Study On The Influence Of Electric Contact Strengthening Process On Brush Plating Coating

In the process of industrial production,wear,fracture,and corrosion are the three main failure modes of metal parts.About 80 % of the equipment damage is caused by various forms of wear of parts.The failed parts can be remelted and processed into new products,but this process imposes a great burden on resources and the environment.Compared with manufacturing new products,remanufacturing technology can save energy by 60 %,save raw materials by 70 %,reduce air pollutant emissions by more than80 %,and the quality of remanufactured products is not lower than that of new products.It can not only produce huge economic benefits but also is of great significance to promote green and low-carbon development.Therefore,the repair of parts has received extensive attention.In this paper,the pre-coating was prepared by direct brush plating on the surface of ductile iron by brush plating technology,and the electrical contact strengthening was carried out.In the process of brush plating,the influence of weak activation time on the thick nickel coating was discussed,and the best weak activation time was found.The 1.6mm thick nickel coating was successfully prepared on the surface of ductile iron.After the preparation of the coating,different strengthening currents were used to strengthen the coating,and the variation of the microstructure and properties of the coating with the strengthening current was analyzed.At the same time,the bonding strength of the thick coating was measured and characterized by the shear device,and the best strengthening process under the experimental conditions was found,which provided a certain technical reference for the repair of parts.The preparation process of direct brush plating thick nickel coating on the surface of ductile iron was explored,and the influence of weak activation time on brush plating nickel coating was discussed.The main research results and findings are as follows :(1)When the weak activation time is too long or too short,the phenomenon of peeling and spalling occurs during the brush plating process,and the thickening cannot continue,which may be related to the interface bonding state between the coating and the substrate;(2)By observing and analyzing the initial interface of the samples treated with different weak activation time,it is found that too short or too long weak activation can not deposit a complete and uniform coating on the substrate surface,which may be related to the surface state before brush plating.(3)The surface of the sample was analyzed.It was found that the carbon black layer remaining on the surface after strong activation could not be completely removed during the weak activation time,resulting in poor bonding between the coating and the substrate.If the weak activation time is too long,the surface of the substrate will be slightly corroded and new corrosion products and larger pits will appear,which will affect the combination of the coating and the substrate,so that the subsequent brush plating process peels off and cannot continue to thicken.(4)After pretreatment,electro-clean for 1min,strong activation for 1min,the suitable weak activation time is 2.5min,and a thick nickel coating with a thickness of1.6mm can be prepared on the surface of ductile iron by single brush plating.In the process of exploring the influence of the electric contact strengthening process on the microstructure and properties of the coating,the electric contact strengthening current is taken as the research object.The main research results and findings are as follows :(1)For the thick nickel coating with a thickness of 1.6mm,the executable range of the electrical contact strengthening current should be less than 32 k A;within the scope of the process,the surface quality of the coating is obviously improved by electric contact strengthening.The surface of the coating is smooth and the gap between particles is reduced.After more than 32 k A,the coating cracks and peels off,which is due to the moment of contact and detachment between the coating and the electrode wheel.The stress caused by the expansion and contraction due to the rapid cooling and heating in this area exceeds the bonding strength between the coating and the substrate.(2)In the process executable range,with the increase of current,the internal defects of the coating are reduced and the structure is dense.The defects near the interface disappear,which is due to the huge heat generated between the coating and the electrode wheel under the action of thermal pressure coupling.The coating is in a plastic state and flows to the defect direction according to the law of minimum resistance so that the defects can be healed.(3)In the executable range of the process,with the increase of the strengthening current,the areas of mutual melting,bridging and mosaic between the coating and the substrate increase.The reason is that the grains of the coating and the substrate are under the combined action of pressure and heat.The degree of plastic deformation is increasing,which further improves the bonding state between the coating and the substrate.However,in the case of an excessive strengthening current,the local area between the coating and the substrate will be separated due to excessive stress.(4)Within the scope of the process,the bonding strength of the coating after strengthening is significantly higher than that before strengthening.With the increase of current,the bonding strength between the coating and the substrate increases first and then decreases slightly.This is due to the instantaneous high temperature in the contact area between the electrode wheel and the coating during the process of electrical contact strengthening.Under the action of heat,the internal structure of the coating is dense,and the inlay between the coating and the substrate is deepened and the bonding is closer.The slight decrease may be due to the separation of the local area between the coating and the substrate.During the shearing process,the coating peels off in a weak place when it is loaded.(5)Under the experimental conditions of workpiece speed of 1r / min,electrode wheel feed speed of 1mm/min,and cylinder pressure of 0.2MPa,the optimum strengthening current of 1.6mm thick nickel coating is 28 k A,and the bonding strength with the substrate is 39.8 % higher than that before strengthening.