RU2753210C1 - METHOD FOR MANUFACTURING RODS FROM SUPERELASTIC ALLOYS BASED ON THE Ti-Zr-Nb SYSTEM - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy and can be used to produce rods from superelastic alloys for medical use. A method for manufacturing long rods with a diameter of 3÷10 mm from a superelastic alloy of the titanium-zirconium-niobium system, including obtaining a semi-finished product by cross-screw rolling from an ingot and rotary forging of the semi-finished product on a rod of the final size. Before cross-screw rolling, the ingot is subjected to multi-axis forging with a true degree of deformation of 0.15÷0.25 of the total true degree of deformation. Cross-screw rolling is performed with a true degree of deformation of 0.45÷0.65 of the total true degree of deformation. Multi-axis forging, cross-screw rolling and rotary forging are carried out with intermediate exposures at temperatures of 950÷1050°С, 900÷950°С and 600÷750°С, respectively.

EFFECT: method makes it possible to obtain bars with high strength and superelasticity at room temperature.

1 cl, 5 tbl, 1 ex

Description

The invention relates to the field of metallurgy and can be used to produce rods from superelastic alloys for medical use.

From the prior art, a method for producing rods from an alloy of the nickel-titanium system with a shape memory effect according to RF patent No. 2162900 is known, according to which an alloy with a shape memory effect is produced by the method of helical rolling or pressing, with further rotary forging at a temperature 450 - 950 ° C.

The problem to be solved when creating the claimed method consists in obtaining long ( l > 2000 mm) rods of small sections (3 ÷ 10 mm in diameter) with a fine structure (grain size 10 ÷ 30 μm) from ingots made of superelastic alloys of the Ti-Zr-Nb system and a low Young's modulus (E = 30 ÷ 50 GPa), while the technical result achieved when solving such a problem consists in an increase in strength (σ _in = 600 ÷ 800 MPa), a high value of reversible deformation (ε _arr = 4.0 ÷ 6.5%) and superelastic behavior at room temperature for their further application in the field of medicine.

To achieve this result, a method is proposed for manufacturing a rod from a superelastic alloy of the titanium-zirconium-niobium system, including obtaining a semi-finished product by cross-helical rolling from an ingot and rotational forging of a semi-finished product onto a bar of final size, while, before cross-helical rolling, the ingot is subjected to multiaxial forging with true the degree of deformation, which is 0.15-0.25 of the total true degree of deformation, and screw rolling is performed with the true degree of deformation, which is 0.45-0.65 of the total true degree of deformation.

Additionally, multi-axis forging, cross-helical rolling and rotary forging are carried out with intermediate holdings at temperatures of 950 ÷ 1050 ° C, 900 ÷ 950 ° C and 600 ÷ 750 ° C, respectively.

The ideology of the claimed method is based on a three-stage high-temperature thermomechanical treatment (TMT) - plastic deformation according to the scheme "multiaxial forging (MC) + helical rolling (PVP) + rotary forging (RC)", observing the sequence and regulation of the shares of each process from the total degree true deformation.

Stage 1. The original ingot (cast billet) from a superelastic alloy of the Ti-Zr-Nb system, by its nature, is characterized by the presence of segregation porosity and looseness, a coarse-grained structure, weakened by intergranular bond, and, as a consequence, very limited deformability. As a consequence, multiaxial forging (MCF) occurs under conditions of uneven volumetric compression with alternating directions of application of deforming forces, which most favorably affects the initial deformation of the cast structure. Thus, the MOC is designed to completely eliminate the porosity and looseness of the structure of the initial ingot, to perform primary crushing of cast crystallites, to increase the strength of grain boundaries and, most importantly, to increase the deformability of workpieces and its ability to deform residually without macrofracture at the next stage 2, under conditions of developed shear deformations. ...

