RU2820090C1 - Method for dry electrolytic polishing of aluminum alloy part - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to the technology of electrolytic polishing of the surface of parts from aluminum and its alloys and can be used for treatment of surfaces of parts of turbomachines to improve their operational characteristics. Method includes preliminary preparation of part surface for electrolytic polishing, immersion of part in conductive medium of ion exchangers granules impregnated with electrolyte solution, providing electrical conductivity of ion exchangers granules and ion entrainment of metal from the surface of the part with removal of micro protrusions when supplying an electric potential of an opposite sign to a part and a conducting medium through an external electrode introduced into it, preliminary preparation of the surface of the part for electro-polishing is carried out by zincate treatment, and electro-polishing is carried out until complete removal of the film formed as a result of zincate treatment.

EFFECT: higher quality and reliability of metal part surface treatment by increased surface treatment homogeneity, decreased probability of defects.

1 cl, 1 ex

Description

The invention relates to the technology of electropolishing the surface of parts made of aluminum and its alloys and can be used to treat the surfaces of turbomachinery parts to improve their performance characteristics.

As the surface roughness of critical parts operating under significant mechanical loads, such as centrifugal wheels, compressor and expander blades, increases, their performance characteristics sharply decrease. The quality of surface treatment of the blade significantly affects their strength characteristics, for example, increasing the class of surface cleanliness helps to increase the endurance limit and static strength of the blades (V.F. Makarov, E.N. Bychina, A.O. Chuyan. Mathematical modeling of the polishing process blades of gas turbine engines // Aerospace technology and technology. No. 8 (85), 2011, p. Increased surface roughness of gas turbine blades leads to a deterioration in the gas-dynamic stability of a gas turbine engine (GTE), to an increase in aerodynamic losses leading to a decrease in efficiency, a loss of power, an increase in specific costs and a decrease in the efficiency of the engine or gas turbine unit.

At the same time, the production and repair of parts of gas turbine engines (GTE) and installations (GTU), due to high requirements for surface quality (Ra≤0.32...0.16 microns), is characterized by significant labor intensity in their finishing. This causes problems when machining the surfaces of turbomachinery parts. In this regard, the development of methods for obtaining high-quality surfaces of turbomachinery parts is a very urgent task.

There is a known method of polishing the surface of a part with a circle, in which the parts exhibit reciprocating movement relative to the tool (A.S. USSR No. 1732604. IPC B24B 19/14. Method of polishing the blades of gas turbine engines with a flap circle. Publ. Bull. No. 1, 2014. ), in which polishing is carried out with deformation of the petal circle.

However, the use of mechanical action in known methods for polishing the surface of a part causes a deterioration in the quality parameters of the surface layer of materials, which leads to a decrease in its performance characteristics, especially in cases of processing such parts as turbine blades with thin feathers.

The most promising methods for processing parts of complex shapes, in particular turbomachinery blades, are electrochemical methods of surface polishing [Grilikhes S.Ya. Electrochemical and chemical polishing: Theory and practice. Influence on the properties of metals. L., Mechanical Engineering, 1987], while the greatest interest for the area under consideration is the methods of electrolytic plasma polishing (EPP) of parts [Patent of the GDR (DD) No. 238074 (A1), IPC C25F 3/16, publ. 08/06/1986].

There is also a known method for polishing metal surfaces, including anodic treatment in an electrolyte [RB Patent No. 1132, IPC C25F 3/16, publ. 1996, BI No. 3], as well as a method of electrochemical polishing [US Patent No. 5028304, IPC B23H 3/08, C25F 3/16, C25F 5/00, publ. 07/02/1991].

However, known electropolishing methods do not allow uniform surface treatment of a metal alloy part, especially parts of complex shapes.

The closest technical solution, chosen as a prototype, is the method of dry electropolishing of a part, which includes preliminary preparation of the surface of the part for electropolishing, immersion of the part in a conductive medium of ion exchange granules impregnated with an electrolyte solution, ensuring the electrical conductivity of the ion exchange granules and ion ablation of metal from the surface of the part. with the removal of microprotrusions when applying an electrical potential of the opposite sign to the part and the conducting medium through an external electrode inserted into it [WO2017186992 - Method for smoothing and polishing metals via ion transport by means of free solid bodies, and solid bodies for carrying out said method. Publ. 02.11.2017].

