Common surface finish of aluminum die casting

June 07,2022

Aluminum alloy die-casting products generally have surface treatments: powder spraying (ie powder spraying), baking paint, oil spraying, oxidation, sandblasting, electroplating, etc. According to the thickness and smoothness of the product surface treatment.


1. Powder coating

The powder coating is sprayed onto the surface of the workpiece by powder spraying equipment (electrostatic spraying machine). Under the action of static electricity, the powder will be uniformly adsorbed on the surface of the workpiece to form a powder coating. The powder coating is baked and leveled and cured at high temperature, and becomes the final coating with different effects (different types of powder coatings). It is better than the spray painting process, and the cost is also lower than the spray paint with the same effect.

Powder spraying can be roughly divided into outdoor powder and indoor powder. The texture can be adjusted to various effects, such as smooth surface, sand texture, foaming, etc.


2. Baking paint

The baking paint process is sprayed with aluminum alloy after phosphating and then baked after spraying. This kind of coating is not only anti-corrosion, but also bright and wear-resistant, and it is not easy to fall off.


(1 ) 1 degreasing; 2 water washing; 3 rust removal; 4 water washing; 5 table conditioning; 6 water washing; 7 phosphating; 8 water washing; 9 water washing; 10 dryings;


(2) Purpose and importance of preprocessing:

The purpose of pretreatment is to obtain a good coating. Since the above stamping parts will have grease, oxide scale, dust, rust, and corrosion on the surface during manufacturing, processing, handling and storage, if they are not removed, they will be directly It affects the performance and appearance of the coating film, so pretreatment plays an extremely important role in the coating process.


(3 ) The meaning of preprocessing:

Coating pre-treatment, coating, and drying are the three main processes of the coating process. Among them, pre-coating is the basic process, which has an important impact on the quality of the entire coating, the service life of the coating, and the appearance of the coating. Through degreasing, The surface of the workpiece after rust removal, phosphating and other processes is clean, uniform and free of grease


3. Fuel injection


It is the name of the surface coating processing of industrial products. Fuel injection and carrying are generally specialized in plastic injection, silk screen printing, and pad printing processing; EVA, rubber and other shoe materials are color-changing and silk-screen printing. It has a spraying line, silk screen printing line, pad printing machine and other equipment, and can produce high-temperature resistance, friction resistance, UV resistance, alcohol resistance, gasoline resistance and other products according to customer requirements. Processing range: electronic products: ordinary spray paint, PU paint, rubber paint (feel paint), (such as U disk, MP3, camera, network peripheral products and other electronic products.), can spray the difficult problems encountered in injection molding processing, such as airlines, welded seams, etc., with experience in spraying rubber paint (hand paint), and rework technology of hand paint.


4. Oxidation

Aluminum alloy surface oxidation, suitable for conductive oxidation, aluminum or aluminum profile, suitable for anodizing. Aluminum alloy oxidation color generally has a natural color, sky blue


1. Anodizing is carried out under the condition of high voltage electricity, it is an electrochemical reaction process; conductive oxidation does not require electricity, but only needs to be soaked in the potion, it is a pure chemical reaction.


2. Anodizing takes a long time, often tens of minutes, while conductive oxidation only takes a few tens of seconds.


3. The film produced by anodic oxidation is several microns to dozens of microns, and is hard and wear-resistant, while the film produced by conductive oxidation is only about 0.01-0.15 microns. The wear resistance is not very good, but it can conduct electricity and resist atmospheric corrosion, which is its advantage.


4. The oxide film is originally non-conductive, but because the film generated by conductive oxidation is really thin, it is conductive...


5. Sandblasting

Spray a layer of fine sand on the surface of aluminum alloy products to enhance the friction coefficient of the contact surface and enhance the reliability of the connection. The sand has different thicknesses and textures.


6. Electroplating


Electroplating is the process of depositing metal or alloy on the surface of the workpiece by electrolysis to form a uniform, dense and well-bonded metal layer, which is called electroplating. A simple understanding is a change or combination of physics and chemistry. The application of the electroplating process is generally used for the following purposes: a. anti-corrosion b. protective decoration c. anti-wear


Aluminum alloy coating pretreatment process:


1 ) Degreasing - water washing - water washing - surface conditioning - phosphating - water washing - (pure water washing), using zinc-based phosphating solution, the method is basically the same as the phosphating of steel parts. If not phosphating, hexavalent chromium passivation can also be used, but this method is not environmentally friendly. Or passivated with trivalent chromium.

