| Based on the specific strength,specific stiffness and low vibration damping of Magnesium alloys as lightweight metal material,it has potential applications in aerospace,military industry,electronic products and so on.However,there are some problems of Magnesium alloys.Firstly,it has the poor mechanical properties,as a structure,it often subject to cyclic loading effects happens fatigue fracture.Secondly,the chemical activity of Magnesium alloys is strong,but also prone to corrosion in the air reacts with water and chloride ions,such localized corrosion fatigue cracks will become the source of Magnesium alloy and accelerate fatigue failure.So,it is very important of studying and protecting the fatigue properties of Magnesium alloys.This work was to generate different thickness of ceramic coatings such as 5 μm,10 μm,20μm,and 30 μm on the squeezed states AZ31,using micro-arc oxidation(MAO)technique.By three-point bending fatigue test,three-point bending test and three-point bending corrosion fatigue static load test,the effect of Micro-arc oxidation technique on the fatigue property of AZ31 Magnesium alloy was discussed combined with ceramic coating phase composition,surface morphology,roughness,density,residual stress and corrosion performance.The result showed that Micro-arc oxidation technique could reduce fatigue limit of AZ31 Magnesium alloy in the air,and with the increasing of thickness of the ceramic coating,the reducing magnitude of fatigue limit of the sample were 9.9%,15.4%,17.1%and 17.1%,and eventually remained at about 83%fatigue limit of AZ31 Magnesium alloy.S-N fatigue curves showed that the overload lasting of all MAO samples were higher than AZ31 Magnesium’ alloy,which indicated that the ceramic coatings could inhibit fatigue crack initiation;the fatigue fracture analysis showed that,unlike the source crack initiation of Magnesium alloys,fatigue crack initiation of MAO samples were on the interface of ceramic coating and substrate,suggesting that hard ceramic coating could improve the position of fatigue crack initiation.Micro-arc oxidation technique to reduce the fatigue limit of Magnesium alloy was mainly due to the increasing residual tensile stress in Magnesium matrix during the growth process of the ceramic coating.The residual tensile stress were mainly composed of two parts.First,due to the PBR of MgO is 0.78,ceramic coating generation would have a strong tensile stress.Secondly,the metal thermal expansion coefficient and the elastic modulus of MgO and the Magnesium matrix respectively was 75%and 88%,which in the growth process of melting and quenching would generate a greater tensile stress on the Magnesium matrix.The stress before and after micro-arc oxidation on Magnesium matrix stress tests showed that,with the increase of the thickness of the ceramic coating,micro-arc oxidation treatment on the tensile stress of the Magnesium matrix value were from 9.5 MPa to 36.6 MPa.Micro-arc oxidation technique could improve the fatigue properties of AZ31 Magnesium alloy in corrosive media.With the increasing the thickness of the ceramic coating,the corrosion fatigue limit of the sample increased gradually increased rate of 0.4%,5.53%,19.37%,and 21.74%in 3.5 wt%NaCl solution.When the thickness of the ceramic coating was about 30 μm,the corrosion fatigue limit of sample is maximum.The stronger the corrosive environment,the better the corrosion resistance of the ceramic coating,Micro-arc oxidation technique on improving the performance of Magnesium alloy corrosion fatigue is better.When the corrosive medium was 2 wt%K2Cr2O7+3.5 wt%NaCl solution,the corrosion fatigue life of the ceramic coating thickness of 30 μm sample was four times AZ31 Magnesium alloy,and compared the same thickness of the sample in 3.5 wt%NaCl solution,the corrosion fatigue life was more than 3671 of cycles.The corrosion current density of MAO samples were smaller than the AZ31 Magnesium alloy,and the corrosion potential was higher than AZ31 Magnesium alloy.The ceramic coating thickness of 30 μm MAO sample in 3.5wt%NaCl solution and 2 wt%K2Cr2O7+3.5 wt%NaCl solution,the corrosion potential were higher than 0.145 V and 0.305 V to AZ31 Magnesium alloy,the corrosion current density was reduced by a and four orders of magnitude.Decreasing degree of corrosion current density was positive correlation of the improvement of corrosion fatigue life.It showed that the main reason for micro-arc oxidation technique to improve the performance of Magnesium alloy corrosion fatigue was to improve the corrosion resistance of Magnesium alloy.The better of corrosion resistance of the ceramic coating,the improvement of fatigue properties of Magnesium alloys was more obvious. |