The true degree of deformation ε _MOK at MOK should be 0.15 ÷ 0.25 of the total true degree of deformation ε _Σ = ε _MOK + ε _PVP + ε _RK . If the fraction ε _MOK <0.15ε _Σ , then the initial study of the structure of the original ingot becomes insufficient for the desired increase in deformability, which leads to a critical increase in the risk of destruction of the original ingot at the second stage. When ε _MOC > 0.25ε _{Σ, the} share of subsequent stages in order to achieve the overall technical result is inadmissibly reduced. The MOC scheme is effectively implemented on relatively short workpieces that retain longitudinal stability during compression deformation along the long axis after the next tilting. With an increase in ε _WOC , the length of the forging increases and the tendency to loss of stability during the next upsetting in the direction of the long axis increases.

The density of the metal after the first stage reaches 100% of the theoretical, the grain size is 100-200 microns.

Stage 2. Cross-helical rolling (PVP) performs the function of the main structure-forming factor, providing intensive crushing of the ingot structure and increasing the plastic and physical-mechanical properties of the metal.

The interval of the true degree of deformation at PVP ε _PVP is 0.45 ÷ 0.65 of the total true degree of deformation at MOC, PVP and RK, i.e. ε _PVP = (0.45 ÷ 0.65) ε _Σ

The value of ε _PVP less than 0.45 is not enough for the required refinement of the structure and an increase in the plastic properties of the metal and leads to the possible formation of breaks during rotary forging at the third stage. If the value of ε _PVP exceeds 0.65 of ε _Σ , then an excessive gradient of structure and properties is formed over the cross section of the resulting rods, and, in addition, it becomes difficult to obtain rods with a diameter of less than 10 mm.

As a result, a gradient structure is formed along the section of the bar with a grain size of 20 to 100 μm. The alloy in this condition has high characteristics of ductility (elongation to fracture of 20-40%) but low strength (σ _in = 500-600 MPa).

Stage 3. Rotary forging (RC) is used as a finishing treatment to flatten the gradient structure and obtain small-section bars (<10 mm). This achieves a refinement of the grain structure to an average grain size of 10-40 microns. The resulting bar semi-finished products have a combination of low Young's modulus (E = 30-50 GPa), sufficiently high strength (σw = 600-800 MPa), high reversible deformation (4.0-6.5%), and superelastic behavior at room temperature.

The following example illustrates the practical aspects of the implementation of the claimed method.

Example. An ingot of Ti-18Zr-15Nb alloy (at.%) Was obtained by vacuum arc remelting (VAR). VAR was carried out 4 times for the most uniform distribution of components throughout the ingot volume.

Multiaxial forging was carried out in three stages with a deformation rate of 6 mm / s:

- at the first stage, the ingot was first heated in a furnace for 2 hours from a temperature of 700 ° C to 1000 ° C, followed by holding for 1 hour at a temperature of 1000 ° C. Further, the billet was upset at 1000 ° C at the end by 25-30%. After that, forging was carried out into a square □ 120 mm with a change in the deformation axis;

- at the second stage, the workpiece was heated to 900 ° C, followed by holding for 40 minutes. Further, similar to the first stage, the billet was upset at 950 ° C at the end by 25-30%. After that, forging was carried out into a square □ 105 mm;

- at the third stage, the workpiece was loaded into a furnace preheated to 950 ° C, then the furnace with the workpiece was transferred to a temperature of 900 ° C, the holding time was 40 minutes. At the moment of unloading the billet for deformation, the furnace temperature dropped to 920 ° C. The sediment at a temperature of 920 ° C at the end was also 25-30%; then forging on a square □ 92 mm and □ 87 mm were carried out, after which the edges were knocked down into a circle, after which the diameter of the workpiece was ∅92 mm. The total relative and true deformations, and the total stretch ratio were calculated based on the initial and final geometric parameters for each forging stage. The resulting billet was turned to a diameter of 85 mm for subsequent helical rolling. The deformation modes of the multi-axis forging process are presented in Table 1.

