Method of producing steel nanostructured surface by laser-induced plasma processing

FIELD: process engineering.

SUBSTANCE: invention relates to metal processing by laser and may be used in machine building. Part to be processed is placed in sealed chamber filled with inert gas and modifying gas. Laser beam with spot power density on part surface making (10⁶-10⁷) W/cm² is used to affect steel part surface to produce optical discharge surface plasma in fused metal vapors. Laser beam is displaced at the speed of 0.1-2 m/s at gas pressure in the chamber equal to 1.5-2 atm.

EFFECT: improved mechanical properties, higher wear and heat resistance.

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The technical field.

The invention relates to processes, and more particularly to the treatment of metals with a laser beam, and can be used to improve the mechanical properties of the surfaces of the parts and to improve the stability with respect to different types of wear, heat resistance, heat resistance, and the formation of surface layers with special physical and chemical properties, and may find application in various engineering industries.

The level of technology.

Nanostructured coatings and materials find wide application in industry.

A well-known method of forming micro - and submicrocrystalline structure in the surface layers of steel (Grigoryants A.G., A.N. Safonov. Methods of surface laser treatment. - M.: Vysshaya SHKOLA, 1987, - 191 C.), consisting of high-speed heating by a laser beam of the surface layers and cooling at a rate considerably in excess of the critical speed quenching to martensite. An example of this method is patent RU 2345148 C2 from 27.01.09, where laser processing is carried out using a continuous laser radiation focused into a light spot in the form of a segment and move along a given trajectory with a constant or variable speed. Moreover, at the initial stage of this process the pre-opredelaetsa the maximum temperature on the surface of the processed material, in excess of the temperature required for structural or phase transformation.

Another known method of producing coatings with fine structure is the method of laser surface alloying [Grigoryants A.G., Shiganov I.N., Mizurov A.I. processes of laser processing. - M.: Moscow state technical University n.a. Bauman, 2006, - 663 S.].

This process is carried out by introducing into specified areas of the surface of the various components, which, mixing with the base material, the melting of the laser beam to form patterns of the desired composition, i.e. in the process of a short laser melting surface treated metal and alloys due to large temperature gradients arise intensive hydrodynamic flows. The processes of mass transfer across the zone melting is accelerating. Education doped zones is accompanied by at least three processes that lead to mixing of alloying elements to molten matrix: mass transfer at a distance of several hundred micrometers in the convective mixing, mass transfer over distances of several micrometers due to diffusion in the liquid and solid phases and mass transfer as a result of thermocapillary forces.

Filing filler components in this technology often assests the Ute from the solid phase. Alloying powders are applied previously in the form of a slurry or served directly to the zone melting. Sometimes as alloying components can be used in liquids and gases.

Currently quite well-known ion implantation (doping) ion beam surface of the processed material, the introduction of which there is a change in the elemental chemical composition and structural-phase state of the surface layers (Bykovsky Y.A. and other Ion implantation and laser metal materials. - M.: Energoatomizdat, 1991,-240 C.).

The disadvantage of this method is primarily the presence of a high vacuum and low productivity.

However, this method, like the previous one, does not allow to obtain nanosized surface structure.

This limited range of physical phenomena:

1. Limitations in the formation of a homogeneous distribution. Diffusive convective transport of atoms of the alloying substances from the coating into the matrix is limited by the lifetime of the melt and leads to a uniform depth concentration distribution profile introducing alloying elements.

2. Thermal contact resistance of the coating matrix.

By laser action on the floor in the form of a coating or deposited layer is oznikaet difficulties because of the increased thermal resistance at the boundary of the coating-substrate interface, for example, the evaporation of the chemical elements.

3. Thermodynamic constraints.

Thermodynamic "limit" to appear in laser melting of the coating and the substrate, composed of chemical elements, is not miscible in the equilibrium conditions in the liquid phase.

In these systems there is no possibility of diffusion of the atoms of the alloying elements in the liquid phase, i.e. you cannot mix these atoms in the melt due to the limitations of thermodynamic nature (immiscibility of the elements in the liquid phase equilibrium diagram).

For the formation of nanostructured layers, you need to meet the following requirements:

- ultra-fast heating of the surface layer and the shallow depth of the molten layer, which allows the cooling mode, the heat conductivity of the surface layer with velocity V(°f/c), leading to the formation of nanostructures, ie,

V<V<Vmax,

where V - critical cooling rate, leading to the formation of supposed (>100 nm);

Vmax is the rate of cooling, leading to the formation of amorphous structures (vitrifying), (Vmax=10⁶...10¹⁰°F/c);

- the availability of high-speed source doping the liquid phase of the surface layer and the uniformity of its completion around the molten volume, creating a high concentration of centers of crystallization.

