Aluminum alloys are prone to form oxide films in the air that are difficult to identify with the naked eye. Due to the different CNC machining methods of aluminum parts, the surface of the workpiece will show different states, and different degrees of dirt or traces, and the method should be selected according to the surface of the workpiece during pretreatment.

CNC machining process

1. Problems that need to be paid attention to before anodizing of CNC machined parts: Although the oxide film on the surface of CNC machined aluminum parts does not last for a long time and is relatively easy to remove, it is greasy and heavy, especially in and around the holes (lubricant and added during machining ), such workpieces must be cleaned with organic solvents first. If directly washed with alkali, not only is greasy than cleaning, but the finely machined surface cannot withstand long-term alkali corrosion, and the result will also affect the roughness of the workpiece surface and the quality of the tie. Tolerance may eventually become waste.
2. Problems that occur during processing before casting. Not all surfaces of casting moldings are machined. The unmachined surface leaves a thick oxide layer formed in the casting process, and some clips and sand layers. First of all, it should be machined. Or sandblasting first removes the original oxide film of this part, or reprocessing after alkali washing, can not only remove the original oxide layer of the unprocessed part but also avoid the change of the tolerance of the machined part.
3. Problems that need to be paid attention to in the pretreatment process of the workpiece after heat treatment or welding process: according to the process requirements, the workpiece needs to be cleaned with organic solvent before entering the heat treatment or welding process, except for the oil stain on the clean surface, but this is generally not the case at present, so the surface of the workpiece needs to be cleaned with an organic solvent. A layer of oil-burning tar is formed. This layer of tar needs to be removed in an organic solvent. If it is soaked in lye, it will cause local corrosion, pitting or unevenness, which will seriously affect the product quality. I use the concentrated nitric acid soaking method for soft coking, the soft coking material is slightly washed in an alkaline solution after cleaning.

Some specific methods are as follows:

①Organic solvent degreasing. Seriously less oil can be soaked in a solvent in a short time; in severe cases, use cotton cloth dipped in solvent to wipe oil, or scrub with a bristle brush. The operation should be safe, and the residual solvent should be properly stored after use.
②Dry it. Regardless of the organic solvent cleaning, the drying process cannot be omitted, otherwise, the meaning of cleaning will be lost.
③ Binding. The wire tie material is suitable for aluminum wire, copper wire and galvanized iron wire are prohibited, and the return wire layer can be used. Slightly larger single-sided binding takes into account the position of the tie and ties it to the hole closest to the edge of the part as much as possible to reduce the impact on the surface of the workpiece. Different kinds of workpieces should not be tied to a rope, because different compositions (grades) of alumina have different processing times. Pay attention to the direction in which the banding artifact is wobbling and avoid dimples facing down. Clean the oil stains on the workpiece surface.
④ Alkaline cleaning.
⑤ Rinse with water. It is best to rinse with hot water after washing, it is a good way to clean the alkaline substances on the surface of the workpiece. The workpieces with blind holes and groove reinforcement positions should be flushed, the residual liquid should be poured out, and the nitric acid should be turned over immediately to avoid oxidation.
⑥ Nitric acid.
If it is mixed aluminum, cast aluminum or 50mL/L should be added to hydrofluoric acid according to this formula to speed up the removal of the alkali attached to the insoluble matter on the aluminum surface.

2. Technical requirements for the oxide film formation process

(1) Oxidation. Solution composition and working conditions:
Immediately after the pretreatment, it is transferred to the oxidation process to prevent the workpiece from staying in the atmosphere for too long to produce a native oxide layer and affect the quality of the oxide layer. Soaking in water is better than exposing it to the atmosphere again, but it should not be soaked for too long. If soaked in 3% dilute nitric acid for L5~30min, it will continue to oxidize, but if the time is too long, it will affect the film, especially if it contains Old nitric acid with impurities such as copper.
The oxidation temperature of the solution is a key process. If the solution temperature is too high, the film-forming speed is fast, and the oxide film is easily pulverized; if the solution temperature is too low, the coating speed is slow, resulting in different depths of the coating film and poor adhesion.
To achieve consistent color on the surface of the same aluminum, it should be treated at the same temperature at the same time.
In a certain range, the temperature is inversely proportional to the time, that is, the higher the temperature, the shorter the time required, and vice versa.
High-purity alumina requires longer processing times. The oxidation treatment time is not enough, the oxide film is too pale; the purity and redox of aluminum or the aging of the oxide film, even affect the conductivity of the film.
In order to obtain a uniform color of the paint film, a large mixed solution can be used for the oxidized small rocks in the solution, or a static state (do not mix the solution, do not shake the workpiece), so as to prevent the edge of the workpiece and the solution from switching over the center of the workpiece, and the color of the film is uneven.

