Metals Knowledge:Friction Stir Welding Technology: Part One

W.M. Thomas, I.M. Norris, D.G. Staines, E.R. Wats
TWI Ltd, Cambridge, United Kingdom

Introduction by Key to Metals

Friction stir welding (FSW) is a relatively new joining process that is presently attracting considerable interest. Friction Stir Welding (FSW) is a patented welding process that was invented by the Welding Institute (TWI) in Cambridge, UK in 1991. Friction Stir Welding (FSW) is a solid state welding process where a machine rotates, plunges, and then traverses a special shaped FSW tool along a joint to form a weld. The rotation action and the specific geometry of the FSW tool generates friction and mechanical working of the material which in turn generate the heat and the mixing necessary to transport material from one side of the joint line to the other. The process has numerous advantages over other joining technologies and can be used to weld numerous materials including, but not limited to aluminum, bronze, copper, titanium, steel, magnesium, and plastic.

Friction stir welding is a process that utilizes local friction heating to produce continuous solid state seams. It allows butt and lap joints to be made in low melting point materials (such as aluminum alloys) without the use of filler metals. This process joins materials by plasticizing and then consolidating the material around the joint line. The process can be summarized in the following steps: First, a hole is pierced at the start of the joint with a rotating steel pin. The pin continues rotating and moves forward in the direction of welding. As the pin proceeds, the friction heats the surrounding material and rapidly produces a plasticized zone around the pin. Pressure provided by the pin forces plasticized material to the rear of the pin where it consolidates and cools to form a bond. No melting occurs in this process, and the weld is left in a fine-grained, hot-worked condition with no entrapped oxides or gas porosity.

Introduction

The basic principle of conventional rotary friction stir welding FSW is shown in Figure1.

Figure 1: Basic principle of conventional rotary friction stir welding

The systematic development of Friction stir welding has led to a number of variants of the technology. The following describes preliminary studies being carried out on Twin stir™ technique, Skew-stir™, Re-stir™, Dual-rotation stir and Pro-stir™ a three-dimensional material processing technique.

Currently, FSW is used particularly for joining aluminum alloys in shipbuilding and marine industries, aerospace, automotive and rail industry. Furthermore, the technology provides significant advantage to the aluminum extrusion industry. Automotive suppliers are already using the technique for wheel rims and suspensions arms. Fuel tanks joined by FSW have already has been launched in spacecraft, and many other space advances are under development; commercial jets welded by FSW have successfully completed flying trials with high volume commercial production forthcoming. Aluminum panels for high speed ferries and panels for rail vehicles are also produced. Moreover, the friction stir welding of 50 mm thick copper material has provided a potential solution for nuclear encapsulation of radioactive waste. Friction stir welding is making an impact as a material processing technique and the prognosis for the successful welding of steel products by FSW looks promising.

Twin stir™ technique

The simultaneous use of two or more friction stir welding tools acting on a common workpiece was first described in 1991. The concept involved a pair of tools applied on opposite sides of the workpiece slightly displaced in the direction of travel. The contra rotating simultaneous double-sided operation with combined weld passes has certain advantages such as a reduction in reactive torque and a more symmetrical weld and heat input through the thickness. In addition, for certain applications, the use of purpose designed multi-headed friction stir welding machines can increase productivity, reduce side force asymmetry and reduce or minimize reactive torque.

The use of a preceding friction pre-heating tool followed in line by friction stir welding tool for welding steel is reported in the literature 1999. More recently a similar arrangement has been reported with two rotating tools one used to pre-heat and one used to weld. This disclosure, however, shows "tandem" technique with the tools rotating in the same direction. A further reference is made to tandem arrangements with tools rotating in the same direction. The use of “tandem” contra-rotating tools in-line with the welding direction and "parallel" (Side-by-side across the welding direction) is also disclosed.

Figure 2 shows the three versions of Twin stir™ welding techniques that are being investigated and developed at TWI.

