Titanium-based alloy and article made of same

Abstract

The invention relates to the field of nonferrous metallurgy and particularly to development of titanium-based alloys that are resistant to high-temperature oxidation, and can be used for manufacture of articles operating under conditions of prolonged exposure to high temperatures, in particular for manufacture of components of vehicle engine exhaust systems. The titanium-based alloy consists of aluminum, zirconium, niobium, molybdenum, silicon, oxygen, nitrogen, iron, hydrogen, and carbon, and is characterized in that the components of the alloy are in the following ratio, in weight %: Aluminum 0.7-1.5 Zirconium 0.5-1.5 Niobium 0.5-1.5 Molybdenum 0.1-0.5 Silicon 0.5 max Iron 0.2 max Oxygen 0.15 max Carbon 0.1 max Nitrogen 0.03 max Hydrogen 0.015 max Titanium and inevitable impurities being the balance; and an article made of this alloy. The technical result achieved in the implementation of the invention is obtaining of a titanium alloy with the set of high mechanical and performance properties including increased level of high-temperature oxidation resistance.

Description

[0001] The invention relates to the field of nonferrous metallurgy and particularly to development of titanium-based alloys that are resistant to high-temperature oxidation, and can be used for manufacture of articles operating under conditions of prolonged exposure to high temperatures, in particular for manufacture of components of vehicle engine exhaust systems.

[0002] Components of vehicle engine exhaust systems operate under conditions of high-temperature oxidation (the operating temperature can reach 900°C), therefore the materials that are used for the above mentioned systems are required to have a high level of such characteristics as corrosion resistance, heat resistance, and strength. Moreover, the material that is used shall possess sufficient ductility as most of the components are manufactured by cold forming of sheet products, and by bending of welded tubes. Austenitic stainless steels have been used for a long time as the material that meets the specified requirements, meanwhile, in recent years, with the increasing demand for their use; they have been superseded by ferritic stainless steels having a small thermal expansion coefficient and a lower cost of raw materials. However, an important task for production of exhaust system components is to reduce weight while maintaining performance. Therefore, for manufacture of exhaust systems with a lower weight that allow obtaining the advantages in decreasing the total weight of vehicle structure, the preference of design engineers is given to titanium alloys with high strength-to-weight ratio that is significantly higher than that of alloyed steels. The advantages obtained from replacing alloyed steels with titanium alloys are very significant, as they allow decreasing weight of parts by at least 1.5 times, minimizing corrosion and operational difficulties.

[0003] It is known that commercially pure (CP) titanium is used in automobile exhaust systems and motorcycle mufflers, in this case when using titanium to replace stainless steel the weight decrease can be up to 44%. However, the components are heated above 500 °C during operation, which results in rapid oxidation of the CP titanium materials used, decrease in strength and consequently reduced service life. Therefore, the use of CP titanium without special surface treatment is limited to certain components of exhaust systems operating at relatively low temperatures. To improve the oxidation resistance, various surface coatings such as films or phased deposition can be applied to titanium components; however, such methods are very expensive and not sufficiently effective.

[0004] The existing developed high-temperature titanium alloys that are based on high-alloyed alloys and intermetallic compounds, in particular titanium aluminides, subject to short-term operation at temperatures up to 900 °C do not ensure compliance with technical requirements for the material due to their low ductility, both in manufacturing thin sheet products and in manufacturing finished components of exhaust systems made thereof, which limits the use of high-alloyed titanium alloys in these products. Therefore, the development of new state-of-the-art low-alloyed titanium alloys with a set of high mechanical and performance properties that ensure reliable and durable operation of components of vehicle exhaust systems, with the increasing demand for their use, is a very urgent technical task.

[0005] There is a known α-titanium alloy that is used for exhaust system components, consisting of: 0.4 to 1.5 wt.% Al, 0.5 to 1.5 wt.% Sn, 0.5 to 2.0 wt.% Zr, 0.1 to 1.0 wt.% Si, ≤0.04 wt.% O, and ≤0.06 wt.% Fe, or, moreover, further to the above, consisting of 0.1-1.5 wt.% Nb (Japanese patent No. 4850662, published on 11.01.2012, IPC C22B34 / 12, C22B9 / 22, C22C14 / 00).

