Valve for an internal combustion engine, its method of manufacturing, and the high-temperature titanium alloy for the valve Field of the Art

The invention is related to machine-building, more specifically to engine-building and can be used in the piston-type internal combustion engines.

Prior art During the period of internal combustion engine development different designs, materials and methods of thermal strengthening for intake and exhaust valves were created. These valves mainly determine the reliability and durability of internal combustion engines. Usually steel of special qualities are used to make internal combustion engine valves. Using steel with high density (p = 7. 63-8.0 g/cm3) for that purpose increases valve weight and, hence, causes higher loads in the links of the valve drive mechanisms and considerable shock loads when the valves set on the seats.

All this reduces the reliability and failure-free performance of gas- distribution mechanism and the engine as a whole. Up to 45 % of combustion engine failures are caused by failures of the gas distribution mechanism, mainly due to defects of exhaust valves. The valve weight is one of the restrictive factors in manufacturing special designed engines which are highly boosted in their rotational speeds, and engines for the sport cars.

Piston-type internal combustion engine valves (particularly exhaust valves) work under high thermal loads. The piston-ICEs, i. e.: industrial stationary, transport (marine, diesel locomotive, tractor, aircraft, automobile, motorcycle) or for engines of special design, the temperatures

in the center of valve heads in the steady state are: for intake valves 500- 650°C, and for exhaust valves 650-900°C. Temperature differentials in the fillet portion reach 200-300°C in the axial direction. In the valve head proper the temperature differentials are up to 150-200°C in the radial direction. This causes high levels of temperature stresses in the valve head, in the transitional zone and rapid destruction of the valves (see: (Dvigateli vnutrennego sgoraniya: Konstruirovaniye i raschet na prochnost porshnevykh i kombinirovannykh dvigatelei). Internal combustion engines: Designing and strength analysis of piston-type and compound engines.

Edited by Orlin A. S., Kruglov M. G. The 4th edition, revised and supplemented, Moscow: Machine-building Publishing House, 1984, pp 247-250,258)/ (Raykov I. Ya. , Rytvinski G. N. Konstruktsiya avtomobilnykh i traktornykh dvigatelei.) Construction of motor-vehicle and tractor engines., Moscow : Higher School Publishing House, 1986, pp. 115- 119).

To reduce valve weight, enhance their high-temperature strength and to create prerequisites for further perfection of internal combustion engine designs with considerably higher performance of fuel economy and exhaust emission, a search for new materials with lower density and higher temperature strength is being conducted.

Of these new materials, alloys made on the basis of titanium and titanium intermetallic compounds of the system titanium-aluminium (Ti- Al) largely possess the requirements mentioned above. They offer higher temperature strength and low density (p = 3,9-4, 2 g/cm3). Under the most active investigation and preparation for practical application are alloys made on the basis of intermetallic compounds and containing °l2 phase, based on the compound Ti3AL, or y-phase based on the compound TiAl.

These compounds and alloys have advantages of high-temperature strength and modulus of elasticity over conventional titanium alloys. Their level of

operating temperatures is 750-900°C. Alloys based on the intermetallic compound TiAl which contains y-phase have very low plasticity at ambient temperature (6 = 0.5-1. 5 %). Parts made from this material are made using casting technologies. The alloys based on y-phase with operating temperature levels of up to 850°C are used to manufacture exhaust valves (see Table 1, pp. 1,2). However valves made with the use of casting technologies usually have porosity caused by the distributed shrink hole and the typical cast structure with crystallization liquation. « Curing » cast porosity is possible only with the use of high-temperature gas isostatic method. This causes high manufacturing costs of internal combustion engine valves made from alloys based on the y-phase (see: (Lukyanychev S. U. et al. Struktur i svoistva polufabrikatov iz splava Ti-48Al-2Nb-2Cr na osnove intermetallida TiAl, poluchennykh metodom fasonnogo litiya.) Structure and properties of semi-finished items fabricated by shaped casting from Ti-48Al-2Nb-2Cr alloy based on intermetallic compound TiAl."Production process for lightweight alloys", 1996 N3, p. 16./A.

Choudhury, M. Blum, P. Busse, P. Lupton, M. Gorywoda. « Herstellung von TiAl-Ventilen Durch Schlendergub in Metallische Dauerformen ».

Symposium 2: Werkstoffe fur die Verkehrstechnik, 12997 by DGM Informationsgesellshaft mbh, p. 49-54).

A group of alloys, based on a, 2-phase and containing 8-14 % by mass of aluminum, is related to alloys of the deformation type with lower technology plasticity. The high-temperature strength of these alloys is slightly inferior in comparison with alloys based on y-phase. They have a decisive advantage of adaptability to manufacture, and the cost of manufacture over y-phase alloys.

A number of high-temperature high-strength titanium alloys have been developed for aerospace technology. The alloys of qualities BT18Y and BT25Y Russia, IMI 829 and IMI 834 (England), Ti 6242 Si and

Timetal 1100 (USA) (see Table 1, pp. 3-8) should be noted among them as having the highest temperature strength. These are complexly-alloyed alloys of the deformation type. The formation of the necessary macro-and microstructures to obtain a set of required properties-strength, high- temperature strength, fatigue resistance etc. , is ensured under very rigid temperature - time multistage conditions of deformation and thermal treatments. At the same time, prolonged operation at higher temperatures for these alloys is restricted within the range of 500-600°C (see: (O. P.

