JP5110199B2 - Titanium aluminide casting and crystal grain refinement method thereof - Google Patents

Description

  The present invention relates to a titanium aluminide cast product and a method for refining the crystal grain thereof, and in particular, a titanium aluminide cast product used for a turbocharger mounted on a truck diesel engine or the like, a rotating member continuously used at a high temperature, and the like. And a crystal grain refinement method thereof.

  Titanium aluminide (TiAl), which is an intermetallic compound of Al and Ti, has characteristics such as light weight and high strength, and is therefore promising for rotating members of aircraft and automobile engines. .

  In order to use titanium aluminide for these members, titanium aluminide satisfies the creep strength (especially high temperature creep strength) and fatigue strength at a high level, that is, the titanium aluminide has creep properties (particularly high temperature creep properties) and fatigue properties. It is required to be good.

  One of the metallographic forms of titanium aluminide is a whole surface lamellar structure, and titanium aluminide having this structure is known to have excellent creep characteristics. Further, it is known that many titanium aluminides to which a β-phase stabilizing element (W, Nb, Ta, Cr, etc.) is added have excellent creep characteristics. This is because when these elements are added, from the balance with the amount of Al component, when the primary crystal starts to solidify in the β phase, the crystal grains become coarse, that is, the metal structure becomes a coarse structure. This improves the creep strength.

JP-A-01-298127 JP-A-56-04344 Japanese Patent Laid-Open No. 04-088140 Japanese Patent Laid-Open No. 08-311585 JP 2000-345260 A

  However, from the viewpoint of improving fatigue characteristics, it is preferable that the crystal grains of the metal structure be as fine as possible. In other words, improvement in creep characteristics and improvement in fatigue characteristics are contradictory requirements, and it has been difficult to achieve both.

  An object of the present invention, which was created in view of the above circumstances, is to provide a titanium aluminide cast product having fine metal structure crystal grains and good creep characteristics and fatigue characteristics, and a method for crystal grain refinement thereof. There is.

To achieve the above object, the titanium aluminide cast product of the present invention is a titanium aluminide cast product containing both Fe and V.
Al: 46-50 atomic%,
Fe and V elements in a total amount of 5 atomic% or less (provided that the Fe content is 17.5-0.3x atomic% or less (x: Al content)),
C: 0.1 to 0.4 atomic%,
The balance: Ti and inevitable impurities,
The average crystal grain size of the metal structure is 50 to 300 μm, and the average particle diameter of the C group precipitates precipitated in the metal structure is 1 μm or less.

  According to the above configuration, it is possible to obtain a titanium aluminide cast product in which there is no fear that the creep characteristics are remarkably lowered even if the metal structure crystal grains are refined to improve the fatigue characteristics.

  On the other hand, the grain refinement method of the titanium aluminide cast product according to the present invention, after casting the molten alloy having the same chemical composition as the above-described titanium aluminide cast product into the mold, when cooling the cast body, In the temperature range of 1500-1100 ° C., cooling is performed at a cooling rate of 150-250 ° C./min.

  According to the above method, the titanium aluminide cast product having the above-described chemical composition can be formed into a complete two-phase (α + γ phase) layered structure and fine crystal grains.

In short, according to the present invention, the following excellent effects are exhibited.
(1) A titanium aluminide cast product that does not cause a significant decrease in creep characteristics can be obtained even if the metal structure crystal grains are refined in order to improve fatigue characteristics.
(2) The titanium aluminide cast product of (1) can be formed into a complete two-phase (α + γ phase) layered structure and fine crystal grains.

It is an optical microscope observation figure of the metal structure of a titanium aluminide casting (1). It is the elements on larger scale of FIG. It is an optical microscope observation figure of the metal structure of a titanium aluminide casting (2). FIG. 4 is a partially enlarged view of FIG. 3. It is a figure which shows the relationship between the crystal grain size and creep strength of a metal structure, and a crystal grain size and fatigue strength. It is a figure which shows the relationship between a creep characteristic and an LCF characteristic. It is an optical microscope observation figure of the metal structure of the titanium aluminide cast product excellent in the high temperature creep characteristic. It is the elements on larger scale of FIG. It is the figure which made Table 1 the chemical composition of each casting in an Example, a comparative example, and a prior art example. It is the figure which made each evaluation result of each casting shown in FIG.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a preferred embodiment of the invention will be described with reference to the accompanying drawings.

