefore, in the present invention, the cooling rate is defined in the range of 1 to 10 ° C./sec, at which such inconvenience does not occur.

The temperature range in which the cooling rate specified as described above is applied also has a significant effect on the morphology of the tissue. On the phase diagram, the α crystal precipitates below Tβ, so that the above cooling may be started from the β single phase region. However, when starting from a temperature lower than Tβ + 50 ° C., the cooling may occur due to component shift or partial segregation. There is a possibility that a coarse α-crystal grows partly before and becomes coarse. When cooling is completed at a temperature exceeding Tβ−100 ° C., α crystals grow and become coarse. Since such coarse α crystals are harmful to the fatigue strength as described above, the cooling start temperature is defined as Tβ + 50 ° C. or more, and the cooling end temperature is defined as Tβ−100 ° C. or less.

The present invention defined as above is realized by a vacuum furnace having a gas cooling function. If such a vacuum furnace is used, an inert gas such as Ar gas is introduced after sintering the compact, and the sintered compact is cooled, thereby forming a reaction layer on the surface without reheating. Without this, a high-density powdered sintered titanium alloy can be produced with good productivity and low cost.

[0016] Even if the heat treatment of the present invention is applied to the ingot, the β grains are coarsened during cooling, and the α crystals grow, resulting in reduced ductility and fatigue strength. Therefore, the heat treatment conditions specified in the present invention are effective only for the powder sintered titanium alloy. This is because, in the case of a sintered body, pores in the sintered body play a role of pinning, and even when cooled at a cooling rate in the above range, which is slow for the ingot from the β region, coarsening of β grains is prevented. Because you can.

The titanium alloy applied to the present invention may be any alloy as long as it has an α + β phase at room temperature, and the composition of the mixed powder before sintering may be a desired alloy composition. May be prepared. A typical example of such an alloy is a Ti-6Al-4V alloy, but in weight percent:
1.0-4.0% Fe, 1.0-4.0% Mo,
More preferably, an alloy containing 3.0 to 7.0% of Al, 2.0 to 4.5% of V, and 0.5% or less of O and the balance of Ti and unavoidable impurities is used. Results are obtained.

Hereinafter, the reason why an alloy having such a composition exhibits good characteristics will be described. A composition that can be densely sintered in a short time at a low temperature without applying a treatment such as HIP was studied. To perform dense sintering at low temperature and short time, a high sintering rate is required. For this purpose, the diffusion rate in Ti is high, and it acts as an alloy element for Ti, and its mechanical properties It was concluded that it is sufficient to add an appropriate amount of an element that does not adversely affect water.

As a result of investigating an element having a high diffusion rate in βTi, it was found that Fe corresponds to such an element. That is, the diffusion rate (diffusion coefficient) of Fe is 950.
10 ^{-11 at} ℃ Order (m ² · Sec ^-1 ),
10 ^-13 which is the self diffusion rate of Ti 100 times greater than Further, Fe contributes to an increase in strength. However, if only Fe is used, pores are likely to be generated on the alloy component side due to the Kirkendall effect, which may hinder high density.

In order to prevent such a phenomenon, the inventors of the present invention have further studied and found that, together with the above-mentioned element having a high diffusion rate, an element having a low diffusion rate, that is, an element which delays sintering is added. I found something to do.

As a result of investigating elements having a low diffusion rate, Mo
Was found to correspond to such an element. Mo diffusion rate is 10 ^-10 And 1/10 of titanium
It is. Mo is completely dissolved in β titanium and functions as an alloy element.

However, although the composite addition of Fe and Mo promotes densification, the addition of only these elements does not make the sintered body sufficiently strong. As a result of further study from the viewpoint of improving the strength of the sintered body, Fe
It has been found that by adding appropriate amounts of Al and V in addition to Mo and Mo and controlling the O content, a sintered titanium alloy having a desired strength can be obtained.

Al is the only α-phase stabilizing element which forms a solid solution with Ti in a substitution type, and shows remarkable solid solution strengthening. Also, Ti
Since the diffusion speed in the inside is high, addition of a composite with Fe and Mo also has the effect of increasing the density of the sintered product. V is a β-phase stabilizing element which is completely dissolved in Ti and has an effect of increasing the strength without forming an intermetallic compound which is an embrittlement phase with Ti. That is, V mainly forms a solid solution in the β phase to strengthen it. O has the effect of forming a solid solution in the α phase to significantly increase the strength.

Research by Majima et al. (Powder and Powder Metallurgy, 19
According to 87), there is a possibility that densification may progress up to the vicinity of the composition that does not enter the solid-liquid coexistence region in the binary system diagram of Fe and Ti. Further, from the binary phase diagram, it can be seen that the composition in which solid-liquid coexistence is exhibited at 1200 ° C. is 15% by Fe (the same applies to wt% or less). However, when this amount is large, ductility (toughness) is reduced. Further, if the amount of Fe is too small, the effect of densification cannot be obtained. Therefore, the Fe content is preferably 1 to 4%.

