Fatigue behavior in A6061/AZ31 dissimilar friction stir spot weld made by a scroll grooved tool

Abstract

Aluminum and magnesium alloys dissimilar friction stir spot welds had been fabricated using a tool without probe whose shoulder surface had a scroll groove with the depth of 0.5mm. The scroll groove was designed to induce material flow in the nugget of spot welds. The tensile strengths of dissimilar welds were comparable to those of the magnesium similar welds made by the same tool. Thin intermetallic compound (IMC) layer consisting of Al₃Mg₂ and Al₁₂Mg₁₇ was formed along the interface between aluminum and magnesium sheets. EDX analysis revealed that the thickness of IMC layer was dependent on the distance from the center of nugget, where the formation of thin IMC layer had resulted in the bonding between the dissimilar alloys. The tensile strengths of dissimilar welds fabricated by the different conditions were dependent on the thickness of IMC layer, and were sensitive to the thickness of IMC layer around the edge of the nugget. Subsequently, tensile shear fatigue tests had been conducted, and the fatigue strengths were nearly the same between dissimilar and magnesium similar welds. Fatigue fracture modes were dependent on the load levels, where fatigue crack propagated along the interface at the higher load levels than 1000N. However, fatigue crack initiated in the lower magnesium sheets at the edge of the nugget and propagated through thickness at the low load level of 750N. Fractographic and EDX analyses revealed that the fatigue crack mainly propagated through the thin IMC layer at the higher load levels.