Corrosion of ultrasonic spot weldbonds of magnesium to steel

Reliable mixed-metal joining, especially between magnesium and steel, is one of the critical technologies in achieving light-weighting vehicle body construction. However, galvanic corrosion between mixed metal joints is inevitable but not well quantified. In a previous study [1,2], 1.6 mm thick magnesium (Mg) AZ31B-H24 was joined to 0.8 mm thick galvanized mild steel by ultrasonic spot welding in lap-shear configuration and went through an automotive cyclic corrosion test including a step of immersion in 5% NaCl solution. In this study, joints of identical material combination made by ultrasonic spot weldbonding (with adhesive) and a different automotive cyclic corrosion test of immersion in 0.5% NaCl solution are analyzed. Corrosion test procedure includes salt liquid immersion, drying, and constant humidity exposure. Good joint strengths were retained at the end of 30-cycle test, with about 45% of the strength of that of non-corroded samples. Failure mode was shown to be interfacial failure of the USW joints with metal-metal bonding between Mg and steel maintained throughout 30 cycles of corrosion exposure. Joint strength of corroded samples decreased due to the corrosion attack at the joint interface and the delamination of adhesive from Mg sheet. Adhesive was found to help in protecting the interior of the Mg surface from corrosion as long as it adheres to the Mg surface.