| With the increasing demand of aerospace and weapon equipment lightweight,the research and development of high-quality and low density materials has been paid more and more attention.Magnesium alloy,as the lightest metal structure material,can meet the needs of the above fields.In recent years,the application of magnesium alloy in high-end weapon equipment has increased year by year,and the scope of application is also expanding.However,magnesium alloy has restricted its application in related fields due to its own shortcomings.The most typical problem is the lack of heat resistance of conventional magnesium alloy.At present,the most effective way to improve the heat resistance of alloy is to add appropriate alloying elements and to make proper surface treatment.The results show that the thermal conductivity of ceramics is very low and has good heat resistance.At present,the method of adding ceramic layer on the surface of magnesium alloy has also been broken through.Generally,there are two ways of micro arc oxidation and plasma spraying.Due to the different structure and application mode of the layer,the application scenarios and life of the ceramics are different.At present,there are many researches and results on micro arc oxidation,which is considered to be an important means to improve the corrosion resistance of magnesium alloy.However,due to the thin coating thickness,the improvement of heat resistance is not the best.Compared with plasma spray thermal barrier coating,because of its double layer structure,it has great potential to improve the heat resistance performance,whether from the structure layer itself or the combination way.As a new super high strength magnesium alloy with complete intellectual property rights,VW75 magnesium alloy is more and more used in aerospace and weapon equipment,especially in some specific aircraft.In order to further expand the application scope of magnesium alloy and enhance its important position in the field of national defense and military affairs,This paper takes VW75 rare earth magnesium alloy as the object,designs orthogonal test scheme by scientific test method,and spray the yttrium oxide partially stable zirconia(YSZ)powder on the surface of VW75 magnesium alloy by plasma spraying technology and optimize the process parameters,In order to improve the anti-oxidation performance of the material,reduce the thermal conductivity of the material.The surface coating properties of VW75 magnesium alloy were studied by means of optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),electrochemical workstation and wind tunnel testing machine.The key performance parameters of plasma spraying thermal barrier coating were effectively controlled,and the failure mechanism of thermal barrier coating was found in the preparation process,The high temperature resistance,corrosion resistance,wear resistance and arc wind tunnel service performance of thermal barrier coating are systematically studied.The failure forms and specific reasons of thermal barrier coating are analyzed.It lays a solid theoretical foundation and obtains scientific and effective test data for further improving the application of VW75 magnesium alloy with thermal barrier coating in Aerospace and weapon equipment.The particle size distribution d(0.5)is 40-80 ?m of YSZ powder was used as raw material for plasma spraying.The particle size distribution d(0.5)is 78.5 ?m NiCrAlY alloy powder was used as the bonding layer material,and the coating was prepared on the surface of VW75 magnesium alloy by atmospheric plasma spraying system produced by Praxair company of the United States.Meanwhile,the spraying manipulator operating system of ABB company of Switzerland was used to control the uniformity of coating thickness.The orthogonal test table with six factors and five levels was constructed by using the orthogonal test method with six parameters of spraying current(A),spraying distance(mm),main gas flow(SCFH),auxiliary gas flow(SCFH),spraying speed(mm/s)and powder feeding rate(g/min)as the factors of orthogonal test,According to the analysis of the comprehensive evaluation results,the order of the influence of the above factors on the comprehensive performance of the coating is determined as follows:the main gas flow is the largest,the spraying current,spraying speed,powder feeding amount and auxiliary gas flow are reduced in turn,and the spraying distance has the smallest influence on the comprehensive performance of the coating,The optimum technological parameters of plasma spraying ZrO2 coating on VW75 magnesium alloy surface were optimized as follows:spraying current 875 A,spraying distance 85 mm,main gas flow rate 75 SCFH,auxiliary gas flow rate 45 SCFH,spraying speed 400 mm/s,powder feeding rate 30 g/min.The hydrogen evolution experiments of plasma spraying,micro arc oxidation and original VW75 magnesium alloy were carried out by means of scanning electron microscope,optical microscope,electrochemical analysis,3.5 wt.