Titanium alloy sheet material and exhaust system components

Abstract

The present invention relates to metallurgy, and more specifically to a sheet material made from a titanium alloy that is heat-resistant and oxidation-resistant, exhibits structural stability under long-term operating exposure to temperatures ranging up to 800°C, and can be used to manufacture automotive exhaust system components. The titanium alloy sheet material for manufacturing components contains 1.5-3.0 wt% aluminum, 0.1-0.5 wt% molybdenum, 0.1-0.6 wt% silicon, 0.2 wt% or less iron, 0.15 wt% or less oxygen, 0.1 wt% or less carbon, 0.03 wt% or less nitrogen, 0.015 wt% or less hydrogen, and the balance titanium. The sheet material has high creep resistance and oxidation resistance values, and also has a stable structure under long-term operating exposure to temperatures ranging up to 800°C. The material is suitable for cold forming.

Description

[Technical Field]

[0001] The present invention relates to non-ferrous metallurgy, and in particular to the manufacture of sheet material from low-alloyed, heat-resistant and oxidation-resistant titanium alloys that have a stable structure during long-term use at temperatures up to 800°C, and can be used for products with extensive high-temperature applications, especially components in automobile engine exhaust systems. [Background technology]

[0002] Titanium-based alloys are used to manufacture components for various commercial applications, such as internal combustion engines and exhaust systems. These components include intake and exhaust valves, housings, turbine impellers, pipes, and tanks. Engine and exhaust system components made from low-alloyed titanium-based alloys operate at temperatures between approximately 500 and 800°C. Therefore, material operating properties such as heat resistance and oxidation resistance are a priority. Furthermore, because components are primarily manufactured by cold forming from rolled sheet and bending of welded tubes, the applied materials must have sufficient technological plasticity. To achieve high plasticity, it is important to create a structure with a spherical morphology of α-phase grains in the material, as a spherical microstructure provides better forming properties than an acicular structure.

[0003] As internal combustion engine designers improve engine efficiency, properties such as boost pressure, compression ratio, and operating temperature increase accordingly. Increasing levels of these properties create demands on materials that can resist distortion (creep) at higher operating temperatures and pressures in the combustion chamber and exhaust system than are currently achievable with conventional low-alloyed titanium alloys. Creep is the tendency of a solid material to move slowly under load or develop residual strain, and occurs when a metal is subjected to a constant tensile load at high temperatures. High creep resistance is important so that the material can be used extensively without distortion of shape and size, while at the same time maintaining the material's inherent property levels.

[0004] This calls for materials that combine high mechanical and operational properties in the best possible way, in addition to low cost.

[0005] Rolled products and exhaust system components are known that are made from an oxidation-resistant, high-strength titanium alloy, consisting of 0.06 to 0.5 wt% iron, 0.02 to 0.12 wt% oxygen, 0.15 to 0.46 wt% silicon, and the balance titanium and unavoidable impurities. The titanium alloy grain size is 15.9 μm or less on average (Patent Document 1, published January 8, 2013, IPC C22C14 / 00).

[0006] The rolled product has high plastic properties but reduced resistance to high temperature oxidation.

[0007] A material for exhaust systems made from a low-alloyed titanium alloy with excellent resistance to high-temperature oxidation and corrosion is known. It contains 0.30-1.50 wt% Al, 0.10-1.0 wt% Si, and 0.1-0.5 wt% Nb (Patent Document 2, published January 23, 2007, IPC B32B15 / 01; C22C14 / 00, F01N7 / 16). This material is prior art.

[0008] Materials made from the above alloys have high strength and plastic properties at room and elevated temperatures, but have an inadequate level of resistance to high temperature creep. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 8,349,096 [Patent Document 2] U.S. Patent No. 7,166,367 Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention is to develop a low-alloyed titanium alloy sheet material with a globular microstructure that will enable the manufacture of a wide range of products, including those used in automotive engine components and exhaust systems. [Means for solving the problem]

[0011] The technical result achieved by practicing this invention is the production of titanium alloy sheet material with a range of enhanced mechanical and operating properties, including improved levels of creep and oxidation resistance, as well as structural stability and the ability to be cold formed under a wide range of operating conditions at temperatures up to 800°C.

[0012] The technical results are achieved when the titanium alloy sheet material according to the invention for manufacturing parts for extensive use at high temperatures comprises the following elements (in wt%): Aluminum 1.5~3.0, Molybdenum 0.1~0.5, Silicon 0.1~0.6, Iron up to 0.2, Oxygen max 0.15, Carbon up to 0.1, Nitrogen: Maximum 0.03 Hydrogen: Maximum 0.015, Ti balance (Balance - Ti).

