Research On The Production And Function Of Al-alloyed Coating On The Surface Of Magnesium Alloys

Based on the analysis of surface modification in the past, two kind of new technologies on surface treatment to magnesium alloys AZ61 are introduced in the paper. 1) hot-dip By commanding the volume ratio of base and coating materials, the dipping temperature and time and some other process parameters, best-performed coating is attained. Making use of SEM, EDS, XRD and some other testing technology, detailed research on the organization, structure, composition of the coating is made, and fundamental reasons on why metallurgical binding is got on the coating is also analyzed based on the diffuse thermodynamics. 2) high frequency induction Through the analysis of the coating's organization, structure and composition of different conditions in the experiment, the most proper process parameters is acquired for the best-performed coating. Then according to the results in the corrosion experiment, calculate the corrosion rate, analyze the corrosion features of magnesium alloys and the corrosion resistance mechanism of the coating, detailed survey on the bond strength of the coating is made.It shows out that when Al is hot dipped by AZ61, and the volume ratio of base and coating materials is 1.2 to 1, the dipping temperature is 800°C, the binding interface is featured by metallurgical bonding when dipping 2s. And what's more, the thickness of the transition region is as much as 1000um, the microstructure in the room temperature is α-Mg solid solution and a few eutectic structures which is the mixture of α-Mg and β-Mg₁₇Al₁₂. The microhardness of the coating is HV200. Beside, because of the high content of Cu in this area, a few ternary compounds of Al₅₀Mg₄₈Cu₂ can be found. Mg-Al binding is achieved owing to the tow comprehensive effects of fuse and diffuse. Through further analysis of diffuse thermodynamics, further statement is made that the coating interface is not only affected by the pure diffuse.After the high frequency induction cladding of Al by AZ61, the sample of improper binding shows out a white band in the interface, while the sample of well binding shows out the natural transition, without the white band, which formation is closely related to contra-flow of Mg and Al and the evenness' degree of diffusing. The coating structure is mainly intermetallic compounds β-Mg₁₇Al₁₂, with a few Al₂Cu. The content of Al is related to microstructure. While Al% < 6%, the structure is mainly α- Mg, while 20% < Al%< 30%, the structure is fir-tree crystal of a- Mg, with a few α-Mg+β-Mg₁₇Al₁₂, while 30% < Al% < 40%, the structure is mainly Mg₁₇Al₁₂, with the same eutectic structure in it. By calculating the electrochemical impedance spectroscopy, the electrochemical AC impedance spectra and the corrosion rate, the result is found that the cladding Al, comparing with the uncladding one, its corrosion rate decreases 2 magnitude, the abundant β in the cladding surface is the key reason for the high corrosion resistance rate. Combining three fractures in the experiments, detailed analysis is made that the good binding strength exists between the coating and the matrix. After the comprehensive analysis of the experiment's result, the process parameter of the most proper coating is attained: heating power is 40 to 60kw; the heating time is 50 to 65s.