Chemical milling of aluminum sheets according to foreign sources. The method of chemical milling of parts. Key Benefits of Chemical Milling

I read about such an interesting processing method. I want to implement it on a CNC machine :)

From the book "Handbook of a process engineer in mechanical engineering" (Babichev A.P.):

Electrochemical dimensional processing is based on the phenomenon of anodic (electrochemical) dissolution of metal when current passes through an electrolyte supplied under pressure into the gap between the electrodes without direct contact between the tool and the workpiece. Therefore, another name for this method is anodic chemical treatment.

The tool electrode is the cathode during processing, and the workpiece is the anode. The electrode-tool will move progressively at a speed Vn. The electrolyte is fed into the interelectrode gap. The intensive movement of the electrolyte ensures a stable and high-performance course of the anodic dissolution process, the removal of dissolution products from the working gap and the removal of heat that occurs during processing. As the metal is removed from the workpiece-anode, the tool-cathode is fed.

The rate of anodic dissolution and processing accuracy are the higher, the smaller the interelectrode gap. However, with a decrease in the gap, the process of its regulation becomes more complicated, the resistance to electrolyte pumping increases, and breakdown may occur, causing damage to the treated surface. Due to the increase in gas filling at small gaps, the rate of anodic dissolution decreases. Should choose

gap size that achieves optimum metal removal rate and shaping accuracy.

To obtain high technological performance of ECM, it is necessary that electrolytes meet the following requirements: complete or partial elimination of side reactions that reduce the current efficiency anodic dissolution of the workpiece metal only in the processing zone, excluding dissolution of untreated surfaces, i. the presence of high localizing properties, ensuring the flow of electric current of the calculated value in all areas of the treated surface of the workpiece.

The most common electrolytes are neutral solutions of inorganic chloride salts, nitrates and sulfates of sodium and potassium. These salts are cheap and harmless to service personnel. An aqueous solution of sodium chloride (table salt) NaCl has been widely used because of its low cost and long-term performance, which is ensured by the continuous reduction of sodium chloride in the solution.

Installations for ECHO must have filters for electrolyte purification.

The roundness of the hole achieved by itself pleases. But the funnel shape is not encouraging.

I will now try to pump the electrolyte through a medical needle.

Modified April 18, 2008 by desti

Chemical milling of concrete is the treatment of a concrete surface with special chemicals in order to improve its adhesion. On concrete surfaces, after hardening, almost all pores are clogged, so paint and sealant adhere to them rather poorly. Chemical milling helps to open the pores in the concrete and prepare it for the application of any coating.

The connection of surfaces of dissimilar bodies depends on their adhesion. From Latin, adhesion is translated as sticking. Thanks to this phenomenon, it becomes possible to apply paint and varnish and galvanic coatings, welding, gluing, etc. Therefore, increasing adhesion is a very urgent problem for modern construction.

When performing concrete work, situations sometimes arise that make it impossible to pour the entire object at one time. During the subsequent pouring, a so-called cold joint occurs at the point of contact between the new and old layers of concreting.

A cold seam causes a loss of strength of the joint and a violation of its water permeability

Another problem is created by the difficulties that arise when finishing work on concrete surfaces (plastering, making self-leveling floors). Indeed, eight hours after its setting, a cement film (a layer of cement milk) is formed on the concrete surface, which prevents the adhesion of the finishing material and concrete. If the cement film is not removed, the connection will be fragile and the likelihood of delamination and destruction of the floor or plaster will increase significantly.

Various methods are used to remove cement laitance, but in recent years, chemical milling has become widespread. This method is equally effective for removing laitance from both old and fresh concrete or brick surfaces. The main purpose of chemical milling is to prepare the surface for applying various coatings on a polymer, cement or gypsum basis.

Preparing the concrete base for coating

The technology of removing cement laitance with penetrating compounds is used when performing waterproofing works, eliminating the “cold joint”, arranging self-leveling floors, as well as other processes that require high-quality adhesion of the concrete base and the applied composition.

It allows you to open the pores, microcracks and capillaries of concrete, which ensures the penetration of the chemically active composition into its pore space, the formation and growth of crystals of materials used for waterproofing concrete surfaces and other purposes.

