RU2470091C1 - Method of ionic implantation of surfaces parts from titanium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to machine building, particularly, to implantation of the surfaces of parts from aluminium alloys. Proposed method comprises processing part surface by laser beam focused to circular spot at specific radiation energy of 200-450 W/mm². Then, said spot is displaced over part surface at the speed of 45-70 mm/s to implant nitrogen ions at dose of (2-6)·10¹⁷ ion/cm².

EFFECT: higher operating performances.

5 dwg, 3 tbl

Description

The invention relates to methods for ion implantation of surfaces of parts made of titanium alloys by ion doping and can be used in the manufacture of products in engineering, aviation and other industries that are operated at high loads and temperatures.

Titanium alloys occupy an important place among modern structural materials, due to the successful combination of physical, mechanical and technological properties. Low density, high corrosion resistance, high specific strength and heat resistance make these alloys indispensable for aviation and space technology. Recently, there has been a tendency to expand the scope of titanium alloys, in particular in medicine for the manufacture of implants and in the automotive industry. However, their wider use is restrained due to unsatisfactory tribotechnical characteristics.

A number of methods are known for modifying the surface of parts made of titanium alloys by ion doping to solve the problem of increasing their wear resistance.

A known method of surface alloying of titanium to increase wear resistance by ion implantation of palladium with an energy of E = 30-40 keV and a dose of D = 1 · 10 ¹⁶ -5 · 10 ¹⁷ ion / cm ² , in addition, a method characterized in that oxygen is preliminarily implanted with an energy of E = 30-40 keV and a dose of D = 5 · 10 ¹⁷ -1 · 10 ¹⁸ ion / cm ² and then palladium with an energy of E = 30-40 keV and a dose of D = 5 · 10 ¹⁵ -5 · 10 ¹⁶ ion / cm ² (USSR Author's Certificate N1086827 MKI C23C 14/48. Method of surface alloying of titanium. Published on April 15, 86).

The disadvantage of this method is the inability to provide wear resistance at the required level due to the small depth of penetration of ions and, as a consequence, insufficient degree of hardening of the modified surface.

A known method of ion implantation of the surfaces of parts made of titanium alloys, which consists in the implantation of nitrogen or carbon ions with an energy of E = 25-100 keV, current density I = 10-100 μA / cm ² , the radiation dose D≥5 · 10 ¹⁷ ion / cm ² (Komarov F.F.Ion implantation in metals. - M.: Metallurgy, 1990. - 216 p.).

The disadvantage of this method is the inability to provide wear resistance at the required level due to the small depth of penetration of ions and insufficient degree of hardening of the modified surface. In addition, the surface layer hardened during ion implantation wears out rather quickly when parts are used under sliding friction with a significant external load.

The closest in technical essence (prototype) to the claimed method is a method for modifying the surface of titanium alloys (RF patent No. 2117073 according to class C23C 14/48. Method for modifying the surface of parts made of titanium alloys. / Guseva M.I .; Smyslov AM; Safin E .V .; Izmailova NF Publ. 10.08.1998), according to which nitrogen ions are implanted and stabilizing annealing is carried out, characterized in that they are pretreated with argon ions with an energy of 250-400 eV, an ion current density of 1-10 mA / ^cm2 and dose (1-2) 10 ¹⁹ ion / cm ^2, and then is implanted with nitrogen ions en rgiey 250-400 eV and an ion current density of 10-40 mA / cm ² and dose (1-2) 10 ¹⁹ ion / cm ^2.

This method helps to improve the operational properties of products made of titanium alloys by increasing the degree of surface hardening.

A significant disadvantage of the prototype is the long cycle time for processing parts, especially the shape of which differs from flat. This significantly increases the complexity of the process as a whole. At the same time, the surface hardened layer wears out quickly during the operation of parts under sliding friction with a significant external load, which often leads to the setting of rubbing parts.

The inventive method of ion implantation of titanium alloys provides increased wear resistance at high fatigue strength.

The technical result, which is achieved by the claimed method, is ensured by the fact that prior to implantation with nitrogen ions, surface laser processing of the parts is carried out by a beam that is focused on the surface to be machined into a circular spot with a specific radiation power of 200-450 W / mm ² and is moved along it with a speed 45-70 mm / s, after which the details are implanted with nitrogen ions with (2-6) · 10 ¹⁷ ion / cm ² .

The inventive method includes the following sequence of operations:

- surface laser processing of parts with a beam that is focused into a circular spot with a specific radiation power of 200-450 W / mm ² and is moved along it at a speed of 45-70 mm / s; laser surface treatment of parts made of titanium alloys in these modes provides the formation of a surface layer whose thickness is 0.01-0.03 of the thickness of the workpiece;

- irradiation of the treated surface of the titanium alloy part with nitrogen ions with an implantation dose of D (2-6) · 10 ¹⁷ ion / cm ² .

