METHOD FOR FORGING TITANIUM ALLOY AND FORGED TITANIUM ALLOY MATERIAL

Abstract

 The invention provides a titanium alloy having a narrow distribution of material properties in the thickness direction, allowing easy to finish the surface of the forged material after forging, during working the product to the final shape. The forged titanium alloy has a low sensitivity for cracking, excellent workability, and favorable ductility and fatigue properties, and provides a method for forging the titanium alloy. In order to attain the forged titanium alloy and the method for forging thereof, forging the titanium alloy, which has Tβ °C of the β - transus, is conducted, while keeping the relation of 400°C ≦ Td and (Tβ - 400)°C ≦ Tm ≦ Tβ at a strain rate of a range from 2 x 10^-4 s^-1 to 1 s^-1, further limiting the chemical composition of the titanium alloy, keeping the relation of [(Tm - Td) ≦ 250°C] during forging, where Tβ (°C) is the β- transus of the titanium alloy, Tm(°C) is the temperature of the work material for being forged, and Td(°C) is the temperature of a die. The titanium alloy, which is produced by the above-described method, has a fine microstructure to be controlled for forming a specified microstructure, has a uniform and homogeneous material properties in the thickness direction, and has an excellent ductility and a fatigue properties.

Description

Background of the Invention

Field of the Invention

[0001] The present invention relates to a method for forging titanium alloy, and also to a preferable titanium alloy forging stock and to a preferable forged titanium alloy.

Description of the Related Art

[0002] Owing to the excellent material properties, titanium and titanium alloy are widely used in chemical plants, power generators, medical instruments, and aircraft components. In particular, α +β type titanium alloy has a light weight and has a high strength so that this type of titanium alloy has widely been used in several fields. For instance, a turbine blade has a tendency to enlarge the size and to reduce the weight, in order to aim at the higher efficiency of facilities. In this field, the titanium alloy has been used. And, this type titanium alloy has been utilized in aircraft components such as landing gears, which request reducing the weight, taking the object of usage into consideration. Furthermore, the α+β type titanium alloy has been utilized in movable machine parts, such as automobile parts including connecting rod and valve, and commercial goods such as golf club head.

[0003] However, generally speaking, titanium alloys have high susceptibility to cracking, compared with steels, which are widely used in the industries at present. And the hot deformation resistance of titanium alloys is relatively higher at a low temperature range, so it is necessary to work at high temperature range for titanium alloys. With regard to these characteristics, they are described in the "TITAN NO KAKO GIZYUTSU" published by Japan Titanium Society. In a hot working process within high temperature range, especially about forging, there are some technical issues such as surface oxidation and grain coarsening in higher temperature range, and cracking caused by brittle α-case when temperature is down. On the contrary, in a working process within a low temperature range, high hot deformation resistance happens as one of the technical issue. Moreover, the temperature drops by contacting with a tooling and consequent deterioration of workability also happen as one of the technical issues. And it occurs a problem that inhomogeneous microstructure is formed by the adiabatic heat during working at the high strain rate.

[0004] As mentioned above, processing window of titanium alloy is very narrow. Furthermore, in case of applying a conventional forging process, the resulted microstructure is different, in comparison with near the surface area, where temperature drop is caused by contacting with die, and the mid-thickness portion
where the temperature drops slowly or the temperature increases by the adiabatic heat. And, particularly near the surface layer, from time to time, working within a low temperature range causes elongated microstructure and the working within a low range causes increase of hardness. As a result, some sorts of problems are apt to happen, concerning the defective material properties.

[0005] However, from the viewpoint of manufacturing process, several times of reheating and repeated forging are indispensable, due to the narrow processing window of titanium. Furthermore, deterioration of material properties such as ductility and fatigue properties, which is caused by grain coarsening, is also one problem, additionally to complicating the forging process. And there arise some sorts of problems, which means, finishing the oxidized surface should be indispensable after forging. Especially in case of dealing with a complex shape of the forged products, taking into consideration that the microstructure is changeable by reheating, the number of repetition cycle to reheat and to forge should be limited. And the forging independently may not always attain a satisfactory requested final shape. In that case, the finishing allowance increases the working load increases, and the yield of the charged material decreases. Furthermore, the oxidized scale and the deteriorated surface layer such as á-case significantly influence on the material properties, so it becomes necessary to remove the deteriorated layer in the actual use of forging. In addition, in case that no satisfactory final shape is obtained, grinding is required to an excessive degree. That is to say, the narrow processing window and the grinding after hot working, bring out the higher cost. Accordingly, concerning the production of the titanium products, the working cost becomes higher, additional to the higher material cost.

[0006] In order to solve these problems as a concrete means, the forging methods, which means, spending more time and spending more labor, in comparison with the prior arts, are adopted in the present invention. One concrete method in the present invention is isothermal forging and hot die forging. In some cases in recent years, there has been described in "Materials Properties Handbook Titanium Alloys", "TITANIUM TECHNOLOGY", and "TITANIUM AND TITANIUM ALLOYS", published by ASM. These methods adopt forging by heating not only the work material for forging but also the die. The work material and the die are heated to the degree of the same temperature with that of the work material for forging. Or, elsewhere, the work material and the die are heated to the degree of the very close temperature with that of the work material for forging.