Reasons for the formation of cement laitance

Laitance is a fragile and loose crystalline structure on a surface created with concrete. The thickness of its layer can be 20-300 microns, but this layer “lives” separately from concrete. It does not have a strong physical connection with the concrete base and at the same time prevents the penetration of any liquids into the concrete. Because of this, a dense and durable crystalline structure is not formed in the surface layer of concrete.

The main source of cement laitance is an aqueous solution of calcium hydroxide, which comes to the surface of concrete along with water. Reacting with carbon dioxide present in the air, it forms a film of calcium carbonate, which is limestone in chemical composition and does not dissolve in water.

The formation of cement milk is also facilitated by:

  • alkali metal salts, which are present in free form in the composition of cement;
  • ash waste thermal power plants, which are added to cement and release alkalis;
  • gravel, crushed stone, sand containing halogen compounds;
  • antifreeze and modifying additives used in the manufacture of concrete mixtures.

Cement milk in its composition is a mixture of carbonates, nitrates, sulfates and chlorides, soluble and insoluble in water

Soluble alkalis, when cement is combined with water, form solutions that chemically bond with cement aluminates and silicates. Upon contact with carbon dioxide, these alkalis are carbonized and form a dense cement laitance insoluble in water. Another reason for the formation of milk can be the water used to mix the cement, if its composition does not meet regulatory requirements.

Cement milk is a loose, fragile structure that fills the pore space of concrete to a certain depth. When applying any coating to concrete with a cement film on the surface, instead of the expected monolithic connection, a three-layer system “surface coating - cement laitance - concrete” is formed. The strength between the layers of this "pie" is half as much as expected.

At the same time, each of the layers works independently of the others and separately perceives mechanical loads. The weakest point in terms of strength is precisely the cement film. Obviously, with an increase in stresses, destruction will occur precisely here. The cement film is a kind of boundary at which compressive shrinkage stresses turn into tensile stresses. That is why the zone of the cold joint immediately becomes prestressed.

Concrete, as you know, works well in compression, somewhat worse in bending, and very poorly in tension. The joint zone due to tensile stresses has much lower strength and density than monolithic concrete. That's why cracks at equal stresses are formed, first of all, along cold seams.

To avoid the effect of "cold joint" and to make the concrete surface able to accept the protective layer of sealant or paint, it is necessary to remove the cement film and open the pores in the concrete. For this, various mechanical and chemical methods are used.

Methods used to remove cement laitance

mechanical cleaning

Mechanical cleaning of concrete surfaces is carried out using mechanical wire brushes, milling and grinding machines. To avoid damage to the underlying layers of the screed, dry mechanical cleaning of hardened concrete can be carried out only after it has gained a certain strength. But with a set of strength, cleaning is significantly more difficult.

The use of milling machines and driven metal brushes is justified only when concrete gains strength no more than 2-3 MPa. When the concrete becomes stronger, the cleaning efficiency will decrease markedly due to a significant increase in processing time and increased tool wear.

Disadvantages of mechanical methods for cleaning concrete from cement laitance:

  • the possibility of cleaning only after the concrete has gained the necessary strength leads to rather long technological breaks;
  • possible occurrence of internal stresses, manifested by microcracks;
  • only a layer of cement milk is removed, and the pores of the concrete do not open;
  • the formation of a large amount of dust, which requires the use of industrial vacuum cleaners;
  • high labor intensity;
  • high cost of equipment.

With mechanical cleaning of chemical milk, it is even difficult to control the quality of the work performed.

Hydro sandblasting

The use of hydrosandblasting makes it possible to remove the cement film and open the pores of concrete only in the surface layer.

The process has such disadvantages as:

  • the impossibility of cleaning before the concrete gains strength of 5 MPa;
  • the occurrence of internal stresses due to the impact of the working jet, as well as their subsequent relaxation, leading to the formation of microcracks;
  • restrictions in use in the current production and internal work;
  • high cost of equipment (high-pressure compressors, abrasive blasting systems, air filtration units).

Cleaning with air or water jet

This treatment is carried out with water or air jets under a pressure of 0.5-0.7 MPa.. This method is the simplest and allows cleaning almost immediately after concrete is poured (with its strength of 0.3 MPa). With such strength, it is possible to walk on the concrete surface, but traces will remain on it.

