equipment and technology for microarc oxidation of metals and alloys

Microarc oxidation is one of the most perspective aspects of superficial treatment (modifications gaining wide recognition in majority of industries for forming of multipurpose ceramic-like coatings.

The project is devoted to actual problem, i.e. to production of serial high-technology equipment for implementation of microarc oxidation technics in the hypersonic field, allowing constructing new generation composites on a surface of valve metals and their alloys (titanium, aluminum, zirconium etc.) of various functionality with heightened physical-mechanical and service properties.

A recent trend in the field were developments of Progress Industrial Systems SA company of Switzerland with application of the multifrequency hypersonic oscillations in electrolyte in a duty cycle of microarc oxidation of metals.

Short description of the proposed operating procedure

Implementation of the designed solution is carried out as follows: an article is immersed in a tank with electrolyte aqueous solution, two-four walls of the tank are attached with piezoelectric transducers with different resonance frequencies, other walls of the tank serve for cooling (maintaining an optimum temperature) of electrolyte solution in the course of the microarc oxidation and further, the electrodes one of which one is the article are fed with electric current that result in formation of microarcs which allow to gain composite coatings, the process of the microarc oxidation goes on at powered piezoelectric transducers operating on different frequencies (15 to 100 kHz) that allows to reshape coating with consistent pattern and permanently to supply fresh electrolyte to allowed area of microarc oxidation. The composite includes not only its own oxides of metals, but also chemical combinations from electrolyte components, ensuring production of the unique complex of physical-mechanical and service properties.

The designed technics of microarc oxidation in hypersonic fields is cost-effective and high capacity one (in comparison with existing analogues, for example, anodic oxidation), ensures high physical-mechanical and service properties of articles of various degrees of complication.

Product line for MAO

Power facilities - specialized power supply sources;
Tanks for surface preparation, treatment and washover;
positioning device for movement of a hanger with pieces (in case of batch production);
metallic structures for location of tanks and positioning device;
auxiliary equipment - a distiller, a pump-filter for water treatment and pumping-over of solutions, auxiliary tanks, quality control instruments of coatings and electrolyte state.

Pulses of a current of the determined shape move from power sources to the terminals of tanks to molding boxes, thus the piece is anode, the cathode is the tank or an additional electrodes, as a rule, of stainless steel.

Operations.

The number of indispensable operations during the MAO is significantly less, than at traditional processes of anodic oxidation. It results out of the absence of numerous preparatory operations and ecological compatibility of applied solutions. After the putting the pieces on a hanger a degreasing is carried out, then the pieces pass for treating. At large scales of production after the coating tank a trapping tank is installed for the more rational usage of chemicals and rinsing water.

Staff – production engineer (higher technical education) and workers (secondary technical education). For a section of 150-200 м2, as a rule, there are enough 2 workers for one shift. Also a periodic monitoring of a sanitary technician and electrician is needed.

Properties of MAO coatings

Structure:

MAO-coatings are ceramics of complicated structure. A coating during the microarc oxidizing is produced at the expense of oxidizing of metal surface thus reshaped oxide and hydroxide molding boxes of this metal. On the other part the coating grows at the expense of actuation in its structure of electrolyte elements. Electrolyte elements are included into the coating in the form of salts, magnesia and hydroxides of the composite complicated structure. If necessary MAO technics allow to inject in coating any necessary chemical element. The more time is required to the treatment of pieces, the the more electrolyte members are accumulated in a surface layer. The underlayer of coating which is fitting to metal-basis, consists predominantly of its oxide joints.

Diagram of variation of contents of aluminum and phosphorus on a surface of the MAO covering against time processing in phosphatic electrolyte.

Width:

Width of coatings is determined by several major factors. It is the nature of electrolyte, a material of an alloy of metal, a treatment schedule and a process time. MAO allows to obtain coatings by width from fractions to hundreds micrometers. The required width of coating depends on assigning and service conditions. For underlayer plotting under colouring, 5-10 microns there are enough for giving Dielectric properties or high passive hardness it is necessary 50-100 microns. Ornamental properties and anticorrosive properties in the atmospheric requirements ensure 20-40 microns of coating.

Porosity:

Porosity of coatings varies in the interval of 5-50 %, sizes from 0,01 to 10 microns. Constitution of holes at width of coating of more than 5-10 micrometer is complicated, derived with many derivations and closed spaces. It is impossible to obtain coatings free of holes that is stipulated by the process nature. If necessary porosity can be lowered by means of impregnation by various materials or by means of plotting of a stratum of a polymetric compound (coloring agent). Most often impregnation by fluoroplastics and plotting of the polymeric powdered is applied colors. In some cases porosity is the positive factor. By operation of coatings on deterioration in the conditions of lubrication, last goes into holes of coating and Ensures gradual invading in friction allowed band. In medicine the bioactive MAO coatings can contain medical products in their holes.

