TWI Industrial Member Report Sumamry 664/1998
W M Thomas, P Woollin and K I Johnson
Friction stir welding (FSW) is a new technique for butt and lap welding metals and plastics. It was first demonstrated in 1991, when it was used to butt weld aluminium plate of 6mm thickness, and the process has been patented by TWI.  A rotating tool with a central probe is pressed into the components to be welded and traversed along the joint line. Provided the components are adequately jigged, a solid phase weld is formed following considerable hot working of the material at the joint.
The technique has a number of advantages over competing, well established, metal welding techniques. These arise because of the hot forged joint structure and the absence of molten weld metal and include: good mechanical properties, low workpiece distortion, and capability of welding metals whose structure and properties would be degraded by melting.
FSW has made an enormous impact on the welding of aluminium extrusions and plates and is already in commercial production. Detailed development work has been conducted on the FSW of various Al alloy sheets and plates of 1.5-12mm thickness under a confidential TWI Group Sponsored Project.  . This work has generated detailed welding schedules and procedures, and the construction of prototype welding equipment. Other work published on FSW of aluminium has been concerned with the use of FSW tools of various geometries, the use of tool inclination, and the optimisation of welding parameters. [3-5]
With regard to the FSW of other materials, the process is most readily applied to those metals and alloys with melting points below about 1000°C because of the high temperatures and stresses (and hence wear) which would otherwise be generated at the welding tool. Welds have been demonstrated at TWI in lead, zinc and copper.
Despite the tool wear problem, materials of higher melting point have been welded by FSW. Progress is currently being made on a project with the objective of welding titanium alloys in thicknesses up to 25mm.  The process has also been demonstrated for the welding of steel. TWI reported the welding of 3mm low carbon steel in November 1997, [7,8] whilst the Edison Welding Institute, USA, has reported the welding of 6mm mild steel in May 1998. 
If FSW can be applied reliably to steel sheet and plate with minimum tool wear and at rates in excess of those achieved by arc welding, then it should be a technically and economically attractive process for numerous applications. These would initially involve the butt welding of long, straight joints such as are required in the fabrication of large transport vessels and vehicles (ships, trucks and railway rolling stock).
This report describes a study of FSW of butt joints in 12mm steel plate. The purpose of the work was to establish the feasibility of making welds of several metres in length, given the potential tool wear problem, and to identify typical weld properties. The steel chosen for these trials was a 12%Cr stainless steel having a martensitic/ferritic microstructure. This material was chosen because it has the advantage of a wide temperature range of ferrite stability which should make it a relatively easy steel to join by FSW because the high temperature strength of ferrite is low. It is also becoming increasingly industrially attractive because it has good corrosion resistance and is relatively inexpensive.
To determine the feasibility of friction stir welding 12mm thick 12% Cr steel plate and to establish the weld properties and microstructures produced.