Aluminum alloy in the air is extremely unstable, easy to generate the naked eye difficult to identify the oxide film. Shenzhen cnc processing plant due to the different methods of aluminum cnc processing, such as casting molding, forging, extrusion or CNC precision machining, or through different processes molding and then through heat treatment or welding, the surface of the workpiece will show different states, different degrees of dirt true or traces, pretreatment according to the workpiece surface selection method.
1. CNC machining parts before anodizing need to pay attention to the problem: Although CNC machining aluminum parts surface exists oxide film time is not long, relatively easy to remove, but greasy heavy, especially in and around the eyelet (machine processing process requires lubrication and added), such workpiece must first be cleaned with organic solvents, if the direct use of alkali wash not only greasy heavy in addition to clean , and fine machining of the surface can not withstand a long time alkali corrosion The result will also affect the roughness of the surface of the workpiece and the resistance of the tie, and may eventually become scrap.
2. Casting before the problems that arise in the process of processing. Casting molding parts are not all the surface is machined, unmachined surface left castings formed in the processing there is a very thick oxide layer, some clips and sand layer, the first should first use mechanical processing or sandblasting method to remove the original oxide film of this part first, or by alkali washing after processing again, so that not only can remove the original oxide layer of the unmachined parts, but also to avoid machine processing parts tolerance size change.
3. heat treatment or welding process after the workpiece in the pretreatment process needs attention: according to the process requirements, the workpiece into the heat treatment or welding process before the need to clean with organic solvents, except for the net surface oil, but at present is generally not that, so the surface of the workpiece formed a layer of oil burn tar, this layer of tar in organic solvents is to be purged, if immersed in alkali solution will cause local corrosion, produce pockmarks or cause unevenness, seriously affecting product quality. I use concentrated nitric acid immersion method for soft tarring, and soft tarred material is slightly cleaned in alkaline solution after cleaning.
Some specific methods are as follows.
① Organic solvent degreasing. Less oil serious can be immersed in solvent for a short time; serious application of cotton cloth dipped in solvent to wipe the oil, or scrub with a pig bristle brush. Operation should be safe, and the residual solvent should be properly stored after use.
② Drying. Regardless of organic solvent cleaning, the drying process can not be omitted, otherwise the meaning of cleaning will be lost.
③Tying. Tie material is suitable for aluminum wire, banned copper wire and galvanized iron wire, available back to the wire layer. Slightly larger single-sided binding consider the location of the tie, as far as possible, tied to the hole closest to the edge of the part to reduce the impact on the surface of the workpiece. Different types of workpieces should not be tied to a single string, because different compositions (grades) of alumina have different processing times. Pay attention to the direction of the strip artifact wobble and avoid nesting the concave part facing downward. Clean the oil from the surface of the workpiece.
④Alkali cleaning.
⑤ Water rinsing. It is best to rinse with hot water after washing, it is a good way to clean the alkaline material on the surface of the workpiece. There are blind holes, grooves to strengthen the position of the workpiece flush, and pour the residual liquid, and immediately turn over nitric acid glow, so as not to suffer from 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 aluminum surface insoluble material when attached to the alkali.
2, the technical requirements of the oxide film formation process
(1) Oxidation. Solution composition and working conditions.
Immediately after pretreatment, transfer to the oxidation process to prevent the workpiece from staying in the atmosphere for too long and producing a primary oxide layer and affecting the quality of the oxide layer. Soaking in water is better than being exposed to the atmosphere again, but it should not be soaked for too long, if immersed in 3% dilute nitric acid soaked L5 ~ 30min will continue to oxidation, but if the time is too long to produce will have an impact on the film, especially the old nitric acid containing impurities such as copper.
The oxidation temperature of the solution is a key process, the solution temperature is too high, film speed, easy to make the oxide film chalking; solution temperature is too low, the film coating speed is slow, resulting in the coating film depth is not uniform, poor adhesion.
To achieve a consistent color on the surface of the same aluminum, it should be treated at the same temperature at the same time.
Within a certain range, temperature and time are inversely proportional, that is, the higher the temperature the shorter the time required, and vice versa the longer the time required.
High purity aluminum oxide requires longer processing time. Insufficient oxidation processing time, the oxide film is too pale; the purity of aluminum and oxidation reduction or aging of the oxide film, or even affect the electrical conductivity of the film.
In order to obtain uniform film color, the solution of oxidation of small pieces of rock, you can use a large mixed solution, or static (not mixed solution, do not shake the workpiece), to prevent the edge of the workpiece and the solution switch over the center of the workpiece, the film color is not uniform.
(2) washing cycle water. For blind holes, the workpiece will be cut, strengthen the parts of the washing, residual solution in the net dumping, to prevent oxidation solution out of oxidation damage.
(3)Self-inspection. After washing circulating water, the workpiece should be seen from the quality, and if defects are found in the alkaline solution, return after slight oxidation. Removal, repair, stripping after drying is more difficult and more likely to damage the substrate.
(4) Drying. Drying is the key to maintain the quality, shake the workpiece surface free water before oxidation in the drying process, and then in the sun. Can also be baked in 45 ~ 50 ℃ drying conditions, the temperature should not be too high to avoid scorching, aging, cracking, the appearance of old color.