Casting surface treatment

Surface treatment is a process of artificially forming a surface layer on the surface of the base material with different mechanical, physical, and chemical properties from the base. The purpose of surface treatment is to meet the corrosion resistance, wear resistance, decoration, or other special functional requirements of the product. For metal castings, our more commonly used surface treatment methods are mechanical grinding, chemical treatment, surface heat treatment, and spraying the surface.

In the process of processing, transportation, storage, etc., the surface of the workpiece often has oxide scale, rust molding sand, welding slag, dust, oil, and other dirt. To make the coating firmly adhere to the surface of the workpiece, the surface of the workpiece must be cleaned before painting. Otherwise, it will not only affect the bonding force and corrosion resistance of the coating and the metal but also make the base metal even in the presence of It can also continue to corrode under the protection of the coating, causing the coating to peel off and affecting the mechanical properties and service life of the workpiece. Therefore, the surface treatment of the workpiece before painting is an important guarantee and measure to obtain a high-quality protective layer and prolong the service life of the product.

Surface treatment should pay attention

To provide a good workpiece surface, the following points should be paid attention to in surface treatment:

  • No oil and moisture
  • No rust and oxide
  • No sticky impurities
  • No acid and alkali residues
  • The surface of the workpiece has a certain roughness

Surface treatment method

Manual processing:

Such as scrapers, wire brushes, or grinding wheels. The rust and oxide scale on the surface of the workpiece can be removed by hand, but manual processing is labor-intensive, low in production efficiency, poor in quality, and incompletely cleaned.

Chemical treatment:

It is mainly to use an acid-base or alkaline solution to chemically react with oxides and oil stains on the surface of the workpiece, to dissolve it in an acid or alkaline solution to remove rust, oxide scale, and oil stains on the surface of the workpiece, and then use nylon. The finished brush roller or 304# stainless steel wire (wire brush roller made of acid and alkali resistant solution) can be cleaned to achieve the purpose.

Chemical treatment is suitable for cleaning thin plate parts, but the disadvantage is: that if the time is not properly controlled, even if a corrosion inhibitor is added, the steel can be over-corrosion. For more complex structural parts and parts with holes, acid solution pickling It is difficult to completely remove the residual acid immersed in the crevices or holes. If it is not handled properly, it will become a hidden danger of corrosion of the workpiece in the future, and the chemicals are volatile, the cost is high, and the chemical discharge after treatment is difficult. serious pollution of the environment. With the improvement of people’s awareness of environmental protection, this kind of treatment method is being replaced by the mechanical treatment method.

Mechanical treatment:

It mainly includes the wire brush roller polishing method, shot blasting method, and shot peening method.

The polishing method is that the brush roller is driven by the motor, and the brush roller rotates at a high speed on the upper and lower surfaces of the strip in the opposite direction to the movement of the rolling piece to remove the iron oxide scale. The brushed iron oxide scale is washed away by a closed circulating cooling water flushing system.

Shot blasting is a method of using centrifugal force to accelerate projectiles and project them to the workpiece for rust removal and cleaning. However, the flexibility of shot blasting is poor, and it is somewhat blind when cleaning the workpiece due to site restrictions, and it is easy to produce dead corners that cannot be cleaned on the inner surface of the workpiece. The structure of the equipment is complex, the wearing parts are many, especially the blades and other parts are worn quickly, the maintenance man-hours are long, and the cost is high.

Shot peening is divided into shot peening and sandblasting:

Surface treatment with shot peening has a strong impact and obvious cleaning effect. However, the treatment of thin-plate workpieces by shot peening is easy to deform the workpiece, and the steel shot hits the surface of the workpiece (whether shot blasting or shot peening) to deform the metal substrate. Since Fe304 and Fe203 have no plasticity, they peel off after being broken, and the oil film is related to it. Therefore, for workpieces with oil stains, shot blasting and shot peening cannot completely remove the oil stains.

Among the existing workpiece surface treatment methods, the best cleaning effect is sandblasting. Sandblasting is suitable for cleaning the workpiece surface with high requirements.

Factories that need to do the blasting process must fully consider the actual production situation when selecting sand conveying equipment and dust removal equipment, and try to choose equipment with greater power than production needs because shot blasting equipment generally wears out faster After long-term use, such or other problems will affect production a lot, and the selection of equipment with higher power will greatly reduce the wasted time and cost of future maintenance. Insufficient dust removal equipment power not only damages the health of workers but also seriously affects the visibility of the sandblasting room. The inability of dust to be discharged will also affect the quality of the sand itself and affect the surface roughness of the workpiece.

