Study On The Microstructures And Properties Of Plasma-sprayed Al/Al₂O_3p Composite Coating On Magnesium Alloy

Magnesium alloy is one of the lightest metals on the earth, which has the characteristics of high specific strength, high specific stiffness, excellent thermal conductivity, superior electrical conductivity, non-magnetic, electromagnetic shielding, good damping and excellent cutting, and so on. As the third type of metallic structural material except for the steels and aluminum alloys, magnesium alloy is widely used in industries such as aviation and aircraft, automobile manufacturing, electronics, national defense technology, and etc. due to these superior properties. However, the applications of magnesium alloy is restricted by its low electrode potential and active chemical properties, forming oxide film with porous structure in the air, the poor corrosion resistance and wear resistance.In this paper, aluminum coating prepared on the surface of the magnesium alloy AZ91D by wired arc spray was investigated firstly to improve the comprehensive properties of magnesium alloy, especially surface properties, and expand its application scope. Homogeneous microstructure and fine grains of the coating were gained. Part of the grain size has reached the nanometer level. The bonding of interface between coating and Mg alloy substrate was excellent. The study showed that there were new phases Mg17Al12 evolved due to the interdiffusion of atoms. There were no oxides and impurities in the coating except for some smooth and nearly circular pores. A large number of plastic and dimple-like fractures were observed. In this experiment, arc voltage and spray gas pressure were two important factors which have obvious effects on the bonding strength of Mg alloy substrate and coatings. The compactness of the coating and bonding strength were satisfied under spray voltage of 30V, spray gas pressure of 0.6MPa, which were 5.18% and 12.84MPa, respectively. The coating without sealing agent and the coating with sealing agent had better corrosion resistance than that of the Mg alloy substrate.Pure Al coating and Al/Al₂O_3p composite coating were prepared on the surface of Mg alloy AZ31 by plasma powder spraying. The optimum powder mixing ratio, obtained by observing surface 3D morphology, SEM morphology, XRD analysis and scratch test of the different ratio mixed sprayed coatings, was 80wt.% Al and 20wt.% Al₂O₃, respectively. Based on the orthogonal experiment of six factors and five levels, the proper parameters of arc spray for preparation of the Al/Al₂O_3p coating were spray current of 700A, spray distance of 90mm, main gas pressure of 0.4827MPa, secondary gas pressure of 0.2069Mpa, spray speed of 400mm/s and powder feed rate of 30.3g/min.The formability of the Al/Al₂O_3p coating was good and there was no obvious lamellar structure in the composite coating. As reinforced phase, Al/Al₂O_3p was uniformly dispersed in the coatings. The matrix Al and reinforced phase Al₂O₃ showed good compatibility and wettability. The interface between the coating and the substrate could be easily observed and the new phases AlMg and Al12Mg17 were found nearby the interface, which indicated that the diffusion of atoms occurred. The mean porosity of the coating was only about 3.80% and plastic fracture surface could be observed. The study revealed that the particles of Al₂O₃ exhibited different shape in the coating such as round, oval or multi-angular shape, which resulted from different melting degree of the Al₂O₃ in the plasma.Due to high thermal density of the plasma, the Al₂O₃ was melted fully and plenty of thermal was gained for the substrate. Flight speed of molten particles in plasma flame was faster than that in arc spray. So greater kinetic energy and percussive force were obtained when molten particles striking the substrate. At the same time, the bonding was more closely between the plat particle layer and substrate, layer and layer, and lamella was getting thinner and flatter with lower porosity. The oxides in the coatings were decreased greatly resulting from the better protection of the inert gas Ar. Generally, the oxygen increase was about 0.1-1% for plasma spray process and 0.5-3% for arc spray, respectively. The high oxygen increase meaned the high oxides content in the coating, which resulted in high porosity.The mean bonding strength of the Al/Al₂O_3p coating and Mg alloy substrate was 16.04MPa. The microhardness increased gradually from the coating surface to the substrate, ranging from HV45-HV55. Based on the rapid wear test and ring block wear test, wear volume and weight loss of the Al/Al₂O_3p coating were lower than those of the Al coating and AZ31 alloy but with highest friction coefficient. This could be explained by that the reinforced phase Al₂O₃ restricted the plastic deformation of Al and changed the interface state. This was also contributed to the effect of the reinforced phase.From the Tafel curves of the pure Al coatings prepared with arc spray and plasma spray, the composite coatings of Al/Al₂O_3p, the electrode potentials of three kinds of coatings were lower than that of the substrate but with higher corrosion currents. The three kinds of coatings mentioned above could be regarded as anodes of the primary battery and aimed to sacrifice the anodes to protect the Mg alloy substrate, which was cathode in the primary battery. Though Al₂O₃ oxides film formed easily on the Al surface, which can protect the substrate from attack of the corrosive media, the Cl- could penetrate the film and attack the Al substrate. Further, the Cl- can adsorb onto the coating surface and substitute O of the Al₂O₃, thus destroy the coating. Meanwhile, the atom O in the solution could cause the depolarization process of metal surface, accelerate anode to dissolve, make volume expansion for infiltration coating salt solution and led to stress corrosion.The corrosion resistance of the composite coating Al/ Al₂O_3p before and after heat treatment showed no obvious difference, which was better in the salt spray corrosion test than that in the soak corrosion test. During the salt spray corrosion test, NaCl hardly settled down in the solution. The corrosion mainly occurred at the surface of the coating, which belonged to the pit corrosion. During the soaking corrosion test, the coating was surrounded by the NaCl solution. The main corrosion, galvanic corrosion, occurred at the interface of the Al and Mg alloy except for the pit corrosion at the surface. In this case, the weight loss due to the corrosion was larger than those of the Mg alloy substrate and the composite coating Al/Al₂O_3p with sealing treatment. The weight loss of the composite coating Al/Al₂O_3p with sealing treatment was very small in either soak corrosion test or salt spray corrosion test, which indicated the coating provided good protection to the Mg alloy substrate and greatly improved performance of the Mg alloy in corrosive environment.According to the characteristics of Al and Al₂O₃ coating, the composite coating Al/Al₂O_3p, based on the Al substrate reinfored with Al₂O₃ particles, became the better choice to increase the wear resistance and corrosion resistance of the coating. The invetigation in this work offered some theoretical foundation for improving the wear resistance and corrosion resistance of the Mg alloy coating.