JP2010270347A - TiAl ALLOY FOR AIR INTAKE/EXHAUST VALVE AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a TiAl alloy for an air intake/exhaust valve, which has a lower specific gravity and a higher rigidity than a conventional Ti alloy and furthermore has an improved toughness compared with the conventional TiAl alloy, to provide a method for manufacturing the same, and to increase a rotation speed of an engine and improve an output power by realizing valve parts with light weight and high rigidity. <P>SOLUTION: This manufacturing method includes subjecting the TiAl alloy having a composition including 8.0-9.5 wt.% Al and the balance Ti to aging heat treatment at 600-900°C for 40 hours or longer. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a titanium alloy used for engine valve parts and a method for producing the same.

  A TiAl alloy is known as a material for engine valve parts. For example, a valve made of a TiAl alloy described in Patent Document 1 has a feature that it is very light compared to a valve made of a general Ti alloy. In addition, the valve is required to have rigidity for higher engine speed and higher efficiency (combustion efficiency improvement, friction resistance reduction, etc.), but TiAl alloy has higher rigidity than Ti alloy, which is a conventional valve material. There is an advantage of having.

  However, compared to conventional valve materials, TiAl alloys have very low toughness, making it difficult to apply from the viewpoint of valve reliability. For this reason, the applicable range of valve parts made of TiAl alloy has been very limited.

JP-A-6-229213

  Since TiAl alloy mainly consists of intermetallic compounds, its toughness is low and it is very difficult to manage the safety factor of valve parts. That is, when designing a valve component using a TiAl alloy, it is necessary to increase the size of the valve component in order to provide a sufficient margin for the component strength. For this reason, the advantage that TiAl alloy is low specific gravity cannot be used efficiently.

  Accordingly, it is an object of the present invention to provide a TiAl alloy for intake / exhaust valves, which has a lower specific gravity and higher rigidity than conventional Ti alloys, and has improved toughness compared to conventional TiAl alloys, and a method for producing the same. Yes. Another object of the present invention is to increase engine speed and improve output by realizing lightweight and highly rigid valve parts.

  The method for producing a TiAl alloy for intake and exhaust valves according to the present invention is characterized in that a TiAl alloy having a composition of Al: 8.0 to 9.5 wt% and the balance Ti is subjected to an aging heat treatment at 600 to 900 ° C. for 40 hours or more. To do.

By aging the TiAl alloy in the range of 600 to 900 ° C. for 40 hours or longer, α-Ti can be generated, and 30 to 50 vol% of Ti ₃ Al can be precipitated in α-Ti. For this reason, the toughness of the material is improved by α-Ti in the base. Further, since Ti ₃ Al is precipitated in the α phase, it is considered that the rigidity of the base can be improved by precipitation hardening with Ti ₃ Al. Therefore, by the production method of the present invention, a toughness is improved as compared with a conventional TiAl alloy, and a TiAl alloy having a lower specific gravity and higher rigidity than a conventional Ti alloy can be obtained.

In the method for producing a TiAl alloy for an intake / exhaust valve according to the present invention, at least one of Fe, Cr, Mo, V, and Nb is further added to the above composition, and the value of wt% of the added amount is set. When used in the following [Equation 1], the Mo equivalent is preferably 5.7 or less.
[Equation 1]
Mo equivalent = Mo + 2.5Fe + 1.25Cr + 0.67V + 0.28Nb

  If at least one of Fe, Cr, Mo, V, and Nb, which are β-Ti stabilizing elements, is added to the TiAl alloy, the β phase can be precipitated on the matrix. Fe, Cr, Mo, V, and Nb have the effect of lowering the precipitation temperature of the β phase, and the machinability of the material is improved by the β phase. However, if the β phase in the base is excessive, the hardness and strength of the material become too low, so the Mo equivalent is set to 5.7 or less in order to adjust the precipitation amount of the β phase. Note that Sn and Zr may be added for the purpose of solid solution strengthening, and Si and Nb may be added for the purpose of improving high temperature strength and creep resistance.