[0007] And when this method are used, the strain rate is strictly controlled as low as at the degree of around 10^-4 to 10^-5 s^-1. For example, isothermal forging of Ti-6Al-4V alloy is done by selecting the temperature of work material within an approximate range from 900°C to 950°C. And the temperature of die is also controlled within an approximate range from 900°C to 950°C. Also when the hot die is forged, the die temperature is controlled to the degree of an approximate range, which is, from 650°C to 800°C. The range is very close to the temperature of the work material. These methods make it possible to suppress the temperature drop of work material. The methods invite the results in attaining favorable metal flow for obtaining a precise shape by way of forging. Furthermore, the number of reheating cycles decreases. The charged weight of the work material is saved. Additionally, the uniform microstructure through thickness can be obtained.

[0008] Since these methods depend mainly on working at a low strain rate, the forging load decreases to some extent. Furthermore, forging in such an atmospheric condition, that is, the titanium is suppressed to be oxidized, for example, making use of an inert gas and making use of a vacuum atmosphere, enable us to suppress the oxidation.

[0009] However, in these methods, the material is kept to be at a high temperature for a long time, because the work material and the die have the limitation to be heated, so that there happens a problem that the grain is coarsened. In addition, the die is heated to a degree of high temperature, whose temperature is as the same as the of the work material. Elsewhere, the die temperature heated is very close to the temperature of the work material. Therefore, the following kind of the die needs to be adopted. For instance, an expensive Ni-base alloy is used, which is durable within a high temperature range, and, which has excellent heat resistance and oxidation resistance, as described in "Materials Properties Handbook Titanium Alloys" of ASM. Additionally speaking, there has a possibility to cause a problem such that the electric discharge machinery is expensive, in order to work the die. Concerning the problem, it is easy to obtain a good metal flow by making use of the isothermal forging method and by the hot die forging method. However, the uppermost layer of the material, which gets contact with the die, receives the friction by the die. And the difference happens in the microstructure between the inner portion and the portion near the surface area occurs about some kinds of titanium alloy.

Summary of the Invention

[0010] The present invention provides a method for solving the problems of material and for carrying out the manufacturing method. Concretely speaking, the object of the present invention is to provide a titanium alloy that has less distribution of the material properties in thickness direction thereof. And another obj ect of the present invention is to provide the titanium alloy, which is requested to have fewer surfaces finishing after forging. And the titanium alloy has low sensitivity for cracking, excellent workability, and favorable ductility and fatigue properties. Simultaneously, the present invention is to provide a favorable forging stock and a method for forging.

[0011] Firstly, the present invention provides a method for forging a titanium alloy, which comprises:

preparing the titanium alloy as the forging stock;

forging the titanium alloy as the forging stock to have a work hardening factor, whose value is 1.2 or smaller, for obtaining a forged titanium alloy having a uniform material properties;

   wherein the work hardening factor is defined as

   wherein, Hv(ini) is the hardness of the titanium alloy as the forging stock before forging, and
      Hv(def) is the hardness of the forged titanium alloy under the reduction of 20%.

[0012] Secondly, the present invention provides the method for forging the titanium alloy according to firstly mentioned method, wherein the difference of the hardness between the thickness center portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.

[0013] Thirdly, the present invention provides a method for forging a titanium alloy, which comprises:

preparing the titanium alloy as a forging stock;

forging the titanium alloy as forging stock, at strain rates from 2 x 10^-4 s^-1 to 1 s^-1 , while keeping the relation of (Tâ - 400)°C ≦ Tm≦ 900°C and 400°C ≦ Td ≦ 700°C, to obtain a forged titanium alloy having a uniform material properties,

   wherein, Tβ(°C) is a β-transus of the titanium alloy,
      Tm(°C) is the temperature of the work material for forging, and
      Td(°C) is the temperature of a die.

[0014] Fourthly, the present invention provides the method for forging the titanium alloy according to the thirdly mentioned method, wherein the temperature of the die, Td(°C), and the temperature of the work material for forging, Tm(°C), are controlled to satisfy the relation of (Tm - Td) ≦ 250°C.

[0015] Fifthly, the present invention provides the method for forging the titanium alloy according to the thirdly and fourthly mentioned methods, wherein the titanium alloy as the forging stock contains Al: 4 to 5%, V: 2.5 to 3.5%, Fe:1.5 to 2.5% , and Mo.: 1.5 to 2.5%, by mass percentage.

[0016] Sixthly, the present invention provides the method for forging the titanium alloy, according to the thirdly, fourthly and fifthly mentioned methods, wherein
   a titanium alloy as the forging stock has an α+β microstructure,
   the aspect ratio of primary α-phase is 5 or less,
   the average grain size of primary α-phase is 10 µm or less, and
   the volume fraction of primary α-phase is within a range from 20% or more to 80% or less,
   wherein the aspect ratio is defined as the following ratio;
   the aspect ratio = the longitudinal length of a grain / the width of the grain, which is perpendicular to the longitudinal direction.

[0017] Seventhly, the present invention provides a forged titanium alloy, which comprises 1.2 or less of work hardening factor defined by Hv (def) / Hv (ini),
   where Hv (ini) is the hardness of the titanium alloy as the forging stock before forging, and,
      Hv(def) is the hardness of the forged titanium alloy under the reduction of 20% within a temperatures range from (Tβ - 400)°C or more to less than 900°C, wherein the β-transus (°C) of the titanium alloy is Tβ (°C).

[0018] Eighthly, the present invention provides the forged titanium alloy according to the seventh material, wherein the difference of hardness between a thickness center portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.