Wear resistance:

The coatings are used as abrasive resistant ones in various units and aggregates of ambulances and Dodges. At the expense of it in many cases it is possible to apply articles of aluminum - metal enough soft and complicated for traditional methods superficial treating (for example, galvanizers). The comparative were held trials of samples with the MAO coating on aluminum and a steel sample with plotted abrasive resistant stratum of chrome. The unit load at the tests constituted 0.64 N/mm2. MAO - coverings have demonstrated smaller deterioration, especially at high temperatures.

Temperature stability:

MAO coatings have an enhanced stability to thermal and thermal-cycle loadings. Coatings can work without limitations at temperatures of -40 to +60°С. At the gradual heat of a detail the MAO coating the temperature stability is limited to melting point of metal of the detail, as these values for metal certainly more low, than for ceramics. The tests have demonstrated that coatings can resist to 280 thermocycles 310-15ВЭС and to 25 thermocycles 500-15ВеС. At such tests the coated sample is heated to the set temperatures in an oven and then launched to cold water.

Roughness:

In the course of treatment the surface roughness raises with time. It is possible to ontain coatings with roughness up to the 8 class. The roughness depends on a material of an alloy, a state of its surface and a treatment schedule.

Corrosion resistance:

MAO method allows to obtain coatings, resistant in atmospheric conditions and in various corrosive medium - chemically aggressive solutions, vapors, sea water and so forth. As MAO coating are ceramics of the composite structure the corrosion stability of a material of coating is great enough. It is possible to ensure metal-basis corrosion prevention with width of coating and regulating of amount and constitution of holes. Additional protection is provided by impregnation of holes by an inert material (more often a fluoroplastic). Tests (GOST V 20.57.304-76, 20.57.304-88) have demonstrated GOST RV that the obtained coatings can be operated as non-corrodible within 15 years.

Dielectric properties:

Tension at which a puncture of coating is produced, as corrosive stability, depends on the width of coating, type and pore sizes. This magnitude can also be essentially augmented by application of the material which is filling the grains. An average voltage of coating puncture is 600 V. The coating with filling up of holes is uo to 2500 V

Adhesion:

MAO coatings have excellent adhesion with metal-ground, which provided by a transition layer on the interface. The transition layer is reshaped both inside the metal, and outside, and also has Profile with assemblage of flexures.

As a result, the adhesion of the coating with metal appears to be more than hardness of the coating and at a loading there do not appear coating breakoff on the interface metal-coating. Counted by results of -Values of adhesion calculated according to Scrach test amounts to 350 MPa.

The structure of the MAO coating

example of the complete equipment set for mao technology

Example: Production plant has substituted a spraying apparatus material of hard metal for aluminum with MAO coating and as a result has gained major passive hardness of a surface and saving of substitution of hard metal for lighter and low-cost aluminum. The monthly program amounted to 5000 details; in total they coated more than 800000 details. The implementation of the microarc oxidation techniques was restrained till now by little publicity of the method and the lack of manufacturing application of MAO industrial lines. Now this situation varies. The amount of the companies carrying out studies of MAO process was augmented.

Our customers are offered any alternative of the equipment delivery - a selective set of equipment up to "turn-key" delivery.

is offered to Customers

Basic set equipment for MAO

specialized power source with computer process control having the following features: supply voltage 380Vх10А; output voltage (adjustable) from 100 to 1000V; output current (adjustable) from 1 to 1000А; operating modes: pulsing or acoustical; monitoring and stabilizing of current and voltage, 100% recurrence of the given operating mode;

technological tank with cooling (maintaining a set temperature), the tank is equipped with a system of ultrasonic transducers working on frequencies from 15 to 100 kHz, power of ultrasonic transducers compounds from 1 to 12 kW (depending on a construction and bosh sizes), the bosh kit includes process devices for attachment of details.

ultrasonic generator: power 1 to 12 kW, frequency range 15 to 100 kHz (frequency and power are chosen depending on the tank dimensions)

QUESTIONNAIRE

# Processing parameters
1. Area to be processed mm²
2. Workpiece material (Al, Ti, Zr, Mg, ...)
3. Drawing of the workpiece (3D)
4. Treatment:
  • The whole area
  • Inside details
  • fragmentary
5. Required Properties:
  • anti-friction
  • anti-corrosive
  • wear-resistant
  • decorative
  • light reflectance
  • electrical isolation
  • thermal protection
  • bio-compatibility
  • x-ray transparency
  • other
6. Thickness cover
7. The porosity of the outer coating
8. Coverage color:
  • black
  • beige
  • red
  • blue
  • other
Comments
Technology Parameters
9. Productivity per year
10. Bipolar pulse
11. The control system of voltage, frequency, working cycle
12. Programming Workflow
13. Automation of the production line
14. Manual control
15. Technological baths:
  • degreasing
  • washing
  • microarc oxidation
  • drying
  • preparation of electrolyte
16. Provision of special equipment for the workpiece
17. Development of electrolytes
18. Embedding technology micro-arc oxidation in the existing production line
Comments

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