High-homogeneity high-toughness titanium alloy and preparation method thereof

Abstract

The titanium alloy comprises the following elements in percentage by mass: 4.5 to 6.0 percent of Al, 4.0 to 5.5 percent of Mo, 6.5 to 8.0 percent of V, 1.5 to 2.5 percent of Cr, 0.5 to 2.0 percent of Zr, 0.5 to 2.0 percent of Nb, less than or equal to 0.15 percent of O, less than or equal to 0.05 percent of N, less than or equal to 0.08 percent of C, less than or equal to 0.015 percent of H, less than or equal to 0.1 percent of single other impurities, less than or equal to 0.3 percent of total impurities and the balance of Ti. The performance of the head, the middle and the bottom of the bar and the edges, the center and the 1 / 2R position on the corresponding cross section all meet the requirements that the tensile strength Rm is larger than or equal to 1300 MPa, the elongation A is larger than or equal to 8%, the fracture toughness KIC is larger than or equal to 35 MPa.m < 1 / 2 >, and the maximum range of the tensile strength of the same furnace batch is not larger than 50 MPa.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of titanium alloy preparation, and particularly relates to a high-homogeneity high-strength-toughness titanium alloy and a preparation method thereof. BACKGROUND

[0002] Titanium alloy is widely used in aviation, aerospace, shipbuilding and weapon industry due to its high specific strength, excellent high-temperature performance and corrosion resistance. In the landing gear of an aircraft, large titanium alloy parts are particularly important for weight reduction. Ti1023 (Ti-10V-2Fe-3Al) titanium alloy was first developed by TIMET company in the United States in 1974 and applied to the landing gear of Beoing 777 aircraft; Ti5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe) titanium alloy was developed in 1990 and applied to the Boeing 787 aircraft, and then the improved Ti55531 (Ti-5Al-5Mo-5V-3Cr-1Zr-0.35Fe) alloy was further developed in Europe and the United States and applied to the Airbus A380 aircraft.

[0003] At the beginning of alloy design, the development focus is concentrated on the guarantee of high-strength-toughness performance of the alloy, and the homogeneity of the material is not obviously emphasized. For example, Ti1023 alloy contains 2wt.% of Fe element which is prone to segregation, and in actual industrial production, β spots are prone to occur in the bar, which ultimately leads to poor performance uniformity and fatigue performance of the alloy. In Ti5553 / Ti55531 alloy, the Fe content is reduced, but 3wt.% of Cr is introduced, and Cr element also belongs to an element prone to segregation. The composition content of titanium alloy affects the precipitation of phases, and then affects the uniformity of the microstructure during the forging process, and also directly affects the uniformity of the microstructure and performance after the subsequent bar heat treatment, and then significantly affects the performance uniformity of the alloy bar. High-strength-toughness titanium alloy generally contains a large amount of alloying elements, and the segregation of these alloying elements during melting and solidification is inevitable. In large-size titanium alloy bars applied to large parts, the influence caused by composition segregation is particularly prominent. In the industrial production process of Ti55531 alloy bar, the maximum strength difference of large-size bars (diameter ≥ 200mm) after heat treatment in the same furnace batch can exceed 100MPa.

[0004] However, there are positive segregation elements (solidification distribution coefficient K<1, such as Cr, Fe, etc.) and negative segregation elements (solidification distribution coefficient K>1, such as Al, Mo, etc.) in titanium alloy. When segregation is inevitable, if the relative content of the two types of elements can be adjusted, the effect of the elements enriched in the segregation area on the increase of strength and the effect of the elements poor in the area on the decrease of strength can be basically offset. This will greatly limit the influence of composition segregation on the strength uniformity of titanium alloy bars.

[0005] Therefore, for further performance optimization and upgrading of high toughness titanium alloy, design can be considered from the perspective of segregation coupling to prepare a high homogeneity high toughness titanium alloy. SUMMARY

[0006] In view of the deficiencies in the prior art, the purpose of the present application is to provide a high homogeneity high toughness titanium alloy and a preparation method. The innovative idea is mainly to control the content of positive and negative segregation elements in the titanium alloy to weaken the influence of the strength difference of the alloy bar caused by the segregation of alloy elements in each region of the large-size bar, and finally obtain a high homogeneity titanium alloy. The diameter of the large-size titanium alloy bar mentioned in the present application is Ф200mm-Ф500mm.

