CORROSION OF ULTRASONIC SPOT WELDED JOINTS OF MAGNESIUM TO STEEL

摘要

Mixed-metal joining, especially between magnesium and steel, isone of the critical technologies in achieving light-weightingvehicle body construction. However, galvanic corrosion betweenmixed metal joints is inevitable but not well quantified. In thisstudy, 1.6 mm thick Mg AZ31B-H24 was joined to 0.8 mm thickhot-dipped galvanized (HDG) mild steel by ultrasonic spotwelding in lap-shear configuration. No specific corrosionprotection was applied in order to study worst-case conditions forcorrosion behavior. The approach used an automotive cycliccorrosion test - Ford Arizona Proving Ground EquivalentCorrosion Cycle (APGE), which includes cycles of dipping in asalt bath, air drying, then holding in constant humidityenvironment. Lap-shear strength of the joints decreased linearlywith the exposure cycles. All the joints were either taken out oftest cycle for mechanical test or they separated within thehumidity chamber before 25th cycle. X-ray diffraction analysisconfirmed the formation of Mg(OH)_2 deposit in the crevicebetween the AZ31 and steel sheets and on the surface of the AZ31.The deposit grew thicker with cycles with exerting enough forceto deform the AZ31 and HDG steel and causing a gradual openingof joints. The corrosion of the AZ31 was localized and nonuniform.The most severe corrosion occurred not at theintersection of AZ31 and the steel but rather 15-20 mm away fromthe spot welds.