Study On The Preparation&Properties Of Microarc Oxidation Coatings On 7N01 Aluminum Alloy

With the rapid development of modern railway transport,lightweight design of aluminum alloy locomotive body has become an inevitable trend for manufacturing rail vehicles.7N01 aluminum alloy is ideal weldable structural materials with moderate intensity,mainly used for the end beam,base,frame of side members and other important components on the high-speed vehicle body.Which were required with high surface hardness,low surface roughness,and less prone to wear and corrosion under their service conditions.Therefore,7N01 aluminum alloy and FSW joints used in locomotive were made surface protective treatment by micro-arc oxidation technology in this paper,in order to improve their surface performance,and provide some guidance and some basic data to expand the application of aluminum alloy in the traffic field.The suitable electrolyte system&solution component ratio were selected by orthogonal design method.Influence of process parameters,such as the working voltage,current density,pulse frequency,duty ratio on thickness and surface characteristics(e.g.surface porosity,pore size distribution,surface roughness and surface profile support ratio),of the layer is discussed by using an eddy current coating thickness,IPP image analysis software,a precision roughness in order to optimize process parameters.The results show that using constant current mode&suitable parameters(5.5 A-dm-2,450 Hz,20%,30 min),the more complete MAO layer with the better appearance quality,the higher hardness,the greater proportion of small pore size distribution,the more less surface porosity,the more excellent the contact stiffness and the more excellent wear resistance can be obtained in the NaA102 electrolyte system containing 5.5 g·L-1 NaAlO2,1 g·L-1 KOH,3 g·L-1 NaF and 0.8 g·L-1 EDTA,etc.The morphology,phase composition,and distribution of elements in the ceramic coating were characterized by using SEM,EDS,XRD,XPS and other methods.There are a lot of discharge holes accompanied by solidified lava and microcracks morphology,etc.on the surface.Interface bonding state is good,and most of the pores do not reach at the interface.The surface of the layer is mainly composed of A1 and O,which present gradient variation in the cross-sectional direction.F-,N Na+,Ca2+ in the electrolyte involved in process,can be adsorbed and retained in the discharge channels.The melt ejected from the discharge microporous more inclined to generate γ-Al2O3 phase.However,the amount of α-Al2O3 phase in the dense layer gradually increased,with higher degree of crystallization.Vickers hardness and thermal shock resistance of the layers were measured by using a hardness tester and by thermal shock method.A WTM-2E tester is used for the friction wear test.Wear scar morphology,debris characteristics,element distribution of MAO layer and some mechanisms were studied by SEM,EDS,etc.The results show that hardness value of the dense layer is the largest with declining gradient toward the substrate.No blistering and peeling phenomenon on the film was found under 50 times thermal shock.The friction coefficient of the layer against GCrl 5 steel balls,about 0.5～0.6,tends to be stable.The loss of the coating under unlubrication friction for 30 min is only about 6.19 mg.Wear mechanism between the two parts principally exhibits the adhesive wear characteristics and slight abrasive wear characteristics.Corrosion resistance of the ceramic layer were tested by using drip corrosion test,salt spray corrosion test(NSS),and electrochemical method.The corresponding corrosion mechanism was deduced.The results suggest that the corrosion rate of the samples in NSS test for 480 h was only 0.019g·m-2·h-1 and only 5.9 × 10-3 g·m-2·h-1 after sealing teatment.The micropores in the surface of ceramic layer formed in the discharge breakdown are composed of mostly blind,which effectively prevent the corrosive medium such as Cl-into the substrate.The corrosive process of the ceramic layer displays the localized corrosion characteristic on the surface.The corrosion resistance on the surface of FSW joints can be significantly improved ascribed to the uniform MAO coatings.The corrosion rate of FSW samples with MAO coating was only 0.03g·m-2·h-1 and 0.02 g·m-2·h-1 after sealing teatment in NSS test for 96 h.