The first article of surface treatment of mechanical design - electroplating, anodizing, phosphating passivation, black and blue, high-frequency quenching, surface carburization and nitriding, metal coloring, sandblasting shot blasting, polishing, wire drawing, rolling!
Surface treatment is an application technology that uses mechanical, physical or chemical methods to change the surface state, chemical composition, structural structure and stress state of the workpiece or apply various covering layers to make the workpiece surface have a certain special performance different from its substrate, thereby achieving specific usage requirements.
It can make the local or entire surface of products and parts have the following functions: ① Improve wear resistance, corrosion resistance, fatigue resistance, oxidation resistance, radiation resistance and self-lubricating properties; ② Realize self-healing properties (adaptive, self-compensation and self-healing) and biocompatibility; ③ Improve heat transfer or heat insulation, conductivity or insulation, magnetic conductivity, magnetic memory or shielding, light enhancement, reflective or wave absorbing, wetting or water repellent, adhesion or non-stickness, oil absorption or dry friction, friction factor increase or decrease, vibration damping, sealing, and decorative or antique artistic.
Speaking of surface treatment, in general, any process treatment on the surface of a workpiece belongs to the scope of surface treatment. Such as surface heat treatment, polishing, electroplating, spraying, oxidation, engraving, silk printing, etc. In mechanical design, you need to choose a suitable material and match the corresponding surface treatment to meet the required process requirements. When designing, especially when drawing, it is necessary to specify the required surface treatment process in the technical requirements entries to meet the design requirements.
In terms of mechanical design components, the purpose of surface treatment is as follows:
A. Mechanical surface treatment
The shape and roughness of metal surfaces are changed by manual or automatic physical means, which is suitable for the preliminary treatment of various metal materials and the improvement of surface finish and aesthetics.
a. Sand blasting, spraying blasting
The principles of sandblasting and shot blasting are similar, but the functions and effects are different. Sandblasting mainly uses high-speed airflow to pressurize the gravel conveyed by the sand-sucking pump to the sandblasting gun, spraying the gravel or metal particles onto the surface of the workpiece, aiming to remove impurities on the surface of the workpiece. After completion, there will be pits of large and small size on the surface of the workpiece. Mainly used for cleaning and roughening.
Shot blasting is also called shot blasting. It mainly uses the high-speed rotation of the motor of the shot blasting machine to shoot metal or non-metallic projectiles to the surface of the workpiece, so as to strengthen the metal surface, improve wear resistance, fatigue resistance, and remove residual stress. Mainly used for surface reinforcement and wear-resistant treatment, more refined.
b. Polishing
Polishing mainly reduces the roughness of the workpiece surface through mechanical, chemical or electrochemical actions to obtain a bright and flat surface. This usually involves the modification processing of the workpiece surface using polishing tools and abrasive particles or other polishing media. Polishing generally can only obtain a smooth surface, and cannot improve or even maintain the original processing accuracy. According to the pre-processing conditions, the roughness value after polishing can reach Ra1.6~0.008mm. According to the implementation principle, it is mainly divided into mechanical polishing and chemical polishing.
Mechanical polishing: Use cutting and plastic deformation of the material surface to remove the polished convex parts to obtain a smooth surface. The surface of the workpiece can be micro-cut or grinded by wheel-shaped machinery, or the vibration box or other friction principle can be used to polish the surface by continuous contact with auxiliary grinding materials.
Chemical polishing: Eliminate wear marks and etching and leveling through the selective dissolution of uneven areas on the sample surface by chemical reagents.
c. Rolling pressure
Rolling is to use a hard roller or roller to press on the surface of the rotating workpiece at room temperature and move in the direction of the bus, causing the surface of the workpiece to plastically deform and harden to obtain an accurate, smooth and strengthened surface. Or the processing technology of rolling specific patterns.
d. Brushed wire
The technical processing method of forcing the metal through the mold under the action of external force, compressing the metal cross-sectional area and obtaining the required cross-sectional area shape and size is called the metal wire drawing process. The tool that changes its shape and size is called a wire drawing die. Wire drawing can be made into straight lines, messy lines, corrugated lines, threads and other types according to decorative needs.
