RU2224047C1 - Method for manufacture of semi-finished sheet products from titanium alloys - Google Patents

Abstract

FIELD: metallurgy, in particular, plastic working of metals. SUBSTANCE: method involves manufacturing semi-finished sheet products from titanium alloys suitable for low-temperature superplastic deformation including preliminary processing of blank; rolling at temperatures below polymorphic transition temperature t_pt and annealing. Preliminary processing is conducted on structure with submicron grain size of, for example, less than 1 micron. Rolling is initiated beginning from temperature selected in the range of t_pt-(150÷500^°C), regulated by required submicron grain size in ready semi-finished product. Preliminary blank processing is provided by various plastic deformation methods. EFFECT: improved superplasticity of semi-finished product at reduced temperatures and isotropy of mechanical properties in rolling plane, and increased strength and fatigue characteristics at room temperatures. 9 cl, 3 dwg, 4 tbl

Description

 The invention relates to the field of metallurgy, mainly to the processing of metals by pressure, in particular, to methods for manufacturing sheet semi-finished products from titanium alloys with a submicrocrystalline structure, suitable for low-temperature superplastic deformation in the manufacture of parts in the automotive industry, engine manufacturing, as well as complex aerospace structures.

The implementation of manufacturing technologies for parts of complex configuration using the phenomenon of superplasticity significantly reduces labor intensity and increases the utilization of metal. At the same time, relatively high temperatures of manifestation of superplasticity in industrial semi-finished products from titanium alloys lead to the necessity of manufacturing tooling from expensive heat-resistant materials and to protect the titanium alloy from intense oxidation. Meanwhile, the temperature conditions of superplasticity are largely determined by the grain size in the material. A significant (by 150-200 ^° C) decrease in temperature and an increase in the rate of superplastic deformation is achieved by the formation of microstructures in titanium alloys with grain sizes in tenths of a micrometer, called submicrocrystalline (SMC).

 The effect of low-temperature superplasticity of semi-finished sheets of titanium alloys with SMC structure can be used in the superplastic molding of parts of complex shape and in the combined processes of superplastic molding and diffusion bonding (SPF / DS), in particular in the manufacture of multilayer cellular structures. Lowering the temperature of these technological processes improves their economic performance due to the cheaper and more durable tooling, as well as obtaining better solid-state compounds in SPF / DS due to a decrease in gas saturation of the connected surfaces of the structure. In addition, the SMC structure has a favorable effect on the formation of a solid-phase compound due to its greater development during superplastic deformation of grain boundary slippage and, as a result, accelerated healing of pores in the joint zone. Additionally, maintaining the submicrocrystalline state, along with the efficiency of the process, provides an increase in quality due to the most complete implementation of the unique complex of mechanical properties of the SMC material, in particular, the increased strength and fatigue characteristics, in the structure itself.

 A known method of manufacturing a sheet of semi-finished products (industrial sheets) of titanium alloys used for superplastic deformation [1], including hot rolling of a slab in the β - and (α + β) -regions, warm rolling of hot-rolled steel and subsequent hot batch rolling of sheets in (α + β) regions. Sheets made from this method from titanium alloy Ti-6Al-4V (VT6) have a uniform globular microstructure with grain size of the α phase within 3-4 microns and a certain fraction of the basic component of the texture in the plane of the sheet. The strength and plastic characteristics of the sheets are high in combination with the permissible value of planar anisotropy.

However, sheets with such a grain size are suitable for superplastic deformation at temperatures not lower than 875-900 ^o C [2], which is a disadvantage of this method.

The prototype of the present invention adopted a method of manufacturing sheet semi-finished products from two-phase titanium alloys [3] by rolling at a temperature of the onset of deformation of 400-550 ^o C below the temperature of the polymorphic transformation at a speed of 10 ^-4 -10 ^-2 s ^-1 and the degree of deformation 5-15 % followed by annealing at a temperature of 400-550 ^o C below the temperature of the polymorphic transformation and the repetition of this treatment cycle to achieve a total degree of deformation of 75-95%. In the method, it is supposed to subject the workpiece to processing in the β- and (α + β) -regions by heat treatment or additional fractional deformation to prepare the material structure before rolling. Subsequent rolling and annealing cycles under these conditions allow the formation of a fine-grained structure in semi-finished products, providing an increase in their mechanical and technological properties. In particular, for the VT6 titanium alloy, there is a decrease in the temperature of the manifestation of the effect of superplasticity to 700 ^o C.

