Preparation And Properties Study Of Ni-P-nano WC Electroless Composite Plating On A356 Alloy

In this paper,the Ni-P/nanoWC composite coating was prepared on A3 56 alloy substrate by electroless plating,and effects of the WC concentration,pH value,concentration of surfactant and complexing agent on the plating rate of composite coating were studied by orthogonal design of four factors four orthogonal test with Li6(44)orthogonal test table.Based on the range analysis of orthogonal experiment results,the best combination of the four factors was obtained,and the optimum process parameters of Ni-P/nanoWC composite coating were determined.The surface and cross-sectional morphologies,composition,phase structure,adhesion strength,hardness,abrasion and corrosion resistance properties of Ni-P/nanoWC composite coatings prepared under the optimum process parameters were investigated by comparing with matrix and Ni-P coatings.The Ni-P/nanoWC composite coatings prepared by the optimum process parameters were subjected to vacuum heat treatment at 200?,300?,400?,500? respectively,the effect of heat treatment on the morphology,phase,hardness,wear resistance and corrosion resistance of Ni-P/nanoWC composite coating was studied by compared with the properties of the as deposited coatings.The results are as follows:(1)The electroless composite coating is well bonded with the A356 alloy substrate after the ultrasonic pretreatment of oil,alkali and acid washing,respectively.The four factors selected by orthogonal test in the order of impacting extent of plating rate is:pH value>surfactant concentration>complexing agent concentration>WC concentration,the best process combination is obtained,including of pH=6,surfactant 0.08g/L,complexing agent 15g/L,WC 6g/L,and the average plating rate of the best process formula is 29.844?m/h.(2)As-prepared Ni-P and Ni-P/nanoWC composite coating are homogeneous and compact,and the Ni-P coating is mainly composed of amorphous structure while the composite coating is amorphous structure.The hardness of composite coating is 917.8HV,which is 6 times and 1.4 times of that of the substrate and Ni-P coating,and the wear amount is 2/5 and 3/5 of the substrate and Ni-P,respectively.The results of polarization curves in 3.5%NaCl,1mol/L H2SO4 and 0.5mol/L NaOH solution show that the order of corrosion resistance is:composite coating>Ni-P coating>A356 alloy.Electrochemical Impedance Spectroscopy tests of composite coatings in 3.5%NaCl solution show that the charge transfer resistance(Rt)of composite coating is 27 times and 7 times higher than that of A356 alloy and Ni-P coating respectively,the corrosion resistance is better.(3)After the heat treatment,the crystalline state of composite coating is transformed changed,and the crystallinity of the coating increases with the increase of heat treatment temperature,when the temperature is above 400?,the coating is completely crystalline.The grain size of the coating reaches the nanometer level after heat treatment,the fastest growth rate of grains in the range of 400?500? and obtaining the grain size presenting at the temperature of 500?.With the increase of heat treatment temperature,the Ni3P phase is precipitated in the coating,the hardness and wear resistance properties of the coating is increase first and then decrease,the wear is 2/5 of the composite coating as deposited,the hardness of heat treated coating reaching to the 1353.6HV under the temperature of 400?,the corrosion resistance properties of heat treated coating in 3.5%NaCl,1mol/L H2SO4 and 0.5mol/L NaOH solution was decreased gradually.