  As titanium aluminide having good high temperature creep characteristics and castability, the present inventor has disclosed a Ti-Al-Fe-V alloy or a Ti-Al-Fe-V-B alloy (Japanese Patent Application No. 10-95172 (Japanese Patent Application Laid-Open No. Hei 10-95172)). 11-269484)), and Ti-Al-Mo-V-based alloys or Ti-Al-Mo-V-Si-based alloys (see Japanese Patent Application No. 11-161073 (Japanese Patent Laid-Open No. 2000-345260)). Filed earlier.

  The Ti-Al-Fe-V-based alloy or Ti-Al-Fe-V-B-based alloy described above is a titanium aluminide having excellent high-temperature creep characteristics even when cast. Moreover, Ti-Al-Mo-V alloy or Ti-Al-Mo-V-Si alloy contains Al more than Ti-Al-Fe-V alloy or Ti-Al-Fe-V-B alloy. By increasing the amount, titanium aluminide is intended to improve mechanical properties, that is, to further improve high-temperature creep strength. In general, when the Al content is low, the crystal grains are fine, and when the Al content is high, the crystal grains are coarse.

  Since both titanium aluminides of the prior application focused on the creep characteristics, as shown in FIG. 7 and FIG. I was trying to improve. However, if the crystal grains of the metal structure are coarse, a high creep strength can be obtained, but the fatigue characteristics are lowered. Therefore, these titanium aluminides are not very satisfactory in terms of fatigue characteristics.

  Therefore, in the titanium aluminide cast product of the present invention, by containing C or B within the specified range, the crystal grains of the metal structure are refined, and the high temperature creep characteristics of both titanium aluminides of the prior application are maintained. However, the fatigue characteristics are improved.

The titanium aluminide cast product (1) which has improved fatigue properties while maintaining good high temperature creep properties contains Mo, V, Si,
The chemical composition is
Al: 46-50 atomic%,
All elements of Mo, V, and Si are 5 atom% or less in total (however, Si content is 0.7 atom% or less, Mo content is 17.5-0.3x atom% or less (x: Al Content)),
C: 0.1 to 0.4 atomic%, more preferably 0.2 to 0.4 atomic%,
The balance: a titanium aluminide casting (1) composed of Ti and inevitable impurities.

The other titanium aluminide casting (2) contains Mo, V, and Si,
The chemical composition is
Al: 46-50 atomic%,
All elements of Mo, V, and Si are 5 atom% or less in total (however, Si content is 0.7 atom% or less, Mo content is 17.5-0.3x atom% or less (x: Al Content)),
B: 0.2 to 1.20 atomic%, more preferably 0.3 to 1.20 atomic%,
Remainder: Titanium aluminide casting (2) composed of Ti and inevitable impurities.

  Here, in the titanium aluminide castings (1) and (2) described above, the V content is an arbitrary value of 1.8 atomic% or less, preferably 1 atomic% or less, particularly preferably 0.8. About 5 atomic%.

  Next, the crystal grain refining method of the titanium aluminide castings (1) and (2) will be described below.

First, the chemical composition is
Al: 46-50 atomic%,
All elements of Mo, V, and Si are 5 atom% or less in total (however, Si content is 0.7 atom% or less, Mo content is 17.5-0.3x atom% or less (x: Al Content)),
C: 0.1 to 0.4 atomic% or B: 0.2 to 1.20 atomic%,
Remainder: After adjusting the addition amount of the metal element so as to become Ti and inevitable impurities, dissolution is performed.

  Next, the molten alloy melt is cast into a mold to form a cast body. When this casting is cooled, it is cooled at a cooling rate of 150 to 250 ° C./min in a temperature range of 1500 to 1100 ° C. As a result, in the metal structure of the as-cast material, a single phase of γ phase does not precipitate, and a complete two-phase (α + γ phase) layered structure is formed with fine grains and fine C at the grain boundaries. The titanium aluminide cast product according to the present embodiment in which the group or B group precipitate is deposited is obtained.