Solonina, S. G. Glasunov. Zaroprochnye titanovye splavy). High- temperature titanium alloys. , Moscow, Metallurgy Publication House, 1976, pp. 61-128. /Titanium 95: Science and Technology. Igor V. Gorynin.

Research and Fabrication and Development of Titanium in CIS. , p. 32).

The low level of operating temperatures (not more than 600 °C) prevent them from application to manufacturing internal combustion engine exhaust valves.

A titanium alloy with a phase content of a + a2 and intermetallic a2- phase based on the compound of Ti3A1 (see Tablel, p. 9) is also known (see the patent RU No. 2081929). To enhance its technological plasticity, a hydrogen technology based on the application of a reversible-alloying of the alloy by hydrogen and by thermal impact is proposed. The technology allows the improvement of the structure and the mechanical properties of titanium alloys for systems 0+02 and °'2 (see: (Mamonov A. M., Kusakina Yu. N. , Ilyin A. A. Zakonomernosti formirovaniya fazovogo sostava i stryktyry v zaroprochnom titanovom splave s intermetallidnym yprochneniem pri legirovanii vodorodom). Mechanism of forming phase content and structure in the high-temperature titanium allow with intermetallic strengthening under hydrogen alloying. "Metals", 1999, No 3, pp. 84-87). High labor needs and the complexity of the technology described above makes it unsuitable for mass production.

Analogues of the proposed invention"The Valve for Internal Combustion Engines"are known designs of combined types (see Table 1, pp. 10-12), manufactured from industrial titanium alloys. To reduce the valve cost, its stem is made of low-alloyed titanium alloys while its head with a stem-head transitional section is made of high-temperature high- alloyed titanium alloys (patent US No 5169460, applications JP No 03- 009008, JP No 62-197 610). The separately-made head and stem then join together by welding, or by making a press fit. However the valves made by this method do not have the required strength and reliability.

Another analogue of the proposed valve is the valve (application JP No 06-184683) manufactured from a rod made of biphase a + (3-titanium alloy. Two different microstructures with grain dimensions of 1-4 llm (in the stem) and 300 u. m (in the head) are formed by deformation treatment.

The valves (see application JP No 06-184683) do not have the required high-temperature strength to satisfy the conditions of its prolonged mechanical and thermal loading.

The closest analogue to the proposed"Internal combustion engine valve"is a titanium valve with a cylindrical stem, disc shaped head and fillet portion which ensures a smooth junction of the stem and the head with different microstructures of the stem, head and fillet portion (the patent US No. 4729546). The valve (patent US No. 4729546) is made of high-temperature strength titanium alloy of Cl+ ß phase content. The microstructure of the stem and transitional section is mainly comprised of zones with thin equiaxial grains of a-phase, and of zones with a microstructure of colony-type and grain dimensions of 5-50 u. m. The valve head has a substantially homogeneous microstructure of the colony type with colony dimensions of 50-300 urn. The valve stem of this design has high tensile and fatigue strength, while the head has resistance to creep,

which allows deformation of not more than 1 % under a stress of 27.5 MPa and a temperature of 760°C within 100 hours.

A disadvantage of this valve (patent US No. 4729546) is that biphased a + (3-titanium alloys used for its production can not ensure the necessary strength properties for internal combustion engine valves operating in conditions of prolonged heat and short-term high-temperature (900°C) loading. The chemical content and microstructure of the alloys shown in Table 1, pp. 13-16, can not provide a valve head with properties of prolonged high-temperature strength at temperatures higher than 600°C.

Titanium alloy high-temperature strength is determined by the a-phase, and only additional alloying makes it possible to enhance it.

Any structural conversions of biphase titanium alloys as was done in the valve (patent US No. 4729546) do not lead to considerable enhancement of high-temperature strength. Additionally, the homogeneous microstructure of the colony-type in the valve head does not provide enhancement of creep resistance of the valve material. The microstructure of the valve stem and in the stem-disc-shaped head transitional section in the patent mentioned before is a mixture of zone of thin equiaxial grains of a-phase and zones with a microstructure of the colony type with grain dimensions of 5-50 p. m. Such microstructure can not ensure enhanced prolonged strength for the transitional section, particularly for exhaust valves operating in the condition of creeping under high temperatures (600- 700°C, see Fig. 15), and can not ensure reliable operation of the valve as a whole. The valve stem operating in the condition of cyclic tensile and bending load, due to the low magnitude of the module of elasticity, hardness and strength under higher temperature, also does not possess the required properties. This causes residual deformation and elongation of valve stem in the process of operation. The indications mentioned above become particularly apparent in the operation of high-performance engines.

The closest analogue to the proposed invention « The Method of manufacturing internal combustion engine valve » in technical essence is the method of manufacturing valves with different specified microstructures (see patent US No 4675964). According to this method, the engine valve including the exhaust valve is manufactured of industrial high-temperature biphase a + (3-titanium alloys (see Table 1, pp. 13-16).

The technical result of the method described above is obtaining a valve with a twin microstructure. The process of manufacturing a stem and a valve head using deformation methods and subsequent thermal processing is conducted subsequent to the task of having different microstructures, which by their properties satisfy the condition of mechanical and thermal loading of a valve in the process of its operation in the internal combustion engine.

The described method of manufacturing the internal combustion engine valve made of high-temperature titanium alloy (see the patent US No 4675964) consists of forming the valve from the rod-shaped billet, by deformation processing and subsequent thermal processing. The head and the stem of this valve have different microstructures. Deformation treatment and the subsequent thermal treatment are conducted in two stages. Stem microstructure is formed in the first stage, while the head microstructure is formed in the second stage.