The TiAl alloy for intake and exhaust valves of the present invention is a TiAl alloy having a composition of Al: 8.0 to 9.5 wt% and the balance Ti, and 30 to 50 vol% of Ti ₃ Al is precipitated in α-Ti. It is characterized by.

In the TiAl alloy, if Ti ₃ Al is less than 30 vol%, the effect of improving the rigidity of the base is small, and if it exceeds 50 vol%, α-Ti is reduced and the toughness of the material is lowered. Therefore, in the present invention, the TiAl alloy to precipitate 30～50Vol% of _Ti 3 Al in alpha-Ti.

In addition, in the TiAl alloy for intake and exhaust valves of the present invention, a TiAl alloy in which at least one of Fe, Cr, Mo, V, and Nb is further added to the above composition is preferable, and the value of wt% of the added amount thereof. Is used in the following [Equation 2], the Mo equivalent is preferably 5.7 or less.
[Equation 2]
Mo equivalent = Mo + 2.5Fe + 1.25Cr + 0.67V + 0.28Nb

Hereinafter, Fe, Cr, Mo, V, and Nb which are β-Ti stabilizing elements will be described.
Fe:
Fe is inexpensive, and the effect of stabilizing the β phase is very large even if the amount of Fe is small. In addition, there is an effect of improving the strength of the material. However, if Fe is added excessively, it combines with Ti to produce TiFe, so that ductility is lowered. For this reason, in this invention, the addition amount of Fe shall be 2 wt% or less which is the range which does not dissolve excessively, but dissolves in (alpha) -Ti.

Cr:
Cr is also inexpensive, and the effect of stabilizing the β phase can be obtained even if the amount of Cr is small. In addition, there is an effect of improving the strength of the material. However, if added excessively, the machinability may deteriorate, so the amount of Cr added is less than 3.0 wt%. In the present invention, from the viewpoint of ensuring high toughness, it is desirable that the content be 1 wt% or less, which is a range in which the solid solution is dissolved in α-Ti and no excessive TiCr ₂ is precipitated.

Mo, V:
Since Mo and V are all solid solution type elements, they do not combine with Ti to produce precipitates. Therefore, in the present invention, it is used to adjust the precipitation amount of β phase.

Nb:
Nb has a low effect of stabilizing the β phase, but an effect of improving creep resistance is also obtained. For this reason, in the present invention, a small amount of Nb is added.

According to the method for producing a TiAl alloy for an intake / exhaust valve of the present invention, a TiAl alloy having reduced rigidity by adding Al and precipitating Ti ₃ Al can be obtained. Moreover, according to the TiAl alloy manufacturing method of the present invention, a TiAl alloy with improved toughness can be obtained by leaving an appropriate amount of α-Ti on the base. When the TiAl alloy of the present invention is used for a predetermined valve part, the valve part can be reduced by about 7% as compared with the case of using Ti6246 which is a conventional material. Further, since the TiAl alloy of the present invention is highly rigid, it is possible to realize further weight reduction of the valve parts and higher engine rotation by design.

It is a graph which shows the compression workability test result of the test piece of each material used in embodiment.

Below, an example of the TiAl alloy of this invention is described. In order to disperse ₃₀ to 50 vol% of Ti3Al, the amount of Al added was 9.5 wt%, and other component elements were appropriately added. Table 1 shows the components of each material. As shown in Table 1, from the viewpoint of workability and strength during hot extrusion, materials added with β-phase stabilizing elements such as Fe, Cr, Mo, V, and Zr are further added to promote solid solution strengthening. Prepared materials. In order to obtain creep resistance, 0.2 wt% of Si was added to each material, and Nb was also added to the material A in particular. In each material, the addition amount of the component elements was set so that the Mo equivalent was 5.7 or less.

