TITANIUM-BASED ALLOY AND ARTICLE MANUFACTURED FROM SAME

Abstract

 The invention relates to nonferrous metallurgy, namely to the development of low-alloyed titanium alloys characterized by high-temperature strength and thermal stability, and can be used for manufacture of articles intended for long-term operation at high temperatures, namely components of exhaust systems of vehicle engines. Titanium alloy containing aluminum, molybdenum, silicon, oxygen, nitrogen, iron, hydrogen, with the alloy components taken in the following ratio, % wt.:
Aluminium
1.5 to 3.0
Molybdenum
0.1 to 0.5
Silicon
0.1 to 0.6
Iron
0.2 max
Oxygen
0.15 max
Carbon
0.1 max
Nitrogen
0.03 max
Hydrogen
0.015 max
Titanium and inevitable impurities - balance,
which in one embodiment additionally contains copper 0.5 to 1.5% wt., and article made thereof.
A technical result of the embodiment of invention is the production of titanium alloy characterized by a combination of high mechanical and performance properties, including a higher level of creep resistance, with capability of cold forming.

Description

[0001] The invention relates to nonferrous metallurgy, namely to the development of low-alloyed titanium alloys characterized by high-temperature strength and thermal stability, and can be used for manufacture of articles intended for long-term operation at high temperatures, namely components of exhaust systems of vehicle engines.

[0002] In various commercial applications, such as internal combustion engines and exhaust systems, titanium alloys are used for manufacture of their components such as intake and exhaust valves, housings, turbine impellers, pipes and tanks. In many of these applications, engine components, particularly exhaust systems, made of low-alloyed titanium alloys are subject to operating temperatures of 500 to 800 °C. Therefore, the performance properties of alloys, such as high-temperature strength and oxidation resistance, are a priority. In addition, the material used shall exhibit sufficient process ductility because the components are mainly manufactured by cold forming of rolled sheet metal and by bending of welded tubes.

[0003] As designers of internal combustion engines improve the efficiency of engines, the characteristics such as boost pressure, compression ratio and operating temperatures improve accordingly. Increasing the level of these characteristics results in the need for materials that will resist (creep) strain at higher operating temperatures and pressures in the combustion chamber and exhaust system than are currently achievable with conventional low-alloyed titanium alloys. Creep, which is the susceptibility of a solid material to slow offset or residual strain under load, occurs when metal is subjected to a constant tensile stress at elevated temperature. High creep resistance allows the material to be used for a long time without distortion of shape and size, while it is important to maintain the level of original material properties.

[0004] Consequently, materials which, in addition to their low price, have the best combination of high mechanical and performance properties are in demand.

[0005] There is a known oxidation-resistant high-strength titanium alloy consisting mainly of (% wt.): 0.2 to less than 0.5 iron, 0.02 to less than 0.12 oxygen, 0.15 to 0.6 silicon, and balance - titanium and inevitable impurities. The alloy additionally contains at least one element selected from the group consisting of Al, Nb, V, Mo, Sn, Zr, Ni, Cr and Ta, with a total content of less than 1.5 (US Patent No. 7767040, published 03.08.2010, IPC C22C14/00).

[0006] The alloy exhibits high plastic properties, but has low resistance to high-temperature oxidation.

[0007] There is a known low-alloyed titanium alloy characterized by excellent resistance to high-temperature oxidation and corrosion, which is used as a material for exhaust system of vehicles or motorbikes, containing (% wt.) Al: 0.30 to 1.50%, Si: 0.10 to 1.0%, and additionally containing Nb: 0.1 to 0.5 (US Patent No. 7166367, published 23.01.2007, IPC B32B15/01; C22C14/00, F01N7/16) - prototype.

[0008] The alloy exhibits high strength and plastic properties at room and elevated temperature, but has insufficient level of high-temperature creep resistance.

[0009] The objective of this invention is to develop low-alloyed titanium alloy enabling the manufacture of a wide range of articles thereof, including those used in engine components and exhaust systems of vehicles.