In the first stage the billet is heated to a temperature lower than the temperature of the complete polymorphous conversion (Tpc, n i. e. ß-transus temperature) (ß-transus temperature) for this alloy, and then deformed by extrusion, thus forming the valve stem. In the second stage the billet section related to the valve head is heated to a temperature which is higher than Tpe for this alloy, and then deformed by forging.

As a result of deforming and thermal treatment, different microstructures are formed in the stem and in the head. They provide

properties close to those needed to operate under conditions of mechanical and thermal loading of the valve in corresponding parts in the engine. In this case the microstructure of the first valve zone (the stem and the section connecting the stem and the head) mainly contains a mixture of thin equiaxial grains of a-phase and zones of the colony type with grain dimensions of 5-50 u. m. This microstructure has high strength and high fatigue strength at ambient temperatures. The microstructure of the valve (head) in the second zone is homogeneous and mainly of the colony type with colony dimensions of 50-300 um. The disadvantages of the described method (see patent US No 4675964) are: * inability to extrude the valve stem if it is made of high- temperature strength alloys with lower technological plasticity; e rapid wearing of press tools made of expensive high- temperature strength steels, particularly in the process of extrusion of titanium alloys in the biphase a+ (3-region ; o deterioration of billet quality in the process of forging, due to undesirable grain growth in the billet section related to the valve head at the time of heating the whole billet before the extrusion of the valve stem in the first stage; e formation of a hard oxide layer of 0.1-0. 3 mm thickness due to heating the whole billet before extrusion; the heating as a rule is conducted in the furnace with an external source of heat over a long period of time (not less than 10 minutes). The presence of the oxide layer causes the wear of pressing tools and creates additional difficulties for subsequent stages. The only method to avoid these difficulties is to take additional measures to ensure billet protection from oxidation. There are complications in the mass production to put into practice the process of thermal treating,

which can ensure high temperature differential between the valve head and the stem; * the valve head in the process of thermal treatment is heated to a temperature higher than-transus temperature and the stem-to a temperature lower than-transus temperature (in this case the temperature gradient is 10-205°C) ; the holding time under these temperatures is from 0.5 up to 8 hours; discrepancy in the properties of the structure formed in the valve stem and the required properties, in particular in the transition section to the head for an exhaust valve due to the low magnitude of the module of elasticity, hardness and strength at higher temperatures; * the required serviceability of the valve is not provided under high temperatures because a homogenous valve head microstructure of the colony type with colony dimensions of 50-300 llm (primary beta- grain in the microstructure exceeds colony dimensions more than 3 times) can not ensure the required resistance to creep and prolonged strength (see Table 1"Relationships between critical microstructural features and mechanical properties of titanium alloys". ASM Handbook, vol. 2,1990, p. 1052. Reviewed by Gerhard Welsch, Case Western Reserve University, and Rodney Boyer, Boeing Commercial Airplane Company).

The closest analogue of the proposed invention"High temperature titanium alloy"is the titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0. 1 Si with phase content Cl+ß, used for manufacturing internal combustion engine valves of the gas distribution mechanism by a hot deformation method (patent US No 4675964). Internal combustion engine valves manufactured from such alloys are capable for prolonged operation under temperatures of 550-600 °C only.

The disadvantage of biphase cc+p-titanium alloys known from patent US No 4675964 (see the Table 1, pp. 13-16) is their inability to ensure properties of prolonged high temperature strength (prolonged strength, resistance to creep) at temperatures exceeding 600 °C. This fact is confirmed by the author of patents US No 4675964, N° 4729546 in the article containing the results of tests of valves made of two-phase titanium alloys (see Allison J. E., Sherman A. M. , Bapna M. R. Titanium in engine valve system. "J. Metals", 1987,39, No 3, pp. 15-18).

Disclosure of the invention The goal of the invention is to increase reliability and non-failure operation of ICE valves, and by these means increase the durability of the engine.

The technical result of improving the properties of a valve material, ensuring serviceability of ICE valves over the range of operating temperatures, is achieved by increasing the long-term high-temperature strength of the valve head (long-term strength, creep resistance) and improving the mechanical properties of the valve stem (high-temperature strength, modulus of elasticity, stiffness).

This enables: a) increasing the operating temperatures of a valve head up to 850°C while the engine is running for a long time, and up to 900°C while high-performance engines with a short life are running; b) reduce the physical dimensions of the valve stem and fillet portion. The offered inventions permit: * ensure prolongation of engine life by increasing reliability and non-failure operation of ICE valves;

reduce the valve weight by 10-20% due to reduction of physical dimensions of the valve stem and the fillet portion, as well as reduce dynamic loads onto the parts of a valve mechanism; reduce the valve spring force resulting in an additional reduction of loads in the valve mechanism, reduction of friction loss and improvement of engine parameters including an increase of fuel economy and reduction both of exhaust and noise emission; boost an engine's rotation speed, raising its efficiency; control the properties of ICE valves by manufacturing them with sufficient high-temperature strength for engines used for different purposes and performance-boosted to different degrees; use deformation processes for manufacturing ICE valves when the technological plasticity of titanium alloys is limited, and by these means achieve high productivity and low cost price under conditions of mass production.

