| With the rapid development of industry, the people pay more and more attention to lightweight, as one of the lightest structural materials, magnesium alloys have a widely application prospect on vehicle, machinery, aerospace, military fields. But magnesium alloy castings have a poor corrosion resistance, especially the castings are easy happening corrosion destroy in a seriously working conditions. At present, the traditional anticorrosion measures have all kinds of limitation for the complicate shape magnesium alloy casting. Therefore, it is an important topic to study on new anti-corrosion technology for expanding the application scope of magnesium alloy castings.The LFC (lost foam casting) process was employed in this paper, in order to improve the corrosion resistance of magnesium alloy, the alloying/ceramic layer with high strength and density was prepared on the magnesium alloy casting surface through spreading one or two layers special coatings on foam pattern surface. Further, the magnesium alloy surface alloying kinetic, metallurgy mechanism were revealed, the formation mechanism of alloying/ceramic composite layer was analyzed. In this study, a depth research has been made in alloying/ceramic formation law, the process route, anti-corrosion effect, the main work was finished as follow:Firstly, the LFC surface alloying technology of magnesium alloy was researched systematically, the metal aluminium powder was chosen as the alloying elements to prepare alloying coating, an average thickness of 300 to 500μm alloyed layer was fabricated on casting surface. The influence of alloying aluminum grain size, binder, vacuum degree and pouring temperature on alloyed layer microstructure were investigated. The results show that the surface tension was overcame when the pouring temperature, vacuum degree more than the critical value, the alloyed layer was formed; It was difficulty for liquid metal to infiltrate into the alloying coating layer when the aluminum particle was small, which caused a bad effect on alloyed layer quality; binder of alloying coatings also had an important impact on the formation process of alloyed layer. A compact alloyed layer was attained when the process parameters as follow:pouring temperature of 780℃, vacuum degree of-0.06 MPa, alloying coating thickness of 0.8 mm, Al powder particle size of 150μm. The XRD analysis shows that the alloyed layer was mainly consists ofα-Mg andβ-Al12Mg17, besides, a small amount of Al3Mg2, Al, (MgZn). The distribution condition of different elements in the alloyed layer section was analyzed, the results show the content of Al and the micro-hardness distribution along the surface to matrix was gradient shape. The electrochemical test of alloyed layer in 3.5Wt.% NaCl aqueous solution was carried out, the corrosion potential was improved 110mv, polarization resistance increased 6～7 times, so corrosion resistance of magnesium alloy was improved by surface alloying processing.Secondly, the magnesium alloy surface ceramic process was studied, in which the PbO-ZnO-Na2O ceramic powder was used to prepare the ceramic coating. A thickness of 40～200μm ceramic layer was fabricated on casting surface. The effect of vacuum degree and pouring temperature on combining interface between the ceramic layer and the matrix was analyzed detailedly. The larger vacuum degree can make coating layer tight and get a more dense ceramic layer, moreover, high pouring temperature make foam pattern decompose into small molecules, these molecules were ejected from sand mould successfully under larger negative pressure, improve the interface bonding strength. The results show that an excellent combining interface was formed when the pouring temperature was 760℃～780℃and vacuum degree was -0.06MPa. The influence of metal liquid mould filling frontier, pouring gate position and LFC coating on the ceramic layer formation process was inspected. By means of microscopic analysis, the line scan of combining interface was carried out, a conclusion was drew that the interface between the ceramic layer and matrix was machinery combination. The test reveals that bonding strength was an average of 4.5 MPa. Electrochemical experiments of ceramic layer in 3.5Wt.% NaCl aqueous solution shows the corrosion potential was improved about 400mv, the corrosion current dropped three orders of magnitude, the anticorrosion properties of magnesium alloys was improved greatly.Based on the above research achievement, the alloying/ceramic composite protective film was prepared on the magnesium alloy casting surface, and an alloying area with higher corrosion resistance was formed between the magnesium alloy substrate and ceramic layer, the alloy has a dual corrosion protection, the thickness of composite layer reaches to 400μm, the anti-corrosion was improved significantly than of sole alloying layer, and was quite with that of sole ceramic layer, but the bonding strength reaches to 5.5 MPa and rise by nearly 20%. Through the wettability test between the ceramic powder and different substrate, it was found that the bonding strength of interface was improved with the increase of Al element content in the substrate. In order to realize the surface modification, at the same time, improve the casting surface quality, research and development of special LFC transfer coating applying to magnesium alloy was carried out.Finally, the formation mechanism of alloying/ceramic layer was investigated, it was point out that the alloying process can be divided into three stages:liquid metal mould filling, particle melting, diffusion reaction, the alloyed layer was formed under heat transfer, mass transfer between alloying coating and liquid metal. The ceramic layer was formed under the melt latent heat, but no diffusion. The surface alloying process dynamic and the formation of ceramic layer model was established by analyzing the formation of alloying/ceramic layer formation process. The results for research of magnesium anti-corrosion technology have an important academic value and practical significance.
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