Table 1

Operation type Ingot m = 13.8 kg Total
relative
deformation
ε,% Complete set of hoods,
λ _full True degree of deformation, ε hн, mm hk, mm Exposure at a temperature of 1000 ° C, 1 hour Forging (drawing) by □ 120 mm 149 105 179 120 132 120 124 120 17 1.21 0.19 Exposure at a temperature of 950 ° С, 40 minutes Forging (drawing) by □ 105 mm 216 152 148 98 164 98 145 98 128 105 115 105 23 1,3 0.26 Exposure at a temperature of 950 ° С, 40 minutes Forging at ∅92 mm 265 186 121 100 133 93 140 85 132 82 128 85 111 87 105 87 99 87 95 87 117 92 123 92 96 92 102 92 103 92 98 92 98 92 40 1.65 0.5 Total values for the entire operation 150 92 65 2.65 0.97

Cross-helical rolling (PVP) was carried out alternately on several mills when the workpiece reached certain geometric parameters. PVP on a three-roll helical rolling mill MISIS-130T was carried out on a billet with a diameter of 85 mm, obtained from the results of multi-axis forging and turning (see above). The billet was heated to a temperature of 950 ° C and held for 40 minutes. Rolling was carried out to a diameter of 54 mm, after which the billet was rolled on a three-roll helical rolling mill MISIS-100T to a diameter of 40.7 mm with preheating in a furnace at a temperature of 950 ° C for 15 minutes. The workpiece was divided into 4 equal parts and further processing was carried out according to the same geometric and deformation parameters. Then the rods were rolled on a 14-40 three-roll minisan to a diameter of 35.7 mm. During the operations, an oxide layer was formed and to remove it, the rods were turned to a diameter of 30 mm. Further processing was carried out on the 10-30 mill. The diameter of the obtained blanks is 19.9 mm before finishing turning. Deformation modes of PVP are presented in Table 2.

table 2

Mill Diameter Relative
deformation,% K-NT hoods,
λ Complete set of hoods,
λ _full True degree of deformation, ε D ₀ D ₁ Misis-130T Exposure at a temperature of 950 ° С, 40 minutes 85.0 78.0 15.8 1.19 1.19 0.17 78.0 74.0 10.0 1.11 1.32 0.11 74.0 67.0 18.0 1.22 1.61 0.20 67.0 61.0 17.1 1.21 1.94 0.19 61.0 54.0 21.6 1.28 2.48 0.24 Misis-100T Exposure at a temperature of 950 ° С, 15 minutes 54.0 47.0 24.2 1.32 3.27 0.28 47.0 40.7 25.0 1.33 4.36 0.29 14-40 40.7 35,7 23.1 1.30 5.67 0.26 Turning 35,7 30.0 - - - - 10-30 Exposure at a temperature of 910 ° С, 40 minutes 30.0 27.3 17.2 1.21 6.85 0.19 27.3 24.0 22.7 1.29 8.86 0.26 24.0 19.9 31.2 1.45 12.88 0.37 Total values for the entire operation 85 19.9 82/56 * 5.67 / 2.27 * 12.88 2.56

* Total parameters of deformation before and after turning, respectively.

Hot rotary forging of Ti-Zr-Nb bars. After PVP, rods with a diameter of 19.9 mm were turned to a diameter of 17.6 mm to remove the surface defect layer. Before forging, the billet was heated in an electric furnace with resistance heaters to a temperature of 700 ° C. To replace the bikes between passes with a smaller diameter, the workpiece was placed in an oven. Temperature control was carried out with an optical pyrometer. With a decrease in the diameter of the workpiece, the longitudinal feed rate was increased, which was of great importance for the metal flow regime in the deformation zone and for obtaining a high-quality surface. After the last pass, the bar was cooled in water. After RC, the bars were straightened on a straightening-polishing machine in several passes. The final diameter of the finishing rods after grinding was 5.5 mm. Table 3 shows the deformation modes of hot rotary forging (RC), in table 4 - the total parameters of deformation modes for all stages of thermomechanical processing.

