| 其他摘要 | The poor corrosion resistance is a serious restriction on the extensive application of Mg alloys. Improving the corrosion resistance of Mg alloys by proper surface alloying is of significance in view of science and technology. Among the numerous alloying elements, aluminum and rare earth (RE) showed good prospect in improving corrosion resistance of magnesium alloys. Research on surface alloying of magnesium base alloys with Al is extensive, while alloying with RE is rare. In the past, study on RE alloying mainly focus on addition of RE intermediate alloy during alloy melting and casting as well as surface RE conversion treatment in domestic and overseas. In the present study, laser surface alloying (LSA) and high-energy micro-arc alloying (HEMAA) have been employed to deposit rare earth (RE) metals onto Mg alloy in an attempt to enhanceing its corrosion performance. To develop a corrosion-resistant magnesium alloy, laser and HEMAA surface rare earth (RE) alloying layer formed on magnesium alloys and their corrosion properties have been studied, which was funded by Natural Science Foundation of China with a name “Study on the surface RE alloying layer formed on the magnesium alloys by HEMAA”(Project No.50371093). The typical ternary RE-free Mg-Al-Zn system ZM5, AZ31 alloys were employed as substrates. Electrodes made by Al-Y, Al-Nd and Mg-Nd alloy were used as RE-source for HEMAA. At the same time, laser surface remelting of RE-containing magnesium alloys and laser surface alloying of the preplaced Al-Y, Al, Mg-Al and Al-Si powders were also performed for comparison. The characteristics of RE alloyed layer, the existent status of RE, the transfer of RE and the formation of RE alloyed layer were evaluated by optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA). Corrosion resistance of the different RE alloyed layers were evaluated by potentiodynamic polarization and long term immersion tests in an artificial seawater. The main results are summarized as following.
1、Laser surface remelting zone on the magnesium alloys containing RE metals has different microstructure to the as-received substrate. The RE-containing phases are significantly refined to nanometers and distributed in the matrix. The RE is enriched at the interface. The interface of substrate/surface remelting zone boundary of RE-containing Mg alloys is much clear to be observed in comparison to that of RE-free Mg alloys. RE-containg Mg alloys showed high resistance to thermal cracking in laser melting process in comparison with the RE-free Mg alloys.
2、An Y-containing alloyed layer composed of Al2Y, Mg17Al12 and Al is obtained by laser surface alloying different preplaced of Al-Y powders on ZM5 Mg alloy. The grain size of this layer is about 1~1.5μm. The relative proportion of Al2Y increases with preplaced thickness of the Al-Y powders, while the relative proportion of Mg17Al12 decreases. Microhardness of the alloyed layer can reach to 300HV. The alloyed layer has three different geometric characteristics depending on the thickness of preplaced powder. From microscopic point of view, the geometric characteristics are related to the dilution. The interface angle shows a linear relation with the thickness of the preplaced Al-Y powder.
3、The preferential formed phases are Mg17Al12, Mg2Si and Al2Y during laser surface alloying the preplaced Al/Mg-Al, Al-Si and Al-Y on ZM5 Mg alloy. It is these preferential formed phases that cause the different dilution degree of the alloyed layer. Difference in melting point of the preferential formed phases brings different intensity of the melt convection, which leads to different dilution. Relative high melting point of the preferential formed Al2Y phase has an ability to suppress the convection, thus the obtained alloyed layer has a relatively lower dilution degree for the Al-Y powder rather than powders of Al/Mg-Al and Al-Si.
4、HEMAA has been employed to deposit three different binary rare earth alloys of Mg-Nd, Al-Nd and Al-Y on AZ31 Mg alloy. During the alloying process, mass transfer between substrate and electrode occurs simultaneously, but the amount of the mass transferred is quite different from each to other.
5、The so called“splash spraying”appearance is observed on the surface of both spots and alloyed layers during HEMAA process. Growing style of the alloyed layer exhibits two different ways mainly depending on either mass gain or mass loss of the AZ31 substrate. When substrate gains a certain amount of mass, the surface of the alloyed layer is higher than the initial surface of the substrate; the other way round, the surface of the alloyed layer is lower than the initial surface of the substrate.
6、Mass transfer intensity from electrode to substrate depends on relative heat flux(Rq) between electrode and substrate when HEMAA has been employed to deposit three different binary Mg-Nd, Al-Nd and Al-Y RE metals on AZ31 Mg alloy. Among the three electrodes of binary RE metals, Mg-Nd electrode has a highest relative heat flux value, which causes a highest amount of mass transferred from electrode to substrate. For example, the maximum value of the mass transfer coefficient (MTC) for Mg-Nd can be amount to 67.56%, while for the Al-Nd and Al-Y electrodes 42.92% and 19.53%, respectively.
Mass transfer coefficient (MTC) depends not only on relative heat flux, but also on discharge energy input and function time. The higher the discharge energy input is, the higher the MTC is in the HEMAA process. The cathode mass gain (CMG) is negative at very low settings (60V, 100Hz, 420W) for all the three electrodes, or positive at both middle (80V, 250Hz, 630W) and higher settings (100V, 525Hz, 1500W) for Mg-Nd and Al-Y electrodes, or negative at middle settings and changed from negative to positive at higher settings for Al-Nd electrode.
7、Laser surface remelting zone of RE-containing Mg alloys, laser Al-Y alloying layer of ZM5 alloy and high-energy micro-arc with Mg-Nd, Al-Nd and Al-Y deposited layers on AZ31 alloy are all superior to their substrates in corrosion properties in 3.5wt% NaCl solution. It is observed that a continuous and compact film formed on laser surface Al-Y alloyed layer and HEMAA Al-Y/Al-Nd layers. The film is stable in the presence of chloride ions as a result of the existance of Al and RE in the film. Also, corrosion resistance of HEMAA Mg-Nd layer is lower than that of the laser Al-Y and HEMAA Al-Nd /Al-Y alloyed layers because of lower Al content in the surface film. On contrast, ZM5 and AZ31 substrates exhibited severe localized corrosion wherein corrosion scale is discontinuous, breached and dissolved in the presence of chloride ions. Obviously, lack of Al in the film is responsible for the low corrosion resistance.
8、The formation process of the alloyed layer during both laser surface alloying and HEMA alloying is different. The convection in laser surface alloying plays a great role, while the “meltdrop spraying” in HEMA alloying is the main factor that controls the layer formation. The layer formed by laser surface alloying can be considered as a mixed layer of substrate with preplaced powder on it, while the layer formed by HEMAA can be considered as an alloy layer identical to the electrode. As a result, the corrosion resistance of the laser surface alloyed layer is inferior to the layer formed by HEMAA.
10、Corrosion resistant mechanism of the three alloying process, i.e. laser surface remelting of RE-containing Mg alloys, laser surface Al-Y alloying of ZM5 alloy and HEMA alloying of AZ31 using Mg-Nd/Al-Nd/Al-Y electrodes, is quite different. The improved corrosion resistance was attributed to the changed existance status of RE compounds by laser surface remelting, to the newly formed RE metals layer by addition of RE compounds in laser surface alloying, and to the alloyed layer with a composition identical to the electrode in HEMAA, respectively. Among the three processes, HEMAA is a technology for that it can deposit an electrode with expected properties on the surface of Mg alloy so as to form an alloyed layer identical to the electrode. |
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