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

Abstract

Bobbin friction stir welding (BFSW) is an innovative variant for the solid state welding process whereby a rotating symmetrical tool causes a fully penetrated bond. Despite the process development, there are still unknown variables in the characterization of the process parameters which can cause uncontrolled weld defects. The entry zone and the exit zone consist of two discontinuitydefects and removing them is one of the current challenges for improving the weld quality. In the present research, the characteristic features of the entry and exit defects in the weld structure and formation mechanism of them during the BFSW processing was investigated. Using stacked layers of multi-colour plasticine the material flow, analogous to metal flow, can be visualised. By using different colours as the path markers of the analogue model, the streamline flow can be easily delineated in the discontinuity defects compared with the metal welds. AA6082-T6 aluminium plates and multi-layered plasticine slabs were employed to replicate the entry-exit defects in the metal weld and analogue samples. The fixed-bobbin tool utilized for this research was optimized by adding a thread feature and tri-flat geometry to the pin and closed-end spiral scrolls on both shoulder surfaces. Samples were processed at different rotating and longitudinal speeds to show the degree of dependency on the welding parameters for the defects. The analogue models showed that the entry zone and the exit zone of the BFSW are affected by the inhomogeneity of the material flow regime which causes the ejection or disruption of the plastic flow in the gap between the bobbin shoulders. The trial aluminium welds showed that the elimination of entry-exit defects in the weld body is not completely possible but the size of the defects can be minimized by modification of the welding parameters. For the entry zone, the flow pattern evolution suggested formation mechanisms for a sprayed tail, island zone and discontinuity-channel. For the exit zone a keyhole-shaped discontinuity is discussed as a structural defect.

AFM Characterization of Stir-Induced Micro-Flow Features within the AA6082-T6 BFSW Welds

“…The weld samples were in a butt joint configuration comprising two similar plates (250 mm × 75 mm × 6 mm). The geometrically full-featured bobbin tool (included threads, flats and scrolls) was made of H13 tool steel with a hardness of 560 HV [5,6,21]. The schematic of the BFSW process is shown in Figure 1.…”

“…Bobbin Friction Stir Welding (BFSW) is a modified variant of Friction Stir Welding (FSW) [1] where the conventional tool is replaced by a bobbin-shaped double-sided configuration [2,3]. The rotating double-shoulder bobbin tool penetrates from the edge through the interface of the side-by-side plates, and mixes the materials into a butt-shaped joint [4][5][6]. The heat input [4] generated by the friction between the rotating tool and the workpiece plasticises the material from both sides of the interface, Advancing Side (AS) and Retreating Side (RS), and stirs them together to form a bonded structure [6].…”

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

“…BFSW offers the potential for in situ grain refinement and recovery of mechanical properties. However this material can be difficult to weld, in that a number of flow-based defects can occur, such as tunnel defects [25,27] and hence the tool design and the process settings can be critical.…”

“…The weldment plates were cut into two plates with dimensions of 250 mm in length and 75 mm in width and set side-by-side for a butt weld. The feed rate and rotational speed were optimized based on [13,20,25,27]. For this aluminum grade, lower speeds imposed more violence on the tool and did not create a stable weld, and higher speeds led to poorer weld quality in the forms of structural defects (e.g., tunnel void or kissing bond discontinuities).…”

“…The T6 is a precipitation-hardening tempering that includes three time-temperature cycles; first, solutionizing of the primary microstructure, quenching of the grains, and finally an artificial ageing by tempering heating [24]. Details of the T6 tempering for AA6082 are available [20,25]. In general, quenching can form a single-phase supersaturated solid phase, this can cause a solid solutionizing strengthening [26].…”