Two surface treatments are applied for die casting part. The first is pre- treatment and the other post-treatment. Pre-treatment is to remove oxidation and oil, this will affect post-treatment adhesion and appearance.

During pre-treatment, the following methods can be used:

Shot blasting, sand blasting, phosphating.

During post-treatment, these methods will be common ways:

Spraying, oxidation, electroplating, electrophoresis

In terms of cost, the pretreatment is shot blasting → sand blasting → phosphating → polishing, spraying → electrophoresis → oxidation → electroplating. After phosphating, only spraying and electrophoresis can be carried out, and oxidation and electroplating can no longer be carried out.

Choose from the aspects of decoration and anti-corrosion. The pretreatment sequence is polishing → phosphating → sand blasting → shot blasting, oxidation → electroplating → spraying → electrophoresis.

The automobile engine shell is generally treated by shot blasting → spraying.

Part 1: Surface Pretreatment

1) Manual processing:

Tools for manual processing include scraper, wire brush, fabric or stone grinding wheel. The rust and oxidization remained on workpiece can be removed by this way, but the manual treatment has high labor intensity, low production efficiency, less persistent quality and incomplete cleaning.

2) Chemical treatment:

It mainly uses the acid-base or alkaline solution to react with the oxide and oil on the workpiece surface to dissolve it in the acid or alkaline solution, so as to remove the rust, oxide scale and oil on the workpiece surface, The brush roller made of nylon is made of 304# stainless steel wire (the purpose can be achieved by cleaning the steel wire brush roller made of acid and alkali resistant solution. Chemical treatment is suitable for cleaning sheet parts, but the disadvantage is that if the time is not properly controlled, even adding corrosion inhibitor can cause over corrosion of steel. For more complex structural parts and porous parts, after pickling with acid solution, the residual acid immersed in the gap or hole is difficult to be completely removed. If it is not handled properly, it will become a hidden danger of future corrosion of the workpiece, and Chemicals are volatile and cost is high. It is difficult to discharge chemicals after treatment. If not handled properly, it will cause serious pollution to the environment. With the improvement of people’s awareness of environmental protection, this treatment method is being replaced by mechanical treatment.

3) Mechanical treatment:

It mainly includes wire brush roller wire drawing method, mechanical polishing method and shot peening method.

a. Wire brush roller polishing method

Driven by the motor, the brush roller rotates at high speed on the upper and lower surfaces of the strip in the direction opposite to the movement of the rolled piece to brush off the oxide scale. The brushed oxide scale shall be washed off by closed circulating cooling water flushing system.

b. Mechanical polishing

The polishing method of obtaining a smooth surface by removing the convex part after polishing by cutting and plastic deformation of the material surface. Generally, the oilstone strip, wool wheel, sandpaper, etc. are mainly used for manual operation. For special parts such as the surface of the rotary body, the turntable and other auxiliary tools can be used. If the surface quality requirements are high, the method of ultra precision grinding and polishing can be used. Ultra precision lapping and polishing adopts special abrasive tools. The polishing liquid contains abrasive material, it will be closely brushed on the machined surface of the workpiece in high-speed rotation. Workpiece surface roughness can reach Ra0 008 μM.

c. Shot blasting is divided into shot blasting and sand blasting

The surface treatment with steel shot or sand particles has great striking force and obvious cleaning effect. However, the treatment of thin plate workpiece by shot blasting is easy to deform the workpiece, and the steel shot strikes the workpiece surface (whether shot blasting or shot blasting) to deform the metal substrate. Because Fe304 and Fe203 are not plastic, they are stripped after crushing, and the oil film is deformed together with its material, shot blasting and sand blasting cannot completely remove the oil stain on the workpiece with oil stain. Among the existing workpiece surface treatment methods, the best cleaning effect is sand blasting. Sand blasting is suitable for cleaning the workpiece surface with high requirements. A large amount of silicon dust produced during sand blasting cannot be removed, which seriously affects the health of operators and pollutes the environment.

Part 2: Surface Finishing

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 its surface under the action of external current is called electroplating. Usually part treated by electroplating are copper, nickel, etc.

2) Oxidation

In the electrolyte solution, the workpiece is an anode, and the process of forming an oxide film on its surface under the action of external current is called anodic oxidation. An aluminum oxide film is formed on the surface of aluminum alloy.

3) Electrophoresis

The workpiece is put into the conductive water-soluble or water emulsified coating as an electrode, and forms a decoupling circuit with the other electrode in the coating. Under the action of electric field, the coating solution has been dissociated into charged resin ions, the cations move to the cathode and the anions move to 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 methods

This method is to form a plating layer on the surface of the workpiece by using the interaction of chemical substances without the action of current. The main methods are:

1) Chemical conversion membrane treatment

In the electrolyte solution, the process of forming a coating on the surface of the metal workpiece without the action of external current and the interaction between the chemical substances in the solution and the workpiece is called chemical conversion film treatment. Such as bluing, phosphating, passivation and chromate treatment of metal surface.

2) Electroless plating

In the electrolyte solution, the workpiece surface is catalytically treated without external current. In the solution, due to the reduction of chemical substances, some substances are deposited on the workpiece surface to form a coating, which is called electroless plating, such as electroless nickel plating, electroless copper plating, etc.

3. Hot processing method

This method is to order material melting or thermal diffusion under high temperature conditions to form a coating on the surface of the workpiece. The main methods are:

1) Hot dip coating

The process of putting a metal workpiece into molten metal to form a coating on its surface is called hot-dip galvanizing, such as hot-dip galvanizing, hot-dip aluminizing, etc.

2) Hot stamping

The process of heating and pressurizing the metal foil to cover the surface of the workpiece to form a coating layer is called hot stamping, such as hot stamping copper foil.

3) Chemical heat treatment

The process of contacting and heating the workpiece with chemical substances and ordering certain elements to enter the workpiece surface at high temperature is called chemical heat treatment, such as nitriding, carburizing, etc.

4) Surfacing

The process of forming a welding layer by accumulating deposited metal on the surface of the workpiece by welding is called surfacing, such as surfacing wear-resistant alloy, etc.

4. Vacuum method

This method is the process of ordering the material to be gasified or ionized and deposited on the surface of the workpiece in a high vacuum state to form a coating.

The main methods are.

1) Physical vapor deposition (PVD)

This is a  process in which metal is gasified into atoms or molecules under vacuum, or its ions are converted into ions, which are directly deposited on the surface of the workpiece to form a coating. Its deposition particle beam comes from non chemical factors, such as evaporation plating, sputtering plating, ion plating, etc.

2) Ion implantation

After the coating ion beam is shot into the target material, it is resisted by the target material, the ion beam kinetic energy is reduced, and finally stays on the surface of the solid material to adhere. This process is called ion implantation. Ion implantation is suitable for making shielding covers for radio equipment.

3) Chemical vapor deposition (CVD)

A process in which gaseous substances generate solid deposition layers on the workpiece surface due to chemical reactions under low pressure (sometimes under atmospheric pressure), such as vapor deposition of silicon oxide, silicon nitride, etc.