Effect Of Alloying Elements On Microstructure And Properties Of Hot Dip Zn-Al-Mg Alloy

In this paper,the influence of alloying elements on the structure and properties of hot-dip Zn-Al-Mg alloy was studied by alloying method.The phase of Zn-Al-Mg alloy was analyzed by the X-ray diffraction（XRD）and the microstructure of alloy was observed and analyzed by the Scanning Electron Microscope（SEM）and Transmission Electron Microscope（TEM）.The hardness property of alloy was tested by Bromwell Hardness tester.And the impedance spectrum and polarization curve of the Zn-Al-Mg alloy sample immersed in 5 wt.%NaCl solution were studied by electrochemical experiment to analyze the corrosion behavior.The self-corrosion rate of Zn-11Al-3Mg-（Ti,Zr）alloy was calculated by the full immersion test in5 wt.%NaCl solution.The effect of Ti,Zr and Sb elements on the microstructure and corrosion resistance of Zn-Al-Mg alloy was discussed,and the mechanism of corrosion resistance was explored,which provided a theoretical basis for the research and development of new corrosion-resistant hot-dip Zn-Al-Mg alloy.The results showed that the solidification structure of Zn-11Al-3Mg-（Ti,Zr）alloy consists of Zn rich phase,Al rich phase,MgZn₂ phase,Zn/Al/MgZn₂ ternary eutectic phase and and the[100]Al₃(Ti_xZr_1-x)phase which was calibrated by TEM.With the addition of Ti and Zr,the Al rich phase changed from dendrite to petal-like phase,and the solidification structure of the alloy was obviously refined.The uniform structure has an important influence on the properties of the alloy.With the increase of Ti and Zr,the microstructure of the alloy was refined and the Rockwell hardness of the alloy was also increased.Adding equal amount of Ti and Zr elements,the impedance value of the alloy sample increased indicated that the interface reaction was more difficult to occur.Moreover,the self-corrosion potential of the alloy moved to the positive in varying degrees indicated that the corrosion current density of each curve moved to the negative direction.The result showed that Zn-11Al-3Mg-0.075（Ti,Zr）alloy has better corrosion resistance.Moreover,the corrosion experiment shows that the type of corrosion products does not change by the addition of Ti and Zr,while the compactness of corrosion products was improved.The corrosion products of alloy samples are mainly composed of Zn₅（OH）₈Cl₂ and Zn₆Al₂（OH）₁₆CO₃.The solidification structure of Zn-11Al-3Mg-Ti-Sb alloy was composed of Zn rich phase,Al rich phase,MgZn₂ phase,Zn/Al/MgZn₂ ternary eutectic phase and Al₃Ti.The structure of the alloy is obviously refined with the addition of Ti and Sb elements.With the increase of Ti and Sb,the microstructure of the alloy is refined and the Rockwell hardness of the alloy also increases.The results of electrochemical test show that the addition of Ti and Sb increases the resistance of charge transfer,which indicates that the interface reaction is more difficult to occur.And the Zn-11Al-3Mg-0.2Ti-0.1Sb alloy has better corrosion resistance,with corrosion potential of-1.36627625 V and corrosion current density of 5.89837E-3 A·cm².Moreover,the compactness of the corrosion product layer was improved by the addition of Ti and Sb.The Zn-11Al-3Mg-0.2Ti-0.2Sb alloy coating with 1 minute immersion had good surface quality.The solidification structure of Zn-2.5Al-2Mg alloy was mainly composed of Zn rich phase Al-rich phase and granular/layered Zn/Al/MgZn₂ ternary eutectic.The fine dispersed Al₃Ti,Al₃Zr and Al₃（Ti,Zr）phases formed by adding Ti and Zr elements could be used as nucleation matrix to refine the alloy structure.The hardness of the alloy with Ti and Zr added in combination was higher than that of the alloy with Ti added alone,but not higher than that of the alloy with Zr added.The addition of Ti and Zr increased the resistance of charge transfer and made the interface reaction more difficult to occur.Moreover,the addition of Ti and Zr improved the corrosion resistance of the alloy,and the alloy with equal addition of Ti and Zr had better corrosion resistance.In this paper,the corrosion mechanism model of Zn-2.5Al-2Mg-Ti-Zr alloy was proposed,and it was found that the passivation film of alloy sample was mainly composed of Zn₅（OH）₈Cl₂ and Zn₂Al（OH）₆Cl,and contained a small amount of Mg（OH）₂.