Effect of shaft shoulder morphology on mechanical properties and thermal processes for ultra-thin aluminum alloy μFSW joint

Abstract

The 6061-T6 ultra-thin aluminum alloy with a thickness of 0.5 mm was welded using micro-joint friction stir welding technology. The effects of three kinds of stirring heads with different shaft shoulder morphology on the forming quality， microstructure， mechanical properties， welding thermal cycle， and force process of the 6061-T6 thin-wall butt joint were studied. The flow behavior characteristics of the plastic metal in the three welding cross-sections were analyzed in sequence. The results show that the forming effect of the weld surface is significantly affected by welding heat input. The hardness distribution trend of the joint cross-section formed by three kinds of stirring heads with different shaft shoulder morphology is a “W” shape. The highest hardness values in the center of the nucleation zone and the lowest hardness values in the thermo-mechanical affected zone of the needle-free three-involute diversion groove shaft shoulder welded joint are the highest among the three shaft shoulder welded joints. The mechanical properties of the joint formed by the three-involute guide groove with the needle shaft shoulder are outstanding， and the tensile strength， yield strength， and elongation after fracture are higher than those of the other two. The three tensile fracture forms are mainly ductile fractures. The process parameters of the thermal cycle and force can accurately reflect the changing trend of the welding state. The energy required to maintain the weld metal softening is from the heat generated by friction between the shaft shoulder and the workpiece and the work done by the axial and forward forces of the shaft shoulder. The axial force and forward force changed with the softening degree of the welded metal， playing a dynamic regulating role in the migration of plastic metal in the weld. The shoulder surface has a strong effect on the upper part of the weld， driving the plastic metal migration between the forward side and the backward side. The needle of the stirring head promotes the interaction between the plastic metal and the backing plate， providing a driving force for the flow of the upper and lower parts of the weld metal. The optimal heat generation potential is the combined result of the stirring needle and the involute groove， which work together to form a well-formed weld.

Keywords

• shaft shoulder morphology
• micro friction stir welding（μfsw）
• ultra-thin aluminum alloy
• processing property
• procedure analysis