A method for preparing a low-molybdenum equivalent ultra-high strength titanium alloy

Abstract

This invention relates to a low-molybdenum (Mo) equivalent ultra-high-strength titanium alloy and its preparation method, belonging to the field of additive manufacturing of titanium alloys. The chemical composition of the low-molybdenum equivalent ultra-high-strength titanium alloy is: C ≤ 0.01%, Al: 5.5–6.5%, V: 3.5–4.5%, O: 0.30–0.55%, H ≤ 0.015%, N ≤ 0.02%, with the balance being Ti and unavoidable impurities. This invention obtains low-molybdenum equivalent ultra-high-strength titanium alloy formed parts using LPBF (Limited Part Forming) technology. The final low-molybdenum equivalent ultra-high-strength titanium alloy has a density of over 99.8%, a tensile strength ≥ 1500 MPa, a yield strength ≥ 1400 MPa, and an elongation ≥ 4.5%. The low-Mo equivalent ultra-high strength titanium alloy of this invention utilizes oxygen as a solid solution element to replace commonly used solid solution elements such as W, Mo, Nb, and Zr, thereby preparing low-Mo equivalent ultra-high strength titanium alloy products. Its strength is significantly better than that of traditional high-Mo equivalent titanium alloy products, greatly reducing the cost of ultra-high strength titanium alloys. It can achieve both low cost and high performance, and is suitable for mass production.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing of metallic materials, and relates to a low molybdenum (Mo) equivalent ultra-high strength titanium alloy and its preparation method. Technical Background

[0002] Titanium and titanium alloys are widely used in aerospace, biomedicine, and mechanical engineering due to their excellent corrosion resistance, low density, good biocompatibility, and high specific strength. However, the strength of pure titanium is usually too low to meet the strength requirements of structural components, thus limiting its application and development. Currently, the most common method for strengthening titanium materials is through alloying to improve strength, such as adding alloying elements like Al, V, Fe, Mo, and Nb. By adding different alloying elements, the microstructure of titanium alloys can be altered, resulting in different types of ultra-high-strength titanium alloys, whose room-temperature tensile strength is typically above 1500 MPa. Among them, β-type titanium alloys, with their age-hardening characteristics and good strength-toughness matching, are the best choice for ultra-high-strength titanium alloys. To determine the β-phase content in different titanium alloys, researchers have established a molybdenum (Mo) equivalent ([Mo]). eq The molybdenum equivalent is an important parameter for measuring the β-phase content and stability of titanium alloys. The commonly used formula for calculating molybdenum equivalent is:

[0003] [Mo] eq = [Mo]+[Nb] / 3.3+[Ta] / 4+[W] / 2+[Cr] / 0.6+[Mn] / 0.6+[V] / 1.4+[Fe] / 0.5+[Co] / 0.9+[Ni] / 0.8, where [X] is the mass fraction of element X in the titanium alloy.

[0004] Generally, a Mo equivalent of 8.0 or higher is required to obtain stable β-type titanium alloys with high strength. Commonly used ultra-high-strength titanium alloys include Ti-11.5Mo-6Zr-4.5Sn (room temperature tensile strength 1413 MPa, Mo equivalent 11.5) and Ti-5Al-4.75Mo-4.75V-1Cr-1Fe (room temperature tensile strength 1475 MPa, Mo equivalent 11.8), all of which are high-Mo-equivalent titanium alloys. Titanium alloys with a Mo equivalent below 8.0 are typically non-β-type titanium alloys, belonging to the low-Mo-equivalent category, and their strength is significantly reduced. Examples include Ti-7Al-4Mo (room temperature tensile strength 1080 MPa, Mo equivalent 4.0) and Ti-4Al-3Mo-1V (room temperature tensile strength 930 MPa, Mo equivalent 3.6). However, high Mo equivalent titanium alloys require the addition of large amounts of alloying elements such as Mo, Nb, Ta, and Fe to the titanium matrix, which greatly increases the cost of titanium alloys and makes them difficult to recycle, resulting in a serious waste of strategic resources. Summary of the Invention