If the aluminum alloy is coated only by degreasing, the adhesion is poor and the corrosion resistance is also poor.


2) Phosphating treatment Phosphating treatment is a surface treatment technology in which the workpiece is dipped in a solution with phosphoric acid or phosphate as the main body or sprayed with a spray gun to produce a complete phosphate protective film layer on the surface. The film-forming property of the phosphating treatment liquid is not as good as that of the chromating treatment liquid, and the surface quality of the workpiece is required to be high, and it is usually not suitable for the surface treatment of thin-walled die castings with poor surface quality (the wall thickness is less than 2mm). The thickness of the phosphating film layer is relatively large, and as the paint bottom layer, the adhesion, moisture resistance and corrosion resistance of the paint film can be improved by dozens to hundreds of times. There are few studies on phosphating treatment of magnesium alloys, and the current application is very limited.


1, The surface of die-casting aluminum alloy is electroplated with color zinc. Aluminum itself is an amphoteric metal, which is unstable in acid or alkaline solutions. In addition, the die-casting aluminum alloy itself has a loose structure, and there are defects such as blisters and pores, which often affect the quality of electroplating. After proper pre-treatment, the electro-galvanizing of die-casting parts becomes easier. Electroplating a zinc layer of about 10um, and then performing passivation treatment can double the corrosion resistance of die-casting aluminum alloys. In order to prevent the color zinc from discoloring, dip coating An organic protective film.


2, The surface of die-casting aluminum alloy is treated with chromate. After sandblasting, the die-casting aluminum alloy can be directly treated with chromate, so that a passivation film can be obtained on the surface. This film can be colorless to yellow as required, and It does not affect the surface resistance. In order to meet the requirements of the three-proof of the product, it can be sprayed after chromate treatment.




Introduction to the types of metal surface treatment


Electroplating: The plated metal or other insoluble material is used as the anode, the workpiece to be plated is used as the cathode, and the cations of the plated metal are reduced on the surface of the workpiece to be plated to form a coating. In order to eliminate the interference of other cations and make the coating uniform and firm, a solution containing metal cations in the coating should be used as the electroplating solution to keep the concentration of the metal cations in the coating unchanged. The purpose of electroplating is to coat a metal coating on the substrate and change the surface properties or dimensions of the substrate. Electroplating can enhance the corrosion resistance of metals (the plated metals are mostly corrosion-resistant metals), increase hardness, prevent wear, improve electrical conductivity, lubricity, heat resistance, and surface beauty.


Electrophoresis: Electrophoresis is an electrophoresis paint applied to the cathode and anode, under the action of a voltage, the charged paint ions move to the cathode, and form insoluble matter with the alkaline effect generated on the cathode surface, which is deposited on the surface of the workpiece. Features of electrophoresis: The electrophoretic paint film has the advantages of fullness, uniformity, smoothness and smoothness of the coating. The hardness, adhesion, corrosion resistance, impact performance and penetration performance of the electrophoretic paint film are obviously superior to other coating processes.


Galvanizing: Galvanizing refers to a surface treatment technology that coats a layer of zinc on the surface of metals, alloys or other materials for aesthetics and rust prevention. Now the main method is hot-dip galvanizing.


The difference between electroplating and electrophoresis: electroplating is the process of plating a thin layer of other metals or alloys on some metal surfaces using the principle of electrolysis. Electrophoresis: The phenomenon in which charged particles (ions) in solution move in an electric field. The phenomenon in which charged particles (ions) in a solution move in an electric field. The technology that uses charged particles to move at different speeds in an electric field to achieve separation is called electrophoresis. Electrophoresis, also known as electrophoresis (writing), swimming paint, electrodeposition. )


Blackening: The surface of steel parts is blackened, which is also called blue. The principle is to rapidly oxidize the surface of steel products to form a dense oxide film protective layer to improve the rust resistance of steel products. There are two commonly used methods for blackening treatment: the traditional alkaline heating blackening and the late-appearing normal temperature blackening. However, the blackening process at room temperature is not very effective for low-carbon steel. Alkaline blackening of A3 steel is better. Ferric oxide formed by oxidation at high temperatures (about 550°C) is sky blue, so it is called bluing treatment. The ferric oxide formed at low temperature (about 350°C) is dark black, so it is called the blackening treatment. In weapon manufacturing, bluing is commonly used; in industrial production, blackening is commonly used.


