TWI Industrial Member Report Summary 694/1999
C J Dawes, D G Staines and E J R Spurgin
Background
Friction stir welding (FSW) has made tremendous progress since its first development, and is now used commercially for a number of applications in the aerospace, land transport and marine industries. It is certain that its use will continue to increase throughout the world.
To date, all commercial welding is used to make straight welds, usually in long lengths, using tool designs based on results generated in the TWI GSP 5651. Although these tools work well in such applications, they do have some drawbacks which restricted the use of the process in many potential applications. These are as follows:
- The tool must be tilted away from the direction of travel by typically 1-3° in order to provide a compressive down force behind the tool, which helps in consolidation of the material as it cools. The use of a tilt makes it difficult to make non-linear welds, as complex steering of either the tool carriage or the base plate is required, in order to keep the angle of tilt parallel to the welding direction. With a zero tilt tool, only a simple X-Y movement on either the carriage or the base plate would be required.
- A high down force is required with conventional tool designs, which must be reacted by a rigid backing plate. This adds to the bulk of the equipment required, increasing its cost, and reducing the prospects of portability. This can be overcome by the use of a bobbin tool, and this idea was first suggested when FSW was in its infancy, but the concept proved difficult. With improved tool designs, and the advent of zero tilt tools, the concept of the bobbin tool can be revisited. The absence of the backing plate means that the tool can be attached to a robot, and therefore the process can be used in a much wider range of applications for making two and three dimensional welds.
- The use of conventional butt welding tools for the manufacture of lap welds usually results in excessive thinning of the upper sheet, severely reducing the strength of the weld. In addition, it is more difficult to disperse the oxide layer in a lap weld, largely due to the changed orientation of the surfaces to be welded with respect to the tool. The use of alternative designs to modify the flow of metal during the process could lead to much improved performance of lap welds, and therefore enhancing the potential of the process.
This report describes some progress made in these areas, and responds to a stated objective of the Core Research Programme to develop the process for non-linear applications.
Objectives
- To develop an FSW tool capable of operating with zero tilt.
- To develop an FSW bobbin tool that can contain the weld metal about the tool pin and react the weld metal forging forces necessary for making sound welds.
- To develop a prototype FSW tool for making lap joints that does not exhibit top sheet thinning or serious oxide related flaws in the weld nugget.