In terms of cost, the pretreatment sequence is shot blasting → sandblasting → phosphating → polishing, spraying → electrophoresis → oxidation → electroplating. After phosphating, only spraying and electrophoresis can be carried out, but oxidation and electroplating cannot be carried out.
Considering the decoration and anticorrosion, the pretreatment sequence is polishing → phosphating → sandblasting → shot blasting, oxidation → electroplating → spraying → electrophoresis.
Generally, the shell of automobile engine is shot blasting → spraying.
Surface pretreatment method
1, manual processing:
Such as a scraper, a wire brush or a grinding wheel. Rust and scale on the surface of workpiece can be removed by hand, but manual treatment is labor-intensive.
High dynamic strength, low production efficiency, poor quality and incomplete cleaning.
2, chemical treatment:
Mainly uses the acid-base solution to chemically react with the oxide and oil stain on the surface of the workpiece, so as to dissolve it in the acid or alkali solution, thereby removing the rust, scale and oil stain on the surface of the workpiece, and then uses nylon or
Cleaning the wire brush roller made of 304# stainless steel wire (acid and alkali resistant solution) can achieve the goal. Chemical treatment is suitable for cleaning thin plate parts, but the disadvantage is that if the time is not properly controlled, even if corrosion inhibitor is added, it will corrode steel. For parts with complicated structure and parts with holes, it is difficult to completely remove the residual acid immersed in the gaps or holes after pickling with acid solution. If it is not handled properly, it will become a hidden danger of future corrosion of the workpiece, and the chemicals are volatile and costly, and it is difficult to discharge after treatment. If it is not handled properly, it will be harmful to the environment. With the improvement of people's environmental awareness, this treatment method is being replaced by mechanical treatment.
3, mechanical treatment method:
It mainly includes wire brush roller coating, mechanical polishing and shot peening.
The wire brush roller polishing method means that the brush roller is driven by a motor, and the brush roller rotates at a high speed on the upper and lower surfaces of the strip in the direction opposite to the moving direction of the rolled piece to brush off the oxide scale. Brush off the oxide scale with closed circulating cooling water flushing system.
B, mechanical polishing is through cutting and plastic deformation of the material surface, remove the convex part after polishing, and get a smooth surface. Generally, it is oilstone strips, wool wheels, sandpaper, etc. Both are adopted, and manual operation is the main method. For special parts such as the surface of the revolving body, auxiliary tools such as turntable can be used. If the surface quality is high, ultra-precision grinding and polishing can be used. Ultra-precision grinding and polishing is a kind of special grinding tool, which is pressed tightly on the machined surface of the workpiece in grinding polishing solution containing abrasive and makes high-speed rotating motion. The surface roughness of Ra0.008μm can be obtained by this technology.
C, shot blasting is divided into shot blasting and sand blasting:
Surface treatment with steel shot or sand particles has great impact and obvious cleaning effect. However, it is easy to deform the thin plate workpiece by shot blasting, and the metal matrix is deformed when the steel shot hits the surface of the workpiece (whether shot blasting or shot blasting). Because Fe304 and FE203 are not plastics, they peel off after crushing, and the oil film is deformed with the material, so shot blasting and sand blasting can not completely remove the oil stain on the greasy workpiece. Among the existing surface treatment methods of workpieces, sandblasting is the best cleaning effect. Sand blasting is suitable for cleaning the surface of workpieces with high requirements. A large amount of silica dust produced during sand blasting cannot be removed, which seriously affects the health of operators and pollutes the environment.
According to the different methods used, surface post-treatment technology can be divided into the following categories.
First, electrochemical methods
This method uses electrode reaction to form a coating on the surface of the workpiece. The main methods are:
1, electroplating
In electrolyte solution, under the action of applied current, the process of forming a coating on the surface of workpiece as cathode is called electricity.
Electroplating. The coating can be metal, alloy, semiconductor or contain various solid particles, such as copper plating and nickel plating.
2. Oxidation
In electrolyte solution, under the action of applied current, the process of forming an oxide film on the surface of the workpiece as an anode is called
Alumina film was formed on the surface of aluminum alloy by anodic oxidation.
