Material Interactions in a Novel Pinless Tool Approach to Friction Stir Spot Welding Thin Aluminum Sheet

Abstract

The requirement for a probe, or pin, in friction stir spot welding (FSSW) leads to an undesirable keyhole and ''hooking,'' which can influence the fracture path and weld strength. Furthermore, the full weld cycle for FSSW is typically longer than ideal for the automotive industry, being 2 to 5 seconds. Here, it is shown that using a novel pinless tool design it is possible to achieve high lap shear strength (~3.4 kN) in thin aluminum sheet (~1 mm thick), with short weld cycle times (<1 second). Several techniques have been exploited to study the material flow and mechanisms of weld formation in pinless FSSW, including high-resolution X-ray tomography, to understand the role of the tool design and weld parameters. Despite the ''simple'' nature of a pinless tool, material flow in the weld zone was found to be surprisingly complex and strongly influenced by surface features on the tool, which greatly increased the penetration of the plastic zone into the bottom sheet. Because of the rapid thermal cycle and high level of grain refinement, the weld zone was found to develop a higher strength than the parent material with little evidence of a heat affected zone (HAZ) after postweld natural aging.

The effect of a paint bake treatment on joint performance in friction stir spot welding AA6111-T4 sheet using a pinless tool

“…The weld formation process, in FSSW with a pinless tool, has already been described in detail in ref. [7]. As discussed in this previous study, the deformation zone increases in depth with welding time, penetrating further into the bottom sheet.…”

“…3 Figs. 2& 3 thus demonstrate that, with an appropriate tool design (see [7]), fully bonded welds can be successfully produced with dwell times of less than 1 second, with the minimum of hooking and few defects at the join line. However, discontinuous patches of oxide trails were seen dispersed within the welds near the edge of the bonded area.…”

“…The welding trials were performed with a friction stir welding machine. A standard 0.2 mm shoulder plunge depth was used in all cases, as this was previously found to be the minimum required to obtain a satisfactory weld [7]. The weld coupons were produced at the centre of a 25 mm overlap between two 25 by 100 mm strips.…”

“…The hook formation is a relatively minor effect with short welding times (e.g. < 1 sec), but can become excessive for longer welding times (see [7]). In Fig.…”

“…In a new development to this process it has recently been demonstrated, by the current authors, that it is possible to produce friction spot welds in thin (1mm thick) 6xxx aluminium automotive sheet in a rapid cycle time of less than one second, when using a tool without a probe, and still achieve good joint properties [7]. A pinless tool approach to FSSW has the advantage of not leaving a keyhole in the weld centre, which causes cosmetic and corrosion issues, and is simpler and faster than the RefillTM welding method [8].…”