JP2010280927A - INTERNAL COMBUSTION ENGINE COMPONENT MADE OF TiAl ALLOY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a TiAl-made internal combustion engine component, particularly a valve or a piston pin, which has high temperature high strength and high rigidity, and also is improved in ductility. <P>SOLUTION: The internal combustion engine component are produced through: an extruding process where a TiAl alloy containing, by atom, 40 to 42% Al and 2.4 to 2.6% Cr, and the balance Ti with inevitable impurities is extruded at 1,100 to 1,150°C at an extrusion ratio of ≥10 so as to obtain a molded body; an annealing process where, after the completion of the extruding process or during the extruding process, the molded body is held at 950 to 1,050°C for 2 to 5 h in a vacuum; and a cooling process where the molded body subjected to the annealing process is cooled to 400°C at a cooling speed of 10 to 60 °C/min in an inert gas. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a TiAl alloy internal combustion engine component having high temperature and high strength and high rigidity, and more particularly to a technique for improving ductility of valves and piston pins.

As a valve for an internal combustion engine, conventionally, a valve made of a TiAl alloy excellent in strength and rigidity at high temperature is known. For example, Patent Document 1 discloses a technique in which a TiAl alloy material is heated to 1000 to 1350 ° C. and extruded into a valve shape. In this technique, in order to prevent oxidation of the TiAl alloy material during heating, the TiAl alloy material is sealed in a metal case, and the case is removed by machining after extrusion. Patent Document 2 discloses a technique for forming a valve shape by compacting and sintering a TiAl alloy powder.
Further, as a piston pin for an internal combustion engine, a steel material that is carburized or nitrided is generally used. On the other hand, techniques using ceramics and composite reinforced metals having low specific gravity and excellent rigidity have been disclosed for the purpose of improving fuel efficiency by reducing inertia weight (Patent Documents 3 and 4).

JP 2000-24748 A JP 2000-355706 A Japanese Patent Laid-Open No. 10-61765 Japanese Patent No. 1809240

  By the way, since a valve for an internal combustion engine repeatedly collides with a valve seat, not only strength and rigidity at high temperature but also ductility, that is, impact resistance is required. For example, there is a demand for impact resistance that does not break when a large impact is applied to the valve by knocking or the like. However, despite the poor ductility of 1% for TiAl alloys, no attempt has been made so far to improve the ductility. As for piston pins for internal combustion engines, ceramics and composite reinforced metals have problems of impact resistance, strength, and cost, and the actual situation is that they have not reached a practical level in place of steel. TiAl alloy is also a promising material because of its low specific gravity and high rigidity, but it also has poor ductility in terms of impact resistance that can withstand the explosion load of an internal combustion engine, and it is necessary to improve this property to make it a practical part. there were.

  Accordingly, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a TiAl alloy-made internal combustion engine component with improved ductility as well as high temperature, high strength and high rigidity.

  The TiAl alloy internal combustion engine component of the present invention contains a TiAl alloy containing Al of 40 to 42 atom%, Cr: 2.4 to 2.6 atom%, and the balance of Ti and inevitable impurities at 1100 to 1150 ° C. Extruding step to obtain a molded body by extruding to 10 or more, annealing step for holding the molded body in vacuum at 950 to 1050 ° C. for 2 to 5 hours after completion of the extrusion step or during the extrusion step, and the annealing step are completed The molded body is manufactured through a cooling step of cooling the molded body to 400 ° C. at a cooling rate of 10 to 60 ° C./min in an inert gas.

Hereinafter, the grounds for limiting the numerical values of the present invention will be described together with the operation of the present invention.
Al content: 40-42 atom%
Al is an element that improves the strength and ductility of the TiAl alloy. If the Al content is less than 40 atom%, the ductility is lowered, and if it exceeds 43 atom%, the ductility is lowered together with the ductility. Therefore, the Al content is set to 40 to 42 atom%.