Alkaline oxidation or acid oxidation is used to form an oxide film on the metal surface to prevent the metal surface from being corroded. This process is called "blue". The oxide film formed on the surface of ferrous metal after "blue" treatment, the outer layer is mainly ferric oxide, and the inner layer is ferrous oxide.


The operation process of bluing (blackening): workpiece clamping → degreasing → cleaning → pickling → cleaning → oxidation → cleaning → saponification → hot water boiling and washing → inspection. The so-called saponification is to soak the workpiece with a soapy water solution at a certain temperature. The purpose is to form a layer of iron stearate film to improve the corrosion resistance of the workpiece. Metal surface coloring Metal surface coloring, as the name implies, is to "paint" the metal surface, change its single, cold metal color, and replace it with colorful colors to meet the different needs of different industries. After coloring the metal, the anti-corrosion ability is generally increased, and some also increase the anti-wear ability. However, the main application of surface color technology is still in the field of decoration, that is, to beautify life and society.



Shot blasting: The principle of shot blasting is to use a motor to drive the impeller body to rotate (directly drive or drive with a V-belt), and rely on centrifugal force to remove projectiles with a diameter of about 0.2 to 3.0 (including cast steel shots, steel wire cut shots, stainless steel shot) Pills and other different types) are thrown to the surface of the workpiece, so that the surface of the workpiece reaches a certain roughness so that the workpiece becomes beautiful, or the welding tensile stress of the workpiece is changed into compressive stress, and the service life of the workpiece is improved. By improving the roughness of the surface of the workpiece, the adhesion of the paint film of the subsequent painting of the workpiece is also improved.


Sandblasting: Sandblasting is powered by compressed air to form a high-speed jet beam to spray materials (copper ore sand, quartz sand, emery sandblasting schematic diagram, iron sand, Hainan sand) to the surface of the workpiece to be treated at high speed. Change the appearance or shape of the outer surface of the workpiece surface. Due to the impact and cutting action of the abrasive on the workpiece surface, the surface of the workpiece can obtain a certain degree of cleanliness and different roughness, so that the mechanical properties of the workpiece surface are improved. It improves the fatigue resistance of the workpiece, increases the adhesion between it and the coating, prolongs the durability of the coating film, and is also conducive to the leveling and decoration of the coating. Sandblasting and its


It has the following characteristics compared to cleaning processes (eg pickling, tool cleaning):

1. Sandblasting is the most thorough, universal, fastest, and most efficient cleaning method.

2. Sandblasting can be arbitrarily selected between different roughnesses, and other processes cannot achieve this. Manual grinding can produce a rough surface but the speed is too slow, and chemical solvent cleaning will clean the surface too smooth, which is not conducive to coating adhesion.


Features of shot peening: 1. Great flexibility in cleaning. It is easy to clean the inner and outer surfaces of complex workpieces and the inner wall of pipe fittings; and is not limited by the site, it can be moved to the vicinity of extra-large workpieces for cleaning. 2. The structure of the equipment is simple, the investment of the whole machine is low, the wearing parts are few, and the maintenance cost is low. 3. It consumes a lot of energy and must be equipped with a high-power air compressor station. 4. The cleaning surface is prone to moisture and embroidery. 5. Low cleaning efficiency, many operators, and high labor intensity.


The characteristics of shot blasting: 1. Poor flexibility. Due to the limitation of the site, cleaning the workpiece is somewhat blind, and it is easy to produce dead corners that cannot be cleaned on the inner surface of the workpiece. 2. No compressed air is needed to accelerate the projectile, and a high-power air compressor station is not necessary. Easy to embroider. 4. The structure of the equipment is relatively complex, and there are many wearing parts, especially parts such as blades, which require many maintenance hours and high costs. 5. High cleaning efficiency, low cost, few operators, easy to realize automatic control, suitable for mass production.


Shot peening: 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 Fe3O4 and Fe203 have no plasticity, they peel off after being broken, and the oil film and the substrate are deformed together, so 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. However, most of the general sandblasting equipment in our country is composed of primitive and bulky sand conveying machinery such as dumpling dragons, scrapers, and bucket elevators.


