Preparation And Properties Of A286 Alloy Electrochemical Phosphating Coating For Fasteners

A286 alloy for high-strength fasteners needs to be lubricated before cold plastic working.Phosphate coating is widely used in metal surface lubrication because of its good wear resistance and strong bond with the substrate.However,A286 alloy has a stable passivation film,and it is difficult to prepare a phosphating coating on its surface.In this paper,phosphate coatings with excellent performance were prepared on the surface of A286alloy by electrochemical induction.With the help of scanning electron microscope,X-ray diffractometer,electrochemical workstation,scratch tester,surface profiler,friction and wear tester and other instruments,the preparation process and microstructure properties of phosphating coatings were studied.The effects of current density on the microstructure,properties and deposition process of zinc phosphating coatings were studied.The results show that the main phase composition of the zinc phosphating coatings is Hopeite[Zn₃（PO₄）₂·4H₂O].With the increase of the current density,the grain size decreases continuously,the preferred orientation of the phosphating coating changes,and the coating weight,coating thickness,bonding performance and wear resistance of the phosphating coating increase.The deposition mode of electrochemical phosphating coatings on the surface of A286 alloy is in the form of layer-by-layer growth-stacking,which is gradually refined and dense with the deposition time.The influence mechanism of the current density on the deposition process is mainly:regulating the deposition sequence,the deposition sequence is transformed from Hopeite single-phase deposition to Hopeite/Zn→Hopeite+Zn→Hopeite;by controlling the electrochemical reaction rate,the supersaturation is affected,and the preferred orientation and grain size of Hopeite is regulated.A heat-resistant phosphating coating was prepared by manganese ion doping,the preparation process and the change law of microstructure and properties of manganese ion doped phosphate coatings were explored.The results show the preparation process parameters of A286 alloy manganese ion doped phosphating film are:the current density is 160 m A/cm²,the temperature is 40℃,the molar ratio of manganese to zinc is 1.25,and the phosphating time is 10 min.,and the phosphating time is 10 min.Manganese ion doping is beneficial to improve the thermal stability of the phosphating coating,and its complete water loss temperature is increased from 311℃to 417℃.With the increase of phosphating temperature,the composition phase of phosphating coating changes from Zn to Zn₃（PO₄）₂·4H₂O+Mn₂Zn（PO₄）₂·4H₂O.With the increase of the molar ratio of manganese and zinc,the grain shape of the phosphating coating changes from massive to elongated and granular,and the grains are gradually refined.The tribological properties of the phosphating coatings of the two systems under different conditions were investigated.The results show that:under dry friction conditions,When the load increases,the wear of the zinc phosphating coating first increases and then decreases,and the wear rate of the manganese ion doped phosphate coating increases.The wear rates of the phosphating coatings in both systems increased with the increase of the rotational speed.Under the condition of saponification treatment,during the process of increasing load and rotating speed,phase transfer occurs,and the friction surface is transformed into the contact between lubricating layer,which leads to the decrease of the wear of phosphating coatings of the two systems with the increase of load and rotating speed.