[0013] If the ratio of Mo to Si (wt%) is equal to 0.4-3, the sheet material contains at least 90% (vol%) of α-phase. The total content of β-phase and titanium silicide intermetallic particles is 0.5-5 vol%. The average α-phase grain size ranges from 5 to 100 μm. In addition, the sheet material is produced in the form of rolled sheets up to a maximum thickness of 6 mm. This technical result is also achieved when components of automobile exhaust systems manufactured from titanium alloy sheet material are proposed, which are used extensively at high temperatures. [Brief explanation of the drawings]

[0014] [Figure 1] A comparative graph of the mechanical tensile properties of the materials in the as-delivered (annealed) condition is shown in FIG. [Figure 2] The results of the evaluation of oxidation resistance compared to prior art alloys are shown in the graph of FIG. 2, which shows the dependence of the weight gain of the alloys on the square root of the oxidation time at a temperature of 560° C. [Figure 3] The results of the evaluation of oxidation resistance compared to prior art alloys are shown in the graph of FIG. 3, which shows the dependence of the weight gain of the alloys on the square root of the oxidation time at a temperature of 625° C. [Figure 4] The results of the evaluation of oxidation resistance compared to prior art alloys are shown in the graph of FIG. 4, which shows the dependence of the weight gain of the alloys on the square root of the oxidation time at a temperature of 800° C. [Figure 5] The results of the creep resistance of the claimed materials compared to the prior art are shown in the graph of FIG. [Figure 6] It is shown that the grain structure of the material with titanium silicide grains and β-phase interfacial layer after annealing at 625°C for 1000 hours remains unchanged compared to the initial one. DETAILED DESCRIPTION OF THE INVENTION

[0015] Alloying elements from various stabilizer groups are added to titanium alloy materials: α stabilizers: aluminum, oxygen, carbon, nitrogen; β stabilizers: molybdenum, silicon.

[0016] Aluminum improves heat and creep resistance and inhibits scaling at high temperatures. The aluminum content in the alloy is assumed to be 1.5-3.0 wt%. To maintain optimum technological plasticity, the maximum aluminum content in the alloy is limited to 3.0 wt%.

[0017] As long as the oxygen, nitrogen, and carbon contents are within the specified range, the strength of titanium increases and the allotropic transformation temperature rises, maintaining a high level of strength and plasticity. Higher concentrations of oxygen, carbon, and nitrogen will reduce the technological plasticity and affect the strength of the alloy.

[0018] Beta stabilizer group (Mo, Si).

[0019] Alloying with molybdenum in amounts of 0.1-0.5 wt% improves strength due to solid solution hardening and the appearance of a β-phase interfacial layer in the structure. This β-phase interfacial layer is an interface boundary that slows the movement of strain dislocations during deformation and prevents the aggregation and growth of α-grains at high temperatures during heat treatment and operation. Molybdenum contents above 0.5 wt% decrease the β-transus temperature of the alloy and increase the proportion of β phase in the structure, thereby reducing heat resistance.

[0020] The presence of silicon in the alloy, in titanium solid solution, improves creep resistance. The silicon content in the alloy is set in the range of 0.1-0.6 wt%. In this range, silicon forms intermetallic compounds with titanium, forming silicides of complex stoichiometric composition (TiSi). The formation of the necessary amount of silicide in the alloy improves heat resistance and creep resistance and prevents alpha grain growth at high temperatures. In addition, silicon significantly improves oxidation resistance, provided its content does not exceed 0.8 wt%. Higher contents result in a decrease in technological plasticity / formability due to the formation of coarse-grained silicides. The absence of Zr and Sn in the alloy lowers the eutectoid transformation temperature for silicide formation, allowing the Si content in solid solution to be maximized, resulting in the greatest improvement in heat resistance.

[0021] Limiting the maximum hydrogen content in the alloy to 0.015 wt% avoids embrittlement of the alloy due to the possible formation of titanium hydrides.

[0022] The iron content in the alloy is limited to 0.2 wt% because higher contents adversely affect creep resistance and short-term heat resistance.

[0023] The main factor in structural stability during extended operation at high temperatures is the presence of grain growth-inhibiting particles: beta-phase particles and silicide particles in the alloy. The presence of both types of particles in the alloy is crucial and is achieved by aligning the Mo and Si contents. The preferred ratio of beta-isomorphous molybdenum to beta-eutectoid silicon, in weight percent, Mo / Si, is in the range of 0.4 to 3. This ratio allows for improved oxidation and creep resistance, as well as improved structural stability during extended operation.

[0024] The composition of elements added to the alloy in the claimed amounts and individually characterized by their beneficial effect on the oxidation resistance of titanium allows an additive effect to be achieved in obtaining high creep resistance values ​​while providing strength, plastic properties in combination with oxidation resistance compared to known low-alloyed titanium alloys.

