ã€China Aluminum Network】 Due to long-term commitment to the production of small unmanned aircraft, the engine cylinder material is cast aluminum alloy. In order to improve the surface hardness of the cylinder body cavity and enhance the wear resistance, the hard chrome plating process has been used for a long time; the properties of the coating are all good. However, recently, there have been a number of cylinder blocks after the chrome plating appeared peeling phenomenon, after the analysis of the process personnel, to find the cause of the failure, and proposed solutions.
1 failure analysis
Because aluminum is easily oxidized in the air and problems occur in every minute step from pre-treatment to electroplating, it is possible to cause spalling of the coating. Therefore, when searching for a cause, it is necessary to go deep into the production site and perform inspections step by step.
During the pre-plating pretreatment, workers were found to use a copper wire hanging cylinder block for a series of operations, so after pickling, copper will be dissolved and displaced to the surface of the aluminum substrate in nitric acid, and then the zinc coating layer will be attached to the replacement copper. On the layer, the bonding force with the substrate decreases, causing the plating to peel off. When the pre-plating pretreatment process is completed before entering the chrome plating tank, the use of a live lower tank does not allow the zinc immersion layer to dissolve in the middle of the chrome plating tank, and the zinc immersion layer is damaged, which seriously affects the bonding strength of the plating layer. In addition, during the pretreatment process, it was found that yellow spots appeared on some surface areas of the cylinder body after zinc dipping, which was inconsistent with the uniform dark gray zinc film required before aluminum alloy chrome plating, resulting in poor adhesion of the chrome plating layer and peeling of the plating layer. Seen from the outside of the cylinder block, both the inner cavity and the outer wall of the coating layer are covered with tiny and inconsistent pores that are gathered at one or more places. According to the data, the hole is irregular in shape, rough in the hole, coarse in grain, and some may even undergo metallographic structure change. As the cylinder block plating process before more, respectively, through different chemical solutions, if the casting shrinkage hole diameter and number exceeds the normal specification, it will inevitably cause the chemical solution into the castings in the basic internal, difficult to clean during the operation, when the chromium plating temperature When it rises, the solution remaining in the pores seeps out, causing the surface film composition of the cast aluminum substrate to change, causing the plating to peel off.
2 solution
First of all, workers are required to operate strictly in accordance with the process specification. Before the pre-plating pretreatment process, aluminum cylinders are used to suspend the cylinder block to avoid copper displacement. After pre-plating pretreatment, the cylinder's residence time in the air is reduced as much as possible, and the lower tank is charged, and after the small current is activated, it is raised to a normal current for electroplating.
In addition, for the small amount of yellow spots that appear after zinc immersion in the cylinder surface, it has been found through practice that if the yellow spot is gently wiped off by hand, plating with good bonding strength can still be plated. At the same time, the cylinder block must undergo strict cleaning and drying after being processed. If the casting defect is serious, the cylinder body can be used for secondary plating. The unqualified plating layer will be retreated by the previous time. After a period of time, a second plating process can obtain a plating layer with good bonding strength.
3 Summary
There are many factors affecting the chrome plating on aluminum substrates. From the selection of racks to the operation between processes, strict compliance with the process specification is required to obtain better plating.
The substrate material has a non-negligible impact on the plating quality of the product. Although the problem of defective casting coating bond strength can be solved by the process, due to the residual chemical solution residues caused by the internal structural defects of the substrate, many microscopic corrosion primary cells are formed, resulting in severe intergranular corrosion, and the physical properties of the product will be affected. , mechanical properties and airtightness have caused adverse effects. This should lead to the attention of product designers and users, and try to avoid or reduce the adverse effects of the material and mechanical design on the quality of the plating during plating.
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