[0007] Based on the component system (Al, Mo, V, Cr, Zr, Fe) of the high strength and toughness titanium alloy Ti55531 for commercial aircraft landing gear, a small amount of β-stabilizing element Nb is used to replace the low content of the β-stabilizing element Fe which is prone to segregation, the content of the element Cr which is prone to segregation is further reduced, and the content of the element V which is the least prone to segregation is increased, so as to weaken the segregation of the alloy while ensuring the strength of the alloy and the matching of the basic strength and toughness. At the same time, most importantly, by coordinating the relative contents of Al, Mo, V, Cr, Zr and Nb, the contents of the positive and negative segregation elements in the titanium alloy are coupled and controlled, so that the uniformity of the theoretical strength reaches the optimum (the theoretical strength adopts the strength calculation method of Al equivalent and Mo equivalent coupling commonly used in titanium alloy).

[0008] In order to achieve high homogeneity and high strength and toughness, the technical scheme adopted by the present application is as follows: A high homogeneity high strength and toughness titanium alloy, characterized in that the nominal composition of the titanium alloy is Ti-5Al-4.5Mo-7V-2Cr-1Zr-1Nb, the positive and negative segregation degrees of each element are considered, the proportions of each element are coupled and designed to reduce the strength difference caused by local component segregation while ensuring the strength and toughness, and the mass percentages of each element satisfy: Al: 4.5%~6.0%, Mo: 4.0~5.5%, V: 6.5~8.0%, Cr: 1.5~2.5%, Zr: 0.5~2.0%, Nb: 0.5~2.0%, O: ≤0.15%, N: ≤0.05%, C: ≤0.08%, H: ≤0.015%, each of other impurities ≤0.1%, total ≤0.3%, and the balance is Ti.

[0009] Further, in order to reduce the influence of local component segregation on the strength difference to a greater extent, the contents of the positive segregation elements Cr and Zr and the negative segregation elements Al and Mo need to control the coupling relationship: (Cr+1 / 9Zr):(1 / 3Mo+1 / 6Al)=0.7~1.2.

[0010] Further, the preparation method of the high-homogenized high-toughness titanium alloy has the characteristics that the following steps are included: ingredients are allocated according to nominal composition, then sponge titanium and intermediate alloy particles are mixed and pressed to obtain an electrode block; the electrode block is used to prepare a long titanium alloy electrode by vacuum welding; the long titanium alloy electrode is melted twice or more than twice in a vacuum consumable arc furnace to obtain a titanium alloy ingot; the ingot is forged and heat treated for multiple times to finally obtain a high-homogenized high-toughness titanium alloy rod.

[0011] Compared with the prior art, the application has the following beneficial effects: The application is based on the traditional high-toughness titanium alloy system (Ti55531 alloy) that has been widely used in commercial applications, and is modified by segregation coupling design. The main point is that the segregation of alloy components is inevitable, and the positive and negative segregation element content coupling adjustment is used to weaken the influence of element segregation in each region on the strength of the alloy rod. Finally, a high-homogenized high-toughness titanium alloy is obtained, which solves the problem of insufficient performance uniformity of large-size rod of current high-toughness titanium alloy. The performance of the head, middle, bottom and corresponding cross-section edge, center and 1 / 2R of the rod all meet: tensile strength Rm≥1300MPa, elongation A≥8%, fracture toughness K IC ≥35MPa·m 1 / 2 , and the maximum range of tensile strength of the same furnace batch is not more than 50MPa. However, in the current industrial production process of commercial Ti55531 alloy rod, the maximum difference of strength of large-size rod (diameter≥200mm) after heat treatment can exceed 100MPa. Therefore, the high-homogenized high-toughness titanium alloy has a wide application prospect in the field of large-size parts such as new-generation aircraft landing gears. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 Tensile strength value distribution diagram of titanium alloy rod prepared for example 1 and comparative example 1. DETAILED DESCRIPTION

[0013] The technical solutions of the application will be further described in detail below in combination with specific embodiments. The application can be realized in many different forms, and is not limited to the embodiments described herein. It should be understood that the purpose of providing these embodiments is to make the understanding of the disclosed content of the application more thorough and comprehensive. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art of the technology to which the application belongs. The terms used in the specification of the application herein are only for the purpose of describing the specific embodiments, and are not intended to limit the application.