3. Electrophoresis
As an electrode, the workpiece is placed in a conductive water-soluble or water-emulsified coating to form a circuit with another electrode in the coating. Under the action of electric field, the coating solution has dissociated into charged resin ions, and the cations move to the cathode and the anions move to the anode. These charged resin ions and adsorbed pigment particles are electrophoresed to the surface of the workpiece to form a coating. This process is called electrophoresis.
Second, the chemical method
This method is current-free, and uses the interaction of chemicals to form a coating on the surface of the workpiece. The main methods are:
1, chemical conversion membrane treatment
In the electrolyte, the chemical substances in the solution interact with the metal workpiece without external current.
The process of forming a coating on its surface is called chemical conversion film treatment. Such as bluing, phosphating, passivation, chromium salt treatment, etc.
2. Electroless plating
In electrolyte solution, the surface of the workpiece is treated by catalysis, and there is no external current. In the solution, due to the reduction of chemicals,
The original function is to deposit some substances on the surface of the workpiece to form a coating, which is called electroless plating, such as electroless nickel plating and electroless copper plating.
Third, the heat treatment method
In this method, materials are melted or thermally diffused at high temperature to form a coating on the surface of the workpiece. The main methods are as follows:
1, hot dip plating
The process of putting a metal workpiece into molten metal to form a coating on its surface is called hot dip plating, such as hot dip galvanizing and hot dip aluminizing.
2. Thermal spraying
The process of spraying molten metal on the surface of workpiece after atomization is called thermal spraying, such as thermal spraying zinc and heat.
Spraying ceramics, etc.
3. Hot stamping
The process of covering the surface of the workpiece with a layer of metal foil and forming a coating by heating and pressing is called hot stamping, such as hot stamping copper foil.
4. Chemical heat treatment
The process of contacting and heating the workpiece with chemical substances to make some elements enter the surface of the workpiece orderly at high temperature is called chemical heat treatment, such as nitriding and carburizing.
5, surfacing welding
The process of depositing metal on the surface of workpiece by welding is called surfacing, such as surfacing wear-resistant alloy.
Fourthly, vacuum method.
This method is a process in which materials are evaporated or ionized in order and deposited on the surface of the workpiece to form a coating under high vacuum.
The main method is.
1. Physical vapor deposition (PVD) is a process in which a metal is vaporized into atoms or molecules, or ionized into ions, and directly deposited on the surface of a workpiece to form a coating. The deposited particle beam comes from non-chemical factors, such as evaporation, sputtering and ion plating.
2. Ion implantation
The process of implanting different ions into the surface of the workpiece under high voltage to modify its surface is called ion implantation, such as boron implantation.
3. Chemical Vapor Deposition (CVD) Under low pressure (sometimes atmospheric pressure), the process that gaseous substances react on the surface of the workpiece to form a solid deposition layer is called chemical vapor deposition, such as the vapor deposition of silicon oxide and silicon nitride.
Verb (short for verb) spray
Spraying is a kind of coating method which is applied to the surface of the coated object by spraying gun or disc atomizer and dispersing it into uniform and fine droplets by means of pressure or centrifugal force. It can be divided into air spraying, airless spraying and electrostatic spraying.
1, air injection
Air spraying is a widely used coating technology in coating construction at present. Air spraying is the use of compressed air flow through the spray hole of the spray gun to form negative pressure. The negative pressure causes the paint to be sucked out of the straw and sprayed out through the nozzle to form paint mist, which is sprayed on the surface of the coated parts to form a uniform paint film.
2, no air spraying
Airless spraying is to pressurize the liquid paint with a booster pump in the form of a piston pump and a diaphragm pump, and then deliver it to an airless spray gun through a high-pressure hose. Finally, the hydraulic pressure is released at the airless nozzle, which is instantly atomized and sprayed on the surface of the coated object to form a coating. Because the coating does not contain air, it is called airless spraying, which is called airless spraying for short.
3. Electrostatic spraying
Electrostatic spraying is a spraying method which uses high-voltage electrostatic field to make negatively charged coating particles move in the opposite direction of the electric field and adsorb the coating particles on the surface of the workpiece.