Extrusion temperature range: 1100-1150 ° C
The β-Ti phase is a highly ductile structure, and the present invention improves the ductility by precipitating a fine β-Ti phase at the grain boundary of the crystal structure as described later. In the Ti-Al equilibrium diagram, when the Al content is 40 to 42 atom%, the structure includes a β-Ti phase at 1150 ° C. or lower. Therefore, a large elongation can be obtained by extruding a TiAl alloy at 1150 ° C. or lower. Moreover, by extruding at 1150 degrees C or less, a structure | tissue refines | miniaturizes and it can obtain high intensity | strength. In other words, in the temperature range exceeding 1150 ° C., it becomes a structure in which the α-Ti phase or the γ phase is added thereto, and sufficient elongation cannot be obtained at the time of extrusion, and the structure is refined to obtain high strength. I can't. On the other hand, in the temperature range below 1100 ° C., the deformation resistance of the TiAl alloy is large and practical operation becomes difficult. The extrusion process is performed in the atmosphere with the TiAl alloy material sealed in a stainless steel case, and after the extrusion process, the case is removed by machining to obtain an internal combustion engine part.

Extrusion processing rate: Extrusion ratio of 10 or more By applying strong processing with an extrusion ratio of 10 or more in the above-described temperature range of 1100 to 1150 ° C., miniaturization is promoted, and the effect of the annealing treatment described later is maximized and large elongation is achieved Obtainable. When the extrusion ratio is less than 10, the fineness is insufficient and a sufficient effect cannot be obtained. The extrusion ratio is the ratio between the diameter of the material filled in the container and the diameter of the material after extrusion. In the present invention, it refers to the diameter of a portion that becomes a valve or a piston pin as an internal combustion engine component.

Annealing temperature: A β-Ti phase (body-centered cubic) having high ductility is precipitated in the TiAl alloy structure by annealing at 950 to 1050 ° C. Moreover, since the minute crack formed by the compressive stress at the time of extrusion is diffusion-bonded by annealing, a product defect is avoided and the yield is improved. In this case, the β-Ti phase contributes to the improvement of the ductility of the entire alloy because impurities are likely to precipitate and precipitate at the crystal grain boundaries having low ductility. Even if the annealing temperature exceeds 1050 ° C., further precipitation of the β-Ti phase cannot be expected, and the crystal grains become coarse and the strength is lowered. On the other hand, if the annealing temperature is less than 950 ° C., the precipitation of the β-Ti phase becomes insufficient. Therefore, annealing time shall be 950-1050 degreeC. In addition, from the viewpoint of promoting oxidation prevention and diffusion bonding of micro cracks on the surface, annealing is performed in a vacuum instead of an atmospheric atmosphere.

Annealing time: 2-5 hours If the annealing time is less than 2 hours, the temperature of the TiAl alloy is not uniform. On the other hand, if the annealing temperature exceeds 5 hours, the crystal grains become coarse and the strength decreases, and the operation time becomes unnecessarily long. Therefore, the annealing time is 2 to 5 hours.

Cooling rate after annealing: 10 to 60 ° C./min up to 400 ° C. In order to leave the β-Ti phase without being transformed into α 2 (Ti ₃ Al) phase or γ phase, cooling is forced after annealing. The atmosphere during cooling is, for example, an inert gas such as Ar, and the cooling is performed by air cooling. When the cooling rate exceeds 60 ° C./min, strain may be generated in the TiAl alloy, and stress may concentrate on the crack diffusion bonding portion to open the crack. On the other hand, when the cooling rate is less than 10 ° C./min, the β-Ti phase is easily transformed into an α2 phase or a γ phase. Therefore, the cooling rate after annealing shall be 10-60 degree-C / min to 400 degreeC.