The specified technical result and the elimination of drawbacks for the offered ICE valve made from a heat-resistant a+p-titanium alloy and consisting of a cylindrical stem, head in the form of a disk9 as well as a fillet portion with various microstructures in its parts ensuring smooth junction of the stem with the head, is achieved by the addition of intermetallic a2-phase based on Ti3Al, from which the valve is manufactured. The intermetallic 0. 2-phase is dispersed in the a-phase. The stem microstructure represents a combination of microstructures of three types (equiaxial, bimodal and lamellar). The above-mentioned microstructures smoothly change from one type to another in the radial direction from the surface to center in the order named above. The microstructure of the head represents a microstructure of two types: (basket weave and lamellar).

The fillet portion has a mixed microstructure consisting of microstructures which are characteristic for both stem and head.

Additionally, the size of the grains for the microstructure of the valve stem is 3-40 urn, but for the microstructure of the head they are 50- 200 um.

The mass fraction of the intermetallic a2-phase in the material of the valve is from 7 to 80 % by mass, and the aluminium content in the alloy is from 7.5 to 12.5 % by mass.

In the offered invention the elimination of the above-mentioned drawbacks is carried out by making a valve with different microstructures of the stem and head. Additionally, the alloy, from which the valve is manufactured, has the intermetallic oc2-phase based on Ti3Al. This intermetallic a2-phase is dispersed in the a-phase. The a2-phase contained in the valve material and dispersed in the a-phase, as well as the combination of the specified microstructures in the valve stem, provides the increase of tensile strength at elevated temperatures for the valve stem. At the same time this provides the increase of modulus of elasticity and stiffness, fatigue initiation and fatigue crack growth, fracture toughness and impact strength. This improves strength and high-temperature properties of the valve. The content of the y-phase in the material of a valve head dispersed in a-phase, as well as a mix of microstructures in the valve head, give long-term high-temperature properties to the valve head by increasing its'long-term strength and creep resistance.

An additional doping of the high-temperature Ti alloy with aluminium transforms the alloy from the biphase state (a+p) into the three- phase (a+i+a2) state. The a2-phase based on Ti3Al is dispersed in the a- phase and isolated mainly during heat treatment. The au-phase isolation in the alloy takes place during a-phase ordering. The a2-phase contains dissolved aluminium in excess of an equilibrium concentration. For example, the solubility limit of aluminum in the a-phase at 550°C is 7.0- 7. 5% by mass (see: (Komilov I. I. Titan. Istochniky, sostavy, svoystva,

metallokhimiya i primineniye) Titanium. Sources, Compositions, Properties, Metal Chemistry and Application, Moscow, Nauka, 1975, p.

187) ). A dispersed isolation of the a2-phase occurs when at this temperature the concentrations of aluminum are considerable. This phase is a solid solution based on Ti3Al. This solution allows an increase in the heat resistance of the alloy up to 750-900°C depending on the a2-phase quantity.

A necessary combination of mechanical and high-temperature properties (depending on operating temperature of the valve) is reached when the content of the a. 2-phase in the alloy is from 7 to 80 % by mass. More detailed corroboration and the validity of the importance of novelty features of the invention"Valve of internal combustion engine"may be found in the section"Embodiment of invention".

The specified technical results and elimination of drawbacks for the offered method of manufacturing of an ICE valve from heat-resisting titanium alloy having (ot+ß) phase composition, lie in the following. A valve is formed from a cylindrical rod by deformation processing and subsequent heat treatment. The valve has different microstructures in its stem and head. The deformation processing is carried out in two stages. The stem microstructure is formed at the first stage and the head microstructure is formed at the second stage. At the first stage deformation acts on the stem billet with preliminary heating of the billet up to a temperature lower than complete polymorphic transformation temperature (ß-transus temperature) for the given alloy, and at the second stage, correspondingly, -on a part of the rod concerning the head, previously heated up to temperature higher than temperature of complete polymorphic transformation (ß-transus temperature) for the given alloy. At the same time the valve head is deformed by forging. The specified technical results and elimination of drawbacks are achieved by the fact that the rod for manufacturing a valve is made of a high-temperature titanium alloy. This high-temperature titanium

alloy is hardened by an a2-phase intermetallic compound based on Ti3Al dispersed in the a-phase. Thus, a three-phase (a+p+az) composition of the alloy is reached. In so doing, the first stage of deformation processing means preliminary heating only of the deformable rod part when it is heated 5-20°C below that temperature of complete polymorphic transformation of the given alloy. In this case the rod processing is carried out by cross and taper rolling to reach a three-type combination of microstructures in the stem. They are: equiaxial, bimodal and lamellar. In the specified order they smoothly pass from one type to another in radial direction, from surface to center. The second stage of deformation processing means preliminary heating of the rod up to temperature 5-50°C above the temperature of complete polymorphic transformation of the given alloy. This temperature corresponds to the beginning of deformation, but the ending of deformation is carried out at temperatures below the temperature of complete polymorphic transformation of the given alloy. During this process a pressed valve is being formed by forging. It has a fillet portion ensuring smooth junction of the stem with the head. The head has a microstructure representing a mix of two-type microstructures :"basket weave"and lamellar.

A mixed microstructure consisting of microstructures characteristic both for the stem and the head is reached for the fillet portion.

At the same time a microstructure of 3-40 u. m of grains in the valve stem and that of 50-200 u. m in the valve head is reached.

Additionally, at the first stage of deformation processing, heating of the deformable rod part is carried out by the electrical contact method.

Thus, heating is carried out at a speed of from 10 to 50°C/s. After this heating cross and taper rolling with a degree of deformation of 30-70% is carried out.

At the second stage of deformation processing, heating of the deformable rod part is carried out by an induction method at a speed of from 20 to 50°C/s. The subsequent forging of the valve head is carried out at a deformation degree of 40-60%.

