Study on Friction Stir Lap Welding of Aluminium to Steel Sheets

Abstract

Friction stir lap welding (FSLW) of 6061-T6 aluminium sheet and DX51D galvanized steel sheet was carried out by adding zinc foil to the lap interface and studying the influence of the zinc foil on the formation mechanisms and mechanical properties. The influence of the thickness of zinc foil, the plunge depth of the shoulder and the shape of the tools on the mechanical properties of the weld are discussed. Zinc foil reduced the generation of brittle intermetallic compounds, such as Fe4Al13. During the welding process, the axial force was small due to the high rotating speed. Liquid zinc was retained at the interface, where eutectic Al–Zn with low melting point and an Fe–Zn compound were generated to achieve the metallurgical combination of aluminium and steel. The fracture was located in the heat affected zone (HAZ) of the 6061-T6 base aluminium. The results showed that when the zinc foil was too thin, there was less zinc content at the interface; the resulting Al–Zn eutectic had low melting point, was not fully spread and had poor continuity, resulting in poor mechanical properties. When the zinc foil was too thick, a large amount of zinc-based solid solution was generated at the interface, and most of the fracture occurred in the zinc-rich layer.