[0019] Ninthly, the present invention provides the forged titanium alloy according to the ninth material, consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of substantially Ti.

[0020] Tenthly, the present invention provides the forged titanium alloy according to the seventh material, wherein
   the titanium alloy as the forging stock has an α + β microstructure,
      the aspect ratio of primary α-phase is 5 or less,
      the average grain size of primary α-phase is 10 µ m or less, and
      the volume fraction of primary α-phase is within a range from 20% or more to 80% or less,
   wherein the aspect ratio is defined as the following ratio;
   the aspect ratio = the longitudinal length of a grain / the width of the grain, which is perpendicular to the longitudinal direction.

Brief Description of the Drawings

[0021] 

FIG. 1 is a graph showing the relationship between the heating temperature and surface oxidation in titanium alloys.

FIG. 2 is a graph showing the relationship between the average grain size of primary α-phase and the elongation.

Fig. 3 is a graph showing the relationship between the average grain size of primary α-phase and the fatigue strength.

Fig. 4 illustrates the forging method of Example 1.

Fig. 5 illustrates the forging method of Example 2.

Fig. 6 illustrates the forged shape of Example 3.

Description of the Preferred Embodiments

[0022] Concerning the detail of the present invention, the preferred embodiments have been described as follows.

[0023] The present invention has a specific technical feature that a mechanism of a grain boundary sliding with diffusional accommodation during being deformed at a given temperature are utilized efficiently, when a titanium alloy is forged. Some kinds of the titanium alloy have such a specific mechanism.

[0024] It is known that a large amount of deformation is attainable, due to the grain boundary sliding with diffusional accommodation, under the condition of the given temperature and under the condition of the given strain rate, when some kinds of titanium alloy are allplied to. In this case, the work hardening does not occur and a homogeneous microstructure can be obtained in the forged titanium alloys.

[0025] In the usual forging method, it is easy to be off the proper condition, due to temperature drop of the work material and the friction by contacting with the die in the conventional forging, even if the initial conditions are fitted to the grain boundary sliding with diffusional accommodation. From the standing point of view, which is, for solve the problems, the present invention provides that the temperature of the work material and that of the die are defined as their optimum range. And the present invention provides that the titanium alloy is forged to get the optimum composition and the optimum microstructure. Consequently, the forging method of the present invention could be found out such as an excellent workability, an excellent material property and an excellent surface property.

[0026] The mechanism of the grain boundary sliding with the diffusional accommodation in the forging process can be verified, by way of the comparison with hardness of work material between before and after forging. As an ideal concept, when the mechanism of the grain boundary sliding with the diffusional accommodation works in the forging, pile up (accumulation) of dislocation (transfomation) does not occur. As a result, the hardness does not increase by forging work. However, in the real method, increase of hardness is unavoidable in the actual forging, due to the ununiform temperature of the work material. Taking the above-mentioned facts into consideration, it is defined that the mechanism of the grain boundary sliding with the diffusional accommodation is working in the forging, when Hv(def)/Hv(ini) is 1.2 or less than 1.2 in this invention. Hv (ini) is the hardness of the titanium alloy as the forging stock before forging, and Hv(def) is the hardness of the forged titanium alloy under the reduction of 20% within a temperatures range from (Tβ- 400)°C or more to less than 900°C, wherein the β-transus (°C) of the titanium alloy is Tβ (°C). Reduction ratio of actual forging is from 20% to 80% although it depends on the final shape. So it is defined that Hv(def) is hardness of work material forged at 20% of reduction ratio.

[0027] When the material is deformed under the mechanism of the grain boundary sliding with the diffusional accommodation, work hardening is slight. Consequently, the difference of the hardness between the thickness center portion of the work material and near the near the surface area of the work material is small. Therefore, a uniform forged material can be obtained. Concretely speaking, there is no difference about the material properties on all of the portions, independent from the different located portions. If the value of the above-described work hardening factor is not more than 1.2, such a kind of the titanium alloy has the material properties, concerning the difference in hardness of Hv 60 or less between the surface layer and the inner portion. This hardness prevents from generating the different material property among each portion such as ductility and fatigue strength. (Note: Here in-above and here in-after, near the surface area is defined as within a range of approximately 5 mm or less distant from the surface of the material after forged, although the distance depends on the size of the forged product.

[0028] The forging condition for getting the work hardening factor 1.2 or less has been described, as follows.

[0029] According to the present invention, the forging is achieved on a titanium alloy, which has the β-transus of Tβ(°C) at a strain rate, whose range is from 2 x 10^-4 s¹ to 1 s^-1 , while keeping the relation of (Tβ - 400) °C≦ Tmβ ≦ 900°C and 400°C≦ Td ≦700°C. Here, Tm(°C) is defined as the temperature of the starting material for forging, and Td(°C) is defined as the temperature of die.

[0030] At first, according to the present invention, it is required to execute forging within the given temperature range and under the given condition about the strain rate, to induce deformation. The deformation is caused by the grain boundary sliding with the diffusional accommodation. Generally speaking, concerning the titanium alloys, the temperature range, which induces deformation caused by the grain boundary sliding with diffusive accommodation, is below the β-transus. Accordingly, the work material temperature Tm is required to be within a temperature range of below the β-transus.

