Friction stir in aerospace - the industrial way - Part I

TWI Bulletin, May/June 2002

Part II was published in TWI Bulletin, July/August 2002

Stephan Kallee gained his Diploma in Mechanical Engineering at the Technical University of Munich, where he specialised in production technology and business administration. After completing a course as a European Welding Engineer, he joined TWI in May 1995, where he works as Collaborative Project Manager for friction and forge welding processes. His recent activities include the application of friction stir welding to automotive lightweight structures, aluminium pipelines and high-speed ferryboats. Now he manages the EUREKA Eurostir ^® project on Industrialisation of Friction Stir Welding.

Dave Nicholas is Business/Technology Development Manager of the Friction and Forge Processes Group in the Electron Beam, Friction and Forge Processes Department at TWI. His interests centre on solid phase welding processes with the focus on friction welding processes. He developed several welding technologies from initial laboratory experiments to their industrial exploitation such as friction surfacing, linear friction welding and, since 1991, friction stir welding. He gained an honours degree in Metallurgy from the University of Wales and has been at TWI since 1967. He is a Chartered Engineer, a Member of the Institute of Metals and a Fellow of The Welding Institute. 

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 has recently gained Eur Ing, CEng and FWeldI status. He is the author of many technical papers and has been responsible for the conception and/or development of a number of emergent technologies.

In the aerospace industry, large tanks for launch vehicles are being produced by FSW from high-strength aluminium alloys. The first Boeing Delta II rocket with a friction stir welded Interstage Module was successfully launched in August 1999 and one with three friction stir welded tanks was launched in April 2001. As Stephan Kallee, Dave Nicholas and Wayne Thomas report all major manufacturers of military and civil aircraft are currently investigating the potential for replacing riveting by FSW, and seven large international collaborative projects have been launched to assess the benefits.

Friction stir welding (FSW) was invented and patented ^[1,2] in 1991 at TWI in Cambridge (UK) and has been developed to a stage where it is now applied in production. Currently 65 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and several of them exploit the process in commercial production. They have filed more than 400 patent applications related to FSW ^[3] .

In macrosections of good quality welds in aluminium alloys a well-developed nugget is visible at the centre of the weld, as shown in Figures 1 and 2. Outside the nugget there is a thermomechanically affected zone, which has been plastically deformed and shows some areas of partial grain refinement ^[4] . The overall shape of the nugget is very variable, depending on the alloy used and the actual process conditions.

Design of Whorl ^TM and MX Triflute ^TM tools

FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. Current state-of-the-art tools permit up to 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool. The design of the friction stir welding tools is the heart of this remarkable and still relatively new welding process.

Instead of using a cylindrical 'pin', a 'probe' can be used (the more generic term 'probe' includes for example, truncated cones, non-round cross sections, conical spirals and whisks). In an investigation of the Whorl ^TM and MX Triflute ^TM families of tools ^[6,7,8] , trials were carried out with the tool configurations shown in Figure 3. The frustum shaped tool probes incorporate a helical ridge profile designed to augur the plasticised weld metal in a downward direction. Some probes also have side flats, or re-entrant features, to enhance the weld metal flow path. The Whorl ^TM concept provides for non-circular probe cross-sections. In this way, the probe displacement volume is less than its volume of rotation, to enable the easier flow of plasticised material. A prototype Whorl ^TM tool is shown in Figure 4 superimposed on a transverse section taken from a 75mm (=3inches) thick weld in aluminium alloy 6082-T6.

Multi-helix tools, which TWI trademarked MX Triflute ^TM, have an odd number of relatively steeply angled flutes and incorporate a coarse helical ridge around the flutes' lands ( Fig.5) ^[9] . These reduce the tool volume further and therefore aid the material flow, the break-up and the dispersion of surface oxides. The tool shoulder profiles under investigation are designed to provide better coupling between the tool shoulder and the workpiece.

MultiStage ^TM tools, - for lap welding

TWI has already developed a number of lap welding tools in its Core Research Programme ^[10] and is now systematically working on advanced probe shapes. One possible feature of FSW tools for lap welds is the application of a second 'shoulder' located at the interface region between the two plates. The lower part of this MultiStage ^TM pin is reduced in diameter and has a pentagonal flattened profile to stir up the oxides and to improve the material flow. In an industrialisation study for Fokker Space ^[11] , TWI conducted pilot trials with 250mm square, 2.5mm thick 7075-T73 clad sheet. The cladding was found to contaminate the weld nugget and there was lifting of the free edge of the top sheet. Trials continued with the cladding removed from the mating surfaces. The metallurgical sections looked better but significant upper sheet thinning occurred, as can be seen in the cross-section ( Fig.6).

The tool design was modified so that less metal moved in the direction of the top-sheet ( Fig.7). Trials then continued with flat 250mm square sheets of 2.4mm thick aluminium alloy 7075-T7351. The results were highly satisfactory. Having demonstrated the feasibility of a high-strength lap weld, it was necessary to move to a more representative configuration. A series of trials was conducted with 1m long flat sheets and a very encouraging microstructure has been achieved ( Fig.8).

New Skew-stir ^TM motion to improve the material flow

The skew-stir ^TM 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 component material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding. This results in an asymmetric shaped probe ( Fig.10). The skew-stir ^TM technique provides an increased flow path, and the width of the weld nugget region is greater than the diameter of the probe. This feature is ideally suited for lap and T-joints and similar welds, where the interface is 90° to the machine axis, ie parallel with the work piece surface.

Friction skew-stir ^TM welding increases the extent of the plasticised material surrounding the probe. The skew-stir ^TM motion, therefore, provides a method of increasing the 'dynamic to static volume ratio' of the probe. Traditionally, the 'dynamic to static volume ratio' is provided by the geometry of the probe because of its re-entrant features. Skew-stir ^TM can be used to advantage, where tools of complex shape cannot be used. Moreover, the greater volume of plasticised material results in better disruption and dispersion of surface oxide layers. This should minimise the risk of undesirable joint features.

The arrangement shown in Figure 9 results in the shoulder face remaining in the same plane as the plate top surface. Tilting the work piece or the machine spindle produces an angle between the workpiece and the tool face that can be varied to suit conditions. It is also possible to assemble tools with the tool shoulder not perpendicular to the spindle axis to increase the forging action.

Control of the tool heel plunge depth

One of the most critical settings to achieve successful friction stir welds is the position of the tool shoulder relative to the work piece surface. TWI has successfully introduced a mechanical position control system, using one or two rollers beside or in front of the tool ( Fig.11). These rollers guarantee that the tool does not plunge too deep into the workpiece and that the plasticised material is sufficiently forged underneath the tool shoulder.

The trend is now towards on-line force and torque measurement for data monitoring and closed-loop control. Especially on robots and transportable machines, which may not be rigid enough to withstand deflection, load cells or more complex systems can be installed. TWI has experimented with a Kistler rotating dynamometer and a force measurement table, both of which use piezo crystals for measuring the forces and torque ( Fig.12).

On rigid machines, the tool heel plunge depth can be kept constant by either position or force control ( Fig.13). On these machines rollers are not necessary for maintaining the tool heel plunge depth. Hydraulically adjustable rollers can be used for pressing the sheets onto the backing bar, to avoid bulging near root of the weld. These are operated with a compliant system for compensating changes in material thickness.

Table 1. Mechanical properties of friction stir welded aluminium alloy specimens

References

End of Part I

Part II in the next issue examines weld quality and aerospace applications