[0014] The application is based on the component system (nominal composition Ti-5Al-5Mo-5V-3Cr-1Zr-0.35Fe) of high strength and toughness titanium alloy Ti55531 for commercial aircraft landing gear. In this system, there are ① positive segregation elements such as Cr, Fe, etc.; ② negative segregation elements such as Al, Mo. First, replace the most easily segregated Fe element with Nb, and increase the content of the least easily segregated V element; further considering that segregation is inevitable, the application creatively couples and adjusts the relative contents of the two types of elements to make the effects of the two types of elements on strength in the segregation region basically offset each other. Example 1

[0015] Ti-5Al-4.5Mo-7V-2Cr-1Zr-1Nb titanium alloy bar According to the composition of Ti-5Al-4.5Mo-7V-2Cr-1Zr-1Nb, the corresponding weight proportion of titanium sponge and intermediate alloy particles are selected, mixed and pressed into an electrode, and then welded and subjected to three times of vacuum arc melting to obtain a Φ680mm titanium alloy ingot with a weight of 1.5t; further, the above ingot is subjected to multi-fire forging to obtain a large-size titanium alloy bar with a diameter of Φ350mm and a length of about 3000mm; the above bar is subjected to conventional heat treatment to obtain a high strength and toughness titanium alloy. Example 2

[0016] Ti-6.0Al-5.5Mo-8.0V-2.5Cr-2.0Zr-2.0Nb titanium alloy bar The same as example 1, except that the nominal composition Ti-6.0Al-5.5Mo-8.0V-2.5Cr-2.0Zr-2.0Nb is used for batching, and finally a titanium alloy bar is prepared. Example 3 Ti-4.5Al-4.0Mo-6.5V-1.5Cr-0.5Zr-0.5Nb titanium alloy bar

[0017] The same as example 1, except that the nominal composition Ti-4.5Al-4.0Mo-6.5V-1.5Cr-0.5Zr-0.5Nb is used for batching, and finally a titanium alloy bar is prepared. Example 4

[0018] Ti-4.5Al-4.0Mo-7.0V-2.5Cr-2.0Zr-1.0Nb titanium alloy bar The same as example 1, except that the nominal composition Ti-4.5Al-4.0Mo-7.0V-2.5Cr-2.0Zr-1.0Nb is used for batching, and finally a titanium alloy bar is prepared. Example 5

[0019] Ti-6.0Al-5.5Mo-7.0V-1.5Cr-0.5Zr-1.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-6.0Al-5.5Mo-7.0V-1.5Cr-0.5Zr-1.0Nb, and titanium alloy rods are finally prepared. Example 6 Ti-5.0Al-4.5Mo-7.0V-2.5Cr-2.0Zr-1.0Nb titanium alloy rods

[0020] The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-5.0Al-4.5Mo-7.0V-2.5Cr-2.0Zr-1.0Nb, and titanium alloy rods are finally prepared. Example 7

[0021] Ti-6.0Al-5.5Mo-7.0V-2.0Cr-1.0Zr-1.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-6.0Al-5.5Mo-7.0V-2.0Cr-1.0Zr-1.0Nb, and titanium alloy rods are finally prepared.

[0022] Comparative Example 1: Ti-5Al-5Mo-5V-3Cr-1Zr-0.35Fe titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-5Al-5Mo-5V-3Cr-1Zr-0.35Fe, and titanium alloy rods are finally prepared.

[0023] Comparative Example 2: Ti-6.1Al-5.6Mo-8.1V-2.6Cr-2.1Zr-2.1Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-6.1Al-5.6Mo-8.1V-2.6Cr-2.1Zr-2.1Nb, and titanium alloy rods are finally prepared.

[0024] Comparative Example 3: Ti-4.4Al-3.9Mo-6.4V-1.4Cr-0.4Zr-0.4Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-4.4Al-3.9Mo-6.4V-1.4Cr-0.4Zr-0.4Nb, and titanium alloy rods are finally prepared.

[0025] Comparative Example 4: Ti-4.4Al-3.9Mo-7.0V-2.6Cr-2.1Zr-1.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-4.4Al-3.9Mo-7.0V-2.6Cr-2.1Zr-1.0Nb, and titanium alloy rods are finally prepared.

[0026] Comparative Example 5: Ti-6.1Al-5.6Mo-7.0V-1.4Cr-0.4Zr-1.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-6.1Al-5.6Mo-7.0V-1.4Cr-0.4Zr-1.0Nb, and titanium alloy rods are finally prepared.

[0027] Comparative Example 6: Ti-5.0Al-4.5Mo-6.0V-2.0Cr-1.0Zr-1.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-5.0Al-4.5Mo-6.0V-2.0Cr-1.0Zr-1.0Nb, and titanium alloy rods are finally prepared.

[0028] Comparative Example 7: Ti-5.0Al-4.5Mo-7.0V-2.0Cr-1.0Zr-3.0Nb titanium alloy rods The process is basically the same as in Example 1, except that the ingredients are prepared according to the nominal composition Ti-5.0Al-4.5Mo-7.0V-2.0Cr-1.0Zr-3.0Nb, and titanium alloy rods are finally prepared.