Straight-printed wire drawing: refers to the processing of straight lines on the metal surface by mechanical friction.
Brushing of random patterns: It is a kind of matte silk pattern without any obvious patterns obtained by moving the metal plate forward, backward, left and right under a high-speed copper wire brush. This processing requires high surface requirements on the metal plate.
Corrugated wire drawing: generally made on a brushing machine or a rubbing machine. Using the axial movement of the upper group of grinding rollers, brushes are applied on the surface of the metal plate to obtain a wavy pattern.
Thread wire drawing: It is a small motor equipped with circular felt on the shaft, fixed it to the table top, at an angle of about 60 degrees to the edge of the table, and also makes a drag plate with fixed metal plates, and a polyester film with straight edges is pasted on the drag plate to limit the progress of the thread. Using the rotation of felt and the linear movement of the drag plate, threaded patterns with the same width are rubbed on the surface of the metal plate.
e. Surface Nanoization
Surface nanoification is to use various physical or chemical methods to refine the surface grains of the material to the nanometer order to prepare a surface layer with a nanostructure. The substrate remains in its original state, thereby improving and improving the surface properties of the material, such as fatigue strength, corrosion resistance and wear resistance.
Traditional nanometal preparation methods, such as metal evaporation and condensation in situ cold press forming method, mechanical grinding method, amorphous crystallization method and electrolytic deposition method, have limited the practical application of nanomaterials in industry due to the complex preparation technology and high cost. In recent years, with the successful development of high-speed and high-precision shot peening projectors, nano-normalization of metal surfaces can be successfully achieved using shot peening technology. Currently, using supersonic shot peening technology, surface nanoification can be achieved on complex workpieces such as flat plates, shafts, and engine blades.
B. Surface heat treatment
a. High frequency surface quenching
Place the workpiece in an inductor wound with hollow copper tubes, and after high-frequency alternating current is passed, an induction current of the same frequency is formed on the surface of the workpiece, the surface or part of the part is quickly heated, and then quickly immerse it in water or oil to cool it to complete the immersion work, so that the surface or part of the workpiece meets the corresponding hardness requirements.
b. Surface alloying
Through physical methods, the additive material is brought into the substrate to form an alloying layer. The typical process of this technology is carburizing and nitriding treatment of metals. Usually, metal and permeate are placed in a closed cavity together, and the metal surface is activated by heating and vacuuming, and carbon and nitrogen enter the metal matrix through decomposition, absorption, diffusion and other actions. Improve surface hardness, enhance wear resistance, improve fatigue strength and maintain cardiac toughness.
C. Surface chemical conversion membrane
The "conversion film" method refers to a method in which insoluble compound coverings are generated on the metal surface by reacting the outer atoms of the metal with an anion of the selected medium. This is a chemical film formation treatment method. This covering film generated by chemical treatment is usually called a "conversion film" or a "chemical conversion film".
a. Oxidation black/blue
Oxidation of ferrous metal is the process of placing the workpiece in a concentrated solution of sodium hydroxide containing sodium nitrate or sodium nitrite to form a very thin oxide film on the surface of the workpiece, also known as blue or black. It needs to be placed in a boiling solution during processing, so some people are used to calling it boiling black.
In order to improve the anti-rust ability of the steel parts, the surface of the steel parts is oxidized into dense and smooth iron tetroxide with strong oxidizing agent. This thin layer of iron tetroxide can effectively protect the interior of the steel parts from oxidation. The iron trioxide oxidized at high temperatures (about 550℃) is sky blue, so it is called blue treatment. The iron tetroxide formed at low temperatures (about 350℃) is dark black, so it is called blackening treatment. In weapons manufacturing, blue treatment is commonly used; in industrial production, black treatment is commonly used.