[0031] If the forging temperature is below [Tβ - 400 (°C)], the work hardening factor becomes excessively more than 1.2. When the titanium goes on being forged, there is one possibility to generate a large amount of crack, even in case that the titanium has excellent workability. Consequently, One kind of difficulty happens, that is, the difficulty influences on producing the primary product and on the secondary product. Additionally, while the titanium alloy is worked, the deformation resistance increases remarkably. From the standing point of the capacity of the forging machine, it is not preferable to encounter with the above-mentioned kind of difficulty.

[0032] On the other hand, in a high temperature range, the oxidation proceeds on to a great extent. Therefore, both from one aspect of spending a lot of time on surface finishing of the forged titanium alloy after forged, and from the other aspect of production-yields after forged, it is an essential condition to forge the titanium alloy below 900°C, in order to suppress the oxide layer to the degree of 100 µm or less. FIG.1 shows the relationship between the heating temperature and thickness of oxidation layer of the titanium alloy. In case of the titanium alloys, as seen in FIG.1, the fact is found out that the oxidation on the surface of the titanium alloy increases rapidly, when heating temperature is over 900 °C. Within a temperature range less than 900°C, the oxidation of the titanium alloy is suppressed. And the thickness of the oxidized layer invites a satisfactory result, which is, sufficiently less than 100 µm, by the reason of suppression. In case that a temperature range for forging is adopted to be 870°C or less, the thickness of the oxidation layer is suppressed, being decreased to the degree of 50 im or less. In this way, the present invention makes it possible to suppress the oxidation layer of the titanium alloy, more and more.

[0033] Additionally speaking, determining the temperature of the die, Td(°C), which is 400°C or more, enables us to suppress the temperature drop of work material by contact with die. And, die temperature control makes it possible to prevent from deteriorating the workability of the forged material. Simultaneously with the above-mentioned results, the following good results are brought out. That is to say, a precise forgeability can be attained, and a crack can be avoided. The precise forgeability and the avoidable crack are adaptable to everywhere. It goes without saying such as the parts, which have a thinner thickness. The higher the die temperature is, the bigger becomes the suppressing temperature drop of work material. However, in case that the die temperature is higher than the β -transus, there happens a problem that the temperature of the work material for being forged has a possibility to rise up to the β-transus or more. Furthermore, even when at a temperature of the β-transus or less, and additionally when the temperatures above 700°C, an expensive material such as Ni-base alloy which has heat resistance and oxidation resistance, is required. So, this problem is not preferable, from the viewpoint of the cost-performance on forging. In addition, concerning the die production, which is made of the above-mentioned material, an expensive production method such as using an electric discharge machining one is required. Higher reheating temperature makes the die oxidize, and the temperature makes the tool oxidize, in addition to the corresponding work material. Then, the oxidation forces the die and the tool to live on for a short life.

[0034] From the other technical standing point of view, that is, the durability, which is mentioned above, it is not preferable to exceed the temperature of 700°C.

[0035] In order to induce deformation caused by the grain boundary sliding with diffusional accommodation during forging, and in order to keep work hardening factor of 1.2 or less, the following strain rate is necessary, which is, within a range of from 2 x 10^-4 s^-1 or more to 1 s^-1 or less. Compared with a strain rate in a conventional forging process, a slightly slower strain rate is determined. And compared with a strain rate in an isothermal forging process, a faster strain rate is determined. That is to say, between 2 x 10^-4 s^-1 or more to 1 s^-1 or less. This strain rate results in avoiding a long extended working time in the isothermal forging, and the strain rate results. in achieving an efficient forging. In addition, the mechanism of the grain boundary sliding with the diffusive accommodation is made utilize of. As a result, a favorable workability and a uniform microstructure after forging are attained. The above-mentioned factor invites, also, the effective result, that is, the material properties are much improved, such as ductility and fatigue properties.

[0036] Furthermore, taking the more chances to make utilize of the mechanism of the grain boundary sliding with diffusional accommodation into the consideration, the preferable strain rate in the forging process is arranged within a range of from1 x 10^-3 s^-1 or more to 0.1 s^-1 or less.

[0037] Furthermore, in order to keep the work hardening factor of 1.2 or less, and in order to keep the difference between the hardness of the thickness center portion of the work material and that of neighborhood of the surface area of the work material, as Hv 60 or less, it is preferable to execute the forging under the condition of, adding to the above-given condition, keeping the relation of [(Tm - Td) ≦ 250°C] between the die temperature, Td(°C) and the temperature of the work material for being forged, Tm(°C). The execution to forge under the relation of [(Tm - Td) ≦ 250°C] brings up the result of having improved the microstructure difference between near the surface area, where the cooling speed is fast, and the thickness center portion, where the cooling speed is slow. Forged products with uniform material properties can be obtained by this way. If the temperature difference between the die temperature, Td, and the temperature of the work material, Tm, is more than 250°C, it is unfavorable, because the ununiform material properties in the forged product is likely to be generated, caused by the temperature difference during forging between near the surface area and the thickness center portion. In case of particularly large-sized forging materials, it spends a longer time to forge the material. And the load for forging increases, too. From this standing point, it is effective to control the temperature by the manner that the temperature of the work material, Tm, and the temperature of the die, Td, come close to each other, such as by the manner to satisfy the relation of [(Tm - Td) ≦ 250°C].