In more detail the essence of the proposed method is illustrated by drawings:

- figure 1 presents a graph of the distribution of microhardness in the surface layer after surface laser treatment of the VT6 alloy at different speeds of movement of the heating spot at a specific radiation power of 350 W / mm ² : 1-70 mm / s, 2-45 mm / s, 3- 30 mm / s;

- figure 2 presents the dependence of the relative elongation (4), temporary tensile strength (5) and yield strength (6) on the ratio of the depth of the hardened layer to the total working thickness of the VT6 titanium alloy sample;

- figure 3 - the microstructure of the surface layer of titanium alloy VT6 after laser treatment in argon with an oxygen content of 0.2% (alpha layer, × 100);

- figure 4 is a change in the wear of implanted parts made of VT6 alloy without surface laser treatment (7), subjected to surface laser treatment (8), implanted with nitrogen ions (9) and implanted with nitrogen ions after surface laser treatment (10);

- figure 5 is the appearance of the friction track of samples of VT6 alloy after implantation with nitrogen ions (a) and after laser treatment and subsequent implantation with nitrogen ions (b) with a friction path of 800 m

Laser surface hardening before ion implantation promotes the creation of a layer on the surface of the part with increased microhardness and fine-grained structure due to high cooling rates. The formation of a layer with increased microhardness is greatly influenced by the parameters of focusing and moving the heating spot of the laser beam.

When focusing the laser beam on the surface of the workpiece into a circle-shaped spot with a specific radiation power of more than 450 W / mm ² , the surface of the parts is melted. When the surface is melted, microcracks are observed in the surface layer, which, when the parts work under conditions of sliding friction under load, lead to increased wear. To prevent surface melting, one has to use high speeds of moving the heating spot, which significantly complicates the processing process and the design of the used technological equipment.

In the case of using a heating spot with a specific radiation power of less than 200 W / mm ² (corresponding to a spot diameter of more than 4 mm with a beam power of 2.5 kW), the probability of fusion of the surface of parts in the entire range of spot movement speeds over the surface is almost completely eliminated. However, in this case, to ensure the required depth of the hardened layer, it is necessary to reduce the processing speed (the speed of movement of the heating spot over the surface of the parts), which significantly affects the level of stresses in the treated layer.

Therefore, the optimum should be considered the range of values of specific power in the heating spot 200-450 W / mm ² , which corresponds to the diameter of the heating spot 2-3.2 mm In this range of specific power values, the danger of melting the surface of the workpieces is completely prevented, but the possibility of carrying out a processing process with high productivity remains.

In addition to the specific power in the heating spot, the microhardness values and the depth of the hardened layer are significantly affected by the speed of movement of the heating spot along the treated surface.

So at a speed of movement of the heating spot less than 45 mm / s there is an increase in the depth of the treated layer over 300 microns (figure 1). At such a depth of the hardened layer, a manifestation of the embrittling effect of laser treatment on the mechanical properties of the VT6 alloy and, especially, on its elongation (FIG. 2) is observed.

When the speed of movement of the heating spot in the range of 45-70 mm / s, the depth of the hardened layer does not exceed 150 microns (figure 1). With such a depth of the hardened layer, an increase in the microhardness value is observed in the surface regions of the part and the mechanical properties are kept at a high level (Fig. 2).

An increase in the speed of movement of the heating spot over 70 mm / s leads to hardening of the surface layer and the crumbling of its individual particles during operation of the parts under sliding friction under external load. Such a high speed of movement of the heating spot creates difficulties in the implementation of the technological process of processing and significantly increases the requirements for the accuracy of mechanisms for moving parts.

Therefore, the optimal range of speeds for moving the spot of heating the laser beam over the surface of the workpiece is 45-70 mm / s.

The effect on surface hardening of a titanium alloy by surface laser treatment during subsequent treatment with low-energy nitrogen ions indicates an important role, along with the formation of a dislocation structure in the surface layer, of surface effects associated with sorption processes that are activated by argon plasma arising in the contact zone of laser radiation with the surface of the part.

The degree of hardening of the surface layer of titanium alloys during laser processing is greatly influenced by the composition of the gas medium in which it is conducted. Inert gas (argon) was chosen as the basis of the gaseous medium. Oxygen was added to argon to form titanium oxide in the surface layer in the area of the laser beam. It was found that when the oxygen content in argon is up to 0.1%, finely dispersed titanium oxides and alloying elements are formed in the surface layer, which lead to an increase in microhardness. When the oxygen content in argon is more than 0.2%, intense oxidation of the surface layer is observed with the formation of an alpha-coated sublayer (Fig. 3), which leads to intensive crushing of particles during sliding friction.