[0005] Based on the above analysis, this invention aims to provide a low-Mo equivalent ultra-high-strength titanium alloy and its preparation method. This invention proposes a low-Mo equivalent ultra-high-strength titanium alloy and its preparation method, abandoning expensive rare alloying elements such as Mo, Nb, Ta, Zr, and W. It utilizes oxygen as a strengthening element and combines it with LPBF (laser-powder bed fusion) technology to prepare a novel ultra-high-strength titanium alloy, achieving low-cost design and preparation of ultra-high-strength titanium alloys. This solves the problems of excessively high cost, difficulty in recycling, and serious resource waste caused by existing methods of strengthening titanium alloys by adding large amounts of rare alloying elements (increasing Mo equivalent). The objective of this invention is mainly achieved through the following technical solutions:

[0006] A method for preparing a low-Mo equivalent ultra-high-strength titanium alloy, characterized in that the chemical composition of the low-Mo equivalent ultra-high-strength titanium alloy, by mass percentage, is: C≤0.01%, Al: 5.5~6.5%, V: 3.5~4.5%, O: 0.30~0.55%, H≤0.015%, N≤0.02%, with the balance being Ti and unavoidable impurities; the particle size range of the powder used for the low-Mo equivalent ultra-high-strength titanium alloy is 15~53μm, and the powder is formed into low-Mo equivalent ultra-high-strength titanium alloy formed parts by LPBF technology.

[0007] The preparation method of the low Mo equivalent ultra-high strength titanium alloy as described above specifically includes the following steps:

[0008] Step 1: The particle size range of the titanium alloy powder used for LPBF is 15–53 μm.

[0009] Step 2: The titanium alloy powder described in Step 1 is formed using LPBF technology to prepare low Mo equivalent ultra-high strength titanium alloy products.

[0010] Furthermore, the LPBF forming parameters in step 2 are: laser power 180-200W, scanning speed 800-1000mm / s, pass spacing 80-120μm, powder thickness 30μm, and the protective atmosphere is high-purity argon (99.99%).

[0011] Technical implementation elements

[0012] 1. Titanium alloy composition design: The oxygen content (x) of the low Mo equivalent Ti-6Al-4V-xO ultra-high strength titanium alloy in this invention is 0.30-0.55%, and the Mo equivalent is only 2.5-3.2.

[0013] However, the Mo equivalent of traditional β-type titanium alloys is usually above 10.

[0014] 2. Titanium alloy preparation process: This invention uses LPBF technology to form low Mo equivalent ultra-high strength titanium alloy. Compared with traditional melting and forging processes, LPBF technology has an extremely fast cooling rate and can obtain a unique solidification structure.

[0015] The technical effects of this invention are as follows:

[0016] (1) Reducing the addition of rare metal elements lowers the Mo equivalent of the titanium alloy system and reduces the cost of titanium alloy.

[0017] (2) By introducing oxygen as a strengthening element into the titanium alloy and combining it with the rapid cooling characteristics of LPBF technology, an ultrafine acicular α' martensite structure was obtained, resulting in a significant increase in strength. Oxygen, as a α-phase stabilizing element in titanium alloys, can significantly increase the α / β phase transformation point of titanium alloys, leading to the formation of a more stable α' phase. Furthermore, the cooling rate during the LPBF forming process is very fast (10... 4 ~10 6 The high oxygen content (K / s) restricts grain growth in titanium alloys, resulting in an ultrafine martensitic structure. Furthermore, oxygen, as an interstitial solid solution element, effectively strengthens titanium alloys through solid solution, improving their tensile strength. In traditional smelting processes, excessively high oxygen content promotes the formation of the brittle α2(Ti3Al) phase, leading to a decrease in the alloy's ductility and toughness; therefore, oxygen content is strictly controlled. In this invention, titanium alloys are formed using additive manufacturing technology. The extremely rapid cooling rate during forming prevents the brittle α2(Ti3Al) phase from precipitating, thus avoiding performance degradation caused by the brittle phase. Additionally, the rapid cooling rate of LPBF technology enriches oxygen in the formed titanium alloy, creating an oxygen-rich phase. This allows the titanium alloy to maintain a certain level of ductility and toughness while achieving high strength. This overturns the traditional understanding that oxygen content and plasticity are inversely proportional, achieving a synergistic balance between strength and plasticity, and providing a new approach for the design of high-performance metallic materials.

[0018] (3) The LPBF forming process reduces material waste, lowers the preparation cost, and can also be used to prepare complex shapes.