The difference between shot peening and shot blasting: shot peening uses high-pressure air or compressed air as power, while shot blasting is generally a high-speed rotating flywheel to project steel sand at high speed. Shot peening has high efficiency, but there will be dead ends, while shot peening is more flexible but consumes a lot of power. Although the two processes have different injection power and methods, they are both aimed at high-speed impacting the workpiece, and their effects are basically the same. In comparison, shot peening is finer and easier to control the accuracy, but the efficiency is not as high as shot blasting, and the shape is complex. For small workpieces, shot blasting is more economical and practical, and it is easy to control the efficiency and cost. The particle size of the shot can be controlled to control the spray effect, but there will be dead corners, which are suitable for batch processing of workpieces with a single shape and surface. The selection of the two processes mainly depends on the workpiece. shape and processing efficiency.


The difference between shot peening and sandblasting: Shot peening and sandblasting both use high-pressure air or compressed air as power, and blow it out at high speed to impact the surface of the workpiece to achieve a cleaning effect, but the effect of the selected medium is different.


After sandblasting, the dirt on the surface of the workpiece is removed, the surface of the workpiece is slightly damaged, and the surface area is greatly increased, thereby increasing the bonding strength of the workpiece and the coating/plating layer. Rough surface, the light is refracted, so there is no metallic luster, and it is a dark surface. After shot peening, the dirt on the surface of the workpiece is removed, the surface of the workpiece is traced without being damaged, and the surface area is increased. The surface of the workpiece is not damaged, and the excess energy generated during processing will lead to the surface strengthening of the workpiece substrate. The surface of the workpiece after sandblasting is also metallic, but because the surface is spherical, the light is partially refracted, so the workpiece is processed. for a matte effect.


Phosphorylation: It is a process of chemical and electrochemical reaction to form a phosphate chemical conversion film, and the formed phosphate conversion film is called a phosphate film. The main purposes of phosphating are: to provide protection for the base metal and to prevent the metal from being corroded to a certain extent; to be used as a primer before painting to improve the adhesion and anti-corrosion ability of the paint film; to reduce friction in the metal cold working process Lubricate use.


Passivation: Its mechanism can be explained by thin-film theory, that is, passivation is considered to be due to the interaction between metal and oxidizing properties, and a very thin, dense, well-covered, and firmly adsorbed metal surface is formed on the metal surface during the action. passivation film. This film exists as a separate phase, usually a compound of metal oxides. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting the corrosive medium, so that the metal basically stops dissolving and forms a passive state to prevent corrosion.


Advantages of passivation

1. Compared with the traditional physical sealing method, the passivation treatment has the characteristics of absolutely not increasing the thickness of the workpiece and changing the color, improving the precision and added value of the product, and making the operation more convenient;

2. Since the passivation process is carried out in a non-reactive state, the passivation agent can be added and used repeatedly, so the service life is longer and the cost is more economical.

3. Passivation promotes the oxygen molecular structure passivation film formed on the metal surface, the film layer is dense, the performance is stable, and it has a self-healing effect in the air at the same time, so compared with the traditional method of applying anti-rust oil, passivation formed. The passivation film is more stable and more resistant to corrosion.


The phenomenon that the chemical stability of the metal or alloy is significantly enhanced by some factors is called passivation. The metal passivation phenomenon caused by some passivating agents (chemicals) is called chemical passivation. Such as concentrated HNO3, concentrated H2SO4, HCIO3, K2Cr207, KMn04, and other oxidants can passivate metals. After the metal is passivated, its electrode potential moves in the positive direction, which makes it lose its original characteristics. For example, the passivated iron cannot replace the copper in the copper salt. In addition, the metal can also be passivated by electrochemical methods. For example, Fe is placed in an H2SO4 solution as an anode, the anode is polarized with an applied current, a certain instrument is used to raise the iron potential to a certain extent, and Fe is passivated. The phenomenon of metal passivation caused by anodic polarization called anodic passivation or electrochemical passivation


Spraying: Use pressure or electrostatic force to attach paint or powder to the surface of the workpiece, so that the workpiece has anti-corrosion and appearance decoration effects


Baking paint: apply primer and topcoat on the substrate, and send it to a dust-free constant temperature baking room for baking every time it is painted.


: It is a microporous (slit) infiltration sealing process. The sealing medium (usually a low-viscosity liquid) is infiltrated into the micropores (slits) through natural infiltration (ie, micropore self-priming), vacuuming and pressure, and the gaps are filled, and then through natural (room temperature), cooling Or heating and other methods to solidify the sealing medium in the gap to achieve the effect of sealing the gap.


spray: spray the paint on the surface of the product and let it dry naturally.


