Details of Mg-Zn reactions in AZ31 to galvanised mild steel ultrasonic spot welds

摘要

For ultrasonic spot welds between sheets of 0.8 mm thick hot dip galvanised mild steel and 1.6 mm thick AZ31B-H24, two Mg-Zn interface reaction layers, totalling around 22 mm in thickness, formed in 0.3 s, replacing the original 9 mm thick galvanised coating on the steel. The layer contacting AZ31 had composition near 25 at-%Zn. The second layer, contacting the steel, had composition near 66 at-%Zn. Mg-Zn alloying formed liquid phases during welding that were progressively squeezed from the joints until they were no longer evident at 1.0 s welding time. At that point a less than 500 nm thick layer of Al_5Fe_2 was identified by electron diffraction and microchemical analysis at the AZ31/steel interfaces. For welding times greater than about 0.5 s lap shear failures propagated through the AZ31 near the welded interfaces. This indicated that the strength of the AZ31-mild steel bonds was greater than that of the AZ31 near the welded interfaces.