However, the known prototype method has low reliability and cannot be used for surface treatment of critical parts made of aluminum and aluminum alloys, since the presence of an electrically insulating oxide film on their surface prevents the occurrence of electrochemical processes of ion ablation of material from the surface of the workpiece.

The problem to be solved by the claimed invention is the creation of a method that improves the quality and reliability of processing parts made of aluminum alloys, especially critical parts of complex shape, such as turbomachinery blades.

The technical result of the invention is to improve the quality and reliability of surface treatment of a metal part by increasing the uniformity of its surface treatment and reducing the likelihood of defects.

The technical result is achieved due to the fact that in the method of dry electropolishing of an aluminum alloy part, which includes preliminary preparation of the surface of the part for electropolishing, immersion of the part in a conductive medium of ion exchange granules impregnated with an electrolyte solution, ensuring the electrical conductivity of the ion exchange granules and ion ablation of metal from the surface parts with the removal of microprotrusions when an electrical potential of opposite sign is applied to the part and the conducting medium through an external electrode inserted into it, unlike the prototype, preliminary preparation of the surface of the part for electropolishing is carried out by zincate treatment, and electropolishing is carried out until the film formed as a result of zincate is completely removed processing.

In addition, the following additional methods of performing the method are possible: an aqueous solution of NaCl with a concentration of 8% to 11% or an aqueous solution of alkali pH 11-14 is used as an electrolyte for impregnation of ion exchanger granules; zincate treatment is carried out in an aqueous solution with a concentration of 50 g/l ZnO and 262.5 g/l NaOH at 60-70°C for a period of time from 1.5 minutes to 3 minutes; NaOH or KOH is used as an alkali; the material used for ion exchange granules is ion exchange resins obtained from the copolymerization of either polystyrene or polyacrylate and divinylbenzene, and the sizes of the ion exchange granules are selected from the range from 0.1 mm to 0.8 mm , and the ion exchange granules are additionally set into vibrational motion ; electropolishing with ion exchanger granules is carried out either by applying a positive electric potential to the part and a negative electrical potential to the ion exchanger granules from 12 to 35 V, or in a pulsed mode with a change in polarity with a pulse frequency range from 20 to 100 Hz, a pulse period from 50 to 10 μs, with a current amplitude of positive polarity during a pulse of +50 A and their duration from 0.4 to 0.8 μs, with a current amplitude of negative polarity during a pulse of -20 A and their duration from 0.2 to 0.4 μs, with a rectangular the shape of the output current pulses and the duration of pauses between pulses from 49.6 to 9.2 μs.

The inventive method for dry electropolishing of an aluminum alloy part, in particular the surface of a blade blade during its polishing, is carried out as follows.

First, preliminary preparation of the surface of the part is carried out for electropolishing using the zincate treatment method.

It is more expedient to carry out the zincate treatment process using a double treatment method with preliminary cleaning of the surface of the part: cleaning in concentrated nitric acid (50-58%) for 10-12 seconds, followed by rinsing in distilled water. Then the workpiece is placed in a bath with a solution for zincate treatment (for example, zinc oxide with a concentration of 15 g/l and sodium hydroxide - 100 g/l). The first zincate treatment is for 15-20 seconds, then the second stage of zincate treatment is for 2-3 minutes, followed by rinsing in water (running water for 4-7 minutes, at a temperature of 40-50°C).

Then the parts are immersed in a conducting medium of ion exchanger granules impregnated with an electrolyte solution (for example, an electrolyte in the form of an NaOH alkali solution, providing a pH of 11-14). As a result of zincate processing, the oxide film is removed from the surface of the workpiece and an electrically conductive metal film containing zinc is formed.

The electrical conductivity of ion exchanger granules should ensure ion ablation of metal from the surface of the part with the removal of microprotrusions when an electrical potential of the opposite sign is applied to the workpiece and the conductive medium through an external electrode inserted into it. In this case, electrical contact must be ensured between the entire polished surface of the part and the ion exchange granules, and between the ion exchange granules and with each other and with the external electrode. In this case, intensive movement of ion exchange granules occurs relative to each other, as well as relative to the workpiece. This creates favorable conditions for the occurrence of electrochemical mass transfer processes, which ensures uniform ion entrainment of material from the workpiece. The latter circumstance leads to an increase in the quality and productivity of electropolishing.