[0038] In the present invention, the titanium alloy, which is used as the forging, stock preferably consists essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass percentage, and the balance of substantially Ti. The term "balance of substantially Ti" referred herein is defined as a material, which contains inevitable impurities and other trace-quantity elements, have a possibility to exist within the specified range showed in the present invention, unless these inevitable impurities and other trace- quantity elements cancel the function and the effect of the present invention.

[0039] Compared with conventional kinds of titanium alloy, the present invention allows the titanium alloy to deform, which causes by the grain boundary sliding with diffusional accommodation in a low temperature range from 700°C to 870°C. Therefore, without thicker oxidation scale, without deteriorating the surface layer, and without deteriorating the formation of α-case, the present invention enables the titanium alloy to be forged. The reason is written up as follows, why it is indispensable to specify the composition of the titanium alloy.

[0040] Al is an essential element for an α+β type titanium alloy, in order to stabilize the α-phase, and the Al has an effect on increasing the strength. If the Al content is less than 4%, the AL content cannot contribute to the degree of the sufficient material strength.

[0041] If the Al content exceeds 5%, the ductility and the toughness deteriorate. Both of the above-mentioned results, which mean, the material strength, the ductility and the toughness, are not preferable.

[0042] V, Mo, and Fe are elements, in order to stabilize the â phase and have an effect to increase the strength. The V content, if less than 2. 5% cannot contribute sufficiently to high strength. In this case, the β phase becomes unstable. On the contrary, if the V content exceeds 3.5%, lowering the β-transus causes the problem to narrow the processing window, and furthermore, adding increase of cost due to addition of a large amount of expensive alloying element.

[0043] Mo has an effect to refine microstructure and has an effect to suppress the grain growth. Fe has high diffusibility in titanium. With respect to these effects, which are caused by Mo and Fe, the precise forgeability increases. On the contrary, the hot deformation resistance during forging decreases. And the above-mentioned results bring up the additional good effects, such as improving the ductility and the fatigue properties after forging.

[0044] If the Mo content is less than 1.5%, a sufficient contribution to strengthening cannot be obtained. And also, the β phase cannot sufficiently be stabilized. If the Mo content exceeds 2.5%, lowering the β-transus causes to narrow the range of the processing window. Furthermore, effects of Mo and Fe are saturated by adding Mo and Fe within the range of 2.5 % or more, and by adding a large amount of an expensive alloying element causes high cost. Supplementary speaking, the β-phase becomes to be too stable. In this case, it is harmful for strengthening by solution treatment and aging. If the Fe content is less than 1.5%, contribution of Fe to strengthening is not sufficient, simultaneously without the β-phase being unstable. Furthermore, regardless with one of the good factors about Fe, which means, Fe has a characteristic to diffuse rapidly in titanium and to improve the workability efficiently, the advantage of such characteristic, which Fe has, cannot be effective on the preferable results. Contrarily, if the Fe content exceeds 2.5%, lowering the β-transus causes narrowing the processing window. Additionally speaking, segregation deteriorates the material properties. Furthermore, by specifying the alloy composition as described above, the mutual quantity ratio of α-phase and β -phase is getting to be closer to each other, within a temperature range of from 700°C to 870°C. It becomes easier to activate the mechanism of the grain boundary sliding with additional accommodation.

[0045] According to the present invention, the titanium alloy, which is used as the forging stock, it is preferable that the microstructure is a α+β type, whose aspect ratio has 5 or less of primary α-phase, has 10 µm or less of the average grain size of primary α-phase, and has from the range of 20 or more to 80% or less, as a volume fraction of primary α phase, where the aspect ratio is defined as the ratio of the following:

a) Longitudinal length of a grain
   To

b) Width of the grain, which is perpendicular to the longitudinal direction thereof.

[0046] That's to say, a)/b).

[0047] More preferably, the titanium alloy has 6 µm or less of the average grain size of the primary α-phase.

[0048] FIG. 2 is a graph showing the relationship between the average grain size of the primary α-phase and the elongation. As shown in the FIG.2, if the average grain size of the primary α-phase exceeds 10 µm, the elongation in the tensile test at high temperature decreases rapidly, whose phenomenon influences on the sensitivity for cracking and on the precise forgeability and the like.

[0049] Furthermore, the grain size of primary α-phase influences on the material properties of the forged product, such as the ductility and the fatigue properties. FIG. 3 shows the relationship between the average grain size of primary α-phase and the fatigue properties. As shown in the FIG. 3 , if the average grain size exceeds 10 µm, the sensitivity for cracking during forging increases, and the precise forgeability deteriorates, additional to a result that the material properties such as the ductility and the fatigue properties deteriorate.

[0050] The shape of the primary α-phase influences on the sensitivity for cracking and the precise forgeability. When the aspect ratio is defined as the ratio of longitudinal length of a grain to width thereof perpendicular to the longitudinal direction thereof, such as mentioned above, and in case that the aspect ratio of primary α-phase exceeds the value of 5, the primary α-phase cannot become into the equiaxed grain. Consequently, the precise forgeability deteriorates.

[0051] Furthermore, fine equiaxed microstructure improves susceptibility to cracking in the hot forging, suppresses the cracking during deformation at high strain rate and improves precious forgeability. An α+β type titanium alloy, generally, consists of primary α-phase and transformed β-phase. However, in case that the volume fraction of the transformed β -phase becomes to come within a range of from 20 or more to 80% or less, that's to say, in case that the volume fraction of the primary α―phase becomes to less than 20% or more than 80%, the sensitivity for cracking during forging increases, too. Not only the problem of the cracking, but the precise forgeability, the ductility and the fatigue properties of the material deteriorates.