  Next, the operation of the above-described titanium aluminide castings (1) and (2) will be described.

  The optical microscope observation figure of the metal structure of said titanium aluminide casting (1) is shown in FIG. 1, the elements on larger scale of FIG. 1 are shown in FIG. 2, and the optical microscope observation figure of the metal structure of titanium aluminide casting (2) 3 is shown in FIG. 3, and a partially enlarged view of FIG. 3 is shown in FIG.

  By containing C in the range of 0.1 to 0.4 atomic% (or B in the range of 0.2 to 1.20 atomic%) in the base material of the titanium aluminide casting, the matrix (α + γ phase first) As shown in FIG. 1, FIG. 2 (or FIG. 3, FIG. 4), C group (or B group) precipitates are precipitated in the complete two phases, and these precipitates become nuclei and the crystal grains become finer. In addition, a titanium aluminide cast product having fine metal structure crystal grains as compared with FIGS.

  At this time, if the content of C (or B) is larger than the specified range, the C group (or B group) precipitates are excessively precipitated, and the excessively precipitated these precipitates are more than necessary. Refinement will lead to a decrease in creep strength. In addition, the C group (or B group) precipitates at the end of the solidification of the crystals become coarse, and these precipitates later become the starting point of destruction. On the contrary, if the content of C (or B) is less than the specified range, it does not contribute to the refinement of crystal grains at all. Therefore, since the C group (or B group) precipitate at the end of the solidification of the crystal grains is preferably as small as possible and the size is preferably as fine as possible, the content of C (or B) is It is specified in the above range.

  In these titanium aluminide castings (1) and (2), by adding C within the above specified range, as shown in FIG. 2, the average grain size is 1 μm or less, preferably 0, at the crystal grain boundaries. A granular C-group precipitate having a particle size of 5 μm or less is deposited. Alternatively, by adding B within the above specified range, as shown in FIG. 4, flaky B group precipitates having an average length of 15 μm or less, preferably 10 μm or less (see FIG. Many black spots) are deposited. The fine C group (or B group) precipitates are precipitated as crystal nuclei, whereby the average grain size of the crystal grains in the metal structure is reduced to 50 to 300 μm, preferably 50 to 150 μm.

  FIG. 5 shows the relationship between the crystal grain size and creep strength of the metal structure and the crystal grain size and fatigue strength. Here, the broken line A1 in FIG. 5 is the creep strength characteristic of the titanium aluminide cast product without addition of C or B, the solid line A2 is the creep strength characteristic of the titanium aluminide cast product according to the present invention, and the solid line B is the titanium aluminide cast product. The fatigue strength characteristics of the product are shown.

  As shown in FIG. 5, generally, with the refinement of crystal grains, the creep strength is remarkably lowered and the fatigue strength is remarkably improved (see broken line A1 and solid line B). This is because when the metal is deformed, if the crystal grains are fine, the grain boundary slips and crystal diffusion creep deformation becomes easy, and as a result, the metal is easily deformed. However, in a titanium aluminide cast product, fine C group (or B group) precipitates precipitated in the crystal grains contribute to the refinement of the crystal grains and also act as dislocation pinning (pinning effect). For this reason, the fatigue strength is remarkably improved, and the creep strength is not significantly reduced (see the solid line A2), that is, the creep strength can be prevented from being significantly reduced, and the mechanical properties of the titanium aluminide casting are improved. Is done. Needless to say, if the precipitated C group (or B group) precipitate is coarse, it does not contribute to the improvement of the mechanical properties of the titanium aluminide casting.

  Here, both C and B may be added. In this case, however, it is necessary to control the addition amount of C, especially C, to a very small amount. If the amount of C added is too small, the entire amount of C may be volatilized when the alloy is melted, and control of C becomes extremely difficult.

  When the above titanium aluminide castings (1) and (2) are applied to automobile and truck turbochargers, rotating members used continuously at high temperatures, etc., mechanical properties, particularly high temperature creep properties and fatigue There is no problem in the characteristics, and the manufacturing cost is equivalent to that of both titanium aluminides of the prior application described above, so there is no risk of a significant increase in cost. That is, it becomes possible to mass-produce high performance, high reliability turbochargers or high temperature rotating members at low cost.

