Chemical methods of metal processing. Chemical milling (contour etching) Threading and spiral grooving

The site outlines the basics of electroplating technology. The processes of preparation and application of electrochemical and chemical coatings, as well as methods of coating quality control are considered in detail. The main and auxiliary equipment of the electroplating shop is described. Information on the mechanization and automation of galvanic production, as well as sanitation and safety precautions is given.

The site can be used for vocational training of workers in production.

The use of protective, protective-decorative and special coatings makes it possible to solve many problems, among which an important place is occupied by the protection of metals from corrosion. Corrosion of metals, i.e., their destruction due to the electrochemical or chemical action of the environment, causes enormous damage to the national economy. Every year, as a result of corrosion, up to 10-15% of the annual output of metal in the form of valuable parts and structures, complex instruments and machines goes out of use. In some cases, corrosion leads to accidents.

Electroplated coatings are one of the effective methods of corrosion protection, they are also widely used to impart a number of valuable special properties to the surface of parts: increased hardness and wear resistance, high reflectivity, improved anti-friction properties, surface electrical conductivity, easier solderability, and, finally, simply to improve the external type of products.

Russian scientists are the creators of many important methods of electrochemical processing of metals. Thus, the creation of electroforming is the merit of Academician B. S. Jacobi (1837). The most important work in the field of electroplating belongs to the Russian scientists E. Kh. Lenz and I. M. Fedorovsky. The development of electroplating after the October Revolution is inextricably linked with the names of scientific professors N. T. Kudryavtsev, V. I. Liner, N. P. Fedotiev and many others.

Much work has been done to standardize and normalize coating processes. The sharply increasing volume of work, mechanization and automation of electroplating shops required a clear regulation of processes, careful selection of electrolytes for coating, selection of the most effective methods for preparing the surface of parts before the deposition of electroplated coatings and final operations, as well as reliable methods for quality control of products. Under these conditions, the role of a skilled electroplating worker increases sharply.

The main objective of this site is to help students of technical schools in mastering the profession of an electroplating worker who knows modern technological processes used in advanced electroplating shops.

Electrolytic chromium plating is an effective way to increase the wear resistance of rubbing parts, protect them from corrosion, as well as a method of protective and decorative finishing. Significant savings are provided by chrome plating when restoring worn parts. The process of chromium plating is widely used in the national economy. A number of research organizations, institutes, universities and machine-building enterprises are working on its improvement. More efficient electrolytes and chromium plating modes are emerging, methods are being developed to improve the mechanical properties of chrome parts, as a result of which the scope of chromium plating is expanding. Knowledge of the basics of modern chromium plating technology contributes to the fulfillment of the instructions of normative and technical documentation and the creative participation of a wide range of practitioners in the further development of chromium plating.

The site developed the issues of the influence of chromium plating on the strength of parts, expanded the use of efficient electrolytes and technological processes, introduced a new section on methods to improve the efficiency of chromium plating. The main sections have been redesigned taking into account the nporpecsivnyh advances in chrome plating technology. The given technological instructions and designs of suspension fixtures are exemplary, guiding the reader in matters of choosing chrome plating conditions and in the principles of designing suspension fixtures.

The continuous development of all branches of mechanical engineering and instrument making has led to a significant expansion of the field of application of electrolytic and chemical coatings.

By chemical deposition of metals, in combination with galvanic metal coatings are created on a wide variety of dielectrics: plastics, ceramics, ferrites, glass-ceramic and other materials. The manufacture of parts from these materials with a metallized surface ensured the introduction of new design and technical solutions, improved quality of products and cheaper production of equipment, machines, and consumer goods.

Parts made of plastics with metal coatings are widely used in the automotive industry, the radio engineering industry and other sectors of the national economy. The processes of metallization of polymeric materials have become especially important in the production of printed circuit boards, which are the basis of modern electronic devices and radio engineering products.

The brochure provides the necessary information about the processes of chemical-electrolytic metallization of dielectrics, the main regularities of the chemical deposition of metals are given. The features of electrolytic coatings during metallization of plastics are indicated. Considerable attention is paid to the technology of production of printed circuit boards, as well as methods for analyzing solutions used in metallization processes, as well as methods for their preparation and correction.

In an accessible and entertaining way, the site introduces physical nature in the features of ionizing radiation and radioactivity, the effect of various doses of radiation on living organisms, methods of protection and prevention of radiation hazard, the possibilities of using radioactive isotopes to recognize and treat human diseases.

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 and varnish 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 both sides, it is necessary either to position the workpiece vertically so as to allow the released gas to freely rise from the surface, or to 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 constancy of temperature 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 cleaning of the surface.

