Variations on a theme - the twists and turns of friction stir welding
TWI Bulletin, July - August 2006
Since the welding world witnessed the advent of a friction stir revolution in the nineties, numerous fresh variants of the process have entered the fray.
Wayne Thomas is a Principal Research Engineer in the Innovation Unit. He joined TWI in 1983. He gained his MPhil from Brunel University (Materials Technology) and is also qualified as Eur Ing, CEng and FWeldI. He is the author of many technical papers and has been responsible for the conception and/or development of a number of emergent technologies.
Peter Oakley originally joined TWI in 1972 with a BSc in Chemistry from Leicester University. Having worked in the Materials and Laser Departments, he left TWI in 1991 and obtained an MBA in 1992. He rejoined the organisation in 2001 as a Business Developer in the Group and European Programme Department, and became Manager of the Department in 2003. His current responsibilities cover TWI's portfolio of Group Sponsored Projects, EU and DTI collaborative projects, and working with the UK regional Development Agencies on the development of TWI's Regional Technology Centres.
David Staines joined TWI in December 1974 after gaining City and Guilds qualifications in machine shop technology and tool making. His work in the machine shop largely involved manufacture of special welding equipment. He spent thirteen years on laser processing and since 1995 has worked in the friction welding department mainly on friction stir. He is a Technician Member of the Welding Institute, registered with the Engineering Council UK, and is currently the laboratory supervisor in the Friction and Forge Processes technology group. He also works with the Innovation Unit.
Edward Watts joined TWI in 1980 as a technician in the Advanced Heavy Section processes department working with the friction welding processes. He studied part time and gained his HND in mechanical engineering. Edward worked mainly on non-rotary friction welding processes developing linear friction welding on a dual rotation orbital machine before the construction of dedicated mechanical and hydraulic machines. His main role in the friction group involved developing the equipment and control systems for all the processes including novel types of FSW heads. After 19 years he became the laboratory manager, responsible for all the labs at TWI.
Friction stir welding (FSW) is now extensively used in aluminium industries for joining and material processing applications. The technology has gained increasing interest and importance since its invention at TWI almost 15 years ago. The first episode in this two part paper by the inventor of FSW, Wayne Thomas and his colleagues Peter Oakley, David Staines and Edward Watts introduces some of the variants of FSW, such as Twin-stir TM Skew-stir TM , Re-stir TM , Dual-rotation stir and the Pro-stir TM near-net shape processing technique.
The basic principle of conventional rotary friction stir welding (FSW) is shown in Fig.1.
Fig.1. Basic prinicple of conventional rotary friction stir welding
Currently, FSW is used particularly for joining aluminium alloys in shipbuilding and marine industries, aerospace, automotive and the rail industry. Furthermore, the technology provides significant advantage to the aluminium extrusion industry. Automotive suppliers are already using the technique for wheel rims and suspension arms. Fuel tanks joined by FSW have already been launched in spacecraft, and many other space advances are under development;commercial jets welded by FSW have successfully completed flying trials, and high volume commercial production has been forthcoming. Aluminium panels for high speed ferries and panels for rail vehicles are also produced. Moreover, the friction stir welding of 50mm 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 TM technique
The simultaneous use of two or more friction stir welding tools acting on a common workpiece was first described in 1991. [1] 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. [2] 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 minimise reactive torque. [3]
The use of a preceding friction pre-heating tool followed in line by a friction stir welding tool for welding steel is reported in the literature in 1999. [4] More recently a similar arrangement has been reported with two rotating tools one used to pre-heat and one used to weld. [5] This disclosure, however, shows a '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. [6] The use of 'tandem' contra-rotating tools in-line with the welding direction and 'parallel' (side-by-side across the welding direction) is also described. [7]
Figure 2 shows the three versions of Twin-stir TM welding techniques that are being investigated and developed at TWI.
