Defects response to precoated adhesive in Al/steel friction stir welded joint

Abstract

Friction stir welding (FSW) has been applied for welding dissimilar materials in advanced manufacturing industries such as shipbuilding, automotive and aerospace. For the broad popularization of adhesive in FSW, the innovative coupling method of friction stir welding-adhesive bonding (FSW-AB) technology was designed to join 5A06 aluminum alloy to 304 stainless steel. The joints were fabricated using a single-component structural adhesive applied manually in the pre-lap zone. The joints were then cured by the heat generated from FSW. Macrostructure, defect generation and tensile shear performances of the joints obtained using three different rotation speeds and different adhesive thickness were studied. The adhesive prevented direct plate contact, but the FSW-AB joint showed more flash. Adhesive expansion occurred on the advancing side due to structural asymmetry and uneven heat dissipation. Compared to conventional FSW, the joints produced by FSW-AB exhibited significantly reduced hook defects in the pin-affected zone, a smoother distribution of hardness across the weld cross-section, and notably elevated hardness on the retreating side of the base metal outside the shoulder. It is attributed to the difference in the degree of crystallization between the two caused by the difference in heat dissipation rate. The ultimate tensile shear load of the FSW-AB joint reached 93.3 % of the conventional FSW joint at 1200 rpm stirring speed and 0.25 mm adhesive thickness. Furthermore, the fracture modes of FSW-AB joints were mainly normal fracture and shear fracture.