Preliminary heating of the deformable rod part of a valve is carried out under temperature control.

Heat treatment of the rod is carried out with the help of hardening and annealing, or with the help of annealing only. The first type of heat treatment considers hardening of the rod immediately after each stage of deformation processing. In this case annealing comes after hardening which follows the second stage of deformation processing. The second variant of heat treatment means that annealing comes after the second stage of deformation processing. The rod annealing is carried out in two steps typical both for the first and second variant. The first step requires heating of the rod to within 650-950°C degrees, 0.1-5. 0 hours held at this temperature, and cooling down to a temperature of 500-650°C. The second step means the rod is held at a temperature of 500-650°C within 5-50 hours with the subsequent cooling.

Thus, the claimed method of manufacturing the valve is based on application of deformation technologies alternating with thermal processing. This method allows production of an ICE valve with a microstructure appropriate to the conditions of its ICE mechanical and long thermal stressing including high-performance engines. More detailed corroboration and validity of the importance of novelty features"Method of manufacturing the valve for the internal combustion engine"may be found in the section"Embodiment of inventions".

The specified technical result and elimination of drawbacks in the offered heat-resisting titanium alloy containing aluminium, molybdenum, zirconium, silicon, and having (azid phase composition, are achieved by

the fact that this alloy additionally has an intermetallide axa-phase compound based on Ti3Al, which is dispersed in a-phase. At the same time the alloy contains yttrium and has the following composition in mass per cent: aluminium 7.5-12. 5, molybdenum 1.6-2. 6, zirconium 1.4-2. 4, silicon 0.1-0. 2, yttrium 0.05-0. 1, and titanium-the rest.

The application of titanium alloys hardened with an intennetallide a-phase based on Ti3Al and dispersed in a-phase ensures improvement of the mechanical properties both for intake and exhaust valves. First of all, this concerns heat-resisting properties ensuring long-term function of those alloys at temperatures of from 600°C up to 850°C in the valve head. At the same time this ensures serviceability during a limited period of time for exhaust valves of high-performance ICEs at temperatures in the valve head of up to 900°C.

Brief Description of the Figures.

The invention is explained by figures, where : - in Fig. 1 the general view of a ICE valve is shown; - in Fig. 2 the initial cylindrical rod is shown; - in Fig. 3 a rod after cross and taper rolling is shown; - in Fig. 4 a billet of a ICE valve after forging is shown; - in Fig. 5 the equiaxed microstructure on the surface of a valve stem is shown; - in Fig. 6 the bimodal microstructure located between the stem surface and the central area of a valve stem is shown; - in Fig. 7 the lamellar microstructure of the central areas of a valve stem is shown; - in Fig. 8-10 the microstructures of a valve stem in the case of being heated for cross and taper rolling up to a temperature above the claimed

(allowable) range of temperatures (temperature of complete polymorphic transformation-5 C), when it is impossible to reach a necessary combination of microstructures in the valve stem are shown; - in Fig. 11 the microstructure of a valve head representing a mix of two types:"basket weave"and lamellar is shown; - in Fig. 12 a fragment of the lamellar microstructure in the valve head on an enlarged scale is shown; - in Fig. 13 a fragment of the"basket weave"microstructure in the valve head on an enlarged scale is shown; - in Fig. 14 the estimated operating temperature distribution at the intake valve by the example of a medium-efficiency engine is shown; - in Fig. 15 the estimated operating temperature distribution at the exhaust valve by the example of a medium-efficiency engine is shown.

Embodiment of the Invention The valve (Fig. 1) consists of stem 1 with a constant diameter and disc-shape head 2 including fillet portion 3 which assures the smooth junction of the stem and the head.

Intake and exhaust valves are made of a titanium alloy with a different aluminum content in the range of 7.5-12. 5 % by mass (see p. 17 in Table 1 and Table 3). Additionally, these alloys are reinforced by the intermetallic ot2-phase which is dispersed in the a-phase and increases the heat resistance of the alloy up to 900° C. Deformation processing and heat treatment used in the manufacture of a valve by the claimed method allows obtaining different microstructures in stem 1, head 2 and fillet portion 3.

A microstructure in stem 1 represents a combination of microstructures of three types (equiaxial, bimodal and lamellar). The above-mentioned microstructures smoothly change from one type to

another in the radial direction from the surface to center in the order named above.

To attain the maximum strength characteristics, a heterogene structure is created in the valve stem. This microstructure consists of three microstructures.

An equiaxial microstructure is created on the surface of the valve stem. The equiaxial microstructure serves to increase the strength, plasticity, fatigue limit, fatigue crack initiation resistance and high-cycle fatigue resistance (see: Kolachev B. A., Pol'kin I. S. , Talalaev V. D.

Titanovye splavy raznykh stran (Titanium alloys of different countries).

VILS, 2000, p. 297). The increased fatigue crack initiation resistance, together with other positive properties of this microstructure (strength, plasticity, endurance), does not require an additional processing of the stem surface (blast cleaning, polishing etc. ) in order to increase its fatigue strength.

A bimodal microstructure is located between the stem surface and the central areas of the stem (Fig. 6). The bimodal microstructure permits an optimal combination of mechanical properties of the alloy and advantages of the equiaxial and lamellar microstructures. Additionally, the creation of the bimodal microstructure in the transition layer between the equiaxial and lamellar microstructures favors the smooth transition from one microstructure to another and the improved mechanical properties of the material of the valve stem.