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.

K.: Technika, 1989. - 191 p.
ISBN 5-335-00257-3
Download(direct link) : sputnik_galvanika.djvu Previous 1 .. 8 > .. >> Next

In electrochemical milling, a coating of any acid-resistant paint applied by a stencil can serve as a protective coating. The pickling solution in this case consists of 150 g/l sodium chloride and 150 g/l nitric acid. Etching occurs at the anode at a current density of 100–150 A/dm2. Copper plates are used as the cathode. After the termination of the process, the cathodes are removed from the bath.

Electrochemical milling is more accurate than chemical milling.

PRE-TREATMENT OF ALUMINUM AND ITS ALLOYS

To ensure strong adhesion of the electrolytic coating to aluminum, an intermediate layer of zinc, iron or nickel is applied to the surface of the latter (Table 21).

CHEMICAL AND ELECTROCHEMICAL POLISHING

A smooth metal surface can be obtained by chemical or electrochemical (anodic) polishing (Tables 22, 23). The use of these processes makes it possible to replace mechanical polishing.

When aluminum is oxidized, mechanical polishing is not enough to achieve a shiny surface; after it, chemical polishing is necessary.

21. Solutions for aluminum pretreatment

Orthophosphoric acid Glacial acetic Orthophosphoric acid

280-290 15-30 1-6

Acid Orange * For:

dye 2

pinned surface

1st intermediate processing

ratu-ra. FROM

4. orthophosphorus!

Triethane! lamin

500-IfXX) 250-550 30-80

Triethanolamine Catalin BPV

850-900 100-150

Orthophs ph rthic acids Chromic thydrnd

* Products ps mining are processed by flushing in the same mine 6A / dm2

trochemical polishing When polishing precious metals by chemical or electrochemical methods, their losses are completely eliminated. Electrochemical and chemical polishing can be not only a preparatory operation before applying electroplating, but also the final stage of the technological process. It is most widely used for aluminum. Electrochemical polishing is more economical than<ими-ческое.

The current density and duration of the electropolishing process are selected depending on the shape, size and material of the products.

COATING PROCESS TECHNOLOGY

SELECTION OF ELECTROLYTES AND PROCESSING MODES

The quality of the metal coating is characterized by the structure of the precipitate, its thickness and uniformity of distribution on the surface of the product. The structure of the precipitate is influenced by the composition and pH of the solution, the hydrogen released together with the metal, the electrolysis mode - dark

polishing

M41
with SS
Density
„|§..
cathodes

From sent
carbonaceous

I-IL
15-18
1,63-1,72
12XI8H9T, over

1-5
10-100

From steel 12X18H97
H:rusty1d

From styles 12X18H9T Aluminum and 3-5 20-50 - (aluminum) stainless

0.5-5.0 20-50 1.60-1.61 From copper or evine Copper

temperature, density of the goka, the presence of swing, filtration and 1. d.

To improve the deposit structure, various organic additives (glue, gelatin, saccharin, etc.) are introduced into electrolytes, complex salts are precipitated from solutions, the temperature is increased, continuous filtration is used, etc. The released hydrogen can be absorbed by the deposit, contributing to an increase in brittleness and porosity. , and the appearance of so-called pitting points. To reduce the effect of hydrogen on the quality of the precipitate, the parts are shaken during the process, oxidizing agents are introduced, the temperature is increased, etc. The porosity of the precipitate decreases with increasing thickness.

The uniform distribution of the precipitate on the surface and delirium depends on the scattering ability of the electrolyte. Alkaline and cyanide electrolytes have the best scattering ability, acidic electrolytes are much less, and chromium electrolytes are the worst.

When choosing an electrolyte, it is necessary to take into account the configuration of the products and the requirements that apply to them. For example, when coating products of a simple shape, you can work with simple in composition electr>-

lantamn that do not require heating, ventilation, filtration; when coating products of complex shape, solutions of complex metal salts should be used; for coating internal and hard-to-reach surfaces - internal and additional anodes, filtration, mixing; to obtain a brilliant coating - electrolytes with complex brightening and leveling additives, etc.

GENERAL SCHEME OF THE TECHNOLOGICAL PROCESS

The coating process consists of a series of sequential operations - preparatory, coating and final processing. Preparatory operations include machining [parts, degreasing in organic solvents, chemical or electrochemical degreasing, etching and polishing. The final processing of coatings includes dehydration, clarification, passivation, impregnation, polishing, brushing. After each operation

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 pattern is carried out by the following methods: manual and mechanized engraving on the overcast (lacquer, wax) layer, xerography, screen printing, offset printing, 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).