To increase the uniformity of processing in the case of processing parts of complex shapes, it is also possible to additionally influence the “electrode-granules-workpiece” system with vibration and/or cause the part to rotate. The vibrational movement of the part relative to the ion exchange granules can be ensured by the reciprocating movement of the part along its longitudinal axis, for example, with a frequency from 30 to 200 Hz, an amplitude from 0.1 to 2 mm.

If it is necessary to increase processing productivity, you can use an electropolishing installation with several external electrodes.

Electropolishing of a part is carried out through electrochemical processes (ionic entrainment of part material 1) between the part and an external electrode through ion exchange granules impregnated with an electrolyte solution, ensuring their electrical conductivity and ion ablation of metal from the surface of the part with the removal of microprotrusions from it. At the same time, the absence of an oxide film on the surface of the workpiece made of aluminum alloy, ensured by preliminary zincate treatment, promotes the occurrence of electrical processes and polishing of the workpiece surface.

Ion exchange resins obtained from the copolymerization of either polystyrene or polyacrylate and divinylbenzene are used as granules. The average sizes of anion resin granules are selected from the range from 0.1 to 0.8 mm.

Electropolishing is carried out either by applying a positive electric potential of 12 to 35 V to the external electrode, or in a pulse mode with a change in polarity with a pulse frequency range of 20 to 100 Hz, a pulse period of 50 μs to 10 μs, and an amplitude of current of positive polarity during a pulse of +50 A and their duration of 0.4 to 0.8 μs, with a current amplitude of negative polarity during a pulse of -20 A, and their duration of 0.2 to 0.4 μs, with a rectangular shape of output pulses current and duration of pauses between pulses from 49.6 μs to 9.2 μs.

When polishing, parts made of aluminum alloy D16 and AA2139 were used.

The polishing process is carried out until a given value of the surface roughness of the part is obtained.

The following studies were also carried out on polishing parts made of aluminum alloy. An unsatisfactory result (N.R.) was considered to be a result in which the polishing effect was not observed on the polished surface or an unacceptable change in the geometry of the blade blade occurred. If there were no defects on the surface of the part, the result was considered satisfactory (UR).

In all cases, the following modes of processing parts turned out to be universal.

When processing according to the conditions and modes according to the prototype method [WO2017186992], practically no polishing occurred due to the presence of an oxide film on the surface of the parts.

The ion exchange granules used are ion exchange resins obtained from the copolymerization of either polystyrene or polyacrylate and divinylbenzene. Brands of anion exchangers based on synthetic resins used in the present invention: Anion exchanger 17-8ChS, Anion exchanger Purolite A520E, Lewatit S 6328 A (based on styrene-divinylbenzene copolymer), “Lewatit M500”, “Lewatit MonoPlus MK 51”, “Lewatit MonoPlus MP 68” ", Purolite C150E, Purolite A-860 (macroporous strongly basic anion exchange resin based on acrylates), anion exchanger sulfonated styrene-divinylbenzene copolymer. The listed ion exchanger granules, impregnated with the electrolyte compositions given below, showed a positive result.

In pulse mode with polarity reversal:

- pulse frequency range from 20 to 100 Hz: 15 Hz (N.R.), 20Hz (U.R.), 40Hz (U.R.), 60Hz (U.R.), 80Hz (U.R.) , 100 Hz (U.R.), 120Hz (N.R.);

- pulse period from 50 μs to 10 μs: 60 μs (N.R.), 50 μs (U.R.), 40 μs (U.R.), 30 μs (U.R.), 20 μs (U. .R.), 10 μs (U.R.), 5 μs (N.R.);

- current amplitude of positive polarity during a pulse of +50 A and their duration 0.4 μs to 0.8 μs: 0.2 μs (N.R.), 0.4 μs (U.R.), 0.6 μs (U.R.), 0.8 μs (U.R.), 10.0 μs (N.R.);

- with a current amplitude of negative polarity during a pulse -20 A and their duration 0.2 µs to 0.4 µs, 0.1 µs (N.R.), 0.2 µs (U.R.), 0.3 μs (U.R.), 0.4 μs (U.R.), 0.5 μs (N.R.);

- with a rectangular shape of output current pulses (UR);

- and the duration of pauses between pulses from 49.6 μs to 9.2 μs - (U.R.) going beyond the range - (N.R.).

