Improvement of interfacial bonding strength in roll-bonded Mg/Al clad sheets through annealing and secondary rolling process

The effect of annealing and secondary rolling process has been investigated in relation to evolution of interface microstructure as well as interfacial bonding mechanisms of roll-bonded two-ply Mg/Al clad sheets in this study. Two types of thin reaction layers, viz., γ (Mg17Al12) and β (Mg2Al3) phase layers, were observed to form along the Mg/Al interface, the thickness of which was found to increase with annealing time. The grains in the γ layer were grown coarsely in a columnar shape, as they grew continuously in coarse-grained Mg substrate alloy side. The thickness of the γ layer was found to reduce significantly after the secondary one-pass rolling. At the same time, the initially columnar γ grains became refined into equi-axed grains by recrystallization caused by large plastic deformation during the secondary rolling. The serrated flow observed in the reaction layers during a nano-indentation test in the γ layer disappeared completely without any sign of micro-cracks after the secondary rolling. This in turn implies that the γ layer, acted as an embrittlement site before the rolling, may not be brittle any longer after the rolling. The grain refinement and disappearance of columnar grains in the γ layer appear to greatly improve the interfacial bonding strength in these clad sheets. This secondary rolling process seems to provide a great advantage in terms of manufacturing, since the same rolling stand for roll-bonding process can also be used without any additional equipment.