| As the lightest constructional material, magnesium alloy has high specific strength, high specific modulus, good damping ability,good magnetic-shielding ability, and good recyclability. Since its chemical character is very active, Mg alloy is prone to oxidize and burn, which adds difficulties to deposit, production, processing and the use of product, and this becomes the important factor to hamper the popularization, application and development of magnesium alloy. The only solution to these issues is to improve the oxidation resistance of magnesium alloys.This topic started from the ignition-proof of the magnesium alloy emerging from foundry practice, concluded ignition-proof methods as solvents covering, gas protection, semi-Solid forming and alloying from fire triangle, contrasted the process feature and shortage of different methods, summarized the advantages of alloying and development prospects, pointed out if suitable alloying elements can be found out, alloying can not only improve the flammability property of magnesium alloy solution, but also improve oxidation resistance at high temperature (not molten state) and corrosion resistance at normal temperature, alloying will have extradionary better ignition-proof effects compared with other methods, it can solve most of difficulties confronted with expanding the application of magnesium alloy with same kind and amount of alloy.The topic selected mixed rare earth (RE), the component of which was optimized, and added as alloying element to magnesium alloy ZM5, found that the right amount of mixed rare earth can not only improve the oxidation resistance at high temperature, but also improve corrosion resistance at low temperature. By the combination of theory and practice, this topic researched the mechanism that the function and effect of improving oxidation resistance at different states by adding RE systematically, and analyzed the active mechanism of RE on different magnesium alloy in depth, established the theoretical and practical foundation for the successful development of RE ignition-proof magnesium alloy.Mixed rare earth can improve the ignition temperature of ZM5 solution. The experiments results indicate that the ignition temperature of ZM5 can be raised greatly by the addition of RE, the effects of ignition-proof was the best when the addition of RE was 0.1%, the ignition temperature of ZM5 reached 830℃, and the ignition-proof magnesium alloy can be melted and poured in atmosphere. The surface morphology of oxide film, phase, distribution of elements of ignition-proof magnesium added RE were detected by scanning electron microscopy (SEM), X-ray diffraction (XRD), spectroscopy (EDS) and other means, found that the oxide film of ZM5-01% was compact, the thickness of it was about 2.5 ~ 3.5μm, the internal atom was not homogeneous, RE elements had a significant phenomenon of surface enrichment. The oxide film had double-layer structure, the external layer was MgO which was not protective, the oxygen can diffuse to internal through external layer continuously. As the RE enriched at the surface of solution, the concentration of rare earth at the surface of solution was increased, which created favorable thermodynamic conditions for the reaction between Mg and oxygen, as the P-B ratio (η) greater than 1, the generated RE2O3 can fill up the hole of MgO, and the compressive stress created by the volume expansion of oxidation make the MgO film to be compact, so the compact internal layer which consisted of MgO,RE2O3 and Al2O3 owned the ability to hold back the diffusion of reactant. By analyzing the temperature oxidation kinetics, It was found that compact internal layer can prohibit the diffusion of reactant effectively and make the oxidation process transfer from controlling by interfacial reactions to controlling by diffusion, oxide film growth follows a parabolic law, indicating the surface of oxide film have protective effect on solution.The oxidation resistance at high temperature of ZM5 and ZM5-0.1%RE was compared by TGA curve. When the ZM5 was heated to 500℃, the oxidation weight of it gains became intense, the temperature of ZM5-0.1%RE can reach 600℃as it had good oxidation resistance at high temperature than ZM5.By exploring the tissue distribution of two alloys after oxidation and combining Mg-Al phase diagram, it was found that the thermal oxidation was start at grain boundary, the important reason was the melting point ofβ-Mg17Al12 which was distributed at grain boundary was very low. When 0.1%RE was added to ZM5, the amount ofβ-Mg17Al12 owned low melting point was decreased, the"breach"of grain boundary oxidation was decreased, and theβ-Mg17Al12 transfer from continuous distribution to discontinuous, the liquid phase formed at high temperature was discontinuous no longer, so the channel that the oxygen diffuse along the grain boundary through continuous liquid phase was broken off, as aluminum-RE phase formed by RE and aluminum and Mg17Al12 were symbiotic on grain boundary, so the inwards infiltration of oxidative reaction can be prevented. So when 0.1%RE was added to ZM5, the high-temperature oxidation resistance of it was improved, the burning behavior was not occurred. At the same time, as the adding of the RE, crystal grain of basal body have been refined, so the ZM-0.1%RE had better mechanical property.Combining the effect of the RE to the ignition temperature of magnesium alloy solution and high-temperature oxidation resistance of magnesium alloy, the"two ignition temperatures"hypothesis of ignition-proof magnesium alloy added RE was put forward, which had theoretical guidance to melting and test of ignition-proof magnesium alloy.Mixed rare earth can improve the corrosion resistance of ZM5 magnesium alloy. By static weight-loss test, contrasted and analyzed the natural corrosion behaviors of ZM5 and ZM5-0.1%RE .The result showed that the adding of RE diminishes the matrix grain of magnesium alloy,β-Mg17Al12 was reduced and become disperse, the degree of segregation and role of galvanic cathode of Al which existed in matrix grain were reduced, so the corrosion resistance of magnesium alloy was improved. |
PDF Full Text Request