Features of aluminum and aluminum alloy:

1. The density of low-density aluminum is about 2.7g/cm3, which is only the second light metal higher than magnesium in metal structural materials, and only 1/3 of iron or copper. 2. Plastic high aluminum and its alloys have good ductility, and can be made into various shapes, plates, foils, tubes and wires by pressing, rolling or drawing. 3. Easy to strengthen pure aluminum has low strength, but it is easy to strengthen it through alloying and heat treatment to make high-strength aluminum alloy, which is comparable to alloy steel in strength. 4. Good conductivity Aluminum is second only to silver, gold and copper in conductivity and thermal conductivity. If the relative conductivity of copper is 100, then aluminum is 64 and iron is only 16. For example, aluminum is almost twice as large as copper in terms of the conductivity of metals of equal mass. 5. Corrosion resistant aluminum and oxygen have very high affinity. Under natural conditions, protective oxides will be generated on the aluminum surface, which has much better corrosion resistance than steel. 6. The melting temperature of easily recovered aluminum is low, 660 ° С The waste is easy to regenerate, and the recovery rate is very high. The energy consumption for recovery is only 3% of that for smelting. 7. Weldable aluminum alloy can be welded by inert gas protection method. After welding, it has good mechanical properties, good corrosion resistance and beautiful appearance, meeting the requirements of structural materials. 8. Easy surface treatment aluminum can be anodized and colored, with high hardness, good wear resistance, corrosion resistance and electrical insulation. Chemical pretreatment can also be used for electroplating, electrophoresis, spraying, etc. to further improve the decorative and protective properties of aluminum

Mechanical pretreatment of aluminum surface:

1. Purpose of mechanical pretreatment

Provide good surface conditions to improve the quality of surface finishing;

Improve product grade;

Reduce the influence of welding;

Produce decorative effect;

Obtain a clean surface. 2. Common methods of mechanical pretreatment Common mechanical pretreatment methods include polishing, sand blasting, brushing, rolling, etc. The specific pretreatment shall be determined according to the product type, production method, initial surface state and final finishing level. 3. The principle and function of mechanical polishing The friction between the high-speed rotating polishing wheel and the workpiece produces high temperature, which is the plastic deformation of the metal surface, thus flattening the convex and concave points of the metal surface. At the same time, the extremely thin oxide film on the metal surface generated instantaneously under the oxidation of the surrounding atmosphere is repeatedly ground down, thus becoming more and more bright. It is mainly used to remove surface defects such as burrs, scratches, corrosion spots, sand holes and air holes on the workpiece surface. At the same time, the slight unevenness on the surface of the workpiece is further removed to make it have a higher luster until the mirror effect. 4. The principle and function of sand blasting is to spray dry sand stream or other abrasive particles onto the surface of aluminum products with purified compressed air, so as to remove surface defects and present a uniform matte sand surface. Main functions: remove burrs, casting slag and other defects and dirt on the workpiece surface; Improve the mechanical properties of the alloy; Achieve uniform surface extinction effect. 5. Principle and function of brushing. Brushing is to remove burrs, dirt, etc. on the product surface with the help of rotary brushing of the brushing wheel. For aluminum alloy, drawing is to conduct wire drawing treatment on the product, which is mainly intended to play the role of decoration. 6. The principle and role of rolling is to put the workpiece into the roller containing abrasive and chemical solution, and use the rotation of the roller to rub the workpiece against the abrasive and workpiece against each other to achieve the polishing effect

Chemical pretreatment of aluminum:

1. Definition and function of chemical pretreatment

The pretreatment process of aluminum surface with chemical solution or solvent can effectively remove the oil stain, pollutant and natural oxide film on the original aluminum surface, so that the aluminum can obtain a clean surface with uniform wetting. 2. Common chemical pretreatment methods include degreasing, alkali washing, ash removal, fluoride sand surface treatment, water washing, etc. According to the use of the aluminum materials to be treated and the requirements for surface quality, different chemical pretreatment processes can be used. 3. Degreasing principle and role. The oil will hydrolyze in the acidic degreasing solution to generate glycerin and corresponding higher fatty acids. With the help of a small amount of wetting agent and emulsifier, the oil will be more easily dissolved, improving the degreasing effect. After degreasing treatment, the grease and dust on the aluminum surface can be removed to make the subsequent alkaline washing more uniform. 4. The principle and function of alkaline washing: put the aluminum into a strong alkaline solution with sodium hydroxide as the main component for etching reaction, further remove the dirt on the surface, completely remove the natural oxide film on the aluminum surface, and expose a pure metal matrix for subsequent anodizing treatment. 5. The principle and function of ash removal After alkali washing, the surface of the product will often be attached with a layer of metal compounds and alkali washing products that are insoluble in the alkali washing bath solution. They are a layer of grayish brown or grayish black hanging ash. The purpose of ash removal is to remove this layer of ash which is insoluble in alkaline solution, so as to prevent the pollution of the tank solution in the next anodic oxidation process. 6. The principle and role of fluoride sand surface treatment The fluoride sand surface treatment is an acid etching process that uses fluoride ions to make the aluminum surface produce highly uniform and high-density pitting corrosion. The purpose is to eliminate the extrusion marks on the product surface and generate a flat surface. However, due to the serious environmental pollution problem of fluoride sand surface treatment process, it has not been widely used.