Nano is tructure are characterized by the features concluded that the processes and make actions happen in the nanometer range spatial dimensions.

Source material are individual atoms, molecules, molecular systems, rather than in the traditional technology of micron and macroscopic amounts of material.

Therefore, unlike traditional technology for nanotechnology typical "individual" approach, in which external control reaches individual atoms and molecules, which allows them nanoscale materials with controlled structure and a radically new physico-chemical properties.

One of the ways to achieve this goal is the technology of nanostructured layers by sintering ultradispersed nanopowders by laser radiation (Shiganov I.N., Mizurov A.I. Modern methods and equipment for three-dimensional shaping parts laser melting of metal powders. Laser Info. 2004, No. 5-6). The advantage of this process is the preservation of the initial parameters of the powder due to the absence of melting.

Recrystallization during sintering is characterized by several essential features, large grains are formed due to the transfer of a substance at a common boundary with grain small grain size larger. P is the postponement of substances takes place by moving the atoms across the grain boundary in the direction of the grain with the least amount of free energy and contact across sites. Driving force for recrystallization is determined by the tendency of the system to move in a more equilibrium state with a smaller total surface boundaries.

An example of a high-speed laser recrystallization deposited on the surface of nano-sized powder of different chemical composition is to work Baranauskas EV, where we used a fine mixture of powders consisting of chemically pure iron and graphite (Baranivskiy E.V., Ipatov, A.G., Microstructure and properties of layers by laser recrystallization of the powder of Fe-based materials. Bulletin of the University. 2007. No. 4, p.88-97).

For the manufacture of the samples was used powders of carbonyl iron grade a-100 and crystalline graphite. Technology of production of carbonyl iron provides chemical purity of the powder with the exception of three elements - oxygen, carbon and nitrogen. For treatment of these items, the original powder was subjected to annealing in hydrogen atmosphere at a temperature of 350°C for one hour with cooling in the furnace. After this, the iron powder was mixed with graphite in proportions necessary to obtain samples of the alloy of iron and carbon with a carbon content of 0.3% by weight. Then, the mixture powder was razulybalas on a vibrating mill. Fractional composition was controlled by the time of grinding, and the size of h is CI powder varied from 100 nm to several micrometers.

Prepared from mixtures of powders of samples were in the double cycle: pressing powder, pre-sintering in a protective atmosphere (dissociatively ammonia) for 2 hours with cooling in the furnace; the calibration samples, the final sintering for 4 hours. The density of the prepared samples was 7.4 g/cm³structure - ferrite, pearlite, with a hardness of 70-90 kg/mm².

For subsequent high-speed laser processing of samples was made technological stand with a CO₂laser Lanthanum-3M operating in generation mode CW beam power of 1 kW. The power density of the laser radiation was set in the range from 2.6·10⁵up to 1.4·10⁶W/cm². The required scanning speed of the laser beam was set frequency n of rotation of the sample in a special fixture and the distance to the center of rotation. The speed range was from 0.1 m/s to 4 m/s

After the laser processing area of the laser recrystallization consisted of two layers: the zone of laser quenching from the liquid phase and zone laser hardening of the solid phase, and due to the high heating rate and cooling the formed heterogeneous in content of carbon fine lamellar martensite.

The disadvantage of this method is the complexity of the slurry is subdisplay mixture of powders and pre-sintering of the samples in a protective atmosphere, and also the limited speed of recrystallization, depending on the radii of curvature of the grain boundaries, i.e. the dispersion of the powder material.

The known method of forming a nanoscale surface coatings in alloys type solid solution (EN 2371380 C1)by the energy impact of laser radiation with a pulse repetition rate of not less than 4 kHz and a maximum power density q<4,88 λ(T)·T_PL/Df and the exposure time of at least 30 sec,

where λ(T) is thermal conductivity of the material; MP - melting point material; Df - aperture laser radiation in the plane.

The disadvantage of this method is the limited class of materials and the duration of the technological process, for samples with a thickness of 2 mm, the exposure time is more than 5 minutes

To overcome the limitations in surface nanostructures, inherent in the classical method of laser doping and ion implantation, and to eliminate the disadvantages inherent in high-speed laser sintering of nanosized powder, will allow the application of the method of laser-plasma treatment of the surface of the material.

