(Friktion rühren Schweißen - Prozeßvarianten und neue industrielle Entwicklungen)
I M Norris, W M Thomas, J Martin and D J Staines
Paper presented at 10th International Aachen Welding Conference, 'Welding and Joining, Key Technologies for the Future', Eurogress, Aachen, 24-25 Oct. 2007.
Friction stir welding (FSW) is now extensively employed in the aluminium using industries for joining and material processing applications. The technology has gained increasing interest and importance since its invention at TWIalmost 16 years ago. The paper will introduce a number of variants of FSW and describes some of the feasibility work that has been carried out to develop 'bobbin stir' welding for welding 25mm thick aluminium alloy material. Inaddition, the recent development of friction stir welding of 12mm thick 12% chromium alloy steel and friction stir welding of titanium alloys is described.
The basic principle of conventional rotary friction stir welding (FSW) is shown in Figure 1.
Fig.1. Basic principle of conventional rotary friction stir welding
The systematic development of Friction stir welding (FSW) has led to a number of variants of the technology. The following describes preliminary studies being carried out on Twin-stir TM , bobbin stir welding, friction stir welding of steel and friction stir welding of titanium.
Currently, FSW is used particularly for joining aluminium alloys in shipbuilding and marine industries, aerospace, automotive and the rail industry.
Automotive suppliers are already using the technique for wheel rims and suspension arms. Fuel tanks joined by FSW have been launched in spacecraft, and many other space advances are under development; commercial jets welded by FSWhave successfully completed flying trials, with high volume commercial production forthcoming. Aluminium panels for high speed ferries and panels for rail vehicles are also produced. Moreover, the friction stir welding of 50mm thickcopper 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 andtitanium products by FSW looks promising.
2. 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.  The concept involved a pair of tools applied on opposite sides of the workpiece and slightly displaced in the direction of travel. The contra-rotating simultaneous double-sided operation with combined weld passes has certainadvantages 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 minimise reactive torque. 
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 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 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.  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 TM welding techniques that are being investigated and developed at TWI.
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
2.1 Parallel Twin-stir TM
The Twin-stir TM parallel contra-rotating variant ( Figure 2a) enables defects associated with lap welding to be positioned on the 'inside' between the two welds. This technique overcomes the problem of 'plate thinning' on the retreating side or hooking on the advancing side ofFSW lap welds.
2.2 Tandem Twin-stir TM
The Twin-stir TM tandem contra-rotating variant ( Figure 2b) can be applied to all conventional FSW joints and reduces reactive torque. More importantly, the tandem technique improves the weld integrity by fragmentation and dispersion of residual oxide remaining within thefirst weld region by the following tool. 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.Use of a friction pre-heating tool for the friction welding of steel may help to minimise probe wear on the following tool.
2.3 Staggered Twin-stir TM
The staggered arrangement for Twin-stir TM ( Figure 2c) means that an exceptionally wide 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 previousweld region. This arrangement is especially useful for lap welds, as the wide weld region produced provides greater strength than a single pass weld. Residual oxides within the overlapping region of the two welds are fragmented anddispersed. 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.
For material processing, the increased amount of material processed will also prove advantageous.
2.4 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 demonstrated the feasibility of Twin-stir TM and showed that welds of good appearance were produced as shown in Figure 3.
Fig.3. Surface appearance of a typical Tandem Twin-stir TM weld, made in 6083-T6
Two exit holes produced in a tandem weld show that a similar footprint was achieved for both the lead and following tool (see Figure 4).
Fig.4. Tandem Twin-stir TM lead and follow exit holes
Metallographic observations reveal a marked refinement of grain size in the weld region and break up of oxide remnants and particles. In lap welds, an upturn on both sides of the weld region is also shown ( Figure 5). Metallographic examination of staggered Twin-stir TM lap welds reveals a weld width of >400% of the sheet thickness as shown in Figure 6.
Fig.5. Macrosection of Tandem Twin-stir TM lap weld in 6mm thick 6082-T6 aluminium
Fig.6. Macrosection taken from a staggered Twin-stir TM lap weld in 3mm thick 5083-H111 aluminium sheet
The advancing sides of the welds 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 followingtool.
3. Bobbin Stir Welding
Self-reacting FSW has been shown to be effective for joining hollow extrusions and lap joints. Essentially there are two types of self-reacting FSW; the 'bobbin tool' and the 'adaptive technique' (AdAPT) .  The bobbin technique provides a fixed gap between two shoulders, while the adaptive technique allows adjustment of the gap between the shoulders during welding. [9-10] Figure 7 shows a fixed bobbin tool with three-sided tapered probe.
Fig.7. Bobbin tool showing two shoulders separated by a pre-set fixed length
Trials using the above arrangement produced good quality welds. A metallurgical section showing the width of the larger diameter (drive side) shoulder and the smaller opposed shoulder is shown in Figure 8. The weld profile is narrower in the mid-thickness that at the shoulder regions. Metallographic examination of the advancing and retreating mid thickness regions revealed no evidence of buried flaws.