  In the present invention, by leaving the β-Ti phase in the TiAl alloy structure, the ductility can be improved as well as high temperature and high strength and high rigidity. % Yield strength and elongation of 2% or more can be obtained. In order to obtain the effects of the present invention with certainty, it is desirable to control the α2 phase having a low toughness in the range of 10 to 20% by volume while containing 20 to 35% by volume of the β-Ti phase and increasing the strength. Further, in order to stably leave the β-Ti phase precipitated by annealing, it is desirable to add at least one kind of Nb and Zr that has little influence on strength and ductility. In this case, Nb is 1.3 to 1.9 atom%, and Zr is 0.1 to 0.2 atom%.

  According to the present invention, it is possible to provide a TiAl alloy-made internal combustion engine component, in particular, a valve and a piston pin, having improved ductility as well as high temperature, high strength and high rigidity.

It is a graph which shows the relationship between Al content and the mechanical characteristic in the Example of this invention. It is a graph which shows the relationship between Cr content and the mechanical characteristic in the Example of this invention. It is a graph which shows the relationship between Nb content and the mechanical characteristic in the Example of this invention. It is a graph which shows the relationship between Zr content and the mechanical characteristic in the Example of this invention. It is a graph which shows the relationship between the annealing temperature and the mechanical characteristic in the Example of this invention.

1. Example 1 A TiAl alloy containing various alloy components was heated to 1150 ° C. and extruded into a sample having a final diameter of 22 mm at an extrusion ratio of 41. This sample was annealed in vacuum at 1000 ° C. for 2.0 hours, and then air-cooled in an Ar atmosphere to 400 ° C. at a cooling rate of 60 ° C./min. The sample was examined for tensile strength, 0.2% proof stress, fatigue strength, and elongation. The results are shown in FIGS.

Verification of Al Content FIG. 1 shows the influence of the Al content on the mechanical properties of the TiAl alloy. As shown in FIG. 1, 0.2% yield strength, tensile strength and fatigue strength decrease as the Al content increases, but when the Al content is 42 atom% or less, the 0.2% yield strength is 800 MPa or more. Thus, the tensile strength is 1000 MPa or more and the fatigue strength is 700 MPa or more. Further, the elongation becomes 2% or more when the Al content is 40 atom% or more. Thus, the grounds for setting the Al content to 40 to 42 atom% were confirmed.

Verification of Cr Content FIG. 2 shows the influence of the Cr content on the mechanical properties of the TiAl alloy. As shown in FIG. 2, when the Cr content is 2.4 to 2.6 atom%, the 0.2% proof stress is approximately 800 MPa or more, the tensile strength is 1000 MPa or more, and the fatigue strength is 700 MPa or more. Further, the elongation is approximately 2% or more. Thus, it was confirmed that the mechanical properties are not significantly affected even if Cr 2.4 to 2.6 atom% is contained.

Verification of Nb Content FIG. 3 shows the influence of the content of Nb, which is an optional component, on the mechanical properties of the TiAl alloy. As shown in FIG. 3, when the Nb content is 1.3 to 1.9 atom%, the 0.2% yield strength is 800 MPa or more, the tensile strength is 900 MPa or more, and the fatigue strength is 650 MPa or more. Further, the elongation is 2% or more. As described above, it was confirmed that the mechanical properties were not significantly affected even when 1.3 to 1.9 atom% of Nb was contained.

Verification of Zr Content FIG. 4 shows the influence of the Zr content, which is an optional component, on the mechanical properties of the TiAl alloy. As shown in FIG. 4, when the Zr content is 0.1 to 0.2 atom%, the 0.2% yield strength is 800 MPa or more, the tensile strength is 800 MPa or more, and the fatigue strength is 700 MPa or more. Further, the elongation is 2% or more. Thus, it was confirmed that the mechanical properties were not significantly affected even when Zr was contained in an amount of 0.1 to 0.2 atom%.