As described above, Mo is an element having a low diffusion rate in βTi, and achieves high density by coexisting with Fe having a high diffusion rate. But M
If the amount of o is less than 1%, the effect cannot be obtained, and if it exceeds 4%, the progress of densification is delayed, and the densification cannot be sufficiently achieved. Therefore, the content of Mo is preferably 1 to 4%.

Al is an element which remarkably strengthens the α phase as described above. However, if the content is less than 3%, the strength of the sintered product cannot be made sufficient. If it exceeds 7% which is an embrittlement phase DO ₁₉ type hcp ordered phase α ₂ (Ti ₃ Al) is precipitated, thereby deteriorating the ductility. Therefore, the content of Al is preferably 3 to 7%.

V has a function of solid solution strengthening the β phase,
If the content is less than 2%, the strength of the sintered product cannot be made sufficient. On the other hand, when it exceeds 4.5%, ductility is reduced. Therefore, the content of V is preferably 2 to 4.5%.

O is brought in from the Ti particles and the master alloy powder (alloy component), and has a function of dissolving in the α phase to significantly increase the strength as described above, but this amount exceeds 0.5%. And harm ductility. Therefore, the content of O is preferably 0.5% or less. If a titanium alloy having such a composition range is used, material properties exceeding those of the Ti-6Al-4V alloy can be obtained. In order to obtain an alloy having such a composition, as described above, the composition of the mixed powder before sintering may be adjusted to a desired alloy composition.

In the present invention, it is preferable to prepare a powder having an alloy composition to be obtained by mixing a titanium powder and a previously alloyed alloy powder. By preparing and sintering the mixed powder after preparing the powder in this way, the uniformity is improved and the densification can be promoted. An example of such a method is described in Japanese Patent Publication No. 2-50172. this is,

(A) A pre-alloy composed of two or more metals, which has a mean particle size of 0.5 to 20 μm by using a pulverizer capable of giving high energy to alloy-forming particles which can be alloyed with titanium. Crush to size,

(B) This and average particle size of 40 to 177 μm
To form a powder mixture in which the weight ratio of the titanium-based metal particles is 70 to 95% and the balance is the alloy-forming particles. The main point is that the green compact is formed into a green compact having a density of 80 to 90%, and (d) the green compact is sintered at a temperature lower than a temperature at which a liquid phase is generated.

By using this method, the theoretical density of 9
A titanium alloy sintered body having a density of 9.0 to 99.8% can be obtained, and a density much higher than 94.5 to 96.5% of a titanium alloy prepared by simply mixing elementary powders can be obtained. It becomes possible. The mechanical properties are almost the same as those of the smelted / forged titanium alloy. In the case of a titanium alloy having a conventionally used composition, sintering can be performed at a sintering temperature of 1260 ° C. for 4 hours by using this method. It becomes possible.

[0033]

Embodiments of the present invention will be described below. (Example 1) In order to obtain a composition of Ti-6Al-4V,
A master alloy powder of 60 Al-40 V having an average particle size of 3.5 μm and a pure titanium powder having -100 mesh (average particle size of about 75 μm) were mixed by a V blender. Then, the mixed powder thus produced was filled in a steel mold, and the pressure was 5.0 ton / cm ^2. With the tensile test piece of FIG. 1 (JIS
Z2550) and the fatigue test piece of FIG.
It molded so that it might become. The density of the obtained compact (compact) was about 85% of the theoretical value.

Next, there is provided a cooling chamber, which is burned by an inert gas.
The formed body was sintered by performing a heat treatment at 1250 ° C. for 4 hours in a vacuum of 10 ⁻⁵ Torr using a vacuum furnace capable of cooling the compact.

After sintering, heating was stopped and cooling was performed under the conditions shown in Table 1. In Table 1, Nos. 1 and 2 are Examples cooled under the conditions specified in the present invention, and Nos. 3 to 6 are Comparative Examples cooled under conditions deviating from the present invention.

The β transformation temperature Tβ of the sintered Ti-6Al-4V alloy was determined by the electric resistance method as 10
It was confirmed that the temperature was 10 ° C. The cooling rate in Table 1 is
Tβ + 50 ° C to Tβ-100 ° C, that is, 1060 ° C
5 shows the average cooling rate between to 910 ° C.

[0037]

[Table 1]

All of the obtained sintered bodies had a density of 99, the theoretical value.
%. These sintered bodies were subjected to a tensile test and a fatigue test without machining. In the tensile test, tensile strength and elongation at a gauge length of 25 mm were determined. On the other hand, the fatigue test was performed using a hydraulic servo tester,
10 ⁷ at a repetition rate of 10 Hz and a stress ratio of 0.1 The times fatigue strength was determined. Table 2 summarizes the obtained results.

[0039]

[Table 2]

As is clear from this table, it was confirmed that in the examples within the scope of the present invention, a powdered titanium alloy having excellent strength-ductility balance and high fatigue strength was obtained. On the other hand, in the comparative example, the strength-ductility balance was poor or the fatigue strength was low.