 The disadvantages of this method are as follows. The billet is subjected to rolling, having, after preliminary β- or (α + β) -treatment, respectively, a coarse-grained lamellar or partially recrystallized globular-lamellar structure. During warm rolling according to the thermomechanical conditions indicated in the method, the processes of recrystallization and globularization of the structure along the cross section of the semi-finished product develop unevenly and not completely. As a result, the resulting semi-finished products have an inhomogeneous microstructure, which is unacceptable in the sheets for further superplastic deformation.

The description of this method shows the results of mechanical tests only for samples cut from a sheet of VT6 alloy along the rolling, but not across the rolling. However, it is obvious that in spite of satisfactory indicators of superplasticity at a temperature of 700 ^° C, microstructural heterogeneity and, accordingly, texture heterogeneity over the cross section of the sheet will lead to anisotropy of the mechanical properties and reduced ability of the sheet to deep draw under superplastic deformation [4].

 The objective of the invention is to provide a method of manufacturing sheet semi-finished products from titanium alloys with a homogeneous submicrocrystalline structure (grain size less than 1 μm), suitable for low-temperature superplastic deformation.

The problem is solved by the method of manufacturing sheet semi-finished products from titanium alloys suitable for low-temperature superplastic deformation, including pre-treatment of the workpiece, rolling at temperatures below the polymorphic transformation temperature (t _pp ) and annealing, characterized in that the pre-treatment is carried out on a structure with submicron grain size, rolling starts from a temperature selected in the range t _PP - (150-500) ^o C, regulated by the required submicron grain size in the finished semi-finished products and those.

The task is also solved if:
- pre-treatment of the workpiece on a structure with a submicron grain size is carried out by deformation at a temperature below t _pp at 150-500 ^o C;
- before deformation, the preform is heated to a temperature of 10-50 ^o C above t _pp and cooled to room temperature at a speed of at least 0.5 ^o C / s;
- rolling is carried out in isothermal conditions;
- rolling is carried out with a strain rate in the range of 10 ^-4 -10 ⁰ s ^-1 ;
- rolling is carried out in several passes with private reductions of 5-20%;
- after reaching a total degree of deformation of 40-65%, intermediate annealing is carried out at a temperature below t _pp at 150-500 ^o C;
- after reaching a total degree of deformation of 40-65%, the rolling direction is changed by 90 ^o ;
- after each pass, the rolling direction is changed by 90 ^o .

The invention consists in the following. In semi-finished products from titanium alloys, including sheet alloys, the formation of a globular fine-grained structure occurs during (α + β) deformation, i.e. at temperatures below t _pp . Moreover, with a decrease in the deformation temperature, the size of recrystallized grains decreases. The formation of a microstructure with a grain size of less than 1 μm is observed at strain temperatures below (0.4-0.5) T _pl , where T _pl is the melting temperature of the material in degrees Kelvin. With decreasing temperature, the degree of deformation necessary for complete recrystallization of the alloy increases, therefore, large plastic deformations are required to obtain a homogeneous recrystallized structure in the entire billet volume at low processing temperatures. The implementation of intensive and uniform plastic deformation of the material over the cross section of the workpiece to obtain a homogeneous submicrocrystalline structure in the sheet prefabricated by rolling alone is not possible. In this regard, in the proposed method for the manufacture of sheet semi-finished products with high homogeneity of the microstructure and submicron grain size as the initial billet for rolling, a billet with previously prepared other methods of pressure treatment of the QMS structure is used. Moreover, rolling is carried out to the required thickness of the semi-finished product.