(2) Washing circulating water. For blind holes, cut the workpiece, strengthen the flushing of the parts, and dump the residual liquid in the net to prevent the oxidizing liquid from flowing out and oxidizing damage.

(3) Self-check. After rinsing the circulating water, the workpiece should be quality-wise, and if defects are found in the alkaline solution, return after slight oxidation. Drying after removal, repair, and peeling is more difficult and more prone to damage to the substrate.

(4) Drying. Drying is the key to maintaining quality, shake the free water on the surface of the workpiece before oxidation during the drying process, and then expose it to sunlight. It can also be baked under the condition of drying at 45~50℃, and the temperature should not be too high, so as to avoid burning, aging, cracking, and old appearance and color.

3. Large area chlorination

(1) Unit. According to the external dimensions of the oxidation (appropriately relax the allowance), make a box with wood or brick, and cover the box with a plastic sheet to form a concave pool, if the height is a plate, it is about 100MM. Only need to shake this. The workpiece on the pool has an oxide film formed on its surface.

(2) Parts. The parts implemented in the working tank (pool) are rapidly changed or rotated in different parts, and finally, the entire workpiece surface is contacted with the solution many times and gradually forms a thicker oxide film.

The above two methods can be prepared by making a large bath and a large amount of solution, which reduces the waste caused by long-term infrequent use, and does not occupy the workshop in the production area.

4. Three possible reasons for the uneven coloring of the oxide film

(1) The area of ​​the workpiece is too large, the operation in the groove is too large, the edge is in contact with the center spot, the large update, the exchange difference, resulting in inconsistent film color…
Preventive measures: Oxidized workpiece swings small, static electricity is OK, but when the temperature is too low, pattern spots are prone to appear, which looks unnatural.

(2) The aluminum layer of the aluminum processing part is damaged and cut off. The aluminum outer packaging is high-quality aluminum, and the inner layer is closed with aluminum mixed. The two kinds of aluminum are different, so there is a “good wind” spot after oxidation. Customers often do not understand this phenomenon, and manufacturers should do more work to explain the reasons and methods to avoid misunderstandings.

(3) Operational aspects.
①The alkaline etching process of the parts is not complete, and the original film and local dirt cannot be removed;
② Immediately after treatment, alkali corrosion is not light, and the surface is alkaline;
③ The workpiece is in contact with foreign objects during the transfer process.

These factors are important in making high-quality anodized aluminum parts, finding the causes of uneven coating color, and taking steps to address them.

5. There are two main reasons why it is difficult to form an oxide film around the hole.

(1) Thoroughly rinse the workpiece after cleaning. When lye was flushed into the holes, the lye was not washed out of the holes to oxidize, thereby corroding the perforations around the film.
(2) Apply butter around the workpiece hole. The aluminum self-tapping screw holes are very astringent, and operators often use butter and lye to lubricate when there is a lack of emulsifier, and the butter is difficult to remove.
Solution: (1) Wash it with gasoline again before alkali washing, and the lotion should be added with an emulsifier;
(3) Alkali washing after workpiece washing.

6. Four points should be paid attention to in the post-processing program

(1) Rinse with hot water. Hot water is designed to age the film. However, the temperature and time must be strictly controlled. If the water temperature is too high, the thickness of the paint film will be reduced and the color will be lightened. Similar problems will occur if the processing time is too long. The appropriate temperature and time are:
Temperature 40~50℃ Time 0.5~1MIN.
(2) Drying. It is better to dry it naturally, wash it with hot water, hang it on the workpiece rack, and let the free water flow vertically downward on the working surface. Use a towel to run the water droplets to the lower corner, and apply this method to dry the color without affecting it, which looks natural.
(3) Aging process. The aging method can be determined according to climatic conditions. It can be exposed to sunlight in summer, and it can be baked in an oven in humid weather or in winter. The process conditions are:
Temperature 40~50℃ 10~15min.