| Wheel hubs,as the important part of the car,can support the entire weight of the car and alsohavethepivotal role ofdiversion?driving and braking.The lightweight of wheel hubs have an in important significance.The magnesium wheelshave attracted attention of many auto industries because of the characteristics of light-weightand fuel saving,improvement of the acceleration and braking performances of vehicles,as well as the damping vibration which can lead more smooth and comfortable indriving.However,magnesium still failed to become the main material of wheel,one of the important factors is the poor corrosion resistance of magnesium alloy.So based on the problems in the road test,magnesium wheel hub was carried out by modifying corrosion resistance and improving the protection ability of wheel hub surface layer.This paper studied the effect of different contents of Ca addition and the thermoplastic deformation conditions on the microstructure,mechanical property and corrosion resistance of magnesium alloy,and also analyzing and comparing effect of three surface treatment(fluorocarbon coating,polyester paint and micro-arc oxidation)on the corrosion resistance,bonding force,ductility and dynamic bending performance of alloy,by means of the metallographic microscope(OM),scanning electron microscope(SEM),differential thermal analysis(DSC),X-ray diffraction(XRD),tensile testing machine,dynamic device and electrochemical workstation.The main conclusions are as following:(1)Research of the corrosion resistance of as-cast Mg-5Al-0.4Mn-xCa magnesium wheel alloy shows that Al2 Ca,(Mg,Al)2Caand Mg2 Ca phases were generated with the addition of 1,2,and 4 wt% Ca,respectively.The corrosion mode of the alloys changed with Ca content.Magnesium substrate is corroded when there is no Ca or little Ca added;and Ca-containing phaseis corroded instead of magnesium substrate when more Ca added.Mg-5Al-Mn alloy with 2 wt% Ca addition has the best corrosion resistance;the corrosion current density is 37.82 ?A/cm2.This phenomenon was attributed to the massive and discontinuous(Mg,Al)2Ca phase,which acted as a sacrificial anode that protected the ?-Mg matrix.And the oxide film formed on the alloy surface inhibits the corrosion medium go deep into themagnesium substrate.(2)Research of the corrosion resistance of extruded Mg-2Zn-1Mn-xCa(ZM21-xCa)magnesium wheel alloy shows that ZM21 alloy has lower corrosion current density,its corrosion resistance is better in the commonly-used magnesium alloy.When different contents of Ca(0,0.1,0.3,0.7 wt %)is added to the ZM21 alloy,grainsbecome refinement and Ca2Mg6Zn3 form,resulting in improvement of strength and corrosion resistance.Addition of excess Ca,such as 1.6 wt%,results in formation of Mg2 Ca phases,which weakened both the mechanical performance and corrosion resistance of the alloy.The ZM21-0.7Ca alloy has the best overall properties,the tensile strength is 260 MPa,ductilityis 21.5%,corrosion current density is 39.74 ?A/cm2 and average weight loss is 0.77 mg/(cm2·d).(3)Research of effect of hot deformation on the corrosion resistance ofmagnesium wheel alloy shows that,after hot deformation,the strength of Mg-5Al-0.4 Mn-2Ca alloy from 201.44 MPa in as-cast alloy increases to 281.01 MPa in extruded alloy.The corrosion current density of alloy with different state has the relationship which isthat the as-cast alloy is the minimum(37.82 ?A/cm2),the extrudedalloy is a little lower(56.08 ?A/cm2),the homogenization alloy is the worst(215.15 ?A/cm2).The corrosion resistance of alloy reduced with the process of hot deformation.(4)After extrusion at different temperature(300 °C,350 °C,400 °C),the corrosion resistance of Mg-5Al-0.4Mn-2Ca alloy differ obviously.Alloy with 350°C extruded has the better resistance than that of 300°C.But when the temperature continued to rise to 400°C,the corrosion current density of alloy matrix increases,leading to the reduced corrosion resistance.The mode of corrosion change with the extruded temperature,there is corrosion pit in the alloy extruded at 300°C and 400°C,the corrosion pit don't occur in the alloy extruded at 350°C which has corrosion resistance layer in the surface.(5)The corrosion resistance of Mg-5Al-0.4Mn-2Ca decreases with the increasing deformation degree.When the extrusion ratio is 9,the corrosion current density and average weight loss of this alloy is 1.26 ?A/cm2 and 19.01 mg/cm2·d,respectively,showing the best corrosion resistance.During the extrusion process,the corrosion resistance of AZ31 change with the microstructure,the grain of area with high degree recrystallization refined,and grains here which have the higher corrosion potential can prevent the corrosion process,leading to the improving corrosion resistance.(6)Comparative study of different surface treatment methods shows that the corrosion current density and binding force of fluorocarbon coating coating is 6.53×10-6?A/cm2 and 33.73 N,respectively,showing the excellent corrosion resistance and bonding force.Fluorocarbon coating coating have low ductility,and its surface generate visible tiny crack after repeated bending 4258 times in the dynamic bending test which simulate the service state of wheel hub.The corrosion current density and binding force of spraying polyester paintis 0.11 ?A/cm2 and 24.05 N,respectively,showing the good corrosion resistance and bonding force.polyester paint have high ductility which reach the maximum value of the static bending test,and its surface generate visible tiny crack after repeated bending 5055 times.The corrosion current density and binding force of micro-arc oxidation coatingis 3.82 ?A/cm2 and 1.88 N,respectively,showing the worst corrosion resistance.Micro-arc oxidation coating has high ductility and its surface generate visible tiny crack after repeated bending 3637 times.(7)Research of spray technology of polyester paint shows that the bonding force and ductility of material after etching process is higher thanwith the material without etching,and its corrosion resistance is a little low than that of without etching.The corrosion resistance and ductilityfirst increase then decrease with the increasingthickness of paint.Material with spraying two layers has the highest corrosion current density of0.11?A/cm2.The component of the paint has no influence on the corrosion performance of material,but colorless painting has a better ductility.Considering the corrosion resistance,ductility of surface coating of magnesium wheel,the optimum spraying process ispolishing—cleaning—etching— spraying two layerscolorless paint— spray coat paint. |
PDF Full Text Request