JPH05311366A - Member made of titanium alloy excellent in fatigue strength and its manufacture - Google Patents

Abstract

PURPOSE:To form the structure of only the inside of a titanium alloy member into a one of rough planar crystals or acicular crystals and to improve its fatigue strength by heating the inside of a titanium alloy member in which the structure of the whole body is formed of a fine equiaxial one while its surface is cooled. CONSTITUTION:A titanium alloy is subjected to hot working at a relatively low temp. to form its structure into a fine equiaxial one. On the other hand, even if working is executed, at the time of executing cooling from a high temp., the structure of acicular crystals or planar crystals is formed by transformation occurring in the process. The inside of a pipe having 20mm diameter is heated to 1150 deg.C for 5sec by a thermocouple, and the outside is charged to a sintered alumina segmental die and is cooled to 550 deg.C, by which the equiaxial structure on the surface layer is hanged into the one of acicular crystals on the inside at the point of 0.15mm, and its fatigue strength is regulated to 720MPa.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titanium alloy machine structural member having excellent fatigue strength.

[0002]

2. Description of the Related Art Titanium alloys typified by Ti-6Al-4V alloys are used for manufacturing various mechanical structural members because of their advantages of light weight and high strength.

Since most of its uses are repeatedly stressed like a connecting rod of a reciprocating engine,
It is desired that the member has high fatigue strength. However, the fatigue strength of titanium alloy members currently manufactured is not always satisfactory.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a titanium alloy member having improved fatigue strength and a method for manufacturing the same.

[0005]

A titanium alloy member having excellent fatigue strength according to the present invention is a member made of titanium alloy and having a desired shape, in which the surface layer has a fine equiaxed crystal structure. The texture of is a rough plate crystal or needle crystal.

The "surface layer portion" refers to a region having a depth of about 50 to 200 μm from the surface of the member.

According to the method of the present invention for producing a titanium alloy member having excellent fatigue strength, the titanium alloy is processed to cause plastic deformation so that the titanium alloy has a desired shape and the whole structure is a fine equiaxed crystal. And a member that is
By heating the inside by energizing while cooling the surface, the inside is transformed into a rough plate-like crystal or acicular crystal structure while maintaining the equiaxed crystal structure of the surface layer portion.

Various means are available for cooling the surface.
The simplest of these is blowing cold air or flowing cold water, but in the case of a member having a relatively complicated shape, in order to cool the entire surface as uniformly as possible, the member should be powdered in an appropriate container. Techniques such as immersing the powder in the body or cooling the powder from the outside of the container are useful. When mass-producing members, prepare a split mold that matches the outer shape of aluminum or copper with high thermal conductivity, and apply a ceramic coating to the surface in contact with the members to insulate them electrically. It is advisable to manufacture and use a cooling jig that has a relatively high thermal conductivity.

The present invention can be applied to all Ti alloys forming an equiaxed (α + β) structure. Among the titanium alloys currently in practical use, those satisfying this condition include Ti-3Al-2.
5V, Ti-5Al-2.5Fe, Ti-5Al-2.
5Sn alloy and the like.

[0010]

FUNCTION These titanium alloys become a fine equiaxed crystal structure by hot working at a relatively low temperature. On the other hand, if the alloy is cooled from a high temperature even if it is processed, a needle-like crystal or plate-like crystal structure is formed by the transformation that occurs in the process. T
Speaking of i-6Al-4V alloy, when forging is started at 900 ° C. and finished at 700 ° C., the structure is an equiaxed crystal as shown in FIG. 1, and when forged at 1100 ° C. and finished at 1000 ° C., The structure becomes acicular crystals as shown in FIG.

Generally, in a titanium alloy, the equiaxed structure is unlikely to cause cracks which are the starting points of fatigue fracture. Therefore, the working conditions are usually selected so as to obtain this structure, but as described at the beginning, the fatigue strength is not sufficiently high. The reason is that once a crack occurs,
The propagation is rapid and easily leads to the destruction of the member. On the other hand, the rough plate-like crystal or needle-like crystal structure is likely to cause cracks but tends to be difficult to propagate.

In the present invention, paying attention to such a difference in behavior of the structure, the surface has a fine equiaxed crystal structure in which cracks hardly occur, and the inside has a rough plate-like or needle-like structure in which cracks hardly propagate. The fatigue strength as a whole is improved by complementing both.

