The surface treatment of aluminum alloy die-casting parts is divided into pre-treatment and post-treatment. Pre-treatment is to remove surface oxide scale and oil stains, increase post-treatment adhesion and improve appearance. The three most commonly used surface pre-treatments for aluminum alloy die-castings are shot blasting, sandblasting and phosphating, and the four most commonly used post-treatments are spraying, oxidation, electroplating and electrophoresis. Other surface treatment methods are only used on products with special requirements due to cost reasons.
Choose from the aspect of cost. The pre-treatment sequence is shot blasting → sandblasting → phosphating → polishing, spraying → electrophoresis → oxidation → electroplating. After phosphating, only spraying and electrophoresis can be performed, and oxidation and electroplating cannot be performed.
Choose from the aspects of decoration and anti-corrosion. The pre-treatment sequence is polishing → phosphating → sandblasting → shot blasting, oxidation → electroplating → spraying → electrophoresis.
Automotive engine casings are generally shot blasted and sprayed.
Surface pretreatment methods
1. Manual treatment:
Such as scraper, wire brush or grinding wheel, etc. Rust and oxide scale on the surface of the workpiece can be removed by hand, but manual processing is labor intensive, has low production efficiency, poor quality, and incomplete cleaning.
2. Chemical treatment:
It mainly uses acid-base or alkaline solutions to chemically react with oxides and oil stains on the surface of the workpiece to dissolve them in acidic or alkaline solutions. solution to remove rust, oxide scale and oil stains on the surface of the workpiece, and then use a brush roller made of nylon or 304# stainless steel wire (a steel wire brush roller made of acid and alkali resistant solution to clean it) The purpose can be achieved. Chemical treatment is suitable for cleaning thin plate parts, but the disadvantage is that if the time is not controlled properly, even if corrosion inhibitor is added, the steel will be over-corroded. For more complex structural parts and parts with holes, it is often After acidic solution pickling, the residual acid immersed in the gaps or holes is difficult to completely remove. If not handled properly, it will become a hidden danger of future corrosion of the workpiece. Moreover, chemicals are volatile and costly. Chemical discharge after treatment is difficult. If treated, Improper treatment will cause serious pollution to the environment. With the improvement of people's environmental awareness, this treatment method is being replaced by mechanical treatment method.
3. Mechanical treatment method:
Mainly. Including wire brush roller drawing method, mechanical polishing method, shot blasting method
a. Wire brush roller polishing method means that the brush roller is driven by the motor, and the brush roller moves in the opposite direction to the movement of the rolled piece. The upper and lower surfaces of the plate are rotated at high speed to remove the oxide scale. The brushed oxide scale is washed away by a closed-circulation cooling water flushing system.
b. Mechanical polishing is removed by cutting and plastic deformation of the material surface. The method of polishing the convex part to obtain a smooth surface generally uses oilstone strips, wool wheels, sandpaper, etc., and is mainly done manually. For special parts such as the surface of the rotary body, auxiliary tools such as turntables can be used. For those with high surface quality requirements, ultra-high-end polishing tools can be used. Precision grinding and polishing method. Super-precision grinding and polishing uses a special grinding tool, which is pressed against the surface of the workpiece in a grinding and polishing fluid containing abrasives, and uses this technology to achieve a surface of Ra0.008μm. Roughness.
c. Shot blasting is divided into shot blasting and sand blasting:
Surface treatment with steel shot or sand has a strong impact and obvious cleaning effect. The processing of the workpiece can easily deform the workpiece, and the steel shot hits the surface of the workpiece (regardless of shot blasting or shot peening), causing the metal base material to deform. Since Fe304 and FE203 have no plasticity, they peel off after being broken, and the oil film deforms together with its material, so For workpieces with oil stains, shot blasting and sandblasting cannot completely remove the oil stains. Among the existing workpiece surface treatment methods, sandblasting has the best cleaning effect and is suitable for cleaning with higher requirements on the workpiece surface. A large amount of silica dust is generated during the sandblasting process, which seriously affects the health of operators and pollutes the environment.
Surface post-treatment technologies can be divided into the following types according to the methods used.
1. Electrochemical method
This method uses electrode reaction to form a coating on the surface of the workpiece. The main methods are:
1. Electroplating
In the electrolyte solution, the workpiece is the cathode, and the process of forming a coating on the surface under the action of external current is called electroplating
The coating can be metal, alloy, semiconductor or contain various solid particles, such as copper plating, nickel plating, etc.