After the laser surface treatment, the fatigue strength and wear resistance of parts made of titanium alloys are significantly affected by the doses of nitrogen ion implantation. When implanting nitrogen ions with a dose of less than 2 · 10 ¹⁷ ion / cm ² , an insignificant increase in the surface microhardness is observed (1.1 ... 1.2 times), which is probably due to the rearrangement of lattice defects by the appearance of additional vacancies, pores, and interstitial atoms that contribute to annihilation imperfections and softening, as well as the manifestation of the undesirable effect of spraying the surface of the processed sample.

Upon implantation of nitrogen ions with a dose of more than 6 · 10 ¹⁷ ion / cm, the surface microhardness after processing increases by 2.5 ... 2.8 times with an indenter load of 0.1 H. Such a significant increase in microhardness leads to the appearance of microcracks in the surface layer at tests for wear resistance and crumbling of individual particles of oxides and nitrides.

Therefore, from the point of view of increasing wear resistance and fatigue strength, the optimal range of doses of nitrogen ion implantation is in the range of (2-6) · 10 ¹⁷ ion / cm ² , in which, along with the formation of a dislocation structure in the surface layer, the occurrence of surface effects associated with sorption phenomena.

To assess the impact of the proposed method of ion implantation on the wear resistance of the VT6 titanium alloy, implantation of flat parts 2 mm thick was performed. Some samples were subjected to laser surface treatment before implantation with nitrogen ions. On the samples, the coefficient of sliding friction without lubrication was determined. The tests were carried out on a special bench, which provides a given force of pressing the counterbody to the surface of the sample and registration of the moment of friction of the counterbody in contact with the test sample.

To determine the fatigue limit during bending, flat samples were made with a thickness of 2 mm in accordance with GOST 25.502-79.

In addition to the mechanical characteristics of the VT6 alloy, after laser surface hardening and implantation, the weight wear of the samples was determined depending on the friction path under load.

The test results to determine the effect of surface laser treatment on the mechanical and tribological properties of the VT6 alloy are presented in table 1.

The test results presented in table 1 show that the implementation of laser surface treatment of VT6 titanium alloy in the claimed range of parameters provides the highest mechanical and tribological characteristics of the samples.

During laser processing of parts made of VT6 alloy at the above optimal parameters, low values of the friction coefficient, 0.08-0.10, are ensured after implantation with copper and lead ions.

Table 2 illustrates the effect of the oxygen content in a protective argon atmosphere during laser surface treatment on the mechanical and tribological properties of VT6 alloy specimens. Laser processing was performed at a heating spot moving speed of 55 mm / s with a specific power of 350 W / mm ² in the spot.

From the obtained test results it follows that the optimal oxygen content in the protective atmosphere of argon during laser treatment of VT6 titanium alloy is 0.1-0.15%.

Table 3 shows the test results of samples of VT6 alloy after laser treatment and subsequent ion implantation with nitrogen. Laser processing was performed at a heating spot moving speed of 55 mm / s with a specific energy power of 350 W / mm ² in the spot with an oxygen content of 0.12% in argon.

Table 3 No. p / p The implantation dose of nitrogen ions D, ion / cm ² Tensile strength σ _in , MPa Elongation δ,% Microhardness HV, MPa Fatigue limit, MPa based on 2 · 10 ⁷ cycles Coefficient of friction Weight wear, 10 ^-5 g one 1 · 10 ¹⁷ 1070 16 5350 525 0.10 220 2 2 · 10 ¹⁷ 1080 16 5750 535 0.08 170 3 4 · 10 ¹⁷ 1080 fifteen 6500 550 0,07 145 four 6 · 10 ¹⁷ 1060 fifteen 6800 550 0.08 150 5 7 · 10 ¹⁷ 1010 13 7200 510 0.16 260

An analysis of the topology of the surface of the bolts after determining the friction coefficient showed that when the depth of the hardened layer is more than 0.03 of the thickness of the sample due to distortions in the structure of the surface layer of the VT6 alloy, surface crumbling is observed.

Wear resistance tests show that the weight wear of parts that underwent surface laser treatment prior to implantation, with a friction path of 200 m, is approximately 1.2-1.8 times less than that of control samples.

From the presented test results, it follows that the use of the proposed method of ion implantation provides increased wear resistance, fatigue limit and reduced friction coefficient of machined parts made of titanium alloys during their operation under sliding friction conditions.

Claims (1)

The method of ion implantation of the surfaces of parts made of titanium alloys, comprising implanting nitrogen ions into the alloy surface, characterized in that before implantation, the surface of the part is treated with a laser beam that is focused into a spot in the form of a circle, with a specific radiation power of 200-450 W / mm ² , after which the spot is moved along the treated surface at a speed of 45-70 mm / s, and nitrogen ions are implanted after laser treatment with a dose of (2-6) · 10 ¹⁷ ion / cm ² .

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