| Magnesium alloy is the lightest metal structure materials in engineering application, which is satisfied the demand of lightweight structure in the areas of automobile manufacturing, aerospace and national defense technology owing to the outstanding performances of light quality, high specific strength and specific stiffness, good thermal conductivity and castability. However, the key factor to block the wide application of magnesium alloy is some problems such as its active chemical properties, low hardness, the poor corrosion resistance and wear resistance. The alloying coating fabricated on the surface of AZ31 magnesium alloy is one of the effective methods to improve its surface properties. Therefore, it has great theoretical significance and practical values to carry out the study on microstructures and properties of surface alloying coating and diffusion layer of AZ31 magnesium alloy.On the basis of the diffusion mechanism and diffusion rule, the concentration distribution equation of diffusional metal element in the diffusion process was established using Fick second law. According to the Arrhenius formula, the diffusion coefficient was obtained, which was determined by the diffusion activation energy and temperature. The diffusion coefficient of Al and Zn elements diffusing in the pure magnesium metal can be obtained under different temperature. The relation of Al and Zn elements concentration distribution and heating temperature was determined, and the relation between the diffusion distance of interface and holding time was found. Results showed that the diffusion coefficient of Al and Zn elements increased with the rising of the heating temperature. And the distance of diffusion interface increased with the rising of the heating temperature and the extension of holding time.The variation of interface topography in Mg matrix was observed by heating the samples of Al and Mg metals to 430℃. The observation results indicated that the variation of the diffusion interface was changed distinctly during the early stage of diffusion, and the rate of Al atom permeating into magnesium matrix was fast. However, the changes of diffusion interface were steady at 60 min. The diffusion rate of interface was slow down from 60 min to 120 min. The concentration of Al element decreased gradually in magnesium matrix. Moreover, the Mg17Al12 and Mg2Al3 of Mg-Al intermetallic compounds and α-Mg solid solution were detected at the diffusion interface. The experimental results of Zn element diffusing in magnesium matrix showed that the diffu-sion distance and rate of interface increased with heating temperature rise. The Mg-Zn intermetallic compounds Mg7Zn3 and Mg Zn phases were formed in the diffusion layer, and the lamellar structure was composed of α-Mg and MgZn phases. The lamellar structure grew into the magnesium matrix along the grain boundary.The Zn-Al alloy coating was prepared on the surface of AZ31 Mg alloy by thermal diffusion method. According to the analysis of experimental results and computational data, the optimal diffusion parameters of heating temperature 360℃ and holding time 60 min were obtained. The microstructure and phase constitution of the Zn-Al alloy coating and magnesium alloy substrate had no changes after diffusion process. An obvious diffusion layer was formed between Zn-Al alloy coating and magnesium substrate. Furthermore, some new phases such as Mg7Zn3 and Mg Zn were generated in the diffusion reaction layer, which indicated that the formation of the diffusion layer was mainly determined by the diffusion of Zn and Mg elements. Compared with the microhardness of magnesium substrate, the microhardness of diffusion layer and Zn-Al alloy coating were 167 HV and 115 HV, respectively, which was improved greatly. When the heating temperature was 360℃ and holding time was 60 min, the average shear strength of sample was the maximum(49.26 MPa). It was shown that the Zn-Al alloy coating was metallurgical bonding to AZ31 magnesium alloy. Some cracks and pores were existed in the diffusion layer with low temperature, while the shear strength of sample decreased with high temperature. The fracture pattern of high temperature sample was brittle fracture, and some cracks were existed in the fracture surface. With the Al element content increasing to 40 wt.%(mass percent) and adding 0.1 wt.% La element, the grain of Zn-Al-La alloy was refined. The Mg17Al12, Mg2Al3, Mg7Zn3, MgZn2 and Al11La3 phases were formed in the diffusion layer of Zn-Al-La alloy, which had the effect of reinforcement to enhance the microhardness and shear strength of diffusion layer. The intermetallic compounds and microhardness of Zn-Al-La alloy diffusion layer increased with the heating temperature rising, but the shear strength decreased due to lots of brittle phase.The Zn-Al alloy coating was directly fabricated on the surface of AZ31 Mg alloy by casting infiltration during the casting process. When the casting temperature was high, a large amount of intermetallic compound was produced. Furthermore, the microstructure of Mg alloy substrate was affected greatly. In the casting infiltration layer, the microhardness increased, but the shear strength value was low. A great deal of brittle structure was existed on the fracture surface. When the casting temperature was low, the casting infiltration layer was not formed fully owing to the lack of diffusion energy in magnesium alloy liquid and little quantity of Mg, Al and Zn elements permeating. The shear strength of interface between the Zn-Al alloy coating and magnesium substrate was bad. The shear strength of sample with the casting temperature of 635℃ was 23.63 MPa, which indicated that the casting infiltration layer had a good bonding. The Mg2Al3, Mg17Al12 and MgZn2 phases were generated in the casting infiltration layer. The Mg-Zn intermetallic compound MgZn2 phase was centralized distributed in the honeycomb structure of the casting infiltration layer, and the Mg-Al intermetallic compounds was distributed widely. In addition, the lamellar structure of α-Mg + β-Mg17Al12Mg17Al12 phases was precipitated along the boundary of magnesium substrate. The microhardness of the casting infiltration layer under the casting temperature of 635℃ and 620℃ had similar value, but it was less than that of under the casting temperature of 650℃.The comparison on the electrochemical corrosion test results of AZ31 magnesium substrate, Zn-Al alloy and Zn-Al-La alloy coating by thermal diffusion, the polarization curves revealed that the corrosion potential of Mg substrate was the lowest(-1.594 V) without passivation stage. But the corrosion potential of the Zn-Al alloy and Zn-Al-La alloy coating with obvious passivation stage increased to-1.0829 V and-1.0496 V, respectively. The corrosion resistance property of Mg alloy was improved by Zn-Al alloy and Zn-Al-La alloy coating.Compared with the wear results of magnesium substrate, the weight loss of Zn-Al alloy and Zn-Al-La alloy coating by thermal diffusion and Zn-Al alloy coating by casting infiltration was very little, and the wear resistance property improved greatly. The worn morphologies showed that the worn degree of magnesium substrate was serious with wide ploughing and fragment. However, the feature of scratches presented on the Zn-Al alloy coating is narrower and shallower without fragments, only some hard particle appearing. The worn morphologies of Zn-Al-La alloy coating exhibited some scratches were existed.According to the experimental results, the corrosion resistance and wear resistance properties improved effectively by Zn-Al alloy and Zn-Al-La alloy thermal diffusion coating and Zn-Al alloy casting infiltration coating. The investigation in this work will supply some valuable foundation for expanding the application of magnesium alloy. |
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