[0010] A technical result of the embodiment of invention is the production of titanium alloy characterized by a combination of high mechanical and performance properties, including a higher level of creep resistance, with capability of cold forming.

[0011] A technical result is achieved by means of titanium alloy containing aluminum, molybdenum, silicon, oxygen, nitrogen, iron, hydrogen, with the alloy components taken in the following ratio, % wt.:

Aluminum	1.5 to 3.0
Molybdenum	0.1 to 0.5
Silicon	0.1 to 0.6
Iron	0.2 max
Oxygen	0.15 max
Carbon	0.1 max
Nitrogen	0.03 max
Hydrogen	0.015 max

[0012] Titanium and inevitable impurities - balance,
which in one embodiment additionally contains copper 0.5 to 1.5% wt., and article made thereof.

[0013] The alloying elements are introduced into the alloy composition from various groups of stabilizers: alpha-stabilizers: aluminum, oxygen, carbon, nitrogen; beta-stabilizers: molybdenum, iron, silicon. In one embodiment of the invention, a beta-stabilizer - copper is introduced into the alloy.

[0014] Aluminum increases high-temperature strength and creep resistance, reducing the scale formation at high temperature. Aluminum content in the alloy is set to contain 1.5 to 3.0% wt. To maintain optimum process ductility, the maximum aluminum content in the alloy is limited to 3.0% wt.

[0015] The content of oxygen, nitrogen and carbon within the specified limits, in addition to strength improvement, increases the temperature of allotropic transformation of titanium and ensures the maintenance of a high level of strength and ductility. Higher concentrations of oxygen, carbon and nitrogen decrease process ductility and impact strength of the alloy.

[0016] A group of beta-stabilizers (Mo, Fe, Si, Cu).

[0017] Molybdenum alloying of the alloy in the amount of 0.1 to 0.5% wt. promotes strength improvement due to the occurrence of β-phase layers in the structure, which act as interphase boundaries and inhibit the dislocation motion during deformation, as well as prevent the collective growth of α-grains at high temperatures. Molybdenum content exceeding 0.5% wt. reduces high-temperature strength, since beta transus temperature of the alloy decreases and the amount of β-phase in the structure increases.

[0018] The presence of silicon in the alloy, which is present in the titanium solid solution, increases the creep resistance. Silicon content in the alloy is set to contain 0.1 to 0.6% wt. Within this range, silicon forms intermetallic compound with titanium - silicide (Ti₃Si). The formation of the required amount of silicides in the alloy increases high-temperature strength, creep resistance, and prevents the growth of α-grains at high temperatures. In addition, silicon significantly increases the oxidation resistance of the alloy up to a concentration of 0.6% wt. At higher concentrations, the process ductility/formability decreases.

[0019] The alloy can be additionally alloyed with copper. Copper, being a eutectoid-forming element and having high solubility in titanium alpha phase, provides the effect of solid-solution strengthening. The formation of Ti₂Cu intermetallic particles, limiting the migration of boundaries at high temperature, helps to increase the high-temperature strength of the alloy, however, the excessive number of Ti₂Cu phase particles reduces the alloy ductility at room temperature, therefore the copper content in the proposed alloy is determined to be 1.5% wt. maximum.

[0020] The maximum hydrogen content in the alloy, limited to 0.015% wt., helps to avoid embrittlement of the alloy due to potential formation of titanium hydrides.

[0021] The composition of elements introduced into the alloy in the specified ratio and individually characterized by a favorable effect on the oxidation resistance of titanium, helps to achieve an additive effect in terms of obtaining high creep resistance values of the alloy while ensuring strength and plastic properties in combination with satisfactory oxidation resistance compared to known low-alloyed titanium alloys.

[0022] Industrial applicability of the invention is proved by the exemplary embodiment.

