Research On Thermal Diffusion Of Aluminum Coating On Magnesium Alloy Based On Surface Nano-crystallization

For a long time, poor corrosion resistance has been one of main bottlenecks to limit magnesium alloys to be widely used. In order to improve the corrosion resistance of magnesium alloys, surface protection is very necessary. In this paper, as surface protective coating for magnesium alloys, aluminum coating was prepared by oxygen - acetylene flame spraying technology. Previous experimental results showed that thermal spraying aluminum coating could supply a certain protection on magnesium alloys. However, formation of thermal diffusion and metallurgical bonding between the aluminum coating and magnesium matrix occurred at higher temperature (usually above 400℃), which may cause microstructure and properties of magnesium alloys change to a certain degree, and the higher temperature is, the larger energy is consumed.In order to low Mg-Al inter-diffusion temperature and improve protective effect of the coating on the matrix, in this paper surface nanocrystallization (SNC) induced by High Energy Shot Peening (HESP) was creatively combined with thermal spraying aluminum coating on magnesium alloy, and two different new combining processes were applied, which are, first surface nanocrystallization of magnesium alloys then thermal spray coating (referred to as Mg/SNC/Al) and first thermal spray coating on magnesium alloys then surface nanocrystallization (referred to as Mg/Al/SNC). The following aspects were creatively studied: effect of HESP on microstructure and properties of magnesium alloys, the thermal stability of nanocrystalline in surface layer of magnesium alloys, effect of surface nanocrystallization on Mg-Al inter-diffusion and the diffusion layer morphology observation with composition analysis, in which three inter-diffusion models of Mg-Al were established.In the experiment, die cast magnesium alloy AZ91D and wrought magnesium alloy AZ31B were used as experimental materials. Surface and cross-sectional morphologies of magnesium alloys after HESP and thermal diffusion treatment with Al coating were observation by OLYMPUS SZ61TR stereomicroscope, OLYMPUS GX51F metallographic microscope (OP), JSM-6460LV scanning electron microscope (SEM), EDS-742 Energy Dispersive Spectrometer (EDS) and SUPRATM 55 field emission scanning electron microscope (FESEM). Through Riguta D / Max-RC X-ray diffraction (XRD) nano-grain size and phase structure on surface layer of magnesium alloy after HESP and heat treatment were analysed. New concepts of equivalent grain size and micro strain were proposed to analyze XRD data, which could avoid calculation errors of XRD and attain relationship between experimental data ingeniously. Microhardness and potentiodynamic polarization curve were measured by use of HX-1000 microhardness tester and electrochemical systems, respectively.The results showed that: grain size on surface layer of two different kinds of magnesium alloys was greatly refined into the nano-scale by HESP, which meant surface nanocrystallinzation could be achieved for magnesium alloy, but refine processes of grain size were different for the two different kinds of magnesium alloy. After surface nanocrystallization microhardness and the corrosion current density on surface layer of magnesium alloys increased significantly. When the heat treatment temperature exceeds 100℃, the nanocrystalline on surface layer began to grow gradually, and when the temperature is lower than 100℃, only recovery occurred with elimination of micro-strain on the surface layer. Nanocrystalline grown up rapidly with heat treatment temperature increasing, and the microhardness and corrosion current density reduced continuously with the growth of nanocrystalline. After surface nanocrystallization, the inter-diffusion temperature between magnesium alloy and aluminum coating reduced so greatly that diffusion layers could be formed at the heat treatment condition of 320℃×2h. Weight ratio of Mg and Al in the diffusion layer was about 6:4, which is similar with the proportion of second-phase Mg₁₇Al₁₂ in magnesium alloy.