[0019] (4) The low Mo equivalent ultra-high strength titanium alloy obtained by the present invention has a room temperature tensile strength >1500MPa, a yield strength >1400MPa, and a fracture elongation greater than 4.5%. Attached Figure Description

[0020] Figure 1 This is a physical image of the LPBF molded part in Embodiment 1 of the present invention;

[0021] Figure 2Here is a SEM image of the microstructure of the LPBF molded part in Embodiment 2 of the present invention;

[0022] Figure 3 This is the room temperature tensile stress-strain curve of the LPBF molded part in Embodiment 3 of the present invention. Detailed Implementation

[0023] The following detailed description, with reference to specific embodiments, further illustrates the composition design and preparation method of a low-Mo equivalent ultra-high-strength titanium alloy. The advantages and benefits of this invention will be better understood by those skilled in the art through the detailed description of preferred embodiments below. These embodiments are for illustrative purposes only, and the invention is not limited to these embodiments.

[0024] This invention provides an ultra-high strength titanium alloy with low Mo equivalent, the chemical composition of which, by mass percentage, is: C≤0.01%, Al: 5.5~6.5%, V: 3.5~4.5%, O: 0.30~0.55%, H≤0.015%, N≤0.02%, with the balance being Ti and unavoidable impurities.

[0025] The composition design of this invention is based on the following principles:

[0026] Carbon: As interstitial solid solution atoms, carbon can improve the overall strength of titanium alloys, but while increasing strength, it also impairs the ductility, toughness, and weldability of titanium alloys. Carbon is generally considered an impurity element in titanium alloys. Taking all factors into consideration, the carbon content of the titanium alloy in this invention is controlled to be less than 0.01%.

[0027] Aluminum: Aluminum is the most commonly used α-stabilizing element in titanium alloys. By strengthening titanium alloys through substitution solid solution, adding an appropriate amount of aluminum can improve the room temperature and high temperature strength, as well as the thermoplasticity. Generally, an appropriate amount of aluminum is added to almost all types of titanium alloys both domestically and internationally. However, if too much aluminum is added, it can easily form a brittle Ti3Al phase in the titanium alloy, significantly reducing its ductility and toughness. Therefore, excessive aluminum addition should be avoided during alloy design. In this invention, the aluminum content of the titanium alloy is controlled within 5.5% to 6.5%.

[0028] Vanadium: Vanadium is the most widely used β-phase stabilizing element in titanium alloys, strengthening the β-phase through substitutional solid solution. Adding vanadium can significantly lower the α-β phase transformation point, increase the hardenability of titanium alloys, and thus enhance the heat treatment strengthening effect. Furthermore, compared with other β-stabilizing elements (Fe), vanadium is less prone to segregation and exhibits good microstructural stability at high temperatures. However, vanadium is a strategic element and is very expensive. Adding excessive vanadium to titanium alloys will significantly increase the cost. Therefore, the cost associated with adding vanadium needs to be considered in alloy design. In summary, the vanadium content of the titanium alloy in this invention is controlled at 3.5%–4.5%.

[0029] Oxygen: Oxygen is an interstitial solid solution element in titanium alloys, playing a role in solid solution strengthening. Simultaneously, the interaction between oxygen and dislocations in titanium alloys can significantly improve their strength. However, most studies report that while the addition of oxygen increases the strength of titanium alloys, it also results in a loss of ductility and toughness, leading to a significant decrease in elongation. Excessive oxygen content can also promote the formation of the brittle Ti3Al phase in titanium alloys, making them brittle. This invention uses oxygen as the main strengthening element for titanium alloys, overturning the traditional understanding that oxygen content and ductility are inversely proportional. It achieves a synergistic balance between strength and ductility, providing a new approach for the design of high-performance metallic materials. The main reason is that the rapid cooling rate of LPBF technology enriches oxygen in the formed titanium alloy parts, forming an oxygen-rich phase, allowing the titanium alloy to maintain a certain level of ductility and toughness while achieving high strength. In summary, the oxygen content of the titanium alloy in this invention is controlled at 0.30–0.55%.