[0052] In the present invention, even after forging, the forged product can have the microstructure, which is similar to that of the forging stock. The way means making use of the grain boundary sliding with diffusional accommodation. Owing to such advantageous characteristics, the present invention is extremely effective on improving the workability and the material properties, even in case of repeating the forging, and even in case of applying such forging process to the working for a complex forged shape.

EMBODIMENTS

[0053] In order to explain the above-described effective functions, there has been described the effects of forging conditions of titanium alloy, the chemical composition of the forging stock. Furthermore, the effects of the microstructure on the forgeability and the material properties after forging have been described to the examples.

[Example 1]

[0054] Cylindrical compression test samples, whose size are 15 mm in diameter and 22.5 mm in height, were cut from material "A01" as shown in Table 1. The sample was forged at reduction of 20% using a die made by SUS310, while varying the forging temperature, the die temperature, and the strain rate. Table 2 shows the forging conditions, the work hardening factor [Hv (def) /Hv (ini)], and the difference in hardness between near the surface area and the thickness center portion. The temperature of the work material, Td, in the formula (Tm - Td), was taken into consideration of temperature range from starting and finishing of forging.

[0055] Nos. 1 to 3 samples were forged under the conditions of the forging temperature, the die temperature, and the strain rate, whose values satisfy the conditions of the present invention. And the results invited a value of 1.2 or less as work hardening factor, and 60 or less as the difference of Vickers hardness between near the surface area and the thickness center portion. Consequently, the hot-forging process under the conditions of the present invention induces the deformation caused by the grain boundary sliding with diffusional accommodation. And the hot-forging in the present invention brings up an excellent results that uniform and homogeneous forged products can be obtained, which means, there is no difference in the each part of the forged material.

[0056] On the contrary, titanium alloys, which were forged under the conditions out of this invention, showed a large work hardening factor, more than 1.2, and showed 60 or more as the differential value in the hardness between near the surface area and the thickness center portion.

[Example 2]

[0057] Using the cylindrical compression samples, which have 15 mm in diameter and 22.5 mm in height, which have the chemical compositions and microstructures given in Table 1, hot forging was performed, as shown in FIG. 4. The hot forging was conducted, under as the same condition as Table 3, using a die of SUS310 and without lubricant. The workability, the condition of the oxidation surface, and the microstructure after forging at near the surface area on a protruded section and at the thickness center portion of the disk shaped section on the lower part, were evaluated. The results are given in Table 3. In Table 3, the mark "○" in the column "Crack" indicates "no crack occurred", and the mark "×" in the column indicates "crack occurred". Nos. 1, 13, and 24 in Table 3 has β-microstructure, so the average grain size of the primary α phase and the aspect ratio were not measured.

[0058] The microstructure of the forging stock and the microstructure of the forged product was evaluated by the average grain size of primary α-phase, the volume fraction of the primary α-phase, and the aspect ratio. The forgeability was evaluated by the precise forgeability in the actual forged result, and by the sensitivity for cracking, mainly by observing the surface condition of the forged product. The precise forgeability was evaluated by such a way as the comparison of the protrusion height, that's to say, how much degree of metal fills existed in the circular holes in the die with metal. (See FIG.4). That is, as illustrated in FIG. 4, the height including the height of the spike like shape protrusion, was defined as H. And the thickness of the disk portion was defined as T. Finally, the precise forgeability was evaluated by the ratio of the value H/T. In order to attain the favorable forgeability, the value of H/T needs 1.5 or more, preferably needs 2.0 or more. Moreover, in order to evaluate the results, concerning to how much degree the surface was finished after the material was forged, the thickness of the layer (oxidation layer), which was caused by the oxidation at the surface layer portion of the forged product, was measured.

[0059] With regard to the Nos. 1 and 13, whose forging temperatures were above the β-transus, cracks were observed. And the parameter value H/T, which evaluated the precise forgeability, was as small as around 1.2. From the judging point of the precise forgeability, it was poor. With respect to the Nos.1, from 13 through 18, and from 20 through 22, whose forging temperatures were above 900°C, the thickness value of the oxidation layer exceeded 100 µm. Concerning the Nos. 8 and 19, whose forging temperatures were low, cracks were observed. Furthermore, the H/T value was as small as around 1.2, resulting in the poor precise forgeability.

[0060] Thinking about the Nos. 6, 8, 18, and 19, whose die temperature were fallen outside of the range of the present invention, the parameter value H/T was as small as 1.5 or less. In a few cases, no crack happened, but in a lot of cases, they have inferior precise forgeability.

[0061] With regard to the Nos. 5 and 17, whose strain rate were fallen outside of the range of the present invention, the parameter H/T, which evaluated the precise forgeability, were smaller than 1.5. In a few cases, no cracks were observed. But, looking at the results in a lot of cases, there was inferior quality, from the standing point of the precise forgeability.

[0062] As described above, in case that the conditions were fallen outside of the range of the present invention, cracks were observed and precise forgeability was deteriorated. In this case, it could be found out that there generated no deformation, which was caused by the grain boundary sliding with diffusional accommodation.

[0063] Secondary, the effects of chemical composition of forging stock, average grain size, volume fraction and aspect ratio of primary a in the forging stock on the forgeability were studied..