In the proposed method, the rolling of the pre-processed workpiece onto a structure with a submicron grain size is carried out in the temperature range of the existence of the QMS of the material structure. Moreover, rolling starts from a temperature selected in the range t _pp - (150-500) ^o C. The upper limit of the temperature range of rolling is governed by the required submicron grain size in the finished sheet semi-finished product. This means that during the rolling process either the initial microstructure of the pre-processed billet is preserved, or it is slightly crushed or enlarged within the submicron range to the required grain size in the finished semi-finished product. The lower limit of the rolling temperature range is limited by the technological ductility of the material processed by the SMC.

 The invention is further developed and clarified using the following techniques.

Pre-processing of the workpiece on a structure with a submicron grain size is carried out by deformation, the temperature interval of which is determined from the dependence of the size of the recrystallized grains of the processed material on the deformation temperature. For the formation of structures with different chemical composition in titanium alloys with a submicron grain size, deformation is carried out at temperatures below t _pp by 150-500 ^o C, corresponding to deformation temperatures below (0.4-0.5) T _pl . For a uniform study of the microstructure along the cross section of the workpiece and obtain a homogeneous QMS structure, various options for intensive deformation processing are used:
- multilateral deformation, including a set of operations of upsetting and broaching with a change in the axis of the workpiece;
- equal channel angular pressing;
- combined loading, combining draft or tension with torsion.

To increase the uniformity of the formation of the SMC structure, the preform is subjected to heat treatment before deformation, including heating in the β-region followed by accelerated cooling. This leads to the formation of a finely dispersed β-transformed structure, which, upon subsequent heating and intense intense deformation, transforms into a homogeneous SMC structure. The heating temperature in the β-region is selected at 10-50 ^o C above the temperature t _{pp of} the alloy being processed. A significant excess of the heating temperature above t _{pp is} not advisable due to the growth of β grains in the single phase region, which negatively affects the uniformity of the subsequent (α + β) deformation. To significantly increase the dispersion of the β-converted structure, the workpiece is cooled at a rate of at least 0.5 ^o C / s. From the point of view of technological feasibility, the workpiece is cooled to room temperature.

 To reduce the undercooking of the workpiece during the rolling process in non-isothermal conditions, rolls are heated or the workpiece is rolled in a shell made of a material that has strength and plastic properties close to the alloy being processed at a temperature of deformation. If the rolls are heated to a deformation temperature, rolling is carried out in isothermal conditions.

The rolling of the billet pre-treated on the QMS structure is carried out with a strain rate in the range of 10 ^-4 -10 ⁰ s ^-1 , which is optimal for achieving the task. The use of the strain rate less than the lower limit is impractical due to the increase in the rolling time to the required thickness of the semi-finished product, and at strain rates above the upper limit the technological ductility of the rolled material is significantly reduced.

Due to the limited technological plasticity of the processed material at low temperatures, rolling to the required thickness of the semi-finished product is carried out in several passes with private compressions of 5-20%, and after reaching a total degree of deformation of 40-65%, intermediate annealing is performed at a temperature below 150 _pp to 150-500 ^o C. Temperature limits of annealing due to the required grain size of semifinished product and the need to develop a material structure return processes to achieve the necessary technological plasticity STI and of further rolling.

Along with the uniformity of the microstructure and high rates of superplasticity, an important characteristic of sheet semi-finished products for superplastic deformation is the value of planar anisotropy of the mechanical properties, which is determined by the crystallographic texture. For titanium alloys, the isotropy of the mechanical characteristics in the sheet plane is ensured by the creation of a basic texture [4], which is formed when longitudinal-transverse warm rolling is applied. To obtain a crystallographic texture of the basic type, rolling is carried out with a change in the rolling direction by 90 ^o either after a total degree of deformation of 40-65%, or after each pass. The choice of one or another rolling option is determined by the dimensions of the manufactured semi-finished sheet, rolling equipment and the rolling method. In the case of using batch rolling of a set of thin sheets placed in a shell, first, rolling in one direction with a total degree of deformation of 40-65% is used to simplify the technological cycle, intermediate annealing, preparation of a new package and subsequent rolling in the transverse direction relative to the previous rolling cycle are carried out. When using card rolling of individual sheets, rolling is carried out with a change in rolling direction after each pass.