In the mode without polarity change: electropolishing was carried out by applying a positive electric potential to the part and a negative electric potential to the external electrode from 12 to 35 V: 8 V (N.R.), 12 V (U.R.), 20 V (U. R.), 30 V (U.R.), 35 V (U.R.), 45 V (N.R.).

Electrolytes used for impregnation of ion exchange granules according to the proposed method:

- first group: aqueous solution of NaCl salt (from 8% to 11% by weight) (6.0% (N.R.), 8.0% (U.R.), 11.0% (U.R.) , 13.0% (N.R.);

- second group: alkalis NaOH and KOH, providing pH 9 (NR); pH 11(U.R.); pH 11(U.R.); pH 12(U.R.); pH 14(U.R.); pH 16(H.R.).

Zincate treatment, composition:

- first option: aqueous solution with a concentration of 50 g/l ZnO and 262.5 g/l NaOH: 50°C - (N.R.), 60°C - (U.R.), 70°C - (U. R.), 80°C - (N.R.). Processing time 1.0 min (N.R.), 1.5 min (U.R.), 2.0 min (U.R.), 3.0 min (U.R.), 4.0 min (N.R.);

- second option: alkaline solution containing, g/l: trisodium phosphate - 20 g/l (N.R.), 30 g/l (U.R.), 50 g/l (U.R.), 70 g /l (U.R.), 80 g/l (N.R.); soda ash - 5 g/l (N.R.), 10 g/l (U.R.), 20 g/l (U.R.), 30 g/l (N.R.);

- third option: zinc oxide with a concentration of 15 g/l and sodium hydroxide – 100 g/l.

Processing time 1.0 min (N.R.), 1.5 min (U.R.), 2.0 min (U.R.), 3.0 min (U.R.), 4.0 min (N.R.).

In comparison with the known polishing method [WO2017186992], when processing a blade blade made of an aluminum alloy using the proposed method, the formation of defects in the form of unpolished surface areas, unacceptable changes in the geometry of the blade blade were practically not observed, while when processing using the known surface polishing method [WO2017186992] the part was practically not carried out due to the presence of an oxide film on the surface, which prevents ion entrainment of the part material.

Thus, the proposed method of dry electropolishing of an aluminum alloy part made it possible to achieve the technical result set in the invention - improving the quality and reliability of surface treatment of a metal part by increasing the uniformity of its surface treatment and reducing the likelihood of defects.

Claims (1)

A method for dry electropolishing of an aluminum alloy part, including preliminary preparation of the surface of the part for electropolishing, immersion of the part in a conductive medium of ionite granules impregnated with an electrolyte solution that ensures the electrical conductivity of the ionite granules and ion removal of metal from the surface of the part with the removal of microprotrusions when applying the opposite sign electrical potential on the part and the conducting medium through an external electrode inserted into it, characterized in that preliminary preparation of the surface of the part for electropolishing is carried out by zincate treatment, and electropolishing is carried out until the film formed as a result of zincate treatment is completely removed, and as an electrolyte for impregnation of granules Ion exchangers use an aqueous solution of NaCl with a concentration of 8 wt. % up to 11 wt. %, and zincate treatment is carried out in an aqueous solution with a concentration of 50 g/l ZnO and 262.5 g/l NaOH, at 60-70°C for a time of 1.5 min to 3 min, and ion exchanger granules are used as the material ion exchange resins obtained on the basis of copolymerization of either polystyrene or polyacrylate and divinylbenzene, and the sizes of ion exchange granules are selected from the range from 0.1 mm to 0.8 mm, while the ion exchange granules are additionally set into vibrational motion, and electropolishing with ion exchange granules carried out in a pulse mode with a change in polarity with a pulse frequency range from 20 to 100 Hz, a pulse period from 50 to 10 μs, with a current amplitude of positive polarity during the pulse of +50 A and their duration from 0.4 to 0.8 μs, with current amplitude of negative polarity during a pulse -20 A and their duration from 0.2 to 0.4 μs, with a rectangular shape of the output current pulses and the duration of pauses between pulses from 49.6 to 9.2 μs.