Fig.8. Bobbin weld in 25mm thick 6082-T6 aluminium
4. FSW Steel welding trials
Friction stir welding of Al alloys is now a well established process and there are a small number of production applications that involve welding of Cu alloys and Mg alloys. However, FSW of harder materials with higher softeningtemperatures remains a challenge due to the combination of high stress and high temperature to which the tool is subjected.
Many researchers have reported work on welding ferrous materials, carbon steels, stainless steels and nickel based alloys using both refractory metal tools and poly-crystalline cubic boron nitride tools and this work has beenreviewed in references  and  . Tool wear and catastrophic tool failure remain issues and friction stir welding of steels over 12mm in thickness and over a few metres in length remains a technical challenge.
TWI has conducted work using composite tools comprising two different coated refractory metal alloys. The material for the shoulder was selected to produce a smooth surface finish and the material for the probe was selected to havehigh strength and to achieve good coupling to the steel at the welding temperature. The tool was made using a Morse taper principle which is an ideal arrangement for securing the two materials, Figure 9.
Fig.9. Friction coupling design for securing different refractory materials
Fig.10. Typical surface appearance of friction stir welded 12% Cr alloy steel plate using a composite tool
Initial results in welding 12mm thick 12%Cr steel have proved encouraging with over 20m of weld being made with a single tool in one metre stages ( i.e with 20 weld starts and stops). The typical surface appearance of a weld is shown in Figure 10 and a representative weld cross section is shown in Figure 11. Initial mechanical tests have shown tensile failure outside the weld region at parent material strength levels and acceptable face, root and side bends achieving 180°.
The concept of composite tools with the shoulder and probe optimised for their particular function is being pursued further with the aim of extending the length of weld that can be achieved and the range of materials that can bewelded.
Fig.11. Metallographic section of weld in 12mm thick, 12%Cr steel
5. Stationary Shoulder Friction Stir Welding (SSFSW) of Ti alloys
Ti alloys present a particular challenge for the conventional friction stir welding process because they have a high softening temperature and low thermal conductivity. This means that it is difficult to generate sufficient heat tosoften the material without causing local overheating and softening resulting in expulsion of material from the weld.
To overcome this problem, a new variant of friction stir welding has been developed termed Stationary Shoulder Friction Stir Welding (SSFSW). In this process, the probe rotates and protrudes through a hole in a stationaryshoulder/slide component. The stationary shoulder adds no heat to the surface so all of the heat is provided by the probe and the weld is made with an essentially linear heat input profile.
Fig.12. SSFSW head design incorporating gas shielding
Equipment has been built that incorporates gas shielding ( Figure 12) and has been tested in welding 6.35mm Ti-6Al-4V alloy, commercially pure Ti and selected other Ti alloys.
Welds of very smooth surface appearance have been produced with significantly improved stability and reproducibility ( Figure 13). A representative weld section is shown in Figure 14.
Fig.13. Butt weld in 6.35mm Ti-6Al-4V alloy showing smooth top surface
Fig.14. Typical weld cross section in 6.35mm Ti-6Al-4V alloy
The process is now being applied to generation of near-net shape components by sequential addition of layers of material to build up parts of complex shapes. Examples are shown in Figure 15 and 16.
Fig.15. Layer build up of T-section using SSFSW:
a) Overview of component after machining to final size;
b) Section showing four sequential passes
Fig.16. Layer build up of corner section using SSFSW:
a) Overview of component after machining to final size;
b) Section showing three pass lap welding of two layers.
6. Concluding remarks
Friction Stir Welding is now a widely used and accepted process for welding of a range of mainly aluminium parts for production applications. The basic process is well understood and has proved robust and reliable in operation andthe incentive exists to extend its use to more challenging applications and alternative materials. This requires development of the process and this paper has described a number of variants that have been and continue to beinvestigated at TWI.
All of the variants described offer potential advantages in specific applications and will continue to be refined, with input from potential end users, with the aim of achieving acceptance and adoption for full scale productiontasks.
The tandem and staggered Twin-stir TM variants will allow for fragmentation and dispersion of oxides in the weld region that experiences passage of both tools. This will result in improved weld integrity and performance which is critical for someapplications.
Additionally, contra-rotating systems will reduce reactive torque during welding and this has benefits in terms of simplification of clamping and jigging for holding parts to be welded. This, in turn, provides an opportunity toreduce machine cost and complexity. Twin-stir TM also offers the possibility of increased weld width in lap welds and increased area coverage in surface processing applications.
The developments in bobbin tool welding are expected to see application in welding of 'enclosed' seams (eg joining of H-section extrusions) and eliminate the need for internal backing bars to support the weld region. The techniqueeliminates any risk of root defects when friction stir welding and this will be important in some critical, high integrity applications.
The principle of composite tools for welding high temperature materials has been demonstrated and further developments in this area are expected to extend the use of FSW in joining of steels, stainless steels and nickel basedalloys.
Finally the SSFSW process has been demonstrated and is continuing to be developed for welding and near net shape fabrication of Ti alloy components. There are a range of potential applications in the aerospace sector. Further workin other materials will extend process capability and open up new opportunities for part design and manufacturing efficiency.
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