 When analyzing the prior art on patent and scientific and technical sources of information regarding methods for manufacturing sheet semi-finished products from titanium alloys with a fine-grained structure suitable for superplastic deformation, a method was not found that is characterized by features identical to all the essential features of the claimed invention. Therefore, the claimed invention meets the condition of "novelty."

 When analyzing the distinguishing features, it was revealed that the claimed invention does not follow explicitly from the prior art. For the first time, an approach has been proposed for the preparation of sheet semi-finished products with an SMC structure from hardly deformed titanium alloys, which together provides their high superplasticity at low deformation temperatures, a high level of strength and fatigue characteristics at room temperature, and isotropy of mechanical properties in the rolling plane. The listed fundamental features of the invention are new and non-obvious. Therefore, the claimed invention meets the condition of "inventive step".

 The invention is illustrated by the following graphic materials.

 Figure 1. The microstructure of the workpiece for rolling from VT6 alloy.

 Figure 2. Microstructure of a 1 mm thick sheet of VT6 alloy.

Figure 3. Fatigue curves for samples cut from the QMS of a VT6 alloy sheet at an angle to the rolling direction of (a) 0 ^o and (b) 90 ^o .

The possibility of carrying out the invention is illustrated by examples. An industrial bar with a diameter of 200 mm was made from (α + β) -titanium alloy VT6, the polymorphic transformation temperature t _pp and the chemical composition of which are given in Table 1. Examples are given for the manufacture of a sheet card with a thickness of 1 mm and a grain size of 0.4-0.6 microns. The proposed examples do not exhaust the capabilities of the method with respect to the manufacture of semi-finished sheet products of various thicknesses, with a larger or smaller grain size from this alloy, as well as other titanium alloys.

 Example 1

Pretreatment in order to obtain a submicro grain size structure in a preform for rolling was carried out by warm deformation according to the multiple draft drawing procedure under thermomechanical conditions, ensuring large plastic deformations in the volume of the preform and the formation of a homogeneous SMC structure. To do this, a 120 mm long billet cut from a bar was heated to a temperature of 1010 ^° C and cooled to room temperature in an aqueous medium, providing cooling of the billet at a rate of at least 0.5 ^° C / s. Then, repeated deformation of the heat-treated workpiece was carried out under conditions close to isothermal, in four stages with a decrease in the deformation temperature from stage to stage in the range of 800-650 ^o C. A deformation temperature of 650 ^o C for VT6 alloy corresponds to a grain size of 0.5 μm recrystallized. For deformation, a hydraulic press was used, equipped with an isothermal unit with flat strikers heated to the deformation temperature. The deformation according to the draft-broaching scheme, as a result of which the preform at the end of each stage acquired an approximately cylindrical shape, allowed repeated deformation cycles. At the final stage of pretreatment, the preform was upset to a thickness of 25 mm. When the selected thermomechanical conditions in the entire volume of the workpiece formed a homogeneous QMS structure with a grain size of 0.5 μm (figure 1).

The experiment found that when a material with the obtained SMC structure is heated to a temperature above 720 ^° C, an intense grain growth occurs to micron sizes, and at temperatures below 550 ^° C the ductility of the material is significantly reduced. For the manufacture of a sheet with a grain size of 0.5 μm, the temperature of heating and rolling the billet with an SMC structure of 650 ^° C was selected, which is 340 ^° C lower than t _pp alloy.