%NaCl immersion test and neutral salt spray test,It is found that the self corrosion potential,self corrosion current density and AC impedance of the two kinds of surface treated samples are much better than those of the original T5 state VW75 magnesium alloy,which can protect the matrix well in the early stage of immersion,and the micro arc oxidation state has better corrosion resistance;After plasma spraying,the corrosion resistance of the sample is improved,but after long-term corrosion,serious corrosion occurs at the interface between the bonding layer and the substrate,resulting in the coating failure.The reason of accelerated corrosion of alloy after plasma spraying in salt spray and immersion test is discussed.It is considered that the corrosion process of ceramic coating in neutral salt spray develops from film corrosion in earlier stage to matrix corrosion in later stage.The corrosion medium is easy to pass through the defects of ceramic coating and reach the bonding layer for storage.After accumulation,it contacts the magnesium alloy matrix through corrosion channel Under the action of Cl-Cr3+ and Ni+ ions,the matrix is seriously corroded and the ceramic layer is completely failed.VW75 magnesium alloy is still insufficient when used above 250?.In order to further improve the heat resistance of the alloy,VW75-T5 alloy,micro arc oxidation alloy and plasma spraying alloy were selected to systematically study the microstructure of the alloy during short-term service and long-term service at high temperature,to explore the creep mechanism of the alloy.The high temperature short-time fracture properties and thermal shock resistance were studied,the results show that the minimum creep rate and the total creep rate of plasma sprayed VW75 magnesium alloy are the best under low stress condition.At 250?/150 MPa,the plasma sprayed VW75 magnesium alloy breaks in 42.8 h,and the creep rate is higher than that of the other two states.At 125 MPa,the plasma sprayed VW75 magnesium alloy shows good creep resistance at all temperature ranges.The creep activation energies of the three alloys are 130.3 kJ/mol,129.8 kJ/mol and 118.4 kJ/mol,respectively,and the stress exponent n is 4,3.77 and 4.84,respectively.It shows that the creep mechanism of the alloy at 250? is close to the dislocation climbing mechanism.The addition of coating inhibits the further plastic deformation of the alloy and has the possibility of maintaining stability.The creep failure behavior of the alloy was analyzed,and the high temperature tensile properties of the alloy under three dimensions of temperature,holding time and coating type were studied.It was found that the performance fluctuation of the alloy by micro arc oxidation and plasma spraying was relatively small.After plasma spraying,the grain size of the alloy grew up,the matrix strengthening phase disappeared and the grain boundary strengthening phase grew up,It is the main reason that the properties of the alloy decrease during short-time tensile.The comprehensive influence mechanism of different types of protection on the high temperature performance of the coating was analyzed.After 158 thermal cycles,the plasma sprayed coating peeled off as a whole,and the micro cracks and pores in the ceramic layer and adhesive layer did not develop into macro cracks.It can be considered that the internal bonding of each layer was quite dense,the coating quality was very high,and the failure position of the thermal barrier coating occurred between the magnesium alloy substrate and the adhesive layer,The difference of thermal expansion performance of each coating layer is an important reason for cracks.In addition,the thickening of TGO is also an important reason for spalling.The results show that the average friction coefficient of plasma sprayed VW75 magnesium alloy is 0.247 at room temperature,0.784 at 100?,0.850 at 200?,and 0.754 at 300?.The average friction coefficient of the uncoated alloy is 0.108 at room temperature,0.578 at 100?,0.621 at 200?,and 0.495 at 300?.The average friction coefficient of coated VW75 magnesium alloy is twice that of uncoated VW75 magnesium alloy at 300?.However,in dry friction and wear test,the wear volume of coated VW75 magnesium alloy is less,and its wear resistance is better than uncoated VW75 magnesium alloy;It is found that the friction coefficient of uncoated alloy changes little at room temperature and fluctuates sharply with the increase of temperature,while the friction coefficient of coated VW75 magnesium alloy does not fluctuate,and the friction coefficient increases gradually with the test;The results show that the wear surface of uncoated VW75 magnesium alloy presents alternate furrow ridge morphology,and the wear scar depth is the largest at 200? compared with other temperatures.The wear surface of coated VW75 magnesium alloy presents island like protruding peak spalling pit morphology.Arc wind tunnel thermal test is one of the most effective methods to test the comprehensive properties of plasma sprayed VW75 magnesium alloy,and it is also an effective means to test the final service effect.The wind tunnel test device controls different samples to keep the same total temperature,total pressure and static temperature as the aerodynamic heat input conditions.The heat insulation effect of different schemes is reflected by the sample back temperature,and the heat resistance performance of different alloys is evaluated.After the wind tunnel test,the adhesion between the coating and the substrate is good,and there is no bulging,shedding and other phenomena;The heating rate of metal substrate inside VW75 magnesium alloy sample decreases from 0.8?/s without coating to 0.39?/s with coating,and the heating rate decreases by about 50%.The wind tunnel test results show that the coating has good bonding performance and remarkable heat resistance effect,which can meet the service environment and requirements in the future.The heating curves and heating rates of different alloys were obtained by wind tunnel test.The structural equation was established by referring to CMC model.The thermal conductivity of VW75 magnesium alloy and ZK60 magnesium alloy covered by TBC was calculated to be 6.44 W/(m·K)and 6.84 W/(m·K),respectively,The relationship between the thermal conductivity and the temperature rising rate of metal matrix in wind tunnel test is verified. |
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