At the same time, concrete has a sufficiently strong structure, so there is no danger of breaking the adhesion of the mortar part and coarse aggregate. The time to achieve this strength is from 4 to 18 hours and depends on the temperature and humidity of the surrounding air, as well as on the properties of the concrete mixture.

The disadvantages of this method include:

  • impossibility of application on vertical surfaces and at negative air temperatures;
  • a cement film remains on the surface of the concrete, insoluble in water;
  • Compressor oil contained in compressed air forms an anti-adhesion film on the surface.

chemical etching

Chemical etching is done with hydrochloric acid. This cleaning process is technically unjustified and even harmful. The use of hydrochloric acid reduces the durability of concrete.

Disadvantages of chemical etching:

  • a slight increase in adhesion strength compared to the untreated surface;
  • surface destruction of not only cement milk, but also cement stone, which causes the destruction of the cold joint between new and old concrete during operation;
  • the need for additional treatment with alkali to neutralize the acid.

The use of hardening retarders

In order to increase the time interval between pouring the concrete mixture and removing the cement laitance, as well as to facilitate the cleaning procedure, various hardening retarders are used, for example, SDB (sulfite-yeast mash). The SDB solution is applied to the concrete surface with a paint sprayer.

The weakened surface layer is removed with power brushes or a high-pressure water jet.

The disadvantages of this method include:

  • the impossibility of processing immediately after pouring concrete. Depending on the air temperature, the start time of processing can be from 2 to 4 days;
  • the need for careful control of the strength of the main concrete;
  • the impossibility of using hardening retarders during concreting in the autumn-winter period.

The low technical level and inefficiency of existing methods for cleaning concrete surfaces from a cement film have led to the search for new ways to solve this problem. As a result of the research, a completely new method for removing cement laitance was developed - chemical milling.

Benefits of chemical milling

The method of chemical milling consists in the sequential treatment of the concrete surface with compounds made on the basis of complex polyfunctional acids. This method completely eliminates the use of mechanical cleaning, shot-, hydro-, sand- and hydro-sandblasting, and in some cases the need to install a plaster mesh.

Chemical milling makes it possible to effectively dissolve cement laitance, open concrete pores, and create a monolith. This method increases the adhesion strength of monolithic concrete layers by 1.5-3 times., gypsum, cement and magnesia screeds, penetrating waterproofing materials, epoxy, polyurethane, acrylate and cement self-leveling floors, tile adhesives, joint sealants, plasters, facade and interior cladding made of artificial and natural stone.

The main advantages of chemical milling:

  • dissolution and removal of cement laitance without destroying the cement stone;
  • elimination of a cold seam, which contributes to the creation of a monolith;
  • increasing the penetration depth of waterproofing materials and other coatings;
  • reducing the labor intensity of cleaning concrete from the cement film;
  • reduction in the cost of work.

Materials used for chemical milling

Concrete during chemical milling is sequentially treated with various compositions, for example, the Kristallizol Himfrez complex. This complex includes two compositions: the acid cleaner Crystallizol Cleaner and the alkaline adhesive activator Crystallizol Active. First, Crystallisol Cleaner is applied to the concrete surface, which dissolves cement laitance. and opens the pores of concrete, but at the same time does not react with the cement stone and does not violate its structure.

An hour later, when foaming stops, Crystallisol Active is applied to the concrete, which enhances adhesion. The use of this complex increases the depth of penetration of active chemicals into the concrete surface.

Chemical milling creates conditions for the organization of a monolithic connection concrete - polymer floor or concrete - waterproofing layer

Advantages of the compositions of Kristallizol Himfrez:

  • compounds are harmless to nature and humans. They meet all environmental safety requirements;
  • do not have a strong smell, so it is convenient and easy to work with them;
  • the chemical composition does not contain acetic, hydrochloric, orthophosphoric, citric acid and other elements that adversely affect the concrete surface;
  • the complex can be used in the construction of any facilities, including food industry enterprises, swimming pools, drinking water tanks.

