Bobbin Tool Friction Stir Welding – An Overview cd.

2. FSW BOBBIN TYPE TOOLS

The bobbin tool, Fig. 1, has two rotating shoulders: upper (above the welded plate) and lower (below the plate) connected by a pin that penetrates the entire plate thickness. Both shoulders prevent plasticized material from expulsion. The lower shoulder also acts as the backing element. The parts to be welded have to be clamped together to prevent from moving them by working forces. Than a rotating tool is slowly moved along the joint line between two pieces of material, which are butted together. The pin is traversed through the joint line while the shoulders are in intimate contact with the top and bottom surface of the workpiece.



The bobbin tool FSW process provides many advantages compared to the classical FSW process. The biggest innovation of the bobbin tool is the ability to eliminate the large stiff structure behind the parts to be welded. There is no need of high axial force as the tool 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. Furthermore all process loads are reduced due to the increased heat production. Also due to increased heat production from two shoulders the travel speed can be increased.

Another benefit is having a bottom shoulder to eliminate the possibility of lack of penetration defect on the root side of the weld. The defect is introduced when the plunge depth on a conventional pin tool is not maintained in the proper location. The lack of penetration defect is especially detrimental in conventional friction stir welding because it is difficult to detect with conventional non – destructive evaluation process.

Bobbin tools can be used when butt and lap welds are mentioned. Those tools are thought to produce welds when there is limited access from one side or restricted possibility to use a fixed backing i.e. tubular structures such as a pair of hollow members or tubes. To perform welds in such conditions bobbin type tools have been proposed and developed. Bobbin type tools can be divided into two groups: fixed and self – reacting tools. Further development of bobbin type FSW tools is induced to the self – reacting tools, as the flexibility of the fixed type bobbin tools is rather poor.