[0025] Further improvements in material properties can be achieved by adjusting the structure, which affects cold forming characteristics. A spherical structure of α-phase grains has higher plasticity and formability values ​​than an acicular structure. For this reason, a homogeneous spherical microstructure with an average grain size of 5 to 100 μm is preferred to improve the formability of sheet material. Obtaining a microstructure with an average α-phase grain size of less than 5 μm requires numerous technological operations and is therefore expensive. In microstructures with an average α-phase grain size greater than 100 μm, the boundaries of large grains become the starting point for fracture. The average diameter of α-phase grains in titanium billet structures is measured according to the practice of the international standard ASTM E112. A scanning electron probe microscope (SEM) is used in backscattered electron mode, and images are processed using software for quantitative analysis of microstructures by elemental contrast to calculate the percentage of β-phase grains and silicides.

[0026] The preferred alpha phase content in the material should be at least 95 vol% to ensure stability of the alpha grain structure during operation. A total content of beta phase and titanium silicide intermetallic particles in the material in the range of 0.5-5 vol% improves creep resistance at high temperatures.

[0027] The industrial applicability of the present invention is confirmed by the examples of specific embodiments thereof. [Example]

[0028] To investigate the properties of the proposed material, an ingot weighing 2,100 kg was melted by vacuum arc remelting using industrial techniques. The chemical composition of the alloy is shown in Table 1.

[0029] [Table 1]

[0030] The ingot was forged and further rolled to obtain 0.9 mm thick coils, the final rolling step being carried out at the beta-transus temperature of 945 °C, which is necessary to produce a spherical alpha grain structure. Test specimens for mechanical characterization of the alloy were obtained in the as-shipped condition. The mechanical properties were evaluated during tensile tests carried out at temperatures of 20 °C, 500 °C, and 700 °C, and a deep drawing test according to Eriksen was carried out to evaluate the formability criteria of the material. The mechanical tensile properties of the material in the as-shipped condition (annealed) are shown in Table 2, and a comparative graph is shown in Figure 1.

[0031] [Table 2]

[0032] To simulate the performance of the material in production, samples were subjected to isothermal annealing in static laboratory air at temperatures of 560°C, 625°C with a holding time of 1000 hours, and 800°C with a holding time of 200 hours. The oxidation resistance was then investigated by calculating the weight gain (mg / cm2) of the samples. The results of the evaluation of oxidation resistance compared to prior art alloys are shown in the graphs in Figures 2, 3, and 4, which show the dependence of the alloy's weight gain on the square root of the oxidation time at temperatures of 560°C, 625°C, and 800°C, respectively.

[0033] Additionally, creep resistance at 500°C was measured for the samples at delivery conditions for 100 hours and is expressed as a function of relative specimen strain at a voltage of 30 MPa. The results of creep resistance of the claimed material compared to the prior art are shown in the graph of Figure 5.

[0034] The average α-phase grain size in the stock material structure in the longitudinal section, measured according to the international standard ASTM E112, is 15 μm. The proportion of α-phase is 98 vol%, and the proportion of β-phase and titanium silicide grains is 2 vol%. The proportion of β-phase and titanium silicide grains was calculated using a scanning electron probe microscope (SEM) in backscattered electron mode and an image analysis program.

[0035] The grain structure of the material with titanium silicide grains and β-phase interfacial layer after annealing at 625°C for 1000 hours remains unchanged compared with the initial one (Fig. 6), indicating the stability of the structure.

[0036] Analysis of test results and research data indicates that the proposed titanium alloy sheet material possesses a range of enhanced mechanical and performance properties, including resistance to high-temperature creep, compared to known low-alloyed alloys. Results of oxidation resistance evaluation of specimens after extended isothermal annealing demonstrate the durability of the material.

Claims

1. 1. A titanium alloy sheet material for manufacturing components for extensive use at high temperatures, comprising: 1.5 to 3.0 wt % aluminum; 0.1 to 0.5 wt % molybdenum; 0.1 to 0.6 wt % silicon; up to 0.2 wt% iron; 0.15 wt% maximum oxygen; 0.1 wt% carbon at most; a maximum of 0.03 wt% nitrogen; A maximum of 0.015 wt% hydrogen is the element content in the titanium alloy; A sheet material comprising the balance being Ti.

2. 2. The sheet material according to claim 1, wherein the alloy contains Mo such that the ratio (wt%) of Mo to Si is 0.4 to 3.

3. 2. The sheet material according to claim 1, characterized in that the average α-phase grain size is in the range of 5 to 100 μm.

4. 2. The sheet material of claim 1, comprising at least 95 vol.% of the alpha phase.

5. 2. The sheet material of claim 1, comprising β-phase and titanium silicide intermetallic particles such that the total content of β-phase and titanium silicide intermetallic particles is 0.5 to 5 vol %.

6. 2. Sheet material according to claim 1, characterized in that it is produced in the form of a rolled product with a thickness of up to 6 mm.

7. 10. An automotive exhaust system component for extensive use at high temperatures and made from titanium alloy sheet material, characterized in that it is made from the sheet material according to any one of claims 1 to 6.

Images