2. Oxidation
In the electrolyte solution, the workpiece is the anode, and the process of forming an oxide film layer on its surface under the action of external current is called
Anodizing forms an aluminum oxide film on the surface of the aluminum alloy.
3. Electrophoresis
The workpiece is placed into a conductive water-soluble or water-emulsified paint as an electrode, and forms a circuit with another electrode in the paint. Under the action of the electric field, the coating solution has been dissociated into charged resin ions. The cations move toward the cathode and the anions move toward the anode. These charged resin ions, together with the adsorbed pigment particles, are electrophoresed to the surface of the workpiece to form a coating. This process is called electrophoresis.
2. Chemical method
This method uses no current and uses the interaction of chemical substances to form a plating layer on the surface of the workpiece. The main methods are:
1. Chemical conversion coating treatment
In the electrolyte solution, the metal workpiece is not affected by external current, and the chemical substances in the solution interact with the workpiece to < /p>
The process of forming a coating on its surface is called chemical conversion coating treatment. Such as bluing, phosphating, passivation, chromium salt treatment, etc. on the metal surface.
2. Electroless plating
In the electrolyte solution, the surface of the workpiece is catalytically treated without external current. Due to the reduction of chemical substances in the solution
Original work The process of depositing certain substances on the surface of the workpiece to form a coating is called electroless plating, such as electroless nickel plating, electroless copper plating, etc.
3. Thermal processing method
This method is to melt or thermally diffuse the material under high temperature conditions to form a coating on the surface of the workpiece. The main methods are:
1. Hot dip plating
The process of placing a metal workpiece into molten metal to form a coating on its surface is called hot dip plating, such as hot plating. Zinc, hot-dip aluminum, etc.
2. Thermal spraying
The process of atomizing molten metal and spraying it on the surface of the workpiece to form a coating is called thermal spraying, such as thermal spraying zinc, thermal spraying
Spray ceramics, etc.
3. Hot stamping
The process of heating and pressing metal foil to cover the surface of the workpiece to form a coating layer is called hot stamping, such as hot stamping Copper foil, etc.
4. Chemical heat treatment
The process in which the workpiece comes into contact with chemical substances, is heated, and a certain element is introduced into the surface of the workpiece at a high temperature is called chemical heat treatment, such as nitriding and carburizing. wait.
5. Overlay welding
The process of welding to accumulate deposited metal on the surface of the workpiece to form a weld layer is called surfacing welding, such as cladding wear-resistant alloys.
4. Vacuum method
This method is a process in which materials are vaporized or ionized and deposited on the surface of the workpiece in a high vacuum state to form a coating.
The main method is.
1. Physical vapor deposition (PVD). Under vacuum conditions, the process of vaporizing metal into atoms or molecules, or ionizing it into ions, and depositing it directly onto the surface of the workpiece to form a coating is called physics. Vapor phase deposition, the deposition particle beam originates from non-chemical factors, such as evaporation plating, sputtering plating, ion plating, etc.
2. Ion implantation
The process of injecting different ions into the surface of a 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 also at normal pressure), the process in which gaseous substances form a solid deposition layer due to chemical reactions on the surface of the workpiece is called chemical vapor plating, such as vapor deposition Silicon oxide, silicon nitride, etc.
5. Spraying
Spraying is dispersed into uniform and fine droplets through a spray gun or disc atomizer with the help of pressure or centrifugal force, and is applied to the surface of the object to be coated. Painting method. It can be divided into air spraying, airless spraying and electrostatic spraying.
1. Air spraying
Air spraying is a coating process that is widely used in paint coating construction.
Air spraying uses compressed air flow to form a negative pressure through the nozzle hole of the spray gun. The negative pressure causes the paint to be sucked in from the suction tube and sprayed out through the nozzle to form paint mist. The paint mist is sprayed onto the surface of the parts to be painted to form a uniform layer of paint. membrane.
2. Airless spraying
Airless spraying uses booster pumps such as plunger pumps and diaphragm pumps to pressurize the liquid paint and then transport it through high-pressure hoses. to the airless spray gun, and finally release the hydraulic pressure at the airless nozzle, instantaneously atomize it, and then spray it onto the surface of the object to be coated to form a coating layer. Since the paint does not contain air, it is called airless spraying, or airless spraying for short.
3. Electrostatic spraying
Electrostatic spraying is a process that uses a high-voltage electrostatic electric field to cause negatively charged paint particles to move in the opposite direction of the electric field and adsorb the paint particles to the surface of the workpiece. A spraying method.