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Development of reliable magnesium (Mg) to steel joining methods is one of the critical issues in broader applications of Mg in automotive body construction. Ultrasonic spot welding (USW) has been demonstrated successfully to join Mg to steel and to achieve strong joints. In this study, corrosion test of ultrasonic spot welds between 1.6 mm thick Mg AZ31B-H24 and 0.8 mm thick galvanized mild steel, without and with adhesive, was conducted. Adhesive used was a one-component, heat-cured epoxy material, and was applied between overlapped sheets before USW. Corrosion test was conducted with an automotive cyclic corrosion test, which includes cyclic exposures of dipping in the 0.5% sodium chloride (NaCl) bath, a constant humidity environment, and a drying period. Lap shear strength of the joints decreased with the cycles of corrosion exposure. Good joint strengths were retained at the end of 30-cycle test.

A new spot joining technology relying on a consumable joining bit has been developed and evaluated on dual phase (DP) 980 steel and a dissimilar combination of aluminum alloy 5754-O and DP 980. This new process, called friction bit joining (FBJ), uses a consumable bit to create a solid-state joint in sheet materials by the action of cutting and frictional bonding. A series of experiments were done in which different welding parameters were employed and lap shear tension testing was carried out to evaluate performance. The best lap shear values averaged 6.5 kN.

Ultrasonic spot welding is a novel solid-state joining technique for automotive body structures. The residual stresses, caused by the inhomogeneous deformation during the welding process, are considered to be a major factor affecting the strength and quality of the weld. Many conventional measurement methods cannot be applied because of the small size of the spot weld. In this study, the ISSR/ring-core cutting method is applied to measure the residual stresses on the ultrasonic spot weld. The ISSR possesses the short gage length and high sensitivity. The ring-core cutting was conducted on the back of the spot weld. The tensile residual stresses were obtained. The measured tensile residual stresses added to the operation stresses could cause the initiation of the cracks.