Figure 2: Twin stir™ variants a) Parallel side by side transverse to the welding direction b) Tandem in – line with the welding direction c) Staggered to ensure the edges of the weld regions partially overlap

Parallel twin-stir™

The Twin stir™ parallel contra-rotating variant (Fig.2a) enables defects associated with lap welding to be positioned on the "inside" between the two welds. For low dynamic volume to static volume ratio probes using conventional rotary motion, the most significant defect will be "plate thinning" on the retreating side. With tool designs and motions designed to minimize plate thinning, hooks may be the most significant defect type. The Twin stir™ method may allow a reduction in welding time for parallel overlap welding. Owing to the additional heat available, increased travel speed or lower rotation process parameters will be possible.

Tandem twin-stir™

The Twin stir™ tandem contra-rotating variant (Fig. 2b) can be applied to all conventional FSW joints and will reduce reactive torque. More importantly, the tandem technique will help improve the weld integrity by disruption and fragmentation of any residual oxide layer remaining within the first weld region by the following tool. Welds have already been produced by conventional rotary FSW, whereby a second weld is made over a previous weld in the reverse direction with no mechanical property loss. The preliminary evidence suggests that further break-up and dispersal of oxides is achieved within the weld region. The Twin stir™ tandem variant will provide a similar effect during the welding operation. Furthermore, because the tool orientation means that one tool follows the other, the second tool travels through already softened material. This means that the second tool need not be as robust.

Staggered twin-stir™

The staggered arrangement for Twin-stir™ (Fig.2c) means that an exceptionally wide "common weld region" can be created. Essentially, the tools are positioned with one in front and slightly to the side of the other so that the second probe partially overlaps the previous weld region. This arrangement will be especially useful for lap welds, as the wide weld region produced will provide greater strength than a single pass weld, given that the geometry details at the extremes of the weld region are similar. Residual oxides within the overlapping region of the two welds will be further fragmented, broken up and dispersed. One particularly important advantage of the staggered variant is that the second tool can be set to overlap the previous weld region and eliminate any plate thinning that may have occurred in the first weld. This will be achieved by locating the retreating side of both welds on the "inside" (see Fig.3).

For material processing, the increased amount of material processed will also prove advantageous. In addition, for welding it would enable much wider gaps and poor fit up to be tolerated.

Figure 3: Arrangement of Staggered Twin stir™ contra-rotating tools with respect to rotation and direction. a) Advancing sides of the "common weld region" are positioned outwards with left-hand tool leading. b) Retreating sides of the "common weld region" are positioned outwards with left-hand tool leading c) Retreating sides of the "common weld region" are positioned outwards with right-hand tool leading d) Advancing sides of the "common weld region" are positioned outwards with right-hand tool leading

Skew-Stir™

The skew-stir™ variant of FSW differs from the conventional method in that the axis of the tool is given a slight inclination (skew) to that of the machine spindle as shown in Fig. 4a, b and c.

The skew-stir™ technique enables the ratio between the "dynamic" (swept) volume and the static volume to be increased by the skew motion of the tool. This can be additional to that provided by the use of re-entrant features machined into the probe. It is this ratio that is a significant factor in enabling a reduction or elimination of void formation and improving process efficiency.

The arrangement shown in Fig. 4a, results in the shoulder face being oblique to axis of the skew tool and square to the axis of the machine spindle. This shoulder face remains in a fixed relationship with respect to the plate top surface. Tilting the plate or the machine spindle will produce a plate to tool tilt that can be varied to suit conditions.

The focal point of a skewed tool affects the amplitude of the orbit of the tool shoulder and probe. With the focal point at the shoulder position, i.e. at the top of the workpiece, the shoulder essentially has a rotary motion with no off-axis orbit. When the focal point is positioned slightly above the top surface of the work piece, or at any position through the thickness of the workpiece, the shoulder contact face has an off-axis orbital movement. In addition, the off-axis orbital motion of the shoulder is dependent on the angle of skew and the distance that the intersection (focal point) is away from the top of the plate. The greater the skew angle and the greater the distance that the focal point is away from the workpiece surface, the greater is the amplitude of the shoulder off-axis movement.

The skew action results in only the outer surface of the probe making contact with the extremities of the weld region. The FSW tool does not rotate on its own axis, and therefore only a specific part of the face of the probe surface is directly involved in working the flow path of material during welding, (see Fig.4a). This probe type is termed A-Skew™.

Figure 4: Details of Prototype A-Skew™ Probe a) Side view b) Front view, showing tip profile c) Swept region encompassed by skew action