Such materials as Lepta Himfrez, Gambit Frez (N-1) Complex, Elakor-MBZ, ArmMix Cleaner, Deoxyl STOP, Tiprom Plus have similar properties. All these materials are manufactured according to the same principle and have an identical physical principle of operation. The chemicals contained in their composition destroy the cement film and open the pores of concrete.. The use of these materials increases the adhesion strength of concrete with self-leveling floors and other coatings by 1.5-3 times.

Technology for applying composition for chemical milling

As an example, consider the technology of applying the Elakor-MBZ composition. This composition is used to remove the cement film, the top contaminated or weakened layer of cement-containing surfaces indoors or outdoors.

General requirements and recommendations:

  • base: concrete surfaces, sand-cement screeds;
  • the humidity of the base should be no more than 6%;
  • air and ground temperature not less than +5 degrees;
  • relative air humidity - not standardized;
  • the curing time of concrete after pouring before processing is at least 14 days.

Stages of the technological process:

  • ground preparation. At this stage, dust, dirt, old paint, oil stains, etc. are removed;
  • preparation of material for work. As a rule, Elakor-MBZ is sold ready for application, but it is also available as a concentrate, which must be diluted with water in a ratio of 1:3. The consumption of the finished composition is 0.4-0.5 liters per square meter;
  • drawing. The composition is evenly applied to the treated surface. This can be done with a roller, brush, pneumatic spray gun or shotcrete method. Shotcrete is the application of solutions using a special shotcrete installation. This installation delivers the solution at a speed of 90-100 m/s. The air pressure in this case is 150-350 kPa;
  • exposure of the composition on the concrete surface until complete dissolution of cement milk and efflorescence;
  • removal of reaction residues with the help of water;
  • exposure before applying plasters, mineral screeds, sealants, tile adhesives, self-leveling floors should be at least one hour.

Safety at work

All chemical concrete milling compounds must be handled with great care. If it comes into contact with the skin, they can leave a rather painful chemical burn.. You should especially be careful not to get these compounds on the face or eyes, as this can lead to disfiguring scars and even permanent blindness.

In case of contact with the skin or eyes, rinse them with plenty of water.

Always wear protective clothing with long sleeves, closed shoes, goggles with a mask and gloves when working with formulations. Avoid breathing chemical vapors as they can cause burns to the throat or mouth.

For the same reason, you should make sure that the workplace is well ventilated. If the fumes are very strong, use a respirator with an acid fumes cartridge to avoid injury. Before using any composition, it is recommended to carefully study the instructions, which are usually indicated on the labels.

The cost of materials for chemical milling of concrete

Estimated cost of compositions used for chemical treatment of concrete surfaces:

Name Purpose Cost, rub/liter
Himfrez Complex Two in one. Removes laitance, opens pores and activates the surface adhesively. 180
Himfrez Cleaner Dissolves cement laitance, opens concrete pores. 140
Himfrez Activator Increases adhesion (adhesion strength) of concrete with cement and polymer coatings. 140
Scraper Gentle cleaning of concrete surfaces from cement deposits and efflorescence. 120
ArmMix Cleaner Cleaning concrete from efflorescence, plaque, cement film, etc., as well as improving adhesion. 65
Deoxyl STOP Cleaning surfaces made of concrete, metal and other materials. The agent bleeds ceramics, enamel, concrete, glass, salt deposits, etc. Safe for tungsten, titanium, chemically resistant plastics. 95
Tiprom Plus Cleaning of facades made of bricks, artificial and natural stone from salt deposits, cement mortar residues, atmospheric pollution. 90

conclusions

Chemical milling is characterized by high productivity, low labor intensity and cost-effectiveness. With it, you can very quickly and quite simply remove cement laitance, the top weakened or contaminated layer of the cement-containing coating from the concrete surface. Experts say that chemical milling is the most effective way to clean concrete from a cement film.

When using mechanical methods for cleaning concrete, care must be taken not to polish the pores of the material with settling dust. Because of this, the surface can become very smooth, and this significantly reduces its adhesion. Chemical milling compounds are high performance, low consumption mortars that are ideal for roughening smooth concrete. They open the pores of concrete and increase its adhesion by 1.5-3 times. Besides, chemical milling is a less time-consuming procedure compared to mechanical milling.