[0064] From the viewpoint of the range of the composition in the present invention, Nos. A01 to A04 is satisfactory, and their microstructure is within the range of the present invention. No cracks were observed in Nos. from 2 to 4, from 7, and from 9 to 12. When the forging goes on, the conditions of the present invention are, absolutely, necessary, in order to obtain the good results. Additionally speaking, the above-mentioned case showed a very excellent forgeability, which can easily be understood by the extremely high value of H/T ≧ 2, as in the cases of Nos. from 2 to 4, 7, and from 9 to 12. Moreover, whichever the objective portion is, for instance, the thickness center portion after forging, or near the surface area after, the result ended up in the same microstructure. In this case, the same microstructure means that the forging stock has 10 µm or less of the average grain size of primary α-phase, 20 to 80% of volume fraction, and 5 or less of aspect ratio. Furthermore, it means, that no remarkable difference in the microstructure appeared between the thickness center portion and the near surface area. Consequently, the fine microstructure, such that no rough surface could generate, was obtained even on the near surface area.

[0065] In case that the forging stock, whose Nos. are from B01 to B06, C01, and D01, and whose chemical compositions are fallen outside of the range of the present invention, are made use of, the following results were shown. That is, in the Nos. 16, from 20 to 22, 25, and 26, except for the temperature of forging stock, the materials were worked, under the control of the same forging conditions as the present invention. In this case, it showed a resultant value of the 1.6 to 1.9 as H/T, which is 1.5 or more, and which is a criteria regarding whether the precise forgeability can be done or not. However, compared with the values of H/T ≧ 2.0 which is attained by using the forging stock according to the present invention, the value of H/T is not satisfactory, and it was revealed that the chemical composition and the microstructure of the forging stock, also, influence on the forgeability. Among these Nos., the Nos. 20 and 26, which used the materials of B02 and D01, respectively, and which satisfied the range of microstructure of the present invention, showed a high H/T value, 1.80 and 1.91, respectively. However, the microstructure after forging, was fallen outside from the range of the present invention. As a resultant problem, a rough surface happened. Not only about the above-mentioned Nos. but also about the Nos. 23 and 25, the microstructure after forging was fallen outside from the range in the present invention. In this case, the same problem, the rough surface happened.

[0066] With regard to the No. 23, the chemical composition and the microstructure went outside from the range in the present invention. Additionally speaking, the forging temperature was lower than the value of the Nos. 16, and from 20 to 22. Although these values were within the range of the present invention, the H/T value was 1.5 or less. More supplementary, the No. 24, which used B06 having the β-microstructure, cracks were observed, and the H/T value was low.

[0067] Since the β-transus of the materials from B01 to B06 was as high as 1,000°C, these materials were possible to be forged in a high temperature range, because the hot deformation resistance was small in the high temperature range. But, such a high temperature forging increases a amount of the oxidized layer to be formed. According to the examples in the present invention, the materials B01 through B04, and B06 adopted 950°C as the reheating temperature, and the starting temperature for forging was 900°C. Compared with the case of A01 through A04, which had 900°C of β-transus, the materials B01 through B04, and B06 adopted higher forging temperature. So, the thickness of oxidation layer became as thick as 150 µm.

[0068] With regard to the material, B05, which adopted 880°C of reheating temperature, in order to suppress the oxidization, and whose starting temperature for forging was 850°C, the low temperature deteriorated the forgeability to invite 1.5 or less of H/T value, although the thickness of the oxidation layer decreased. Furthermore, these examples that had compositions, which were not satisfying the range of the present invention, invited the result that there were difference about the microstructure, between near the surface area and the thickness center portion after forging. And there generated the rough surface, caused by the coarse grains and the elongated grain structure.

[Example 3]

[0069] Using the forging stock, A01 and B01 given in Table 1, whose size was 30 mm in width, 60 mm in height, and 70 mm in length, the hot-forging illustrated in FIG. 5 was conducted, under the conditions in accordance with Table 4. The resultant forged products had the size of approximately 30 mm in width, 20 mm in height, and 210 mm in length. From each of the forged products, samples were cut and prepared. The mechanical properties of these samples are evaluated from the judging points of the Vickers hardness, the tensile properties, and the fatigue properties of the flat plate test piece. The results are shown in Table 4.

[0070] The No. 1, which satisfied the temperature of the material for being forged, the temperature of the die, and the strain rate, according to the present invention, invited the difference (Δ Hv) of 60 or less as the Vickers hardness between two portions. That is, one portion means near the surface area, where the temperature drop by contact with die is significant. The other portion means the thickness centers portion, where the cooling speed is comparatively slow. In this case, the difference (Δ Hv) is 60 or less value, whose value is in accordance with a recommendable condition in the present invention. Changing the viewpoint from the tensile properties and the fatigue properties, the difference between these portions became smaller. The result brought up a excellent and possible method to produce the forged product that has a uniform and a homogeneous material properties. On the other hand, the No. 2, which was forged under the forging condition fallen outside from the range of present invention, invited 60 or more of ΔHv. In case of the No. 2, the difference in hardness happened, between near the surface area and the thickness center portion. And more kinds of the difference happened, that are, the material properties such as the static strength, the ductility, and the fatigue strength between these portions. The result is not preferable, from the standing point of the uniform and homogeneous material properties. As described above, it can be found out definitely that the forging conditions of the present invention are extremely important, from the high technological viewpoint of producing the forged product, which has a uniform and a homogeneous forged material.