The oxide film is mainly composed of magnetic iron oxide and has certain protection capabilities. The film layer is very thin and does not affect the dimensional accuracy of the workpiece. Oxidation does not have hydrogen embrittlement, but sometimes alkali embrittlement occurs. In order to improve the corrosion resistance, wear resistance and lubricity of the film, it can use its good adsorption properties to soak in hot soapy water and oil (spin oil, engine oil or transformer oil).
After completion, the film layer is black and shiny, with protective and decorative effects. Widely used for protective decoration on various precision instruments, optical instruments, mechanical parts and various weapons. Oxidation is also used in non-ferrous metals and alloys such as aluminum, copper, magnesium to improve corrosion resistance or as paint base. However, the composition, color and properties of the treatment solution and the film vary with the alloy.
b. Phosphorylation
Phosphation is the process of placing the workpiece in a phosphate solution containing manganese, iron and zinc to form a phosphate film that is hard to dissolve in water on the surface of the workpiece, also known as phosphate treatment phosphation. According to the operating temperature, it can be divided into three types: high temperature, medium temperature, and low temperature (cold) phosphating.
The operating temperature of the phosphated film is generally below 150°C, but it can withstand a short-term baking of 400 to 500°C. If the temperature is too high, the corrosion resistance will decrease and the mechanical properties, strength, magnetic properties of the matrix will remain basically unchanged after phosphating. However, the film itself has low hardness and strength, and has a certain degree of brittleness.
It is used as a protective layer and paint base for general mechanical parts and products; used for friction reduction and crack prevention during cold stamping and cold heading; silicon steel sheets used for electromagnetic devices such as motors and transformers and steel parts that require insulation to improve insulation without affecting magnetic permeability; it can also be used as an anti-adhesive protective layer for steel molds for casting motor aluminum rotors. In the national defense industry, it can be used as a protective layer and lubricating layer for various weapons; the conduits of fuel and lubricating oil systems on aircraft engines, the cavity of high-pressure gas cylinders on aircraft control systems, landing gear wheel shafts and other similar parts are also commonly used as protective layers for phosphating not only for ferrous metals, but also for non-ferrous metals such as zinc, cadmium, aluminum and other non-ferrous metals and alloys.
c. Passivation
Passivation is the process of placing metal in a nitrite, nitrate, chromate or dichromate solution to form a chromate passivation film on the surface of the metal, also known as chromate treatment. The principle is to use the oxidizing substances of the passivation liquid to promote the active metal ions on the metal surface to be transformed into the passivation state, which is not easily oxidized, and achieve the effect of metal anti-rust.
It is often used as a post-treatment for zinc and cadmium plating to improve the corrosion resistance of the plating; it is used as a protection for aluminum alloys, magnesium alloys, copper and copper alloys; it is also used instead of aluminum anodized film in the aviation industry and other departments; for ferrous metals, it is rarely used alone, mostly used to seal the phosphating layer and enhance corrosion resistance; it is also used to protect the metal from rusting before anti-corrosion construction and improve the adhesion of the paint film.
Metal coloring treatment is a process of forming a colored film or interfering film through surface conversion. Generally, the thickness of the colored film layer is 25-55 nm. Its color is related to the treatment method and film thickness. Usually, colors such as yellow, red, blue, green, and rainbow, spots, etc. can be obtained. The generation of variegated colors is due to the influence of uneven film thickness on the light reflection process. Metal coloring treatment methods include chemical conversion method and electrochemical conversion method (colored films can also be formed through heat treatment or chemical replacement reaction). Metal coloring treatment uses pigments to make them chromatic through adsorption and chemical reactions on the metal surface, or to co-deposit metal ions with dyes through electrolysis to produce color.
D. Surface anodization (aluminum and aluminum alloy)
Anodizing is a process in which an oxide film is formed on an aluminum product (anode) by applying an external current under specific acid electrolyte and process conditions. As an anode, an aluminum is energized, and an oxide film layer with anti-corrosion, wear resistance, insulating and decorative properties is formed on the surface. Before anodizing, it must be pretreated such as polishing, degreasing, cleaning, etc., and then rinsing, coloring and sealing.