Chemical treatment of concrete is used to eliminate the effect of a "cold joint", to activate the action of dedusting compounds and waterproofing materials of penetrating action, to create a monolithic connection between a concrete base and a self-leveling floor. Such processing has practically no restrictions. It can be used to remove the cement film from both old and fresh grout. with porous and dense, wet and dry concrete surfaces, both indoors and outdoors.

More information about chemical milling of concrete is shown in the video:

The essence of the process of chemical milling is the controlled removal of material from the surface of the workpiece by dissolving it in the etchant due to a chemical reaction. Sections of the workpiece that are not subject to dissolution are covered with a protective layer of chemically resistant material.

The removal rate of many materials is up to 0.1 mm/min.

Process Benefits:

high productivity and quality of processing,

· the possibility of obtaining parts of complex configuration, both small and large thickness (0.1-50) mm;

low energy costs (mainly chemical energy is used);

short cycle of preparation of production and simplicity of its automation;

· non-waste due to the regeneration of the process products.

During processing, material removal can be carried out from the entire surface of the workpiece, to various depths or to the entire thickness of the part (through milling). Chemical milling includes the following main stages: preparation of the workpiece surface; applying a protective layer of the picture; chemical etching; removal of the protective layer and quality control of products (see fig. 3.1).

Surface preparation is cleaning it from organic and inorganic substances, for example, using electrochemical degreasing. The degree of purification is determined by the requirements for subsequent operations.

The application of the protective layer of the picture is carried out by the following methods: manual and mechanized engraving on the overcast (lacquer, wax) layer, xerography, screen, offset, and photochemical printing.

In instrumentation, the most widely used method is photochemical printing, which provides small sizes of products and high accuracy. In this case, to obtain a protective layer of a given configuration, a photomask is used (an enlarged photocopy of the part on a transparent material). As a protective layer, liquid and film photoresists with photosensitivity are used. Liquid, the most mastered in the industry, require high quality cleaning of the surface of the workpieces. To apply them to the surface, one of the methods is used: immersion, watering, spraying, centrifugation, rolling, spraying in an electrostatic field. The choice of method depends on the type of production (continuous application or on individual blanks); requirements for the thickness and uniformity of the formed film, which determine the accuracy of the dimensions of the pattern and the protective properties of the resist.



Rice. 3.1. General scheme of the technological process of chemical milling.

Photochemical printing of a protective pattern, in addition to the operation of applying a photoresist and drying it, includes the operations of exposing the photoresist layer through a photomask, developing the pattern, and tanning the protective layer. During development, certain areas of the photoresist layer dissolve and are removed from the surface of the workpiece. The remaining photoresist layer in the form of a pattern defined by a photomask, after additional heat treatment - tanning - serves as a protective layer during the subsequent chemical etching operation.

The chemical pickling operation determines the final quality and yield of the product. The etching process proceeds not only perpendicular to the surface of the workpiece, but also sideways (under the protective layer), which reduces the accuracy of processing. The amount of etching is estimated through the etching factor, which is equal to , where H tr is the depth of etching, e is the amount of etching. The dissolution rate is determined by the properties of the treated metal, the composition of the etching solution, its temperature, the method of supplying the solution to the surface, the conditions for removing the reaction products, and maintaining the etching properties of the solution. Timely cessation of the dissolution reaction ensures the specified accuracy of processing, which is approximately 10% of the depth of processing (etching).

Currently, etchants based on salts with an amine, an oxidizing agent, are widely used, among which chlorine, oxygen compounds of chlorine, bichromate, sulfate, nitrate, hydrogen peroxide, and fluorine are most often used. For copper and its alloys, covar, steel and other alloys, solutions of ferric chloride (FeCl 3) with a concentration of 28 to 40% (weight) and a temperature in the range of (20 - 50) C, which provide a dissolution rate of (20 - 50) µm/min.

Among the known etching methods, there are immersion of the workpiece in a calm solution; in a stirred solution; spraying solution; solution spraying; jet etching (horizontal or vertical). The best processing accuracy is provided by jet etching, which consists in the fact that the etching solution is supplied under pressure through nozzles to the surface of the workpiece in the form of jets.