The manual sandblasting room should be designed according to the actual situation to be more spacious than the workpiece. It should not be too restrained or it will affect the manual work of the workers. At the same time, the lighting conditions must be good. For drier areas, sandblasting can be carried out outdoors.

Depending on the method used, surface treatment techniques can be divided into the following categories:

1. Electrochemical method

This method uses an 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 flow is called electroplating. The plating layer can be metal, alloy, semiconductor, or containing 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 on the surface under the action of an external current is called anodization, such as the anodization of aluminum alloys.

The oxidation treatment of steel can be carried out by chemical or electrochemical methods. The chemical method is to put the workpiece into an oxidizing solution and rely on chemical action to form an oxide film on the surface of the workpiece, such as the bluing treatment of steel.

2. Chemical methods

This method is no current action, using chemical interaction to form a coating layer on the surface of the workpiece. The main methods are:

(1) Chemical conversion coating treatment

In the electrolyte solution, the metal workpiece has no external current, and the chemical substances in the solution interact with the workpiece to form a coating on its surface, which 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. In the solution, due to the reduction of chemical substances, certain substances are deposited on the surface of the workpiece to form a coating process, which is called electroless plating, such as electroless nickel plating, Electroless copper plating, etc.

3. Thermal processing method

In this method, the material is melted or thermally diffused under high-temperature conditions to form a coating on the surface of the workpiece. Its 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-dip galvanizing, hot-dip aluminizing, 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 aluminum, 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 aluminum foil.

(4) Chemical heat treatment

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

(5) Surfacing welding

By welding, the deposited metal is accumulated on the surface of the workpiece to form a welding layer.
Process, called surfacing, such as surfacing wear-resistant alloys.

4. Vacuum method

The process of forming a coating by vaporizing or ionizing materials on the surface of the workpiece under vacuum conditions. Its main methods are:

(1) Physical vapor deposition (PVD)

Under vacuum conditions, the metal is vaporized into atoms or molecules, or ionized into ions, and deposited directly on the surface of the workpiece to form a coating process, which is called physical vapor deposition. The deposition particle beam comes from non-chemical factors, such as evaporation Sputtering, ion plating, etc.

(2) Ion implantation

The process of implanting different ions into the surface of the workpiece under high voltage to modify the surface is called ion implantation, such as boron implantation.

(3) Chemical Vapor Deposition (CVD)

Under low pressure (sometimes also normal pressure), the process in which gaseous substances form a solid deposition layer due to a chemical reaction on the surface of the workpiece is called chemical vapor plating, such as vapor deposition of silicon oxide, silicon nitride, etc.

5.Other methods

Mainly mechanical, chemical, electrochemical, and physical methods. Among the main methods are:

(1) Coating

The process of applying paint (organic or inorganic) to the surface of the workpiece to form a coating is called painting, such as spray painting, brush painting, etc.

(2) Impact plating

The process of forming a coating layer on the surface of the workpiece by a mechanical impact is called impact plating, such as impact galvanizing.

(3) Laser surface treatment

The process of irradiating the surface of the workpiece with a laser to change its structure is called laser surface treatment, such as laser quenching, laser remelting, etc.

(4) Super hard coating technology

The technology of preparing a superhard film on the surface of the workpiece by physical or chemical methods is called superhard film technology. Such as diamond film, cubic boron nitride film, and so on.

(5) Electrophoresis and electrostatic spraying

  • Electrophoresis
    The workpiece is put into the conductive water-soluble or water-emulsified paint as an electrode and forms a solution circuit with the other electrode of the paint. Under the action of the electric field, the coating solution has dissociated into charged resin ions, the cations move to the cathode, and the anions move to the anode. The charged resin ions, together with the adsorbed pigment particles, are electrophoresed to the surface of the workpiece to form a coating, a process called electrophoresis.
  • Electrostatic spraying
    Under the action of a DC high voltage electric field, the atomized negatively charged paint particles are directed to fly to the positively charged workpiece to obtain a paint film, which is called static spraying.
  • The common application of wire brush is the cleaning of castings, including ferrous and non-ferrous; the cleaning of spatter and welding slag during welding; as long as the strength of the base material is enough to withstand brushing, dust, corrosion, and paint. Wire brushes produce a beautiful surface and can also be used to create decorative surfaces.

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