To carry out the rolling, a card with a thickness of 8 mm and dimensions in the plan of 100x100 mm ^{2 was} cut from a pre-processed billet. The billet for rolling was heated in an electric resistance furnace to a temperature of 650 ^o C. The rolling was carried out on a mill whose rolls were heated to 350-400 ^o C. The rotation speed of the rolls was selected from the condition that the deformation rate was 10 ⁻² s ⁻¹ at a compression rate of 10 % per pass. After each pass, the billet was heated in an oven to a temperature of 650 ^° C and the rolling direction changed by 90 ^° . After reaching a total degree of deformation of 50%, intermediate annealing was carried out at a temperature of 650 ^o C for 1 h. Further rolling to obtain a sheet card with a thickness of 1 mm was also carried out with a change in the direction of rolling at each pass, annealing at a total reduction of 40-65%.

 During the rolling process, a homogeneous microstructure with a submicron grain size was preserved in the sheet semi-finished product (Fig. 2) and the formation of a basic crystallographic texture was observed.

 To assess the mechanical properties of a sheet with a QMS structure (grain size 0.5 μm), samples were cut along and across the rolling direction at the final pass and tensile tests were performed. Tables 2 and 3 present, respectively, at room and elevated temperatures, the test results of the QMS sheet and for comparing the industrial sheet of VT6 alloy with a fine-grained structure (grain size 3 μm).

 To assess the fatigue fatigue endurance limit, we tested the samples on an electrodynamic bench with their cantilever fastening and pulsating loading. The test results - the values of the number of cycles N before the formation of cracks at the amplitude of the stress of the cycle σ - are given in table 4 and figure 3.

As follows from the data obtained, the mechanical properties of the SMC of the VT6 alloy sheet at all test temperatures are weakly dependent on the direction of specimen cutting, which indicates their isotropy in the sheet rolling plane. The results obtained also indicate an increased by 20% strength and 30% fatigue properties of the SMC sheet, in comparison with the industrial sheet. Tests at elevated temperatures showed high characteristics of the superplasticity of the QMS sheet in the temperature range of 650-750 ^o C, which is significantly lower than the temperature range of 875-925 ^o C, typical for superplastic deformation of industrial sheets with a grain size of 2-5 microns.

 Thus, the results of a study of the structure and mechanical properties allow us to conclude that the titanium alloy prefabricated sheet manufactured by the proposed method has a uniform QMS (grain size 0.5 μm) structure, exhibits low temperature superplasticity, has a high level of strength and fatigue resistance in combination with isotropic mechanical properties at room and elevated temperatures.

 Example 2

To produce a sheet with a grain size of 0.6 μm for rolling, a card 8 mm thick was used, cut from a pre-processed workpiece obtained in Example 1, with a uniform structure and grain size of 0.5 μm. The grain size of 0.6 μm for VT6 alloy corresponds to a deformation temperature of 675 ^o C. The card blank was heated to a temperature of 675 ^o C, which is 25 ^o C higher than the temperature of the final stage of deformation of the initial blank, and rolled on rolls heated to 400-450 ^o C a strain rate of 10 ⁻³ s ⁻¹ and a compression of 10% per pass. After each pass, the billet was heated in the furnace to a temperature of 675 ^o C. Until the total deformation of 50% (sheet thickness 4 mm) was achieved, the billet was rolled without changing the direction of rolling. Then, after intermediate annealing at a temperature of 675 ^° C and changing the rolling direction by 90 ^° , rolling was carried out without changing its direction after each pass until a total deformation of 50% was reached (sheet thickness 2 mm). Then, again conducting an intermediate annealing and changing the rolling direction, we got a sheet card 1 mm thick. Studies have shown that, as in example 1, the resulting sheet has a homogeneous submicrocrystalline structure with a basic component of the texture. Due to an increase in the temperature of heating and rolling to 675 ^{° C.,} the grain size was 0.6 μm.

 Example 3

To produce a sheet with a grain size of 0.4 μm for rolling, a card with a thickness of 8 mm was used, cut from a pre-processed blank obtained in Example 1, with a uniform structure and grain size of 0.5 μm. The size of the recrystallized grains of 0.4 μm for VT6 alloy corresponds to a deformation temperature of 630 ^° C. The card blank was heated in a continuous furnace to a temperature of 630 ^{° C.} and rolling was carried out in isothermal conditions on a mill whose rolls were also heated to 630 ^° C. Rolling was carried out with a strain rate of 10 ⁻⁴ s ⁻¹ and a reduction rate of 20% per pass to obtain a sheet card with a thickness of about 1 mm. Moreover, the rolling was carried out without intermediate annealing, but with a change in its direction by 90 ^o after a total strain of 65%.

