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Investigation on Surface Defects Produced in Pretreatment of Die-casting AM60B Magnesium Alloy

DING Pei-pei,YE Cong-wang,LING Guo-ping(Institute of Metallic Materials,Zhejiang University,Hangzhou 310027,China)  
It is found that the surface defect like black stripes appeared in certain surface regions of die-casting wheel hub AM60B magnesium alloy after acid pickling in the anti-corrosion treatment.The microstructure and composition were observed by means of stereomicroscopy,metallomicroscopy and scanning electron microscopy combined with energy spectrum analysis.The formation mechanism of the defect was discussed.The results show that the microstructure and composition on the edge of die casting AM60B magnesium alloy are nonhomogeneous.Such nonhomogeneity is attributed to the crack which originates under stress at later solidification is feeded by surrounding low melting point Al-rich molten metal.Al-rich molten metal contains certain content of low potential element Mg,and local galvanic cell is formed between Mg and ambient composition in corrosive environment.Mg is selective corroded accordingly.The ambient composition around the crack is easier to be eroded for a great deal of high potential eutectic structure existing at the same time crack,ultimately forming the morphology of black stripes.
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