In order to realize such a combination of structures, first, the whole structure is made into a fine equiaxed crystal by plastic working, then the surface structure is preserved as it is, and only the inside is rough plate-like crystals or needle-like crystals. In order to crystallize, the procedure of heating to high temperature and cooling is included. Electrical heating while cooling the surface is a means of achieving this.

As will be readily understood from the above description,
The temperature that the inside should reach by the electric heating must be such that transformation occurs in the process of cooling from there, and the surface must be kept within a temperature range where the original structure does not change. Since the temperatures thereof are slightly different depending on the alloy composition, the conditions of actual operation should be determined depending on the case by conducting some experiments if necessary.

[0015]

Example: By weight, Al: 6.04%, V: 4.07
%, O: 0.14%, N: 0.002% and H: 0.
Ti-6Al- containing 002% and the balance being Ti
The 4V alloy was produced by plasma melting followed by vacuum arc secondary melting. The ingot is rolled and forged to a diameter of 2
It was a 0 mm round bar. Forging temperature (start → end) 900
When the temperature was changed to 700 ° C. (sample “A0”), the structure was as shown in FIG. 1, and when the temperature was changed from 1100 to 1000 ° C. (sample “B0”), the structure was shown in FIG. 2. ..

The above round bar was machined to a diameter of 8 mm. Taking several samples A0, opening a hole from one end thereof toward the center in the axial direction, inserting a thermocouple inside and contacting the thermocouple with the surface, while cooling the surface by the method shown in Table 1, Electricity was applied to heat the inside to a high temperature. Table 1 shows the temperature of the surface and the center and the heating time together with the cooling method.
Shown in.

Table 1 No. Cooling method Surface temperature Center temperature Heating time A1 Sintered alumina split mold 550 ° C. 1150 ° C. 5 seconds A2 BN embedded in powder 650 ° C. 1150 ° C. 10 seconds A3 air blow 750 ° C. 1110 ° C. 30 seconds A4 air blow 1050 ° C. 1220 ° C. 60 The above samples A0 and B0 cut into a diameter of 8 mm,
The surface of each of the samples A1 to A4 was polished into smooth test pieces, and the Ono-type rotary bending fatigue test was performed. Table 2 shows the structure and the transition position of the structure (depth from the surface) together with the value of fatigue strength.

Table 2 section Sample No. Surface structure Internal structure Transition position Fatigue strength mm MPa Example A1 equiaxed needle-like crystal 0.15 720 A2 equiaxed needle-like crystal 0.10 680 A3 equiaxed needle-like crystal 0.05 650 Comparative example A equiaxed equiaxed-585 B needle Needle-like crystals-505 A4 Needle-like crystals Needle-like crystals-495

[0019]

INDUSTRIAL APPLICABILITY The titanium alloy member of the present invention has a structure in which the surface layer portion is a fine equiaxed crystal and a crack which is a starting point of fracture is hard to occur, and the inside is a rough plate crystal or needle crystal. Since the structure is such that cracks do not easily propagate, the fatigue strength is improved as compared with the conventional member having an equiaxed crystal structure as a whole.

The improved member can be manufactured by a simple and easily controlled method of internal heating by energizing under surface cooling.

The present invention, like the connecting rod described above,
It can be applied not only to members to which repeated stress is applied, but also to products such as rolling bolts, which is useful for increasing the reliability and durability of the machine.

[Brief description of drawings]

FIG. 1 is a copy of a microscope field of view (magnification: 480) showing the structure (equiaxed crystal) of the surface layer of a titanium alloy member of the present invention.
Times).

FIG. 2 is a copy of a microscope field of view (magnification: 480 times) showing the internal structure (acicular crystals) of the titanium alloy member of the present invention.

Claims (2)

[Claims] 1. A member made of a titanium alloy and having a desired shape, wherein the surface layer has a fine equiaxed crystal structure and the internal structure has a rough plate crystal or needle crystal structure. Titanium alloy member with excellent fatigue strength. 2. A titanium alloy is processed to cause plastic deformation to obtain a member having a desired shape and having a fine equiaxed crystal structure as a whole, and this member is energized while cooling its surface. A method for producing a titanium alloy member excellent in fatigue strength, which comprises heating the inside to change the inside into a coarse plate-like or needle-like structure while maintaining the equiaxed crystal structure of the surface layer portion.