The Use of Bobbin Tools for Friction Stir Welding of Aluminium Alloys

Abstract

The use of a double sided friction stir welding tool (known as a bobbin tool) has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design and cost. However, the response of these tools in generating fine microstructures in the nugget area has not been established. The paper presents detailed metallographic analyses of microstructures produced in 25mm AA6082-T6 aluminium wrought alloy, and examines grain size, texture and mechanical properties as a function of processing parameters and tool design, and offers comparison with data from welds made with conventional tools.

Thermomechanical Grain Refinement in AA6082-T6 Thin Plates under Bobbin Friction Stir Welding

“…The initial design was for a single-shoulder tool, the conventional-FSW (CFSW). TWI then introduced a further improvement in the form of a tool with two shoulders that penetrated through the whole weld section [2][3][4]. This bobbin FSW (BFSW) process is a cheaper manufacturing technique with potentially higher productivity.…”

Formation Mechanisms for Entry and Exit Defects in Bobbin Friction Stir Welding

“…Furthermore, two shoulders create and retain more heat within the working region [18]. These features can enable the BFSW process to join plates with a thickness range of 1.5 mm up to 25 mm [10,19,20]. Also, in the BFSW process a symmetrical joining pattern is expected in the cross section of the weld perpendicular to welding direction, as the fully contained pin can eliminate the incomplete root penetration of CFSW [10].…”

“…These features can enable the BFSW process to join plates with a thickness range of 1.5 mm up to 25 mm [10,19,20]. Also, in the BFSW process a symmetrical joining pattern is expected in the cross section of the weld perpendicular to welding direction, as the fully contained pin can eliminate the incomplete root penetration of CFSW [10]. Among the different bobbin tool designs, the fixed gap between the shoulders, with scrolled surface patterns, allows a relative flow between the shoulders to make a constant force in the stirring zone which can noticeably reduce the material distortion and residual stresses during the process [21].…”

“…Most of the published studies are focused on the process setup [9,10], tool development [10,15] or metallurgical and mechanical properties of the bobbin welds [22][23][24]. A predictive approach regarding the potential discontinuity defects (i.e., crack or void) is one of the necessities for process improvement which has not been described in the literature.…”

“…To eliminate the defects and reduce the difficulties of the FSW process, the TWI introduced an innovative variant by modifying the tool to a bobbin geometry [10,11]. The bobbin tool is a symmetrical rotating component consisting of two shoulders, one on each side of the workpiece connected by a pin fully embedded into the workpiece joint [12].…”