[Example 4]

[0071] Using the forging stock, which was the No. A01 given in Table 1, and whose size was 150 mm in diameter and 750 mm in length, the hot forging was adopted, in order to obtain a shape shown in FIG. 6. The hot-forging was conducted under the condition of 800°C of heating temperature of the forging stock, 780°C of the starting temperature of forging, 670°C of the finishing temperature of forging, the die temperature range within from 650°C to 620°C during forging, and 2.3 x 10^-3 of the strain rate. In this case, the forgeability regarding a large-sized forged product was evaluated. Adaptable samples were cut and prepared from the forged shape at each position given in FIG. 6.

[0072] And the tensile strength as the material properties was evaluated. Furthermore, the fatigue strength as the material properties, while using the specimen that was prepared by the rotation-bending, test was evaluated. The results are shown in Table 5.

[0073] By making use of a forging stock, which has a chemical composition and a microstructure that satisfy the conditions of the present invention, it was found out that forging large-sized member of the titanium alloys can be attained. And, even when such forging is adapted to the titanium alloys, which has a difficulty to be worked as the property, the same attainable results could be found out. And in the present invention, it was found out that the material properties corresponding to the obtained forged product are extremely favorable.

The effectiveness of the present invention

[0074] As described above, the present invention makes it easily possible to provide a high strength forged product from the titanium alloy. The characteristics of the high strength forged product of the titanium alloy have a narrow distribution of the material properties, towards the thickness direction. This invention make it easily possible to remove the oxidation layer and the invention make it possible to finish the surface of the forged product, after forging, during being worked in order to obtain the final figure and shape. Furthermore, the invention makes it easily possible to obtain a less sensitivity for cracking, possible to obtain an excellent workability of the forged titanium alloy, a good quality about the ductility and about the fatigue strength. Finally, the present invention invites an excellent and a fine forged titanium alloy, whose strength is extremely high. Thus, the present invention has a big deal of effectiveness on the industrial and the applicable usage.

Claims

1. A method for forging a titanium alloy comprising the steps of:

preparing a titanium alloy as a forging stock, wherein the titanium alloy has a thickness center portion and near the surface area;

forging the titanium alloy as the forging stock to have a work hardening factor, whose value is 1.2 or less, for obtaining a forged titanium alloy having a uniform and homogeneous material properties, wherein the work hardening factor is defined as,

   where, Hv(ini) is the hardness of the titanium alloy as the forging stock before forging,    and,
      Hv (def) is the hardness of the forged titanium alloy under the reduction of 20%.

2. The method according to claim 1, wherein a difference of hardness between the thickness centers portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.

3. A method for forging a titanium alloy comprising the steps of:

preparing a titanium alloy as a forging stock;

forging the titanium alloy as the forging stock, at a strain rate within a range from 2 x 10^-4 s^-1 or more to 1 s^-1 or less, while keeping a relation of (Tβ - 400)°C ≦ Tm ≦ 900°C and 400°C ≦ Td ≦ 700°C, to obtain a forged titanium alloy having a uniform and homogeneous material properties;

   where, Tβ (°C) is a β-transus of the titanium alloy,
      Tm(°C) is a temperature of the work material for being forged, and
      Td(°C) is a temperature of a die.

4. The method according to claim 3, wherein the temperature of the die, Td (°C) , and the temperature of forging stock, Tm (°C), are controlled to satisfy the relation of (Tm - Td) ≦ 250°C.

5. The method according to claim 3, wherein the titanium alloy as the work material for being forged contains Al: 4 to 5%, V:2.5 to 3.5%, Fe : 1.5 to 2.5%, and Mo : 1.5 to 2.5% , by mass percentage.

6. The method according to claim 3, wherein,
   the titanium alloy as the forging stock has an α+β microstructure, an aspect ratio of primary α-phase is 5 or less, an average grain size of primary α-phase is 10 µm or less, and
   a volume fraction of primary α-phase is within a range from 20% or more to 80% or less,
   wherein the aspect ratio is defined as the following,
   the aspect ratio = longitudinal length of the grain/width of the grain, which is perpendicular to the longitudinal direction.

7. A forged titanium alloy comprising:

a thickness center portion and near the surface area;

a work hardness factor of 1.2 or less, wherein the work hardening factor is defined by Hv(def)/Hv(ini), where,
   Hv (ini) is the hardness of the titanium alloy as the forging stock before forging, and,
   Hv (def) is the hardness of the forged titanium alloy under the reduction of 20%,

   wherein the forging stock within a temperatures range from (Tβ - 400)°C or more to less than 900°C, and Tβ(°C) is a β- transus (°C) of the titanium alloy.

8. The forged titanium alloy according to claim 7, wherein difference of the hardness between the thickness center portion of the forged titanium alloy and near the surface area of the forged titanium alloy is 60 or less of Vickers hardness.

9. The forged titanium alloy according to claim 7 consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of substantially Ti.

10. The forged titanium alloy according to claim 7, wherein,
   the titanium alloy as a before forging has an α+β microstructure,
   an aspect ratio of primary α-phase is 5 or less ,
   an average grain size of primary α-phase is 10 µm or less, and
   a fraction of primary α-phase is within a range from 20% or more to 80% or less,
   wherein the aspect ratio is defined as the following ;
   the aspect ratio = longitudinal length of the grain / length of the grain, which is perpendicular to the longitudinal direction.

Drawing