  These titanium aluminide castings can be used as metal parts as they are, but these castings are as-cast materials and may have casting defects. For this reason, if necessary, these castings may be subjected to heat treatment such as HIP processing or homogenization processing to remove casting defects. As heat treatment conditions at this time, heat treatment is performed at a temperature range of 800 to 1100 ° C. or at a temperature of ({1220 + 25 (Al atom% −44)} + 10) or more, and cooling is performed at a rate of 100 ° C./min or more.

  Next, an embodiment of the present invention will be described.

  The titanium aluminide castings (1) and (2) described above were Ti—Al—Mo—V alloys. In contrast, in the titanium aluminide cast product according to the embodiment of the present invention, at least a part of Mo is replaced with Fe in the chemical composition of the titanium aluminide cast product (1) or (2), and Ti—Al— Fe-V type.

Specifically, the titanium aluminide casting according to the embodiment of the present invention contains Fe and V,
The chemical composition is
Al: 46-50 atomic%,
The total amount of both Fe and V elements is 5 atomic% or less (however, the Si content is 0.7 atomic% or less, the Fe content is 17.5-0.3x atomic% or less (x: Al content) )),
C: 0.1 to 0.4 atomic%, more preferably 0.2 to 0.4 atomic%,
The balance: Ti—Al—Fe—V alloy composed of Ti and inevitable impurities.

  It goes without saying that the titanium aluminide cast product according to the present embodiment can obtain the same effects as the titanium aluminide cast products (1) and (2). Moreover, since the titanium aluminide cast product according to the present embodiment does not contain Mo as compared with the titanium aluminide cast products (1) and (2), the manufacturing cost can be reduced.

  Below, titanium aluminide cast products (1) and (2) and the titanium aluminide cast product of the present invention will be described as Examples 1 to 19 together with Comparative Examples 1 to 6 and Prior Examples 1 and 2.

  After melting and producing a molten alloy consisting of Al, Mo or Fe, V, Si, C or B, the balance Ti + inevitable impurities, cooling while maintaining a cooling rate of 150 to 250 ° C / min in a temperature range of 1500 to 1100 ° C Thus, 28 types of cast bodies having different chemical compositions were produced (Examples 1 to 19, Comparative Examples 1 to 7, and Prior Examples 1 and 2).

  The chemical composition of each casting is shown in Table 1 of FIG.

Next, the structure observation, creep characteristics, and fatigue characteristics of each cast body were evaluated. Moreover, comprehensive evaluation was performed based on these evaluations. Each evaluation result is shown in Table 2 of FIG.
(Tissue observation)
As a structure observation method, each cast body is cut, and the cut surface is subjected to a microstructure analysis by an optical microscope and a backscattered electron image, and the size of the C group or B group precipitate and the presence or absence of the effect of grain refinement are observed. did. Here, the case where the effect of refining the crystal grains was remarkable was marked with 、, the case where it was present with ○, and the case where there was no crystal grain with ×.
(Creep property evaluation)
As a method for evaluating creep characteristics, each cast body is processed into a test piece having a parallel part φ6 × 30 mm, and a creep rupture test is performed in the atmosphere under a load of 240 MPa and a temperature of 760 ° C. to measure a creep rupture life (hr). did.
(Fatigue property evaluation)
As a method for evaluating fatigue characteristics, a triangular wave having a frequency of 60 Hz is used, and a fatigue test is performed on each cast body at room temperature, with a stress amplitude R (minimum stress / maximum stress) of 0.1 and an average stress of 192.5 MPa. And the number of cycles (breaks) until rupture was measured. At this time, those with good fatigue characteristics were marked with ◯, and those with the same or better than the previous examples 1 and 2 were marked with △.

  As shown in Table 2 of FIG. 10, in the castings of the first and second examples without addition of C or B, the creep rupture life is as long as 500 hours or more, and the high temperature creep characteristics are excellent, but the crystal grains are coarse. Therefore, fatigue characteristics were not good. Overall evaluation based on these was only ○.