Quality control of parts includes visual inspection of their surface and measurement of individual elements.

The process of chemical milling is most beneficial in the manufacture of flat parts of complex configuration, which in some cases can also be obtained by mechanical stamping. Practice has established that when processing batches of parts up to 100 thousand, chemical milling is more profitable, and more than 100 thousand - stamping. With a very complex configuration of parts, when it is impossible to manufacture a stamp, only chemical milling is used. It should be taken into account that the process of chemical milling does not allow the production of parts with sharp or right angles. The radius of rounding of the inner corner must be at least half the thickness of the workpiece S, and the outer corner - more than 1/3 S, the diameter of the holes and the width of the grooves of the parts must be more than 2 S.

The method has found wide application in electronics, radio engineering, electrical engineering and other industries in the production of printed circuit boards, integrated circuits, in the manufacture of various flat parts with a complex configuration (flat springs, raster masks for kinescopes of color TVs, masks with a pattern of circuits used in thermal spraying processes , nets for razors, centrifuges and other parts).

Concrete milling is actually the removal of a layer of concrete to a predetermined depth.

Application area:

Concrete milling is a rather aggressive action and is used in the following cases:

1. leveling the concrete surface;
2. removal of the top layer of a monolithic slab, foundation or floor located above the required level;
3. removal of the old polymer coating, or laid in violation of technological requirements;
4. removal from concrete of linoleum or tiles laid on glue;
5. cleaning the contaminated surface from stains and various types of glue;
6. on pedestrian areas, the device of anti-slip zones. Treatment of the road surface in order to improve the grip of the surface with the tires of vehicles on ramps and ramps;
7. as a preparatory procedure before applying various materials to concrete that increase interlayer adhesion.

Milling the concrete floor allows you to get a more even surface with increased adhesive properties. In addition, the use of this technology can eliminate the stage of creating an additional screed in the installation of concrete floors.

Types of milling:

1. mechanical, with the help of special equipment equipped with diamond-coated cutting tools. Processing is carried out by a milling element "drum". The principle of operation of the milling unit is to rotate the drum. Under the influence of torque, the cutter is ejected, as a result of which it hits the work surface. The higher the impact force, the greater the layer is removed from the surface.

The advantage of mechanical cleaning methods is their use where it is impossible to use dusty and wet and expensive processes of sandblasting and hydrosandblasting.
Effective notching of the surface, increasing the area of ​​stress transfer. However, the use of impact tools (perforators, jackhammers) for removing the film and subsequent notching should be excluded, due to possible damage to the top layer of concrete of the joint surface.

The disadvantages of mechanical methods of concrete surface preparation include the following:
- the possibility of cleaning only after the concrete has gained strength of 1.5 MPa leads to long technological breaks;
- only the top layer of the cement film is removed and the pores of the concrete do not open;
- possible occurrence of microcracks;
- dust formation requires cleaning with an industrial vacuum cleaner;
-high cost of equipment and labor intensity;
- the complexity of the organization of quality control of work.

2. The method of chemical milling is based on sequential brushing, roller or spray treatment of the concrete surface with water-based compounds made from complex polyfunctional acids and bases (without the use of polymers). At the same time, the compositions do not contain hydrochloric, acetic, citric, orthophosphoric acids and substances that destroy concrete.

Chemical milling completely eliminates the use of manual mechanical cleaning, including in places that are not accessible to the mechanical milling method. This method effectively dissolves the cement film, opens the pores and increases by 1.5-3 times the adhesion strength of layers of monolithic concrete, cement, gypsum and magnesia screeds, waterproofing cement materials of penetrating action, cement, epoxy, polyurethane and acrylate self-leveling floors, as well as suture sealants, plasters, tile adhesives, internal and facade cladding made of natural and artificial stone.

Compositions for chemical milling are odorless, do not have a harmful effect on humans and the environment.

The Lepta Himfrez composition based on inorganic acids is used for chemical milling, cleaning of efflorescence (white spots on the facade), cement mortar residues, cement laitance and atmospheric pollution for concrete and brick surfaces before applying penetrating waterproofing, plaster, paint.