Example 2 In this example, a titanium alloy was used.
An experiment was conducted in which the composition of was changed. Here, Fe
1.0-4.0%, Mo 1.0-4.0%, Al
3.0-7.0%, V 2.0-4.5%, O 0.5
% Was prepared. In this range of composition
So that master alloy powder and sponge titanium powder
And mixed under the same conditions as above to produce a mixed powder
did. Then, the mixed powder was filled in a steel mold, and a pressure of 5.
0 tonf / cm^Two And a tensile test with a shape as shown in FIGS.
Obtained specimens and fatigue specimens were vacuum-baked at 1150 ° C.
Performed. As a result, anything within this composition range
The total density is 99.5% in less than 140 minutes
Reached. By the way, 1150 for Ti-6Al-4V
Must be fired at 20 ° C for 20 hours.
Was confirmed to have extremely good sinterability.

Next, a compact (pressure) containing 2% of Fe, 2% of Mo, 4.5% of Al, 3% of V, 0.2% of O and 0.2% of O and the balance of Ti in the above composition range. The powder was subjected to a heat treatment at 1150 ° C. for 2 hours in a vacuum of the order of 10 ⁻⁵ Torr in a vacuum furnace having the above-mentioned cooling function, thereby sintering the compact.

After sintering, heating was stopped and cooling was performed under the conditions shown in Table 3. In Table 3, Nos. 7 and 8 are Examples cooled under the conditions specified in the present invention, and Nos. 9 to 12 are Comparative Examples cooled under conditions deviating from the present invention.

The β transformation temperature Tβ of the alloy having this composition
Was 905 ° C. as a result of measurement by an electric resistance method. The cooling rate in Table 3 indicates the average cooling rate between Tβ + 50 ° C. and Tβ−100 ° C., that is, between 955 ° C. and 805 ° C.

[0045]

[Table 3]

All of the obtained sintered bodies had a density of 99, the theoretical value.
%. Then, by the same method as in the first embodiment,
The tensile properties and fatigue strength of the sintered body were determined. Table 4 summarizes the obtained results.

[0047]

[Table 4]

As is clear from this table, it was confirmed that in the examples within the scope of the present invention, a powder sintered titanium alloy having excellent strength-ductility balance and high fatigue strength was obtained. On the other hand, in the comparative example, the strength-ductility balance was poor or the fatigue strength was low. Next, the results of Examples 1 and 2 are collectively shown in FIGS.

FIG. 3 shows the relationship between tensile strength and elongation. As is clear from this figure, the sintered body manufactured under the conditions within the range of the present invention is a Ti-6Al-4V alloy having a tensile strength of 97 kgf / mm ^2. Above, elongation 12% or more, Ti
-2Fe-2Mo-4.5Al-3V-0.2O alloy with a tensile strength of 109 kgf / mm ² As described above, it was confirmed that the composition had excellent strength-ductility of 10% or more.

FIG. 4 shows the relationship between the cooling rate and the fatigue strength. As is clear from this figure, the sintered body manufactured under the conditions within the scope of the present invention is Ti-6Al-4V
37kgf / mm ^{2 with} alloy As described above, Ti-2Fe-2Mo-
40kgf / mm ² with 4.5Al-3V-0.2O alloy It was confirmed that it had the above high fatigue strength.

As is clear from any of the figures,
It was confirmed that the Ti-2Fe-2Mo-4.5Al-3V-0.2O alloy exhibited better characteristics than the Ti-6Al-4V alloy.

[0052]

According to the present invention, there is provided a method for producing a high-density powder sintered titanium alloy which can produce a high-density powder sintered titanium alloy excellent in ductility and fatigue strength at low cost and with high productivity. You.

[Brief description of the drawings]

FIG. 1 is a diagram showing a tensile test piece used in an example of the present invention.

FIG. 2 is a view showing a fatigue strength test piece used in an example of the present invention.

FIG. 3 is a diagram showing a relationship between tensile strength and elongation.

FIG. 4 is a diagram showing a relationship between a cooling rate and fatigue strength.

────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification symbol FI C22F 1/00 630 C22F 1/00 630K 687 687 692 692A 692B (56) References JP-A-5-140708 (JP, A) Kohei 2-50172 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 1/04 B22F 3/10-3/24 C22F 1/18

Claims (3)

(57) [Claims] 1. A powder having an alloy composition to be obtained is prepared.
This powder is formed into a compact, and then the compact is sintered into a β single phase in an α + β alloy, and then, when the β transformation temperature of the alloy is expressed as Tβ, Tβ + 50
The temperature range from T.degree. C. or higher to T.beta.
A method for producing a high-density powder sintered titanium alloy, comprising cooling at a rate of 0 ° C./sec. 2. The powder of the alloy composition has a content of 1.0% by weight.
-4.0% Fe, 1.0-4.0% Mo, 3.0-4.0%
7.0% Al, 2.0-4.5% V, and 0.5%
The method for producing a high-density powdered titanium alloy according to claim 1, wherein the following O is contained, and the balance is made of Ti and unavoidable impurities. 3. The high-density powder sintered titanium alloy according to claim 1, wherein the powder of the alloy composition is a mixed powder obtained by mixing a titanium powder and a pre-alloyed alloy powder. Manufacturing method.