Aluminum materials are extremely unstable in the air, and oxide films that are difficult to recognize with the naked eye are easily formed. Due to the different processing methods of aluminum parts, such as casting, direct cutting of rolled plates, fine machining, heat treatment or welding after forming by different processes, the workpiece surface will show different states and different degrees of dirt or traces. Therefore, in the pretreatment process, the pretreatment process method must be selected according to the actual situation of the workpiece surface.
(1) Problems to be paid attention to in the pretreatment process of fine machined parts: Although the natural oxide film on the surface of fine machined parts is just formed, which is easy to remove, it is heavy greasy, especially in and around the holes (added due to the need for lubrication during machining). Such workpieces must be cleaned with organic solvent first. If they are directly washed with alkali, it will not be possible
However, the greasy surface is difficult to remove, and the fine machined surface can not withstand strong alkali corrosion for a long time. As a result, the roughness and tolerance matching of the workpiece surface will be affected, and it may eventually become a waste product.
(2) Problems needing attention in the pre-treatment process of cast parts. Not all the surfaces of the cast parts are machined, and the non machined surfaces have excessive oxide layers formed in the casting process, and some have sand layers. At this time, the original oxide film of this part should be removed first by machining or sandblasting, or processed after alkali washing. Only in this way can the original oxide layer of the unprocessed part be removed and the change of the tolerance dimension of the machined part be avoided.
(3) Problems 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 it is transferred to the heat treatment or welding process to remove the oil stain on the surface, but this is generally not possible at present, so a layer of oil stain sintered coking material is formed on the surface of the workpiece, which is difficult to remove in the organic solvent. If it is immersed in alkali solution, it will cause local corrosion, Pitting or unevenness will occur, which will seriously affect product quality. This layer of coking material is soaked in concentrated nitric acid to soften it. After the coking material is softened, it can be completely removed by a little cleaning in the alkali solution.