Table 3

Operation type Diameter Relative
deformation,% K-NT hoods,
λ Complete set of hoods,
λ _full True degree of deformation, ε D ₀ D ₁ Hot rotary forging at 700 ° C Exposure at a temperature of 700 ° C, 30 minutes 17.6 17.0 6,7 1.07 1.07 0.07 17.0 16.0 11.4 1.13 1.21 0.12 16.0 15.4 7.4 1.08 1.31 0.08 15.4 14.0 17.4 1.21 1.58 0.19 14.0 13.0 13.8 1.16 1.83 0.15 13.0 12.0 14.8 1.17 2.15 0.16 12.0 11.0 16.0 1.19 2.56 0.17 11.0 10.3 12.3 1.14 2.92 0.13 10.3 9.8 9.5 1.10 3.23 0.10 9.8 8.9 17.5 1.21 3.91 0.19 8.9 8.0 19.2 1.24 4.84 0.21 8.0 7.2 19.0 1.23 5.98 0.21 7.2 6.5 18.5 1.23 7.33 0.20 Total values for the entire operation 17.6 6.5 86.3 7.33 7.33 1.99

Table 4

Operation type Diameter Relative
deformation,% Full c-nt of the extract, λ _full True degree of deformation, ε D ₀ D ₁ Multi-axis forging 150 92 65 2.65 0.97 PVP 85 19.9 82/56 * 12.88 2.56 RK 17.6 6.5 86.3 7.33 1.99

* Total parameters of deformation before and after turning, respectively, during the operation

As a result of all operations, a long bar with a diameter of 5.5 mm was obtained. To study the structural characteristics in the longitudinal and cross sections, 6 samples with a height of 4 mm were cut from different parts of the bar and embedded in a two-component fast-curing casting resin based on methyl methacrylate in the form of powder and liquid. The smooth surface of the samples after pouring was obtained by multistage grinding and polishing preparation on a grinding machine.

The average grain size was determined by the random secant method for all prepared samples. Using a light microscope, 9 micrographs were obtained for each prepared sample. On each image, 15 parallel lines were drawn at the same interval. The lengths of the secant sections falling on one grain were measured, converted into micrometers, for further determination of the average grain size. As a result of the study, it was revealed that the average grain size for the cross section is 19.5 microns, for the longitudinal section it is 24.7 microns. The resulting bar has a fairly fine-grained structure, which has a positive effect on the functional properties of the material.

In order to study the mechanical properties of the obtained rods, samples were prepared for mechanical tests with a working part length of 20 mm and a diameter of 3 mm. Static mechanical tensile tests were carried out on a tensile testing machine with a strain rate of 1 mm / min. Diagrams "stress - deformation" were constructed, from which the following values were identified: phase yield stress σ _f , intersection of tangent lines of the elastic region and the yield region; Young's modulus E at the stage of elasticity, the tangent of the angle of inclination to the tangent to the elastic region; tensile strength σ _in , maximum stress on the obtained stress-strain diagram; deformation before fracture ε, the maximum value of the sample deformation during the test. Cyclic tests were carried out according to the scheme: "deformation by 1% - unloading" in each cycle with the accumulation of deformation until the destruction of the sample. According to the obtained diagrams "stress - strain", the following parameter was calculated: the maximum reversible strain ε _{arr, the} difference between the induced and permanent deformations. The values obtained by mechanical tests after treatment are shown in Table 5. The resulting semifinished Semifinished possess a combination of low Young's modulus (E = 42,6 GPa), a sufficiently high strength (σ _in = 641 MPa), high values of reversible deformation (6,2 %) and superelastic behavior at room temperature.

Table 5

σ _f , MPa σ_v, MPa E, GPa ε,% ε _sample,% 258 641 42.6 14.6 6.2

Claims (4)

1. A method of manufacturing long rods with a diameter of 3 ÷ 10 mm from a superelastic alloy of the titanium-zirconium-niobium system, including obtaining a semi-finished product by cross-helical rolling from an ingot and rotary forging the semi-finished product into a bar of final size, characterized in that before cross-helical rolling, the ingot is subjected to multi-axis forging with a true deformation amounting to 0.15 ÷ 0.25 of the total true deformation, and helical rolling is performed with a true deformation ratio of 0.45 ÷ 0.65 of the total true deformation. degree of deformation. 2. The method according to claim 1, characterized in that multi-axis forging, cross-helical rolling and rotary forging are carried out with intermediate holdings at temperatures of 950 ÷ 1050 ° C, 900 ÷ 950 ° C and 600 ÷ 750 ° C, respectively.