[0023] Two compositions of ingots weighing 2100 kg were melted according to the industrial process using vacuum arc remelting method to test the properties of the proposed alloy. Chemical composition No. 1 and chemical composition No. 2 of the alloy are given in Table 1.

Table 1

Alloy compo sition	Sampling area	Content of elements, % wt.
		Ti	Al	Mo	Si	Cu	Fe	O	C	N	H
No. 1	Ingot top	base	1,92	0,25	0,41	-	0,034	0,098	0,003	<0,003	<0,003
	Ingot bottom	base	1,91	0,25	0,39	-	0,033	0,095	0,003	<0,003	<0,003
No. 2	Ingot top	base	2,37	0,23	0,31	0,9	0,038	0,121	0,003	<0,003	<0,003
	Ingot bottom	base	2,39	0,22	0,22	0,79	0,033	0,120	0,003	<0,003	<0,003

[0024] Ingots were hot worked by forging and subsequent rolling to produce coils with a thickness of 0.9 mm. Samples in delivery condition were taken to evaluate the mechanical properties of the alloys. Tensile tests at temperatures of 20 °C, 500 °C, 700 °C were performed to analyze the mechanical properties; Erichsen deep drawing cup tests were performed to evaluate the material formability criterion. The values of tensile properties of the alloy in delivery condition (as-annealed) are given in Table 2 and comparative graph shown in Fig. 1.

Table 2

Alloy composi tion	Test temperature,°C	Sampling direction	Mechanical properties			Erichsen test criterion, Average indentation depth, IE, mm
			Proof stress σ0.2, MPa	Tensile strength σB, MPa	Elongation δ, %
No. 1	20 °C	Longitudinal	500	621	21	5,7
		Transverse	542	597	21,5
	500 °C	Longitudinal	212	332	21
		Transverse	209	316	21,6
	700 °C	Longitudinal	91	119	>30
		Transverse	102	117	>30
No. 2	23 °C	Longitudinal	496	614	22,4	5,8
		Transverse	540	588	23,9
	500 °C	Longitudinal	240	399	18,4
		Transverse	253	352	28
	700 °C	Longitudinal	99	109	>30
		Transverse	103	113	>30

[0025] In order to simulate the material performance during operation in the article, isothermal annealing of samples of both compositions was performed in static laboratory air at temperatures of 560 °C and 800 °C with a holding time of 100 and 200 hours respectively. After that, the oxidation resistance was evaluated by calculating the increase in weight of the samples expressed in mg/cm². The results of evaluations of oxidation resistance in comparison with the prototype alloy are shown in the graphs of alloy weight increase versus the square root of oxidation time at 560 °C and 800 °C shown in Fig. 2 and Fig. 3 respectively.

[0026] In addition, creep resistance expressed as a function of relative strain at a stress of 30 MPa was determined on samples of alloy in the delivery condition at 500 °C for 100 hours. The results of creep resistance of the claimed alloy in comparison with the prototype alloy are shown in the graph given in Fig. 4.

[0027] Analysis of test results and evaluation data showed that the proposed alloy exhibits a combination of high mechanical and performance properties, including high-temperature creep resistance compared to known low-alloyed alloys. The results of evaluation of oxidation resistance of alloy samples after long-term isothermal annealing demonstrate the durability of the material.

Claims

1. Titanium alloy containing aluminum, molybdenum, silicon, iron, oxygen, carbon, nitrogen, hydrogen, characterized in that the alloy components are taken in the following ratio, % wt.:

Aluminum	1.5 to 3.0
Molybdenum	0.1 to 0.5
Silicon	0.1 to 0.6
Iron	0.2 max
Oxygen	0.15 max
Carbon	0.1 max
Nitrogen	0.03 max
Hydrogen	0.015 max

Titanium and inevitable impurities - balance.

2. The alloy according to claim 1, characterized in that it additionally contains copper 0.5-1.5% wt.

3. Titanium alloy article, characterized in that it is made of the alloy under claim 1 or 2.

Drawing