Advantages:
1. Increases the depth of penetration of chemically active particles of waterproofing materials.
2. Cleans the surface from efflorescence.
3. Removes the cement film without damaging the concrete.
4. Increases the adhesion of old concrete to new.
5. Eliminates the need for mechanical cleaning of concrete.
6. Does not change the color and appearance of the surface.
7. Odorless.
8. Safe for people.

Chemical methods of processing materials are called, in which the removal of a layer of material occurs due to chemical reactions in the processing zone. Advantages of chemical processing methods: a) high productivity, provided by relatively high reaction rates, primarily the lack of dependence of productivity on the size of the treated surface area and its shape; b) the possibility of processing especially hard or viscous materials; c) extremely low mechanical and thermal effects during processing, which makes it possible to process parts of low rigidity with a sufficiently high accuracy and surface quality.

Dimensional deep etching (chemical milling) is the most common chemical processing method. It is advisable to use this method for processing surfaces of complex shapes on thin-walled parts, obtaining tubular parts or sheets with a smooth change in thickness along the length, as well as when processing a significant number of small parts or round blanks with large; the number of processed places (perforation of cylindrical surfaces of pipes). By local removal by this method from excess material in unloaded or lightly loaded aircraft and missiles, the overall weight can be reduced without reducing their strength and rigidity. In the United States, the use of chemical milling has reduced the weight of a supersonic bomber wing by 270 kg. This method allows you to create new structural elements, such as sheets 1 of variable thickness. Chemical milling is also used in the manufacture of printed circuits for electronic equipment. In this case, the sections specified by the scheme are removed from the panel of insulating material, covered on one or both sides with copper foil, by etching.

The technological process of chemical milling consists of the following operations.

1. Preparation of parts for chemical milling to ensure subsequent tight and reliable adhesion of the protective coating to the surface of the part. For aluminum alloys, this preparation is carried out by: degreasing in B70 gasoline; light pickling in a bath with caustic soda 45-55 g/l and sodium fluoride 45-55 g/l at a temperature of 60-70 ° C for 10-15 minutes to remove the clad layer; washing in warm and cold water and clarification in nitric acid, followed by washing and drying. For stainless and titanium alloys, parts are prepared by pickling to remove scale in a bath with hydrofluoric (50-60 g/l) and nitric (150-160 g/l) acids or in a bath with electric heating up to 450-460 ° C in caustic soda and sodium nitrate (20%) followed by washing and drying, degreasing and light pickling followed by repeated washing and drying.

2. Application of protective coatings to the places of the workpiece that are not subject to etching. It is produced by installing special overlays, chemically resistant adhesive-type templates or, most often, by applying paint coatings, which are usually used as perchlorovinyl varnishes and enamels, polyamide varnishes and materials based on non-prene rubbers. So, for aluminum alloys, PKhV510V enamel, RS1 solvent TU MHP184852 and KhV16 enamel TU MHPK-51257, R5 TU MHP219150 solvent are recommended, for titanium alloys - AK20 glue, RVD thinner. For better adhesion of these coatings to metal, anodizing of the surface is sometimes preliminarily performed. The application of paint and varnish coatings is carried out with brushes or spray guns with preliminary protection of the places of etching with templates or by immersion in a bath; in the latter case, the contour is marked on the dried protective film, then it is cut and removed.

3. Chemical dissolution is carried out in baths in compliance with the temperature regime. Chemical milling of aluminum and magnesium alloys is carried out in solutions of caustic alkalis; steels, titanium, special heat-resistant and stainless alloys - in solutions of strong mineral acids.

4. Cleaning after etching of parts made of aluminum alloys with an enamel protective coating is carried out by washing in running water at a temperature of 50 + 70 ° C, soaking the protective coating in hotter running water at a temperature of

70-90 ° С and subsequent removal of the protective coating with knives manually or with soft brushes in a solution of ethyl acetate with gasoline (2: 1). Then produce clarification or light etching and drying.

The quality of the surface after chemical milling is determined by the initial surface roughness of the workpiece and the etching modes; usually it is 1-2 classes lower than the cleanliness of the original surface. After etching, all defects previously present on the workpiece. (risks, scratches, irregularities) retain their depth, but broaden, acquiring greater smoothness; the greater the depth of etching, the more pronounced these changes. The quality of the surface is also affected by the method of obtaining blanks and their heat treatment; rolled material gives a better surface than stamped or pressed material. Large surface roughness with pronounced irregularities is obtained on cast billets.