E. Electroplating
Electroplating is a process of using electrolytic principles to plating a thin layer of other metals or alloys on certain metal surfaces. During electroplating, the coating of metal or other insoluble materials is used as the anode and the workpiece to be plated is used as the cathode. The cations of the plated metal are reduced to form a plating layer on the surface of the workpiece to be plated. Taking nickel plating as an example: immerse the metal parts in a solution of metal salt as the cathode, and the metal nickel plate as the anode. After turning on the DC power supply, a metal nickel plating layer will be deposited on the parts.
Coating selection
When choosing a metal plating layer, you must pay attention to the following points. 1. Correctly analyze the working conditions of the parts and determine the working requirements for the electroplating layer; 2. The type of metal of the electroplating part and the stability of the metal plating layer in the medium; 3. The tolerances of the structure, shape and size of the electroplating part and the possibility of electroplating on the surface of the part and achieving the required uniform thickness; 4. The bonding force between the plating layer and the surface of the plated part.
F. Special electroplating
a. Brush plating
Brush plating is a special method of electroplating. It does not require a plating tank, but uses a plating pen immersed with a special plating solution to move relative to the plating parts, and obtaining the plating layer through electrolysis. During operation, the workpiece is connected to the negative electrode of the power supply, the plating pen is connected to the positive electrode of the power supply, and wipes it on the surface of the workpiece by wrapping the anode soaked in the solution. The metal ions in the solution are discharged and crystallized at each point when the surface of the workpiece is in contact with the anode, and continue to grow to form a plating layer. If the workpiece is connected to the positive electrode and the plating pen is connected to the negative electrode, the same brush plating device can also perform deburring, etching and electropolishing.
The characteristic of brush plating is that the plating pen can be made into various shapes to adapt to the surface shape and working requirements of the workpiece. The metal ion concentration in the plating solution is high, and it is convenient to store, safe to operate, simple equipment, and has less electricity and water consumption. The same set of equipment can obtain dozens of single metals, alloys and composite plating on various substrates, and the surface of the substrate can also be electro-purified and activated. It allows the use of current density several times to dozens of times larger than tank plating (maximum up to 500 A/dm2), so the plating speed is fast, 5 to 50 times higher than that of general tank plating. The uniformity of the thickness of the coating can be controlled, and mechanical processing is generally not required after plating. This method is suitable for field and on-site repairs, especially for large parts, parts that are not easy to disassemble, and accessories that are not suitable for immersion in tank liquid. It is particularly economical and convenient to use. The disadvantage is that it is not suitable for processing large-area or large-scale parts.
b. Rolling plating
Rolling plating is an electroplating process in which the workpiece is placed in a special roller and in an indirect conductive manner in a rolling state, metal or alloy coating is deposited on the surface of the part in order to achieve surface protection decoration and various functional purposes. It can save labor, have good surface quality, and have small fluctuations in the thickness of the coating.
c. Composite plating
Composite electroplating is a process of co-depositing metal (or alloy) with solid particles (or fibers) by electrochemical methods to obtain composite materials, also known as dispersed electroplating. This composite material layer is called composite coating or dispersed coating. It consists of two parts: a metal or alloy that forms a coating through electrochemical reaction, usually called a matrix metal, which is a uniform continuous phase; the other part is insoluble solid particles or fibers, which are usually dispersed discontinuously in the matrix metal, forming a discontinuous phase, also known as a dispersed phase. Therefore, the composite coating is a metal-based composite material. The phase interface between the matrix metal and the insoluble particles is basically clear and there is almost no diffusion. It is a mechanical mixture in form, but the obtained composite coating has the comprehensive properties of two types of substances: matrix metal and solid particles.
d. Nanocomposite brush plating
Nanocomposite brush plating technology is a new technology developed on the basis of brush plating technology. It is a combination of nanotechnology and traditional technology. It not only maintains the advantages of brush plating, but also greatly broadens the application range of traditional technology and improves its application effect. It is not only a surface treatment technology, but also the key to parts remanufacturing.