  On the other hand, Examples 1-4, 11-13 of titanium aluminide castings (1) to which C is added within the specified range, Examples 5-7, 14-16 of titanium aluminide castings (2), In Examples 5 to 7 and 17 to 19 of the invention, the size of the C-based precipitates is fine or slightly large, and the effect of refining crystal grains is observed in all castings, and the fatigue characteristics are all It was good. Further, the creep rupture life was about 300 to 500 hr, and the creep rupture life was not significantly reduced as compared with the first and second examples. The overall evaluation based on these was as good as ◎. On the other hand, in the castings of Examples 8 to 10 and 14 to 16 in which B was added within the specified range, the size of the B-based precipitates was coarse, and the effect of refining crystal grains was observed in all castings. The fatigue properties were all good. Further, the creep rupture life was 400 hr or longer, and the creep rupture life was not significantly reduced as compared with the preceding examples 1 and 2. The overall evaluation based on these was as good as ◎. Here, in the casts of Examples 1, 11, 17 or Examples 8 and 14, although the effect of crystal grain refinement is obtained, the content of C or B is slightly less, so the fatigue characteristics are the first example. , 2 equal to or better than △, the overall evaluation based on these was only ○.

  Note that the size of the B group precipitate is “coarse” compared to the size of the C group precipitate, and is “fine” on the basis of the size of the B group precipitate.

  In the castings of Comparative Examples 3 and 7, or Comparative Examples 2, 5, and 6, although the effect of crystal grain refinement is obtained, the content of C or B is larger than the specified range, so the content of C or B However, since the size of the C-group or B-group precipitate is large (large or extremely coarse), the creep rupture life is reduced. Overall evaluation based on these was only ○.

  In the cast body of Comparative Example 1 or Comparative Example 4, since C or B is not added itself, the creep characteristics and fatigue characteristics are the same as those of the cast bodies of the preceding Examples 1 and 2, and based on these, Evaluation remained ○.

  Next, the relationship between creep characteristics and fatigue characteristics is shown in FIG. Here, the horizontal axis represents creep rupture life (hr [760 ° C., 240 MPa]), and the vertical axis represents LCF (Life of Cycles to Failure) characteristics.

  As shown in FIG. 6, by adding Si to the Ti-Al-Mo-V alloy indicated by Δ in the figure and increasing the Al content, creep strengthens due to precipitation strengthening by Si and grain coarsening. The fracture life is improved from about 300 hr to about 500 hr, that is, the high temperature creep characteristics are improved, and a Ti—Al—Mo—V—Si based alloy indicated by □ in the figure is obtained.

  By adding C or B to this Ti-Al-Mo-V-Si alloy, the crystal grains are refined and the LCF characteristics are improved, and the titanium aluminide cast product LCF according to the present invention indicated by black circles in the figure. Characteristics are obtained. At this time, if the amount of C or B added is too small, there is no effect of crystal grain refinement and the LCF characteristics will not be improved, as indicated by the circles in the figure. Moreover, when the addition amount of C or B is large or small, the size of the precipitated C group or B group precipitate becomes coarse or fine. At this time, if the precipitated C group or B group precipitate is too coarse, it is impossible to prevent a significant decrease in the high temperature creep characteristics.

  As mentioned above, it cannot be overemphasized that embodiment of this invention is not limited to embodiment mentioned above, and various things are assumed in addition.

Claims (2)

In titanium aluminide castings containing both Fe and V, the chemical composition is
Al: 46-50 atomic%,
Fe and V elements in a total amount of 5 atomic% or less (provided that the Fe content is 17.5-0.3x atomic% or less (x: Al content)),
C: 0.1 to 0.4 atomic%,
The balance: Ti and inevitable impurities,
A titanium aluminide cast product, wherein the average crystal grain size of the metal structure is 50 to 300 μm, and the average grain size of the C group precipitates precipitated in the metal structure is 1 μm or less.   When the molten alloy having the same chemical composition as the titanium aluminide cast product according to claim 1 is cast into a mold and then cooled, the cast body is cooled in a temperature range of 1500 to 1100 ° C at 150 to 250 ° C / A method for refining crystal grains of a titanium aluminide cast product, characterized by cooling at a cooling rate of min.