Preparation And Properties Of Ni-P Amorphous Coatings On SLM Aluminum Alloy

Selected area laser melting（SLM）technology is one of the most advanced additive manufacturing technologies in the last decade.The Si Cp/Al base metal mirrors produced by this technology have low volume ratio,low thermal expansion coefficient and high material strength,and are widely used in aerospace and aviation fields.Therefore,it is necessary to modify the surface of the mirror by chemically coating the surface with a nickel-phosphorus amorphous film,which,after optical processing,can significantly improve the reflectivity of the mirror surface and thus obtain a high-quality optical surface.In this paper,SiCp/Al-based composites formed by SLM technology were used as the matrix material,and the surface coating of Ni-P coating was modified by chemical plating on its metal surface.By using SEM,OM,XRD,TMA,micro-Vickers hardness tester,electrochemical workstation and other testing methods,the chemical plating process conditions and plating solution components on the organization and performance of nickel-phosphorus coatings were studied.The main findings are as follows:（1）The effects of bath pH,application temperature,amount of double complexing agent and application time on the surface morphology,thickness,deposition rate,phosphorus content,hardness and phase structure of the plated layers were obtained.With the increase of the pH value of the plating bath,the deposition rate of the Ni-P coating accelerated,and the phosphorus content and hardness of the coating gradually decreased;with the increase of the application temperature,the deposition rate of the coating accelerated,and the phosphorus content and hardness gradually increased;the complexing agent was chosen as a combination of lactic acid and citric acid,which could not only improve the deposition rate of the coating,but also significantly increase the phosphorus content and hardness of the coating;with the increase of the application time,the thickness of the coating gradually increased,and the hardness showed the effect of the coating time.As the plating time increases,the thickness of the layer gradually increases,the hardness first increases and then stabilises,the nickel phosphorus particles on the surface of the substrate gradually increase,and when the substrate surface is completely covered,the nickel phosphorus particles begin to gather and grow,eventually forming a cell-like organisation.（2）The preparation process of Ni-P coating on the surface of Si Cp/Al matrix composite was optimized.The bath composition of Ni SO₄·6H₂O（30g/L）,Na H₂PO₂（35g/L）,C₃H₆O₃（20ml/L）,C₆H₈O₇（5g/L）,C₂H₃Na O₂（15g/L）and Na F（1g/L）was adopted.When the temperature is controlled at 85±1℃and the pH value is controlled at 4.5±0.1,the amorphous high phosphorus coating with dense surface and continuous thickness of 122μm can be obtained.The hardness of the plating is 3 times higher than the substrate,and the bond between the plating and the substrate is good.（3）The influence of the chemical plating process conditions and the composition of the plating solution on the thermal expansion properties of nickel-phosphorus plating layers is revealed.The thermal expansion of amorphous nickel-phosphorus plating layers increases with increasing temperature and is approximately linearly related to temperature.At the same time,the thermal expansion coefficient of nickel-phosphorus plating is related to the phosphorus content of the plating,and the phosphorus content of the plating is inversely proportional to the thermal expansion coefficient.Comparing the coefficient of thermal expansion of the base material with that of the nickel-phosphorus coating,it can be seen that the coefficient of thermal expansion of the base and the coating match at low temperatures,and as the temperature increases,the coefficient of thermal expansion of the nickel-phosphorus coating is smaller than that of the base material.（4）The corrosion resistance of Ni-P coating in corrosive environment is described.Ni-P coating has excellent corrosion resistance in 3.5wt%Na Cl solution.The corrosion rate of Ni-P coating in strong acid,strong base and neutral salt from fast to slow is:hydrochloric acid,sodium chloride,sodium hydroxide.The impedance of the Ni-P coating is better than that of the substrate,and the impedance of the coating decreases with the time of soaking in 3.5wt%Na Cl solution.