| With the rapid utilization and massive consumption of energy in th e world,in the 21 st century,human beings are paying more and more attention to energy saving and emission reduction,so they propose measures such as lightweighting of automobiles.Magnesium alloy have high hopes for its excellent specific strength and stiffness,but its shortcomings limit application in industrial production.Poor corrosion resistance is one of the important shotcoming.In this dissertation,AZ31 B magnesium alloy is selected as the substrate,and alumina-reinforced copper-zinc alloy composite coating and copper-zinc mixed powder composite coating are prepared on the surface by low-pressure cold spraying technology.Metallographic microscope,stereomicroscope,laser confocal microscope,scanning electron microscope,energy spectrum analyz er,X-ray diffractometer,etc.were used to characterize the morphology,structure,phase composition and corrosion morphology of the cold sprayed coatings.The dimensional microhardness tester,friction and wear tester and universal tensile testing machi ne were used to test the mechanical properties of the coatings,and the electrochemical workstation were used to study the corrosion behavior of the coatings.The main research contents are as follows:The optimal preparation process parameters of the cold spray coatings of the corresponding system were found through a series of experiments.The working pressure of cold spraying was 0.6 ～ 0.8MPa,the gas was compressed air,and the spraying distance was set at 12 mm.By adjusting the gas temperature,powder supply position,nozzle movement speed and alumina content,the thickness and preliminary density test were used as indicators to explore the optimal process parameters for coating depositi on.In this dissertation,the original powders adopt spherical copper powder,spherical-like zinc powder,spherical copper-zinc alloy powder and irregular polygonal alumina powder.Among them,alumina reinforced phase powder mainly plays the role of improving powder fluidity and tamping the coatings.Through a series of experiments,the following rules were obtained for powder materials: spherical copper powder had good fluidity,high powder deposition rate,but large coating porosity;copper-zinc alloy powder had good fluidity,but was not easy to plasticize deformation,so adding 50% by weight of alumina content improves powder deformation ability,increases deposition rate,and also increases coating density;zinc powder had poor fluidity.After adding spherical copper powder and alumina powder,the fluidity was improved,and the thickness and density of the copper-zinc mixed powder composite coating were also improved.Copper-zinc alloy composite coating and copper-zinc mixed powder composite coating can provided certain corrosion protection for the substrate.The copper-zinc alloy composite coating and the copper-zinc mixed powder composite coating had greatly improved the surface hardness of the magnesium alloy,and after the tamping of the alumina,the two coatings had good density and low porosity.When the copper-zinc mixed powder composite coating was rubbed,the metal abrasive debris enters the wear scar to accumulate,which reduced the further wear of the coating and showed better wear resistance than the copper-zinc alloy composite coating.The bonding strength of the two kinds of alumina reinforced composite coating and the substrate were good.Among the test of corrosion resistance,the pure copper coating was used to compare with two composite coatings.The results showed that the pure copper coating had the worst corrosion protection effect.Due to the absence of the alumina reinforcing phase,the density of the pure copper coating was low,and the self-corrosion current density at the initial stage of immersion was close to that of the magnesium substrate.After soaking in 3.5wt.% Na Cl solution for 360 h,the surface of the coating had obvious cracks appear and the coating resistance had also decreased.Alumina reinforced copper-zinc alloy composite coating and copper-zinc mixed powder composite coating have played a good role in corrosion protection for magnesium alloy.The initial self-corrosion current density of both was lower than that of the magnesium substrate in the early stage of immersion.The copper-zinc alloy composite coating showed obvious pitting corrosion behavior.The pitting product was denser Zn O.And the coating had higher density.No obvious cracks appeared in the coating.The copper-zinc mixed powder composite coating showed obvious layer corrosion,and accumulated more corrosion products of zinc on its surface with the soaking time goes by.After immersion for 720 h,the corrosioned depth of the coating was only 80 μm.Combined with the analysis of wear scar morphology,zinc also changed the wear mechanism of the coating,reduced the wear rate of the copper-zinc mixed powder coating. |