For the five types of materials shown in Table 1, ingots with a diameter of 38 mm and a height of 18 mm were produced, and six button-type test pieces each having a diameter of 8 mm and a height of 12 mm were cut out. After aging heat treatment, density measurement and compression processing A sex test was performed. The aging heat treatment was performed at 600 to 900 ° C. for 40 hours or more, and Ti ₃ Al was sufficiently precipitated, followed by cooling by water cooling. Then, alpha-phase by X-ray diffractometry, Ti 3 _Al phase, a check of the phase ratio of the β phase was carried out. The peak angles used for the evaluation are α phase: 35.3 °, 38.4 °, 40.4 °, Ti ₃ Al phase: 25.8 °, and β phase: 39.2 °. In addition, since the peak of the Ti ₃ Al phase overlaps with the _three peaks of the α phase, the phase ratio was calculated in consideration of the influence. These results are shown in Table 2. Table 2 also shows the estimated Young's modulus obtained based on the phase ratio.

From the results shown in Table 2, it was confirmed that about 30 to 50 vol% of Ti ₃ Al was precipitated in α-Ti in each material. From the estimated value of Young's modulus shown in Table 2, it can be inferred that the material showing the highest rigidity is J. From the β phase ratio, it is estimated that M is the most advantageous material for workability.

  Next, in order to compare the workability of the materials, a compression workability test was performed on the five types of materials shown in Table 1 using button-type test pieces. In the compression test, a test piece is fixed to a compression test machine in which a heating furnace is installed, held at each test temperature for 30 minutes, and then until the amount of compressive strain reaches around 60% at a constant speed of 0.1 mm / sec. Compression was performed and the compression yield strength at that time was measured. Assuming primary extrusion, the test temperatures were 800 ° C., 900 ° C., and 1000 ° C., which are temperatures below β transus (β transformation point). The result of the compression workability test is shown in FIG.

  From FIG. 1, it can be seen that the deformation resistance of the material M is clearly smaller than the other materials in each temperature range of 800 to 1000 ° C. From this, it can be understood that the compressive yield strength can be reduced and the workability of the material can be improved by containing a considerably large amount of β phase effective for workability.

  Based on the above results, the materials J and M were further examined. Under the conditions shown in Table 3, materials J and M were each cast by skull melting, subjected to primary extrusion and heat treatment, and after obtaining the phase ratio in the same manner as described above, a tensile test was performed. The results are shown in Table 3. For comparison, an example of physical property values of a conventional Ti valve material (Ti6246, TiAl) is shown in Table 3.

  As can be seen from Table 3, the materials J and M of the present invention have significantly improved elongation values compared to TiAl, which is a conventional material, and the elongation values are equal to or greater than those of Ti6246. Therefore, it can be said that the toughness has been improved to a practical level. Further, compared with Ti6246, the materials J and M have a low density and a high Young's modulus, and it is considered that low specific gravity and high rigidity have been achieved. Therefore, it was confirmed that the TiAl alloy of the present invention has lower density and higher rigidity than the conventional Ti alloy, and further improved toughness than the conventional TiAl alloy.

Claims (4)

  A method for producing a TiAl alloy for intake and exhaust valves, characterized by subjecting a TiAl alloy having a composition of Al: 8.0 to 9.5 wt% and the balance Ti to an aging heat treatment at 600 to 900 ° C for 40 hours or more. When at least one of Fe, Cr, Mo, V, and Nb is added to the composition and the value of wt% of the added amount is used in the following [Equation 1], the Mo equivalent is The method for producing a TiAl alloy for an intake / exhaust valve according to claim 1, wherein the TiAl alloy is 5.7 or less.
[Equation 1]
Mo equivalent = Mo + 2.5Fe + 1.25Cr + 0.67V + 0.28Nb Composition Al: 8.0~9.5wt% and in TiAl alloy and the balance Ti, intake and exhaust valves for TiAl alloy, wherein the 30～50Vol% of _Ti 3 Al in alpha-Ti is precipitated . A TiAl alloy in which at least one of Fe, Cr, Mo, V, and Nb is added to the composition, and when the value of wt% of the added amount is used in the following [Equation 2] The TiAl alloy for intake and exhaust valves according to claim 3, wherein the Mo equivalent is 5.7 or less.
[Equation 2]
Mo equivalent = Mo + 2.5Fe + 1.25Cr + 0.67V + 0.28Nb

Images