 Example 4

Similar to example 1, except that the preliminary processing to obtain in the billet for rolling the QMS structure was carried out by two-component loading, combining the draft with simultaneous torsion. The initial billet with a diameter of 100 and a length of 300 mm, cut from a bar and subjected to heat treatment (heating at 1010 ^° C, cooling in water), was deformed under isothermal conditions in two stages at temperatures of 750 and 650 ^° C on a specialized rolling mill SRD-800. After deformation and creation of a homogeneous SMC structure with a grain size of 0.5 μm, the preform was deposited on a press along a generatrix to a thickness of 25 mm, a card was cut from it with a thickness of 8 mm, and rolled to a thickness of 1 mm.

 Example 5

Similar to example 1, except that the preliminary processing to obtain in the billet for rolling the QMS structure was carried out by equal channel angular pressing. The initial billet with a diameter of 20 and a length of 100 mm was cut out of the bar, subjected to heat treatment (heating at 1010 ^° C, cooling in water) and multi-cycle isothermal pressing at temperatures of 650-600 ^° C in a tooling placed on a hydraulic press. After the angular pressing of the SMC was completed, the workpiece was deposited along the generatrix to a thickness of 10 mm, machined to obtain a plate with a cross section of 8x28 mm ² , length 85 mm, and rolled into a strip 1 mm thick.

Sources of information taken into account
1. Zaitsev A.V. Development and research of technology for manufacturing sheets of alloyed titanium alloys used for superplastic deformation. - Technology of light alloys, 2001, 1, p. 55-59.

 2. Kaybyshev O.A. Superplasticity of industrial alloys. M .: Metallurgy, 1984. 264 p.

 3. RF patent 2058418, MKI C 22 F 1/18. Publ. 04/20/96, Bull. eleven.

 4. Anisotropy of the physical properties of metals / Adamesku R.A., Geld P.V., Mityushov E.A. M .: Metallurgy, 1985.136 s.

Claims (9)

1. A method of manufacturing a sheet of semi-finished products from titanium alloys suitable for low-temperature superplastic deformation, including pre-treatment of the workpiece, rolling at temperatures below the polymorphic transformation temperature t _pp and annealing, characterized in that the pre-treatment is carried out on a structure with a submicron grain size, rolling starts from the temperature selected in the interval t _PP - (150 ÷ 500 ° C), regulated by the required submicron grain size in the finished semi-finished product. 2. The method according to claim 1, characterized in that the pre-treatment of the workpiece on a structure with a submicron grain size is carried out by deformation at temperatures below t _pp by 150 ÷ 500 ° C. 3. The method according to claim 1 or 2, characterized in that before deformation the preform is heated to a temperature of 10 ÷ 50 ° C above t _pp and cooled to room temperature at a speed of at least 0.5 ° C / s. 4. The method according to any one of claims 1 to 3, characterized in that the rolling is carried out in isothermal conditions. 5. The method according to any one of claims 1 to ⁴ , characterized in that the rolling is carried out with a strain rate in the range of 10 ^-4 ÷ 10 ⁰ s ^-1 . 6. The method according to any one of claims 1 to 5, characterized in that the rolling is carried out in several passes with private reductions of 5 ÷ 20%. 7. The method according to any one of claims 1 to 6, characterized in that after reaching a total degree of deformation of 40 ÷ 65%, intermediate annealing is performed at a temperature lower than t _pp by 150 ÷ 500 ° C. 8. The method according to any one of claims 1 to 7, characterized in that after reaching a total degree of deformation of 40 ÷ 65%, the rolling direction is changed by 90 °. 9. The method according to any one of claims 1 to 6, characterized in that after each pass, the rolling direction is changed by 90 °.

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