The lamellar microstructure is located in the central area of the stem (Fig. 7). The lamellar microstructure allows increasing the fracture toughness, fatigue crack growth resistance, impact strength, creep resistance, and long-term strength. The creation of fine-grained lamellar microstructure (grains in size not more than 40 u. m) allows obtaining increased fracture toughness, impact, strength and fatigue crack growth

resistance in conformity with high-frequency cyclic tensile shock loads acting on the valve stem. The creation of this microstructure has a beneficial effect on that part of the exhaust valve which is adjacent to the hot head and operates in a temperature range of 500-560° C (see Fig. 15), i. e. under conditions of creep and increased long-term strength limit. This also favours an increase of strength properties of the valve stem.

Such a combination of microstructures will allow creating a valve stem with high values of reliability and durability.

From the point of view of reliability and durability of the valve stem, the best technical result is achieved when the size of the grains for the microstructure of a valve stem are 3-40 pm. The existence of grains with a size less than 3 um results in the occurrence of internal stresses and microcracks during strain conditions. The grain size exceeding 40 J. m degrades the strength and fatigue properties of the valve.

The microstructure of valve head 2 represents a mix of microstructures of two types:"basket weave"and lamellar ones (Fig. 11).

As a consequence, this microstructure has all the advantages of the mentioned above microstructures. The lamellar microstructure in the valve head (Fig. 12) provides an increased fracture toughness, impact strength, fatigue crack growth resistance, creep resistance, as well as stress-rupture strength. The"basket weave"microstructure (Fig. 13) provides an increase of stress-rupture strength and creep resistance. The mix of microstructures of"basket weave"and lamellar types formed in the valve head has a chaotic character without clearly defined zones. To the greatest extent, this fact promotes the association of positive properties inherent to both microstructures. More than others the"basket weave"microstructure provides an increased long-term rupture strength with a sufficient creep resistance. A large-grained lamellar microstructure (grain size is 50-200 pm) provides to the greatest extent an increased creep resistance and other

positive properties characteristic for the lamellar microstructure. Such a mix of microstructures is in full conformity with the valve head loading (high frequency alternating shock loads in a temperature range of 600- 850°C, and in some cases up to 900°C). This results in an increase of reliability and non-failure operation of ICE valves, as well as increased durability of internal combustion engines.

The best technical result is achieved when the size of grains for the microstructure of a valve head lies in the range from 50 to 200 urn.

Increasing the grain size of more than 200 llm results in a decrease of durability, due to the loss of grain boundaries. Reducing the grain size to less than 50 Am results in a decrease of creep resistance at high temperatures.

A smooth transition from one type of microstructure associated with the valve stem to the other kind of microstructure associated with the valve head takes place in the fillet portion. This allows a reliable operation of the fillet portion of a valve in the temperature range from a high operating temperature in the valve head up to the normal operating temperature in the valve stem (see Fig. 14 and 15).

The application of titanium alloys, additionally hardened by ar- phase intermetallic compound based on Ti3Al which is dispersed in a- phase with an increased heat resistance both for intake and exhaust valves, provides a long-term operation of ICE valves designed for various purposes. This application is also suited to valves for internal combustion engines of different performance levels at temperatures in the valve head of from 600°C up to 900°C, whereas the known titanium alloys have a heat resistance level which does not exceed 600°C. Increasing stiffness and modulus of elasticity in the valve stem and its fillet portion is provided by an phase dispersed in an a-phase. This allows reduction of design dimensions of a valve stem and its fillet portion, resulting in a reduction of

the valve weight by 10-20% in comparison with a prototype alloy.

According to data given in Table 3, points 8 and 9, the superiority of properties of the claimed alloy over properties of the alloy concerning patent US 4729546 is obvious. So, the values of specific parameters for durability (CyB/p) and rigidity (E/p) exceed the values of parameters typical of the prototype alloy. This means that for cB/P (at 800°C) these values are from 2 to 4 times better, and for E/p (at room temperature) they exceed the prototype ones by 11-27 %.

The solubility limit of aluminium in a-phase is variable from 6 to 7. 5 % by mass. When the content of aluminium in the alloy is above this range a2-phase precipitates. This a, 2-phase is a solid solution based on intermetallic Ti3Al compound which additionally raises the high resistance of the alloy and its modulus of elasticity. At the same time this results in a reduction of technological plasticity. The required level of technological plasticity is achieved by the relation of a-and (3-phases in the alloy. This relation can be adjusted by the quantity of oc-and p-stabilizing elements in the alloy. The most effective a-phase stabilizer is aluminium. Molybdenum is one of the most effective (3-phase stabilizers. In view of this fact, the developed Cl+ß-titanium alloy dispersionally hardened by a2-phase consists of the following elements: aluminium, raising the alloy heat resistance and determining the 02-phase content; * molybdenum, stabilizing the (3-phase and influencing the alloy plasticity (relative reduction of the molybdenum content in the alloy when the content of aluminium in this alloy is increased reduces additionally plasticity and technological effectiveness);

* zirconium, expanding the a-phase homogeneity region and influencing the phase precipitation and its quantity, especially when the aluminium content is low.

It is known that the content of dissolved oxygen in the alloy reduces its plasticity considerably. An introduction of ittrium into the alloy permits initiating the process of internal deoxidation in the alloy. Ittrium being dissolved in the alloy during melting interacts with dissolved oxygen, forming Y203 oxide. This oxide reduces the content of oxygen dissolved in oc and ß phases, and raises the technological plasticity of the alloy.