Unlike other types of discharge (electric arc, high frequency, microwave) laser plasma optical discharge has a record unattainable for other discharges) values of power density, nested in the category (up to 1000 kW/cm²), which provides a high temperature plasma (up to 27000 K), electron density (10 cm^-3), due to this laser plasma optical stationary discharge is a powerful source ion (Kozlov GI Continuous optical discharge - laser-plasma source of ions and radiation. New Russian developments in laser science, engineering and technology. The collection of scientific and practical articles. Issue 1. Ed. Corr.-Corr. RAS Panchenko VA - Kaluga: in ACF "Polygon", 2005, p.45-51).

Application of laser-plasma technology allows you to:

1. To get the temperature in the center of the surface laser plasma optical discharge, which is several hundred microns from the treated surface, reaching 27000 To that provides high-speed heating of the surface layer and its high-speed processing.

2. Easy to control the chemical composition of the laser plasma, which opens up great potential to vary in a wide range of chemical composition of the surface layer.

3. Laser plasma will perform an essential function, as high speed modifier liquid phase of the melt processed surface.

4. Manage the energy pumped surface laser plasma, which will atomize almost the Eski all chemical elements.

5. To overcome thermodynamic and thermo-physical constraints at the nanostructuring of the surface layers by creating a high degree of nonequilibrium of the formed alloys in the surface layers.

6. Changing the position of the focal length of the optical system relative to the workpiece, it is quite easy to control the geometric position of the energy center of the plasma relative to the workpiece and thereby changing the depth of the structured layer.

The invention

The objective of the invention is the creation of nanostructured surfaces on critical areas of parts made of steel, which will increase the hardness of the working surface, and therefore increase the life of parts.

This object is achieved in that in a method of producing nanostructured surface of the steel by the method of laser-plasma processes, including the effects of roaming laser beam on the workpiece surface, the influence of the laser beam on the workpiece surface is carried out in a sealed chamber filled with an inert gas such as argon, and a gas-modifier, such as nitrogen, while the laser power and the diameter of the laser spot is chosen so that the power density was above a threshold power density, no is required for the formation of plasma optical discharge in metal vapour, and the movement of the spot of the laser beam on the surface of the parts are with the speed of ensuring stable combustion surface plasma optical discharge.

Moreover, the influence of the laser beam on the workpiece surface is performed with the power density of the laser spot on the workpiece surface, is equal to (10⁶-10⁷) W/cm²and speed 0.1-2 m/s when the pressure of the gas in the chamber is equal to 1.5 to 2 ATM.

This embodiment of the method allows to obtain an abnormally high increase in the hardness of the processed material over the entire treated surface.

List of figures in the drawings.

The invention is illustrated by figure 1, which presents the scheme of laser-plasma technology nanostructuring showing:

1 - laser beam;

2 - focusing θ-lens;

3 - surface plasma with modifier;

4 - focus plane;

5 - liquid phase of the molten metal;

6 structured surface;

7 - the workpiece;

8, 11 - swivel mirror three-axis scanner;

9 - sealed Luggage;

10 - protective environment with gas-modifier;

12 - zoom;

F is the focal length;

ΔF is the amount of defocusing;

D_n- diameter surface of a laser plasma.

The implementation of the invention.

Sposobnosti performed as follows.

1. A focused laser beam serves on the surface of a workpiece, which is located in a sealed chamber with a protective and modifying gas.

It uses a beam of ytterbium fiber laser in a continuous mode.

To protect the treated surface from oxidation process is carried out in a sealed chamber filled with an inert gas AG or helium to a pressure of 1.5 ATA and gas-modifier, such as butane, bringing the total pressure up to 2 ATM.

For alloying of any item, such as nitrogen, into the chamber sleuth N2.

In our case, as the alloying element is a mixture of ions of carbonic gas and AG, coming to the surface of the liquid phase of the metal from the surface of the laser plasma and saturating the surface layer part in the diffusion and convective mixing.

2. The laser beam focus on the structured surface (the surface of a workpiece in the form of a spot of a laser beam with a diameter of 60 to 100 μm. Thus the laser power and the diameter of the laser spot is chosen so that the power density was above a threshold power density required for the formation of non-equilibrium near-surface plasma optical discharge in a metal vapour. The power density of the laser radiation is chosen equal to W=(10⁶-10⁷W/the m ²depending on the wavelength of the laser radiation (for steel, W_CR=2×10⁶W/cm²for λ=10.6 μm).