[0006] The alloy has the increased resistance to oxidation as against CP titanium and high plastic properties; however, it has a low level of strength properties at room and elevated temperatures.

[0007] There is a known low-alloyed titanium alloy that has excellent resistance to high-temperature oxidation and corrosion resistance, and is used as a material for vehicle or motorcycle exhaust systems, consisting of, in weight percentages: Al: 0.30-1.50%, Si: 0.10-1.0%, and further consisting of Nb: 0.1-0.5% (US patent No. 7166367, published on 23.01.2007, IPC B32B15 / 01; C22C14 / 00, F01N7 / 16) prior art.

[0008] The alloy has high strength and plastic properties at room and elevated temperatures, as well as the high-temperature strength; however, its level of high-temperature oxidation resistance is not sufficient.

[0009] The invention aims at development of a titanium alloy that can be used for manufacture of a wide range of articles, including those used in vehicle exhaust system components.

[0010] The technical result achieved in the implementation of the invention is obtaining of a titanium alloy with the set of high mechanical and performance properties including increased level of high-temperature oxidation resistance.

[0011] The technical result is achieved by using the titanium-based alloy provided herein consisting of aluminum, zirconium, niobium, molybdenum, silicon, oxygen, nitrogen, iron, hydrogen, and carbon with the weight percentages of the alloy components: Aluminum0.7-1.5Zirconium0.5-1.5Niobium0.5-1,5Molybdenum0.1-0.5Silicon0.5 maxIron0.2 maxOxygen0.15 maxCarbon0.1 maxNitrogen0.03 maxHydrogen0.015 max

[0012] Titanium and inevitable impurities being the balance; and an article made of this alloy.

[0013] The alloy contains alloying elements from various groups of stabilizers: α-stabilizers: aluminum, oxygen, carbon; β-stabilizers: molybdenum, iron, niobium, silicon; neutral strengthener: zirconium.

[0014] Group of α-stabilizers (Al, O, N, C,).

[0015] Aluminum is the most effective strengthener in titanium alloys that improves strength properties and high-temperature strength of the alloy. Conventional content of aluminum in the alloy is 0.7-1.5 wt.%, since aluminum content below 0.7 wt.% doesn't result in the increase in strength to the required level, and the content above 1.5 wt.% decreases ductility at room temperature. Contents of oxygen, nitrogen and carbon within the specified limits, along with the increase in strength, increase allotropic transformation temperature of titanium and ensure maintaining a high level of strength and ductility. Higher concentrations of oxygen and carbon decrease ductility and impact strength of the alloy.

[0016] Group of neutral stabilizers (Zr).

[0017] Zirconium introduced into the alloy as a neutral element forms a range of solid solutions completely miscible with α-titanium, and with its melting point and the density relatively similar to those of α-titanium, reduces the negative effect of gas impurities, in particular oxygen, and refines the structure. Alloying with zirconium within the range of 0.5-1.5 wt.% ensures solid solution hardening and increase of high-temperature strength.

[0018] Group of β-stabilizers (Nb, Mo, Fe, Si).

[0019] The alloy is alloyed with niobium in the amount of 0.5-1.5 wt.% which increases the oxidation resistance, since niobium is one of the most strongest among all titanium alloying elements that greatly increase this characteristic.

[0020] Compared to the prior art, one more β-stabilizer is introduced into the alloy - molybdenum in the amount of 0.1-0.5 wt.% which contributes to the increase of high-temperature strength of the alloy. With the aim of increasing oxidation resistance, the alloy contains silicon in the amount of 0.5 wt.% maximum which increases creep resistance.

[0021] The presence of iron in the alloy, which is a low-cost β-stabilizing element, increases the strength of the alloy without decreasing ductility. The iron content above 0.2 wt.% results in a decrease in high-temperature strength.