Polishing: The modification of the workpiece surface using flexible polishing tools and abrasive particles or other polishing media. Polishing does not improve the dimensional accuracy or geometric accuracy of the workpiece but aims to obtain a smooth surface or specular gloss, and sometimes it is also used to eliminate gloss (matting). Usually, a polishing wheel is used as a polishing tool. The polishing wheel is generally made of multiple layers of canvas, felt, or leather, and the two sides are clamped by metal discs. During polishing, the high-speed rotating polishing wheel (circumferential speed above 20 m/s) presses against the workpiece, so that the abrasive can roll and micro-cut the surface of the workpiece, so as to obtain a bright machined surface, and the surface roughness can generally reach Ra0.63 ~0.01 microns: When a non-greasy matting polish is used, matt glossy surfaces to improve appearance. When the requirements for the surface of the product are slightly lower, the method of drum polishing is often used. During rough polishing, a large number of abrasives and products are placed in a pot-shaped drum. When the drum rotates, the products and abrasives are randomly rolled and collided in the drum to remove the convex front and reduce the surface roughness.


Ultrasonic cleaning : It uses the cavitation, acceleration and direct flow of ultrasonic waves in the liquid to directly and indirectly affect the liquid and dirt, so that the dirt layer is dispersed, emulsified and peeled off to achieve the purpose of cleaning.



Aluminum and aluminum alloy surface treatment methods

( Method 1 ) Degreasing treatment. Wipe with absorbent cotton dampened with solvent, remove oil stains, and then wipe several times with clean cotton cloth. Common solvents are: trichloroethylene, ethyl acetate, acetone, butanone and gasoline.


( Method 2 ) chemical treatment in following solution after degreasing: concentrated sulfuric acid 27.3 potassium dichromate 7.5 water 65.2 after 60-65 ℃ of immersion 10-30min, take out and rinse with water, dry or dry below 80 ℃; Or wash and then dry in the following solution: phosphoric acid 10 n-butanol 3 water 20 This method is suitable for phenolic-nylon glue, etc., and the effect is good


( Method 3 ) chemical treatment in the following solution after degreasing: ammonium bifluoride 3-3.5 chromium oxide 20-26 sodium phosphate 2-2.5 concentrated sulfuric acid 50-60 boric acid 0.4-0.6 water 1000 dipping at 25-40 ° C for 4.5-6min , that is, washing and drying. This method has high bonding strength, and is bonded within 4 hours after treatment, and is suitable for epoxy glue and epoxy-butyronitrile glue.


( Method 4 ) chemical treatment in the following solution after degreasing: phosphoric acid 7.5 chromium oxide 7.5 alcohol 5.0 formaldehyde (36-38%) 80 dipped at 15-30 ° C for 10-15min, then washed at 60-80 ° C, dry.


( Method 5 ) After degreasing, carry out anodization treatment in the following solution: 22g of concentrated sulfuric acid/ immersion for 10-15min under the direct current intensity of 1-1.5A/dm, then in saturated potassium dichromate solution, at 95-100 Immerse at °C for 5-20min, then wash with water and dry.


( Method 6 ) chemical treatment in the following solution after degreasing: potassium dichromate 66 sulfuric acid (96%) 666 water 1000, immersed at 70 ° C for 10min, then washed with water and dried.


( Method 7 ) After degreasing, chemically treat in the following solution: nitric acid (d=1.41) 3 hydrofluoric acid (42%) 1 immersion at 20°C for 3s, that is, rinse with cold water, and then heat at 65°C Wash with water, rinse with distilled water, and dry. This method is suitable for cast aluminum alloys with high copper content.


( Method 8 ) after sandblasting or grinding, anodize in the following solution: chromium oxide 100 sulfuric acid 0.2 sodium chloride 0.2 at 40 ° C in 10min The voltage is raised from ov to 10v, maintained for 20min, and then in 5min from 10vV was raised to 50v, kept for 5min, then washed with water and dried at 700C. Note: The concentration of free chromium oxide should not exceed 30-35g/1.


( Method 9 ) chemical treatment in the following solution after degreasing: sodium silicate 10 nonionic detergent 0.1 is immersed at 65 ° C for 5 min, then washed with water below 65 ° C, washed with distilled water and dried. Suitable for bonding aluminum foil.


( Method 10 ) chemical treatment in the following solution after degreasing: sodium fluoride 1 concentrated nitric acid 15 water 84 dipped at room temperature for 1min, washed with water and then processed in the following solution; concentrated sulfuric acid 30 sodium dichromate 7.5 water 62.5 at room temperature Immerse for 1 min, wash with water, and dry.