Parallel Twin-stir TM
The Twin-stir TM 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 minimise plate thinning, hooks may be the most significant defect type. The Twin-stir TM method may allow a reduction in welding time for parallel overlap welding. Owing to the additional heat available, the use of increased travel speed or lower rotation process parameters will be possible.
Tandem Twin-stir TM
The Twin-stir TM 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 TM 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.
Fig.2. Twin-stir TM 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
Staggered Twin-stir TM
The staggered arrangement for Twin-stir TM ( 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).
Fig.3. Arrangement of Staggered Twin-stir TM 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
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.
Welding trials
A series of preliminary welding trials has been carried out using an experimental Twin-stir TM head at TWI in order to investigate the characteristics of welds made in a variety of configurations. The welding trials were carried out with the prototype Twin-stir TM head as shown in Fig.4.
Fig.4. Twin-stir TM prototype head assembly
The welding trial demonstrated the feasibility of Twin-stir TM and showed that welds of good appearance were produced as shown in Fig.5.
Fig.5. Surface appearance of a typical Tandem Twin-stir TM weld made in 6083-T6 aluminium alloy
The two exit holes produced in a tandem weld showed that a similar footprint was achieved for both the lead and following tool (see Fig.6).
Fig.6. Tandem Twin-stir TM lead and follow exit holes
Metallographic observations revealed a marked refinement of grain size in the weld region and comminution of oxide remnants and particles. This is consistent with the microstructural features previously observed in conventional rotary stir welds in aluminium alloys. In lap welds, an upturn on both sides of the weld region is also shown ( Fig.7). All sections were prepared in the direction looking towards the start of the weld.
Fig.7. Macrosection of Tandem Twin-stir TM lap weld in 6mm thick 6082-T6 aluminium alloy
Metallographic examination of Staggered twin-stir TM lap welds revealed a 'common weld region' that measures 430% of the sheet thickness as shown in Fig.8.
Fig.8. Macrosection taken from the 'common weld region' of a Staggered Twin-stir TM lap weld in 3mm thick 5083 - H111 aluminium sheets
The tool arrangement used to produce this Staggered Twin-stir TM weld is that illustrated in Fig.3a; whereby the advancing sides of the 'common weld region' are positioned outwards. Consequently, both retreating sides face inwards with the lead weld retreating side receiving further friction stirring treatment from the retreating side of the follower tool.
Skew-stir TM
The Skew-stir TM 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.9a, b and c.
Fig.9. Details of Prototype A-Skew TM Probe
a) Side view
b) Front view, showing tip profile
c) Swept region encompassed by skew action
The Skew-stir TM 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.9a, results in the shoulder face being oblique to the 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, ie 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 substrate material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding, (see Fig.9a). This probe type is termed A-Skew TM .
The Skew-stir TM technique provides an easier material flow path than conventional FSW and a weld nugget region of width greater than the diameter of the probe. In addition the skew action provides an orbital forging action at the root of the weld, which improves weld quality in this region.
Work has been undertaken to establish the fatigue performance of welds made using the Skew-stir TM technique and a fatigue-tested sample is shown in Fig.10.
a) Macrosection
b) Detail of fracture, bottom sheet retreating side
Fig.10. Lap weld made with the retreating side near the top sheet edge (RNE configuration) using Skew-stir TM with an A-Skew TM probe (8.25mm in length) in 6mm thick 5083-H111 aluminium alloy at a welding speed of 3mm/sec (180mm/min)
b) Detail of fracture, bottom sheet retreating
c) Detail of fracture top sheet advancing side
d) Detail of the form of the notch at the edge of the weld - advancing side
Typically these Skew-stir TM lap welds gave good fatigue performance when compared with an artificial weld made from parent material of similar geometry as shown in Fig.11.
Fig.11. Fatigue results of welds carried out with different lap configurations with an 8.25mm long probe
Part two of this examination of friction stir welding variants will take a close look at the Reversal stir welding process, known as Re-stir TM , the Dual rotation process and Pro-stir TM .