(Electro) chemical polishing and chemical conversion of aluminum:

1. The role of chemical polishing or electrochemical polishing

Chemical polishing is an advanced finishing treatment method, which can remove the slight mold marks and scratch stripes on the surface of aluminum products, remove the friction stripes, thermal deformation layers, oxide films, etc. that may be formed in mechanical polishing, make the rough surface smooth, so as to obtain a nearly mirror bright surface, and improve the decorative effect of aluminum products. 2. The principle of chemical polishing is to control the selective dissolution of the aluminum surface, so that the micro convex part of the aluminum surface is preferentially dissolved than its concave part, so as to achieve the goal of smooth and bright surface. The principle of electrochemical polishing is tip discharge. Other chemical polishing are similar. 3. Effect of chemical conversion Chemical conversion is mainly used to protect aluminum and its alloys from corrosion. It can be directly used as a coating or as the bottom layer of organic polymers. It not only solves the adhesion between the coating and aluminum, but also improves the corrosion resistance of organic polymer coatings. 4. The principle of chemical conversion is the chemical treatment process in which the metal aluminum surface in the chemical treatment solution reacts with the chemical oxidant in the solution to form a chemical conversion film. Common chemical conversion includes chemical oxidation treatment, chromate treatment, phosphor chromate treatment and chromium free chemical conversion. 5. Chemical conversion Introduction: Aluminum can get dense protective chemical oxide film in boiling water. This method is called chemical oxidation treatment, but because of the film forming speed and performance, it does not have mass production; The chromated film formed by chromate treatment is the aluminum chemical conversion film with the best corrosion resistance at present. It is not only commonly used as the bottom layer of spraying, but also can be directly used as the final coating of aluminum alloy, but its disadvantage is that the environment is seriously polluted; Phosphochromate treatment can meet the requirements of spraying bottom layer and trivalent chromium is non-toxic, which is widely used in 3C products at present; At present, the industrial production of chrome free chemical conversion mainly adopts the chrome free treatment of fluoride complexes containing titanium or (and) zirconium. The chrome free treatment requires strict chemical pretreatment. At the same time, the chrome free film is colorless and transparent, and the actual effect of chemical conversion cannot be determined by the naked eye. Therefore, it is more dependent on the strict control of reliable processes and processes. To sum up, the chemical conversion most commonly used for 3C products is phosphorous chromate treatment.

Anodizing of aluminum alloy:

1. Definition of anodic oxidation

Anodic oxidation is a kind of electrolytic oxidation. In this process, the surface of aluminum alloy is usually transformed into an oxide film, which has protective, decorative and other functions. 2. Classification of anodic oxide film The oxide film is divided into two categories: barrier type oxide film and porous oxide film. The barrier type oxide film is a thin, dense, pore free oxide film close to the metal surface, and its thickness depends on the applied voltage, which generally does not exceed 0.1um. The porous oxide film consists of a barrier layer and a porous layer. The thickness of the barrier layer is related to the applied voltage, and the thickness of the porous layer depends on the electricity passing through. Porous oxide film is most commonly used. 3. Characteristics of anodic oxide film a. The structure of the oxide film is porous honeycomb junction. The porosity of the film makes it have a good adsorption capacity. It can be used as the bottom layer of the coating and can also be dyed to improve the decorative effect of the metal. b. The hardness of the oxide film is high. The hardness of the anodic oxide film is very high, which is about 196-490HV. Because the high hardness determines that the wear resistance of the oxide film is very good. c. Corrosion resistance of the oxide film. The aluminum oxide film is stable in the air and soil, and has strong adhesion to the substrate. Generally, after oxidation, it will be dyed, sealed or sprayed to further enhance its corrosion resistance. d. The adhesive force of the oxide film, the adhesive force of the oxide film to the base metal is very strong, and it is difficult to separate them by mechanical methods. Even if the film bends with the metal, the film still keeps a good bond with the base metal, but the plasticity of the oxide film is small, and the brittleness is large. When the film is subjected to a large impact load and bending deformation, it will crack. Therefore, the oxide film is not easy to be used under mechanical action, and can be used as the bottom layer of the paint layer. e. The insulation of the oxide film. The impedance of the anodic oxide film of aluminum is high, and the thermal conductivity is also very low. The thermal stability can be as high as 1500 degrees. The thermal conductivity is 0.419W/(m ? K) - 1.26W/(m ? K). It can be used as the dielectric layer of electrolytic capacitors or the insulating layer of electrical products.

Formation process of aluminum alloy oxide film:

1. The first stage of anodic oxidation

In the formation stage of the non porous layer, the voltage increases sharply within the power on and off time (several seconds to tens of seconds) of section ab, reaching the critical voltage (the maximum voltage), indicating that a continuous, non porous film layer is formed on the anode surface at this time. The resistance of the non porous layer is large, which hinders the continuous thickening of the film. The thickness of the non porous layer is proportional to the formation voltage, and the dissolution rate of the oxide film in the electrolyte is inversely proportional. The thickness is about 0.01~0.1 μ m. 2. The second stage of anodic oxidation is the formation stage of porous layer. In the bc section, holes will be dissolved at the thinnest part of the film first, and the electrolyte can reach the fresh surface of aluminum through these holes. The electrochemical reaction can continue, the resistance decreases, the voltage decreases (the descending range is 10~15% of the highest value), and a porous layer appears on the film. 3. In the third stage of anodic oxidation, the porous layer is thickened. In the cd section, the voltage rises steadily and slowly. At this time, the non porous layer is continuously dissolved into a porous layer, and new non porous layers are growing. In this way, the porous layer is continuously thickened. When the generation rate and dissolution rate reach a dynamic balance, the film thickness will not increase, and the reaction should stop.