The aluminium content in the offered heat-resisting titanium alloy is 7. 5-12. 5 % by mass. Thus, an increase of the aluminium content of more than 6-7.5 % by mass, without changing the content of other elements, results in an additional reduction of the technological plasticity of the alloy.

By varying the content of aluminium, molybdenum, zirconium, and ittrium in the alloy, it is possible to change the heat-resisting properties of the alloy, preserving technological plasticity at a level which permits strain working of the alloy.

The heat-resisting titanium alloy was reached by double vacuum arc melting. An ingot of the alloy of diameter 350 mm was extruded into a bar of diameter 50 mm and then rolled into rods of diameter 16-22 mm. After being rolled the rods were subjected to annealing.

The rods were cut into cylindrical billets of 4 measured lengths depending on sizes of valves to be manufactured (Fig. 2).

In order to reach the offered microstructure different to the valve stem and head, two stages of strain working were carried out, with preliminary heating before deformation and subsequent heat treatment.

The billet heating for cross and taper rolling was carried out by electric contact. The power supply was provided through water-cooled

copper contacts. In the contact area the billet remained cold, but its central part was heated up to preset temperature by alternating or direct current.

An infrared pyrometer provided the temperature control. When the preset temperature was achieved, the pyrometer sent a signal to a microprocessor controller. This controller transmitted that signal to actuating mechanisms equipped with pneumatic drives. So, the heated up billet was brought to the rolling zone with minimum time delay.

At the first stage of strain working, a billet of intermediate form was obtained from a billet of cylindrical form by cross and taper rolling. The billet of intermediate form was obtained for the subsequent forging of a valve head (Fig. 3). In the course of cross and taper rolling the structure of a valve stem was formed. A distinctive feature of cross and taper rolling is heating the billet only in zone 5, which is subjected to rolling. Heating only a deformable part of the fillet portion permits avoiding undesirable growth of grains in its undeformable part 6 and facilitates formation of the valve head during forging. This fact is of great importance for alloys of difficult deformation.

Cross and taper rolling was carried out at a degree of strain 30-70%.

It has been experimentally established that in this range of degrees of strain a combination of three-type microstructures is formed. These microstructures are: equiaxial, bimodal and lamellar ones ensuring the achievement of a required technical result. At a degree of strain less than 30% it was impossible to accumulate enough metal in the part of a billet to be pressed for the subsequent formation of a valve head. When the billet was rolled at a degree of strain more than 70% its breakage took place.

Cross and taper rolling was carried out at a temperature 5-20°C below the temperature of complete polymorphic transformation for the given alloy, i. e. it was carried out in (azid region of the alloy. It has been experimentally established that in this range of temperatures cross and taper

rolling ensures a combination of three-type microstructures. These three- type microstructures are: fine-grained equiaxial, bimodal and lamellar ones necessary to achieve the technical result. When the rolling temperature is below the lower limit (temperature of complete polymorphic transformation-20°C) a billet breakage takes place. When the rolling temperature is above the upper limit (temperature of complete polymorphic transformation-5°C) the required combination of microstructures is not reached (Fig. 8-10).

Grain dimensions of 3-40 llm of the valve stem microstructure has been reached during cross and taper rolling. As the deformation effect of cross and taper rolling decreases from surface to center, a transformation of the initial structure of a rod billet into an equiaxial one takes place in the near-surface layers. At the same time, when the deformational effect is minimal a lamellar microstructure is reached in the central zone. This lamellar microstructure is characteristic for a non-deformable alloy subjected to heating.

Electric-contact heating of the deformable part of a billet for cross and taper rolling was carried out at a speed of 10-50 °C/s. The direct electrical heating allows such heating speeds and ensures uniformity of temperatures over the cross-section and length of a billet. When the heating temperature is more than 50 °C/s, the central part of a stem billet is heated up at lower temperatures than its surface. This results in deterioration of the rolling quality. When speeds are less than 10 °C/s oxidation of the surface, an undesirable oxide layer of a billet occur.

Cross and taper rolling allows a high forging efficiency of billet production. This forging efficiency allows about 4-5 billets per min. At the same time deformation tool (rolls) expenses are minimal. The durability of taper rolls is 10-20 times higher than durability of molding matrixes.

Hot forging of the valve head was carried out with the help of a mechanical crank press.

Heating for forging was carried out by an induction method. This heating was applied only to that part of a billet which was going to be subjected to deformations during formation of the valve head. The speed of the heating was 20-50 °C/s. The valve head was forged after heating to a preset temperature (Fig. 4).

Forging was begun at temperatures of deformation 5-50 °C higher than the point of complete polymorphic transformation for the given alloy.

The final stage of formation of the valve head took place at temperatures below the point of complete polymorphic transformation. Thus, due to forging, the head 7 of a disk valve with fillet portion 8 smoothly passing to stem 9 was formed. It has been experimentally established that such temperature conditions of forging results in two-type microstructures: "basket weave"and lamellar ones with size of grains 50-200 J. m, what is necessary to ensure heat resistance of the valve head. For the fillet portion a mixed microstructure was obtained. This microstructure consists of the following elements : Microstructures characteristic for the stem and head, providing smooth transition of microstructures and properties of the valve from one part (stem) to the other (head).

When overheating of a billet to be forged occurs, i. e. its heating above temperature (temperature of complete polymorphic transformation +50°C) an undesirable growth of grain exceeding 200 pm takes place. This growth of grain results in a decrease of the durability of the valve head, due to the loss of grain boundaries.