[0030] Nitrogen and hydrogen: Nitrogen and hydrogen are generally considered impurity elements in titanium alloys. They can improve the strength of titanium alloys but significantly reduce their ductility and toughness. Hydrogen's effect on the mechanical properties of titanium is also reflected in hydrogen embrittlement. When the hydrogen content reaches a certain level, it will increase the notch sensitivity of the titanium alloy, thereby drastically reducing the impact toughness and other properties of the notched specimen. Therefore, the nitrogen and hydrogen content in titanium must be strictly controlled. In this invention, the nitrogen and hydrogen contents of the titanium alloy are controlled to be within 0.02% and 0.015%, respectively.

[0031] Example 1

[0032] Powder with the following composition: C: 0.008%, Al: 6.3%, V: 4.0%, O: 0.36%, H: 0.013%, N: 0.018%, balance Ti and unavoidable impurities, and a Mo equivalent of 2.85 was used for LPBF forming. The printing process was carried out in a high-purity argon atmosphere, with the following printing parameters: laser power 180W, scanning speed 800mm / s, pass spacing 100μm, and powder thickness 30μm. Figure 1 This is a photograph of an LPBF-formed titanium alloy part. The microstructure of the LPBF-formed titanium alloy sample was observed, and then it was machined into tensile bars for room temperature mechanical property testing. The tensile strength was 1560 MPa, the yield strength was 1450 MPa, and the elongation after fracture was 5.2%.

[0033] Example 2

[0034] Powder with the following composition: C: 0.007%, Al: 6.1%, V: 3.9%, O: 0.40%, H: 0.012%, N: 0.002%, with the balance being Ti and unavoidable impurities, and a Mo equivalent of 2.78, was used for LPBF forming. The printing process was carried out in a high-purity argon atmosphere, with a laser power of 200W, a scanning speed of 866mm / s, a pass spacing of 110μm, and a powder thickness of 30μm. The microstructure of the LPBF-formed titanium alloy sample was observed, such as... Figure 2 As shown, the microstructure of the low-Mo equivalent ultra-high-strength titanium alloy formed by LPBF consists of acicular α' martensite interspersed within the primary β grains. α' martensite is a non-equilibrium phase; the rapid cooling during the LPBF process leads to a non-equilibrium phase transformation from the primary β phase to α' martensite. Numerous studies have shown that the acicular α' microstructure can effectively improve the strength of titanium alloy formed parts. The formed parts were then machined into tensile bars for room temperature mechanical property testing. The tensile strength was 1540 MPa, the yield strength was 1430 MPa, and the elongation after fracture was 6.0%.

[0035] Example 3

[0036] A titanium alloy powder with the following composition: C: 0.005%, Al: 6.5%, V: 3.5%, O: 0.38%, H: 0.010%, N: 0.019%, with the balance being Ti and unavoidable impurities, and a Mo equivalent of 2.5, was used for LPBF forming. The printing process was carried out in a high-purity argon atmosphere, with a laser power of 180W, a scanning speed of 866mm / s, a pass spacing of 120μm, and a powder thickness of 30μm. The microstructure of the LPBF-formed titanium alloy sample was observed, and then it was machined into tensile bars for room temperature mechanical property testing. The tensile strength was 1580MPa, the yield strength was 1450MPa, and the elongation after fracture was 4.5%. Its stress-strain curve is shown below. Figure 3 As shown.

Claims

1. A method for preparing a low-Mo equivalent ultra-high-strength titanium alloy, characterized in that, The chemical composition of the low-Mo equivalent ultra-high-strength titanium alloy, by mass percentage, is: C≤0.01%, Al: 5.5~6.5%, V: 3.5~4.5%, O: 0.30~0.55%, H≤0.015%, N≤0.02%, with the balance being Ti and unavoidable impurities, and a Mo equivalent of 2.5~3.

2. The particle size range of the powder used for the low-Mo equivalent ultra-high-strength titanium alloy is 15~53μm. The powder is formed into low-Mo equivalent ultra-high-strength titanium alloy parts using LPBF technology. The cooling rate during the LPBF forming process is 10. 4 ~10 6 K / s; The low-Mo equivalent ultra-high strength titanium alloy has a room temperature tensile strength >1500 MPa, a yield strength >1400 MPa, and a fracture elongation >4.5%. The LPBF forming parameters are: laser power 180-200W, scanning speed 800-1000mm / s, pass spacing 80-120μm, powder thickness 30μm, and a protective atmosphere of 99.99% high-purity argon.

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