[0029] Sampling was performed at the same locations on the bars from all the above embodiments and comparative examples. The sampling locations were the head, middle, bottom, and the edge, center, and 1 / 2R of the corresponding cross-section of the bar. Tensile properties and fracture toughness were tested after sampling. The various mechanical properties of the alloy bars are shown in Table 1 below.

[0030] Table 1. Results of Mechanical Properties Experiment

[0031] Note: The formula for calculating the ratio of the two types of elements is as described above (Cr + 1 / 9 Zr): (1 / 3 Mo + 1 / 6 Al); however, when calculating the ratio of the Ti55531 alloy in Comparative Example 1, the degree of Fe segregation is significantly greater than that of Cr when it contains Fe. Therefore, based on the theoretical relationship, the formula is changed to (Cr + 1 / 9 Zr + 2 Fe): (1 / 3 Mo + 1 / 6 Al).

[0032] The data in Table 1 shows the following: In Examples 1-3 and Comparative Examples 2-3, as the alloy content increased, the strength of the alloy bars increased, while the plasticity decreased. This is a normal "seesaw" phenomenon generally present in an alloy system. Therefore, the titanium alloy composition of the present invention has a reasonable range, allowing the properties to simultaneously meet certain strength and plasticity application requirements.

[0033] Meanwhile, it can be seen from Examples 1, 4-7, and Comparative Examples 4-5 that as the proportion of segregated elements deviates from the equilibrium design requirements, the resulting difference in bar strength is greater and the uniformity of bar performance is worse.

[0034] Comparative Example 6 further clarifies that V is one of the most commonly used elements in titanium alloys and has a good affinity with titanium. In this composition system, excessive reduction of V content will not only reduce the strength of the alloy, but also significantly reduce the fracture toughness of the entire system, which is not conducive to the design requirements of the toughness of the main load-bearing structure.

[0035] Comparative Example 7 further clarifies that while excessively increasing the Nb content slightly increases the strength of the entire system, it significantly reduces the plasticity. This may be because a small addition of Nb is beneficial to the alloy system, but excessive addition to replace Fe will disrupt the stability of the mature Ti-5Al-5Mo-5V-3Cr-1Zr-0.35Fe system, resulting in a significant reduction in the material's plasticity.

[0036] It is important to note that although some comparative examples or similar components have been reported with varying properties by different researchers, and the mean values ​​for some properties may be close to or even better than those in this table, the properties will fluctuate due to differences in experimental conditions, processing parameters, final product dimensions, and heat treatment regimes. The experimental examples and comparative examples in this study were processed under identical conditions, and the process parameters were those of standard industrial production processes for titanium alloy bars. This essentially meets the requirements for comparing and evaluating the comprehensive properties of titanium alloys. Furthermore, it is particularly important to note that these reports generally lack consideration of the control of alloy property homogeneity, focusing only on local properties while neglecting the overall homogeneity of the material.

Claims

1. A highly homogeneous, high-strength, and high-toughness titanium alloy, characterized in that, The titanium alloy contains the following elemental percentages: Al: 4.5%~6.0%, Mo: 4.0~5.5%, V: 6.5~8.0%, Cr: 1.5~2.5%, Zr: 0.5~2.0%, Nb: 0.5~2.0%, O: ≤0.15%, N: ≤0.05%, C: ≤0.08%, H: ≤0.015%, other impurities ≤0.1% individually, ≤0.3% in total, with the balance being Ti.

2. The high-homogeneity, high-strength, and high-toughness titanium alloy according to claim 1, characterized in that, The content of positively segregating elements Cr and Zr, and negatively segregating elements Al and Mo, should be controlled to maintain their coupling relationship: (Cr + 1 / 9 Zr): (1 / 3 Mo + 1 / 6 Al) = 0.7~1.

2.

3. A method for preparing a high-homogeneity, high-strength, and high-toughness titanium alloy, characterized in that, Includes the following steps: Step 1) According to the nominal composition, the sponge titanium and intermediate alloy particles are mixed and pressed to obtain the electrode block; Step 2) Prepare a long titanium alloy electrode by vacuum welding the electrode block; Step 3) The titanium alloy long electrode is melted twice or more in a vacuum consumable arc furnace to obtain a titanium alloy ingot. Step 4) The ingot is subjected to multiple forging and heat treatment to finally obtain a high-homogeneity, high-strength and high-toughness titanium alloy bar.

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