Under the influence of the focused laser beam the surface melts and metal vapour from the absorbed energy of the laser radiation is formed near-surface plasma optical discharge in pairs molten metal

3. Carry out the movement of the spot of the laser beam on the workpiece surface with the speed of ensuring stable combustion surface plasma optical discharge, but no less speed, which determines the exposure time on the processing surface for the existence of the liquid phase of the melt surface layer and diffusion of ions modifier to a depth of nanostructures, i.e. ≤100 nm. The speed range for data execution conditions ranging from 0.1 m/s to 2 m/s

4. Changing the position of the focal length of the optical system relative to the workpiece, it is quite easy to control the geometric position of the energy center of the plasma relative to the workpiece and thereby changing the depth of the molten layer that allows for speed cooling mode thermal conductivity less than the rate of cooling, leading to amorphization of the surface layer, but more speed Oh what ardenia, leading to the formation of supposed.

Thus, selecting the desired power density of laser radiation, the exposure time on the surface and the position of the focal plane, you can manage the structural-phase state of the surface layer, including forming surface of the nanostructure, which allows to obtain an abnormally high increase in the hardness of the processed material over the entire treated surface.

The nanostructuring of the surface of the steel and its alloys by laser plasma is as follows.

Ray ytterbium fiber laser LK-300 (λ=1,07 μm) 300 W power in continuous mode (position 1) emerges from the collimator (figure 1) is not specified and gets on controlled three-axis of the scanner with a rotating mirror (item 8, 11) and focusing θ lens (pos.2), then focuses on the structured surface (pos.6) of the workpiece (pos.7) in the form of a spot of a laser beam with a diameter of 80 μm, which creates a power density of laser radiation W=6×10⁶W/cm²sufficient to generate plasma optical discharge in pairs of metal (steel W=2×10⁶W/cm²). The workpiece itself (pos.7) is located in a sealed chamber (position 9) with the inert gas and gas-modifier (10) and can be moved by means of a manipulator 5-coordinate with the Anka by coordinates X, Y and Z and can rotate around the Z-axis and the y-Ray laser radiation using the zoom (pos.12) can move deeper into the surface of the workpiece to a depth of Δz=±15 mm, which allows you to change the amount of defocus ΔF and thereby to control the depth of the modified layer.

Under the influence of the focused laser beam the surface melts and metal vapour from the absorbed energy of the laser radiation is formed near-surface plasma optical discharge in pairs of molten metal, which can be moved into small sections ~300×300 mm²using a 3-axis scanner with θ lens, which focuses the radiation on a given field with a constant diameter of the focused spot 80 μm, and the workpiece can be moved by means of a manipulator 5-axis machine for two or more axes, which together allows you to handle all must be structured surface details.

To protect the treated surface from oxidation process is carried out in a sealed chamber (position 9), filled with inert gas AG or He (pos.10) and gas-modifier (butane, N₂).

As an alloying element in this case is a mixture of ions of carbonic gas and AG, coming to the surface of the liquid phase of the metal from pripoverhnostnyh the nd laser plasma and saturating the surface layer by diffusion and convective mixing, that allows you to get an abnormally high increase in the hardness of the processed material over the entire treated surface.

For alloying of any item, such as nitrogen, into the chamber additionally let N2.

Based on the threshold power density required for the formation of plasma is calculated, the laser power and the diameter of the focused spot, and choosing the speed of movement of the spot on the treated surface, we determine the time of exposure on the treated surface, i.e. the lifetime of the liquid phase of the melt surface layer, which must satisfy the condition:

t≤100 nm/V,

where V - diffusion rate modifier in the liquid phase of the molten metal.

To carry out the proposed method can be used the following device.

Five-axis machine is used as a pointing device to move the processed sample and the basis for the installation and attachment 3-axis scanner type Fokusschifter" company "Raylase" and the sealed chamber with the part. The inlet of the scanner optically coupled to the collimator ytterbium fiber laser, the radiation which is transmitted through the fiber cable and connector ends, the United QBH plug with a collimator.

Further, the radiation falls first on the TRANS is ocator three-axis scanner, who has the ability to automatically move the program along the optical axis by the value of ±15 mm, thereby buries focal plane on the workpiece by the value of AG=±15 mm, then turning mirrors that deflect under the program of the beam at the maximum angle of ±22° in mutually perpendicular planes, forming a space of size 300×300 mm²when focusing θ-lens with a focal length of F=566 mm θ-lens-3-axis scanner is used to equalize the diameter of the focused spot across the expanded field.

