Preparation And Corrosion Resistance Of Composite Coating On The Surface Of Magnesium Alloy Friction Stir Welded Joint

The magnesium alloy does not melt during the friction stir welding of magnesium alloy,and the welding area is not filled with filler materials.Compared with traditional welding,it overcomes the metallurgical problems associated with it and obtains high-quality welds.However,the inherent poor corrosion resistance of magnesium alloys and their weldments remains a significant limitation.It is very meaningful to investigate the corrosion characteristics and corrosion protection of magnesium alloy after friction stir welding.In this study,the most suitable welding parameters were found on the basis of previous experiments,and a small static gantry friction stir welding device was used to complete the welding of the AZ31B magnesium alloy.After the completion of welding,observe the microstructure of the welded joint and divide the welded joint into heat affected zone?HAZ?,base metal zone?BM?,the stir zone?SZ?and the thermomechanically affected zone?TMAZ?.Micro-arc oxidation?MAO?was performed on the friction stir welding joint,and electroless plating?EN?was performed on the surface of the micro-arc oxidation coating to obtain a micro-arc oxidation/electroless plating?MAO/EN?composite coating.The change of corrosion behavior in different zones of welded joints was studied and the reasons were investigated.The surface and cross section of the coating in each area of the welded joint were characterizing.Combined with phase analysis,electrochemical test analysis and immersion corrosion experiment,the effect of composite coating on corrosion resistance of FSW joint was studied and analyzed.The corrosion protection mechanism of composite coating on welded joint was discussed.The results show:?1?AZ31B magnesium alloy friction stir welding joints have different microstructures in different areas.The Al-Mn-Fe phase in the base material area is in the form of thin strips.the average grain size in the heat-affected zone is about twice that of the base material's grain size,and the Al-Mn-Fe phase is bulk shape in this zone.The average grain size in stir zone is9?m,and the Al-Mn-Fe phase is granular in this zone.The morphologies of Mg₁₇Al₁₂phases in all areas of the welded joints are relatively similar and are granular.?2?AZ31B magnesium alloy friction stir welding joints have different corrosion resistance in different areas.The corrosion potential is the highest in the stir zone,and the lowest in the heat-affected zone.Corrosion current density decreases in the order of heat affected zone,base metal zone,and stir zone.Electrochemical AC impedance spectroscopy shows that the value of the charge transfer resistance in the heat affected zone is the smallest.The overall appearance of the welded joint is pitting corrosion.From the micro-corrosion morphology,the corrosion in the heat affected zone is heavier,and the corrosion degree in the stir zone is the lightest.?3?The micro-arc oxidation coating obtained by micro-arc oxidation treatment for 9min has the best corrosion resistance.The main constituent phases of the micro-arc oxidation coating are MgAl₂O₄ and MgO phases.The micro-arc oxidation coating has similar micro-morphology and thickness in all areas of the welded joint.?4?The micro-arc oxidation coating improves the corrosion resistance of AZ31B magnesium alloy friction stir welding joint to a certain extent.After micro-arc oxidation treatment,the corrosion current density of each area of the welded joint is reduced to a certain extent,the corrosion potential is increased,and the corrosion rate is reduced.?5?The microstructure of the micro-arc oxidation/electroless nickel plating composite coating is relatively uniform in all areas of the welded joint.The electroless plating coating is composed of microcrystalline nickel,the thickness of each area of the welded joint is basically the same,and the surface shows a dense cell structure.The electroless plating coating is tightly fitted with the porous micro-arc oxidation coating,and has the function of sealing the micro-arc oxidation coating.?6?MAO/EN composite coating improves the corrosion resistance of welded joints.After MAO/EN composite treatment,the corrosion current density of each area of the welded joint is reduced by about an order of magnitude,the corrosion potential is increased by 0.8 V,and the corrosion rate is greatly reduced.The composite coating provides good corrosion protection for friction stir welding joints.