2. Second Example A Ti-42Al-2.5Cr (atom%) alloy was heated to various temperatures and extruded into samples having a final diameter of 22 mm at various extrusion ratios. This sample was annealed at various temperatures in vacuum for 2.0 hours, and then air-cooled in an Ar atmosphere to 400 ° C. at a cooling rate of 60 ° C./min. The volume% of each phase in the structure of this sample was measured by semi-quantitative identification (peak separation) by X-ray diffraction, and the tensile strength, 0.2% proof stress, fatigue strength, and elongation were investigated. The result is shown in FIG. In FIG. 5, “as extruded” means not annealed.

FIG. 5 shows the influence of the extrusion temperature, extrusion ratio and annealing temperature on the structure and mechanical properties of the TiAl alloy. In FIG. 1-No. 6 is a comparative example. 7-No. 8 is an example. As shown in FIG. 5, sample No. 1 was extruded at 1150 ° C. to an extrusion ratio of 10 or more and annealed at 1000 ° C. 7-No. In 8, the volume ratio of the β-Ti phase reached 26 to 35%, and the α2 (Ti ₃ Al) phase became 13 to 19% by volume. As a result, the elongation reached 3.5%. The 0.2% proof stress was 850 to 860 MPa, the tensile strength was 1030 to 1060 MPa, and the fatigue strength was 860 to 870 MPa.

On the other hand, sample No. No. 11 and 6.4, respectively, without annealing were used. 1 and sample no. In No. 2, since the ratio of the β-Ti phase was 9 to 18% by volume, the elongation was around 1.0. Similarly, sample No. 5 with an extrusion ratio of 5 that was not annealed. 3, the proportion of the β-Ti phase is 23% by volume, but since the α2 (Ti ₃ Al) phase is 35% by volume, the elongation is still around 1.0%. On the other hand, this sample No. 3 was annealed at 900 ° C., 1000 ° C., and 1100 ° C., respectively. In 4-6, the β-Ti phase increases to about 28% by volume as the annealing temperature rises, and the α2 (Ti3Al) phase is reduced to about 15% by volume, whereby the elongation is 1.5-2.0. Improve to about%. However, sample no. In No. 6, since the annealing temperature exceeded 1050 ° C., the crystal grains became coarse. As a result, the 0.2% proof stress was less than 800 MPa.

  As described above, in the present invention, it was confirmed that the 0.2% yield strength is 800 MPa or more and the elongation is 2.0% or more. Therefore, the internal combustion engine parts made of TiAl alloy according to the present invention, particularly valves and piston pins, have impact resistance that does not break even when a large impact is applied due to knocking or combustion pressure.

  Since the TiAl alloy-made internal combustion engine component of the present invention has high ductility and high ductility, it is extremely promising for use in valves and piston pins for internal combustion engines.

Claims (4)

  Extrusion step of obtaining a molded body by extruding a TiAl alloy containing 40 to 42 atom% Al and Cr: 2.4 to 2.6 atom%, the balance being Ti and inevitable impurities at an extrusion ratio of 10 to 1150 ° C. And after the end of the extruding step or during the extruding step, the annealing step of holding the compact in vacuum at 950 to 1050 ° C. for 2 to 5 hours, and the compact after completion of the annealing step in an inert gas 400 A TiAl alloy internal combustion engine component manufactured through a cooling step of cooling to 10 ° C at a cooling rate of 10 to 60 ° C / min.   2. The internal combustion engine made of TiAl alloy according to claim 1, wherein the TiAl alloy further contains at least one of Nb: 1.3 to 1.9 atom% and Zr: 0.1 to 0.2 atom%. parts. The molded body that has undergone the cooling step contains 20 to 35% by volume of a β-Ti phase and 10 to 20% by volume of an α2 (Ti ₃ Al) phase. A TiAl alloy internal combustion engine component as described.   The internal combustion engine part made of TiAl alloy according to any one of claims 1 to 3, wherein the internal combustion engine part is a valve or a piston pin.

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