| Aluminum alloys have many excellent properties,which have broad commercial application prospects in many fields such as aviation,automobile and electronics,etc.Nowadays,many higher performance requirements were put forward for aluminum alloys,which were used in harsh aviation environments.However,due to the active chemical properties of aluminum,it was easily oxidized in the dry air and then formed an oxide film,which has poor corrosion resistance.Aluminum alloys also have many disadvantages such as low hardness and poor wear resistance.In order to overcome the shortcomings of aluminum alloy in surface performance,surface treatment technology is extremely important to solve various problems in the application of aluminum alloy.As a common aluminum alloy surface treatment process,electroless nickel plating was mainly studied in this work,which can affect improving the surface properties of aluminum alloy.The conclusion is as follows:In this work,a combination electroless nickel plating process of “low phosphorus/medium phosphorus/high phosphorus” was used after determining the optimal operating conditions of each pretreatment(zinc immersion,alkaline preplating,acidic medium/high phosphorus processes)on 2A11 aviation aluminum alloy.We got a multi-coating gradient electroless nickel-phosphorus coating with high corrosion and wear resistance.The morphology,chemical composition,and phase structure of each coating are characterized by the application of SEM,EDS,and XRD,and the bonding force and corrosion resistance,and wear resistance of the coating are tested in this work.A metallographic microscope was used to observe the conditions of the coatings directly,which can study the influence of various process parameters such as plating time and temperature on the coating.The results show that this composite gradient coating has very excellent performance on the surface of 2A11 aluminu m alloy.(1)The zinc immersion process was mainly studied in the pretreatment of electroless nickel plating,As a step that can improve the bonding force between the coating and the substrate,the zinc immersion operation process is extremely important.The surface morphology of the zinc immersion was observed through a microscope,and the scratch method and bending are used to test the bonding force of the coating.The results show that zinc immersion is a reliable guarantee fo r obtaining excellent adhesion.The critical load of coating scratches reaches 81.2 N,and the coating of the bent sample was not skinned or fallen off when the first zinc immersion was 60 s and the second immersion was 30 s at 20-25?.(2)Alkaline electroless nickel pre-plating was a low-phosphorus coating.The best plating temperature was 40?,and the plating time was 10 min for alkaline electroless nickel plating.The best plating temperature is 90?,and the plating time was 1 h for acidic medium-phosphorus electroless nickel plating;the best plating temperature is 90 ?,and the plating time was 2 h for acidic high-phosphorus electroless nickel plating.Low-phosphorus and medium-phosphorus coatings are used as a "soft transition" coating in the middle of the combined coatings,adhesion and toughness of the coating have great improvement with this “soft transition”.The corrosion resistance and wear resistance were greatly improved of the coating with the continuous increase of phosphorus content.Therefore,the good application of aluminum alloy parts in the complicated and harsh aviation environment can be guaranteed due to the three-layer Ni-P combined gradient coatings with high compactness,high corrosion and wear resist ance. |
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