[0022] The maximum content of hydrogen in the alloy which is limited to 0.015 wt.% allows to avoid the alloy embrittlement due to possible forming of titanium hydrides.

[0023] The composition of the elements that are introduced into the alloy in the claimed ratio and are individually characterized by the favorable impact on the oxidation resistance of titanium allows to achieve an additive effect in terms of obtaining high values of oxidation resistance of the alloy while ensuring the strength and plastic properties, as compared to the known alloys.

[0024] Industrial applicability of the invention is confirmed by the example of its specific implementation.

[0025] To evaluate the properties of the claimed alloy the ingot was melted by the method of vacuum arc remelt, with the weight of 2100 kg. Chemical composition of the alloy is given in Table 1. Beta transus temperature was determined by metallographic method, and was equal to 903°C. Table 1Sampling areaContent of elements, wt.%TiAlZrNbMoSiFeOCNHIngot topbalance0.960.90.680.210.0110.0440.1010.003<0.0030.003Ingot bottombalance1.021.030.710.200.0120.0480.0950.003<0.0030.004

[0026] The ingot was subject to deformation by forging and subsequent rolling to manufacture the sheet products in coils, with the thickness of 0.9 mm. To conduct the evaluation, sampling was performed in delivery condition. To analyze mechanical properties, tensile testing was carried out at 20°C, 700°C, and 800°C. To evaluate the criterion of material forgeability, the Erichsen deep-drawing test was carried out. The results of tensile testing and the criterion of the Erichsen test are shown in Table 2.

[0027] To simulate the performance of the material during the article operation, the specimens were subject to isothermal annealing in the static laboratory air at 625°C with the soaking time up to 1000 hours, at 700°C with the soaking time up to 300 hours, and at 800°C with the soaking time up to 200 hours. During the annealing, the specimens were subject to intermediate control of weight, and to unloading. After that, the oxidation resistance was examined by calculating the weight gain of the specimens, expressed in mg / cm 2< , and the average thickness of the oxide on the surface of the specimens was determined on microslices in longitudinal section. The results of examination of the oxide thickness, compared to that of the prior art alloy, are given in Table 3. The results of examination of the oxidation resistance, compared to that of the prior art alloy, are given on the graphs of the dependence of the alloy weight on the square root of the oxidation time at 625°C, 700°C, and 800°C that are shown in Fig. 1, Fig. 2 and Fig. 3, correspondingly. Photograph of the oxide on the surface of the specimen in longitudinal section after isothermal annealing at 800°C during 100 hours (at 550-fold magnification) is shown in Fig. 4. Table 2Condition of specimen during testingSampling directionTest temperature, °CMechanical propertiesThe criterion of the Erichsen test, mmYield strength, σ 0.2 , MPaTensile strength, σ B , MPaElongation, δ, %AnnealedTransverse20°C480529258.3700°C6779>40800°C4050>40 Table 3 Alloy typeThe average thickness of the oxide on the surface of the alloys after high-temperature oxidation, µm625°C / 1000 h700°C / 200 h800°C / 200 hThe alloy according to the proposed invention2.852.8512.4Prior art alloy4.063.6712.8

[0028] Analysis of the test results and the data obtained from examinations revealed that the claimed alloy has the set of high mechanical and performance properties including strength properties and ductility at room temperature, high-temperature strength, as well as oxidation resistance, that are, on average, 10-15% higher than those of the known similar alloys.

Claims

1. A titanium-based alloy consisting of aluminum, zirconium, niobium, molybdenum, silicon, oxygen, nitrogen, iron, hydrogen, and carbon, characterized in that the components of the alloy are in the following ratio, in weight %: Aluminum0.7-1.5Zirconium0.5-1.5Niobium0.5-1.5Molybdenum0.1-0.5Silicon0.5 maxIron0.2 maxOxygen0.15 maxCarbon0.1 maxNitrogen0.03 maxHydrogen0.015 max The balance is titanium and inevitable impurities.

2. An article of the titanium-based alloy, characterized in that it is made of the alloy according to claim 1.

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