A focused laser beam hits the surface of a workpiece, which is located in a sealed chamber with a protective and modifying gas, and the item has the ability to move with the arm of the machine coordinates X, Y and Z, and also to rotate around the Z-axis and Y-axis, thereby enabling the nanostructuring both flat and more complex surfaces.

An example of using the proposed method

One of the areas in which you can successfully use this method, is agregatostroyeniya, which at the present time for the local hardening of parts are widely used for a long, time-consuming and not automate the processes of chemical-heat treatment and plating.

So it is important experiment in the development of technology of laser-plasma structuring were selected part type "Cam" and a spool of steel 12HN3A.

The workpiece is clamped in the clamping device, five-axis manipulator. The treatment area is protected sealed chamber. In the chamber was filled with a protective gas AG to a pressure of 1-1,5 ATM, and then modifying the gas-butane to a total gauge pressure of 2 ATM. Then switched fiber laser with a power of 300 W, the radiation of which optic cable coupled with a 3-axis scanner, comes first on the zoom, and then controlled by a program mirrors, split the laser beam in mutually perpendicular directions, forming a flat area the size of 300×300 mm². Focusing θ-lens with focal length F=566 mm (the diameter of the focused spot d=80 μm) aligns the diameter of the laser spot across the expanded field. The combination of the movements of the manipulator and an expandable platform scanner allows you to handle all be processed surface.

Focused θ lens the laser beam melts the processed metal and simultaneously melting over the target surface occurs plasma optical discharge in a metal vapour. Ions of carbon from the surface of the plasma are absorbed in the liquid phase of the metal and saturate the surface layer by diffusion and convective mixing.

Processing is performed on the developed the th program, after work the camera rethermalized and processed the item is removed from the manipulator.

1. First there was the influence of the laser beam on the workpiece surface without the creation of near-surface plasma optical discharge in a metal vapour. Initial hardness before processing the surface layer was 3700 MPa, after processing in the quench zone of the laser beam hardness was increased to 4800 MPa.

2. To significantly increase the hardness and wear resistance of the material has been injected into the treatment area of the alloying additives, which was used for the carbon input from coating the treated surface (soot thickness of ~30 μm). During this process, the maximum hardness of the machined surface was reached 8000 MPas at a concentration of alloying component, equal to 0.5%, the layer thickness of h≈0.2 mm when the width of the track is ~1 mm, and the penetration depth of up to 0.5 mm.

Histograms of the surface layer showed a reduction in the dispersion of the crystalline formations in the surface layer. But the grain size did not reach the nanoscale range (≥100 nm).

3. Laser-plasma surface treatment of steel SA was held in reflow mode. Alloying element, in our case, carbon ions, are absorbed by the surface of the liquid phase and saturate the surface layer by diffusion and convection-what about mixing with the surrounding plasma optical discharge in metallic vapor and in the camera butane gas. The diameter of the focused spot was equal to ~80 microns, with an average laser power of approximately 300 watts.

Moving the plasma centre was carried out using the scanner over an area of about 200 cm²with a speed of 6.5 m/min and depth of penetration h=0.1 mm, when ΔF=0.

The hardness of the surface layer treated in this way amounted to ~12000 MPa.

Study of the structure of the surface layer, conducted atomic force microscope model Solver PRO-M, showed the presence in the surface layer of nanostructures with different dispersion in the center and at the edges of the machined surface, the same was confirmed by the histogram of the surface layer.

Thus, abnormally high hardness of the machined surface due to the nanostructuring of the surface layer laser-plasma processing.

1. Method of forming a nanostructured surface of the steel parts of the laser-plasma treatment, including the effects on the processed surface of the workpiece with a laser beam, which is moved along the workpiece, with the formation of the vapor of the molten metal surface plasma optical discharge, characterized in that the item is placed in a sealed chamber filled with an inert gas and gas-modifier, and the influence of the laser beam on the surface of Westlaw with power density of the laser spot on the workpiece surface, equal (10⁶-10⁷) W/cm²moreover , the laser beam is moved with a speed of 0.1-2 m/s, when the pressure of the gas in the chamber is equal to 1.5 to 2 ATM.

2. The method according to claim 1, characterized in that used as the inert gas argon and the gas quality-modifier - nitrogen.