An unsufficient heating of a billet below temperatures of complete polymorphic transformation +5°C results in cracking of the valve head during forging.

Fast and one-time heating of the deformable part of a billet prevents the formation of hammer scale and oxide layers on its surface. This promotes more a favorable course of deformation processes during forging, reduction of strain resistance, and attainment of a high-quality forged surface free of cracks, folds and wrinkles.

Heating at a speed of more than 50 °C/s resulted in high heterogeneity of heating of a billet. Due to this fact its central part had no time to get warm up to preset temperatures, while its surface was overheated. This resulted in low quality of forging. Heating at a speed below 20 °C/s degraded the forging productivity.

When the stain working was finished, heat treatment of a valve billet took place. Heat treatment was carried out with the help of hardening and annealing, or with the help of annealing only.

The first variant of heat treatment consisted in hardening immediately after termination of each stage of strain working. After cross and taper rolling hardening of a billet took place. For example, it could be water hardening. Thus, the structure fixation of the valve stem took place in the (a+ß) region. Simultaneously, the process of OC2 precipitation was approaching completion. The microstructure fixation of the valve head took place during hardening after forging According to the phase diagram titanium-aluminum, the C12 phase precipitation begins at temperatures of 500-650°C (see Titanium. Sources, Compositions, Properties, Metal Chemistry and Application. Kornilov I. I., M. , "Nauka", 1975, p. p. 187-107). The higher the process temperature, the more the a2 precipitation. The best results of a2 dispersed precipitation are obtained from diffusion processes at temperatures of 500-650 °C. In this case the 02-phase of 20-50 nm is precipitated. When the temperature is below 500 °C diffusion processes are very slow. When the temperature is above 650 °C the a2 size is great. Annealing in the claimed range of

temperatures results in an increase of heat resistance. Heating a billet up to temperatures of 650-950 °C is aimed to remove internal stresses.

The second variant of heat treatment consisted of annealing after the valve had been formed, i. e. after the second stage of strain working.

Annealing was carried out for the whole valve billet.

Annealing for the first and second variants of heat treatment was carried out in two steps. The first step of annealing meant the billet heating up to temperature 650-950 °C, holding within 0.1-5. 0 hours at this temperature, and cooling down to a temperature of 500-650 °C.

The second step of annealing meant holding the billet at temperatures of 500-650 °C within 5-50 hours, with subsequent cooling.

The first variant of heat treatment ensured the best result when the az-content in the alloy was 7-25% by mass. In that case the increase of mechanical characteristics took place due to the strengthening mechanism of a+P-alloys and dispersed precipitation of the a2-phase. The achieved effect of strengthening as a result of hardening and annealing exceeds the effect of strengthening due to the ccz-phase precipitation, in quantities of 7- 25% by mass. As a result of this heat treatment, ICE valves (mainly intake ones) ensuring long-term operation over a temperature range of 600-650 °C (see Table 2) were obtained.

According to the second variant, it is expedient to carry out heat treatment when the a. 2-phase content is 25-80% by mass. In this case the increase of mechanical properties is provided, basically, by the y-phase dispersed precipitation. Heat treatment according to the second variant is preferable when an alloy has aluminum contents of from 9.5 up to 12.5% by mass.

For engines with a long-term service life (see Table 2) valves (both intake valves and exhaust valves) made of alloys with an aluminium content 9.5-10. 5 %, 10.5-11. 5 % and 11.5-12. 5 % by mass provide a long-

term period of work in the temperature ranges of 650-700°C, 700-800° C, and 800-850° C correspondingly. For high-performance engines, (see Table 2) valves with an aluminium content of 11.5-12. 5 % by mass provide a limited period of work at temperatures up to 900° C.

When making an ICE valve of an alloy with an aluminium content of 7-12.5 % by mass, deformation processing and heat treatment under the claimed conditions provide required mechanical properties of ICE devices shown in Table 3.

As can be seen from Table 3, the developed alloy with an aluminium content of 7.5-12. 5 % by mass, which is reinforced by the intermetallic CC2- phase, has the following improved mechanical properties at temperatures up to 900° C (as compared to prototype alloys): - strength is 3.5 times greater for an alloy with an aluminium content of 7.5-9. 5 % by mass at temperatures 700-760° C. It is 4 times and more greater for an alloy with an aluminium content of 9.5-10. 5 % by mass; -with the increase of aluminium content from 7. 5-9. 0 to 12. 0- 12.5 % by mass, the strength of the alloy increases by 2 times, in the case that the valve heat operates at a temperature of 800° C and under stresses from 260 to 520 MPa; - with the increase of an aluminum content from 7.5 to 12.5 % by mass, the specific strength (Cib/p) is 2-4 times greater at temperatures from 760° C and the specific stiffness (E/p) is greater by 11-27 % at room temperature (this permits to make a valve with lesser physical dimensions of the stem and fillet portion and thereby reduce the valve mass by 10-12 %); - creep resistance of the alloy at temperatures 600-800° C far exceeds the creep resistance of the prototype.

A valve billet made by the method given above was mechanically processed using known techniques, such as turning and polishing. After mechanical processing, the external surface of the valve was hardened, e. g., by nitriding to a depth of 50-100 um.

The offered method for making valves permits obtaining ICE valves matching thermal loads in engines which are used for various purposes and boosted-performance to different degrees. When making valves, it is possible to control properties of the valves by varying the properties of a material, primarily its heat resistance. The offered method is based on the use of highly efficient deformation technologies and can be employed for mass production of ICE valves.