Control of the Nugget Nanostructure of Mg<sub>96</sub>Zn<sub>2</sub>Y<sub>2</sub> Welded by Resistance Spot Welding

Abstract

Recently, Mg96Zn2Y2 alloy was widely studied due to its excellent mechanical properties. These properties were considered to relate to the nanostructure of the alloy such as α-Mg matrix and bent long period stacking ordered (LPSO) structure. In this study, as the first step to the mechanical property control, we applied various spot welding methods to the Mg alloy to control the structure of the joints. The specimen used was an extruded Mg96Zn2Y2 alloy. The sheets were cut from this alloy rod for resistance spot welding. Three kinds of spot welding methods were applied to join the Mg alloy, such as spot welding with cover plates, long-time spot welding, and water-cooled spot welding. In the base metal, secondary phases extended along extruded direction at approximately 30μm intervals were produced in the α-Mg matrix. The secondary phase contained 14H-type LPSO structure. In the nugget of the joint welded with cover plates, a secondary phase was produced like a net in the α-Mg grain which was approximately 30μm in diameter. Both 14H-type and 18R-type LPSO were observed in the secondary phase. In the nugget produced by the long-time spot welding, a secondary phase tended to segregate to the grain boundary of the α-Mg grains. The structure of LPSO was 18R-type. In the water-cooled welding, strong directionality of the secondary phase texture was observed around the edge of the nugget. These results showed that the nugget nanostructure of Mg96Zn2Y2 dramatically varied depending on the above adopted methods of spot welding.