The surface roughness is affected by the structure of the material, grain size and orientation. Hardened aluminum sheets subjected to aging have a higher surface finish class. If the structure is coarse-grained (for example, the metal is annealed), then the finished surface will be with large roughness, uneven, bumpy. The fine-grained structure should be considered the most suitable for chemical processing. Carbon steel blanks are best treated by chemical milling before hardening, since in the case of hydrogenation during pickling, subsequent heating helps to remove hydrogen. However, it is desirable to harden thin-walled steel parts before chemical treatment, since subsequent heat treatment can cause them to deform. The surface treated by chemical milling is always somewhat loosened due to pickling, and therefore this method significantly reduces the fatigue characteristics of the part. Given this, for parts operating under cyclic loads, it is necessary to carry out polishing after chemical milling.

Chemical milling accuracy ±0.05 mm po. depth and not less than +0.08 mm along the contour; the radius of curvature of the cutout wall is equal to the depth. Chemical milling is usually performed to a depth of 4-6 mm and less often up to 12 mm; with a larger milling depth, the surface quality and machining accuracy deteriorate sharply. The minimum final thickness of the sheet after etching can be 0.05 mm, therefore, chemical milling can process parts with very thin bridges without warping, and perform conical processing by gradually immersing the part in the solution. If it is necessary to pickle from two sides, you must either position the workpiece vertically so as to allow the released gas to freely rise from the surface, or pickle in two steps - 1 first on one side and then on the other. The second method is preferable, since with a vertical arrangement of the workpiece, the upper edges of the cutouts are processed worse due to gas bubbles entering there. In the manufacture of deep cuts, special measures (for example, vibrations) should be used to remove gas from the machined surface, which prevents the normal process from being carried out. Depth control, etching during processing is carried out by immersion Simultaneously with the preparation of control samples, direct control of dimensions by thickness gauges such as an indicator bracket or electronic ones, as well as by automatic weight control.

The productivity of chemical milling is determined by the rate of material removal in depth. The rate of etching increases with an increase in the temperature of the solution by about 50-60% for every 10 ° C, and also depends on the type of solution, its concentration and purity. Mixing of the solution during the pickling process can be done with compressed air. The etching process is determined by an exothermic reaction, so the supply of compressed air cools it somewhat, but basically the temperature constancy is ensured by placing water coils in the bath.

Etching by immersion has a number of disadvantages - the use of manual labor, partial breakdown of protective films on untreated surfaces. When processing a number of parts, the jet etching method is more promising, in which alkali is supplied by nozzles.

A means of increasing the productivity of chemical milling is the use of ultrasonic vibrations with a frequency of 15-40 kHz; in this case, the processing productivity increases by 1.5-2.5 times - up to 10 mm/h. The process of chemical treatment is also greatly accelerated by the influence of infrared radiation of directional action. Under these conditions, there is no need to apply protective coatings, since the metal is subjected to strong heating along a given heating circuit, the remaining areas, being cold, practically do not dissolve.

The etching time is set empirically on control samples. The pickled workpieces are removed from the pickling machine, washed in cold water, and to remove the emulsion, paint and BF4 glue, they are treated at a temperature of 60-80 ° C in a solution containing 200 g/l of caustic soda. The finished parts are thoroughly washed and dried in a stream of air.

Improving the conditions for roughing workpieces by cutting by first removing the crust by etching is another example of the dissolving action of a reagent. Before pickling, the workpieces are blown with sand to remove scale. Etching of titanium alloys is carried out in a reagent consisting of 16% nitric and 5% hydrofluoric acids and 79% water. According to foreign literature, for this purpose, etching in salt baths is used, followed by washing in water and then repeated etching in acid etchants for final surface cleaning.

The chemical impact of the technological environment is also used to improve conventional cutting processes; methods of processing materials based on a combination of chemical and mechanical effects are increasingly being used. Examples of already mastered methods are the chemical-mechanical method of grinding hard alloys, chemical polishing, etc.

 

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