Bobbin Tool Friction Stir Welding – An Overview cd.

3. Self - Reacting Bobbin Tools

The self – reacting bobbin tools work on the same principle as fixed bobbin tools and have two rotating shoulders: upper and lower connected by the pin. However, as the simplest – fixed bobbin tools have no control of the forging force, at the self - reacting units a defined force can be applied onto the workpiece. Furthermore adjustable bobbin tools can weld elements with varying thickness. This can be achieved because the pin can be moved up and down through the upper shoulder. The lower shoulder is mechanically connected to the pin.

Based on the TWI principles other companies continue the development of bobbin tools. The AdAPT tool developed by MTS is a unique working multi purpose friction stir welding apparatus. In Europe ESAB Sweden is a leader in bobbin tool technology development. Also German GKSS-Forschungszentrum works on bobbin tool development.

3.1. MTS Systems Corporation Adaptable, Adjustable, Pin Tool

MTS Systems Corporation introduced an advanced friction stir welding system - Adaptable, Adjustable, Pin Tool (AdAPT) – a unique invention that expands the capabilities of the Auto Adjustable Pin Tool for Friction Stir Welding developed at NASA’s Marshall Space Flight Centre [2].

The patented MTS AdAPT welding tool employing Marshall’s retractable pin tool technology enables quick mode switching between fixed, adjustable and self-reacting welding modes to accommodate both linear and complex, non-linear contour weld configurations [3].

In the self – reacting mode the AdAPT head accommodates a new concept that uses a tool with a bottom shoulder on the backside of the workpiece (Fig. 2). The bottom shoulder is mechanically attached to the probe that penetrates the entire plate thickness. The probe is linked to an actuator that provides the capability to adjust both the load and position of the bottom shoulder. A separate axis is used to control the load that the top shoulder induces on the workpiece. This combination of independently controlled top and bottom shoulders results in a self – reacting system in which the forging force on the topside of the workpiece is equal to the forging load on the backside of the workpiece. Therefore, the net force applied to the workpiece (normal to the weld surface) is essentially zero.



The innovative AdAPT welding head provides many advantages compared to the classical FSW process. The biggest innovation on the welding head is the ability to eliminate the anvil – the large stiff structure behind the parts to be welded. The welding head pushes against the top of the part with the same load as it pulls against the bottom of the part resulting in a small net on the part. The both shoulders are independently controlled to provide a balanced, self – reacting process load while following the part profile. With the need for the anvil eliminated, the cost and complexity of the workpiece fixture is reduced. An innovation of the welding head is the integration of a spinning actuator with position feedback inside the weld head assembly. The overall result of this design is the ability to weld thicker materials than conventional FSW. When welding 25mm thick plate, loads on the structure in case of self – reacting welding configuration are more than 50 % lower. This reduction in load occurs because the bottom shoulder adds extra heat to the weld and eliminates a path for heat to escape from the weld. The efficiency of the heating process is increased. The power input required for welding is reduced; thereby the travel speeds can be increased.

The MTS System Corporation demonstrated that 2195 and 2219 aluminium alloys in thickness range from 4mm up to 25mm could be successfully joined using self – reacting FSW [4, 5]. This includes dissimilar alloy welds (2219/2195) and tapered thickness (8mm to 16,5mm over 610mm length) configurations. AdAPT head was also proved to perform self – reacting butt – welds on 4mm to 30mm thick aluminium alloys (2219, 2519, 2195, 5083, 6061, 2024 and 7020). Also dissimilar joints configurations using 8mm thick 2195 and 2219 alloys were performed. Acceptable weld quality was obtained with various pin tool configurations, demonstrating the robustness of the process. The performance of the welds exceeded that of typical fusion welds and met or exceeded that of conventional FSW. Two aerospace companies (Airbus Bremen, Germany and Eclipse Aviation Corporation of Albuquerque, USA) use systems with self – reacting mode – the ISTIR 10 plants as a research and applications development tool for the fabrication of aircraft structures [6].

3.2. ESAB Adaptive Pin Tool and the bobbin tool technique

ESAB the world leader in FSW machines has delivered to The Boeing Company Huntington Beach two SuperStir TM machines covering bobbin tool welding. One lab unit for straight welds and one production unit for circumferential welds intended to be used for welding dome to cylinder on Delta II tanks with a diameter of 2,5m in aluminium alloy 2014 and 2219 around 12,5 mm thickness (two 6,4 mm plates in overlap) [7]. The production unit was delivered and all tests for the machine were successful but as Delta II is going to be terminated the production has never been changed into bobbin tool FSW.

In ESAB SuperStir TM machine bobbin tool welds are carried out starting from a drilled hole and then producing sound butt welds. Figure 3 shows the ESAB production station for the FSW of circumferential seams with a bobbin tool for Delta II tanks.



In ESAB SuperStir TM machine both the retractable tool pin technique (the ESAB name is Adaptive Pin Tool) and the bobbin tool technique are used. To cover all the items, the plant includes a friction plug – welding unit for filling the end hole and a milling unit to be able to machine the end surfaces. The yellow part is the welding head and the two blue rings are fixtures. The elements to be joined are placed vertically as the welding position has no influence on the solid-state process friction stir welding.

Under the auspices of the U.S. Department of Energy, the University of California and Lawrence Livermore National Laboratory examined use of bobbin tool friction stir welding for joining of thick walled aluminium alloy pressure vessels [8]. Extensive friction stir welding parameter studies were conducted on 25mm and 38mm thick 2219 Al alloy plate. Welds were made in flat plates as well as circumferential welds that would join two hemispherical forgings with a 1 meter inside diameter using an ESAB friction stir welding machine.

The work demonstrated that it is possible to make defect – free friction stir welds in a high – strength Al alloy in thickness and topology that had not been previously demonstrated. It is planned to extend the use of bobbin – tool friction stir welding to the development and manufacture of similar pressure vessels in other high – strength Al alloys.

3.3. GKSS-Forschungszentrum bobbin tool

The GKSS-Forschungszentrum bobbin tool, Figure 4, allows for a defined force to be applied onto the workpiece. This can be achieved because the unit contains a hydraulic cylinder where the piston moves the pin up and down through the upper shoulder. The second shoulder is mechanically connected to the pin. By controlling the pressure on the piston, a defined force can be applied.



The GKSS bobbin tool is a prototype, which was supposed to be a portable unit that could be handled by every system with the required payload capacity, for example of a robot. Tests conducted with the NEOS tricept 805 robot indicated a need of a very rigid handling system for the tool. Rigid fixing of the tool and traversing the workpiece achieved this. This allowed performing “bead on plate welds” as well as overlap welds. Results of preliminary mechanical tests have shown satisfactory results. Further development of the GKSS bobbin tool includes the welding system, the tool and the pin and shoulders revision. The final unit must be capable of performing both butt and lap joints.