In friction stir welding (FSW) a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces butted together. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. Frictional heat is generated between the wear resistant welding tool and the material of the work pieces. This heat causes the latter to soften without reaching the melting point and allows traversing of the tool along the weld line. The maximum temperature reached is of the order of 0.8 of the melting temperature of the material. The plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool probe and is forged by the intimate contact of the tool shoulder and the pin profile. It leaves a solid phase bond between the two pieces. The process can be regarded as a solid phase keyhole welding technique since a hole to accommodate the probe is generated, then filled during the welding sequence.

The non-consumable tool has a circular section except at the end where there is a threaded probe or more complicated flute; the junction between the cylindrical portion and the probe is known as the shoulder. The probe penetrates the work piece whereas the shoulder rubs with the top surface. The tool has an end tap of 5 in 6 mm diameter and a height of 5 to 6 mm (may vary with the metal thickness). The tool is set in a positive angle of some degree in the welding direction. The design of the pin and shoulder assembly plays a major role on how the material moves during the process.

 Advantages of FSW

• The process is environment friendly since no fumes or spatter is generated and no shielding gas is required.

• A non consumable tool is used

• Since the weld is obtained in solid phase, gravity does not play any part and hence the process can be done in all positions(vertical, horizontal, overhead or orbital)

• No grinding, brushing or pickling is required.

• Since the temperature involved in the process is quite low, shrinkage during solidification is less

• One tool can be typically used for up to 1000 metres of weld length (6000 series aluminium alloy)

• No fusion or filler materials is required

• No oxide removal necessary as in fusion welding.

• The weld obtained is of superior quality with excellent mechanical properties and fine micro structure.

• The process is cost effective since mechanical forming after welding can be avoided.

Such has been the interest in FSW, which was patented not so long ago that considerable effort is being made in transferring the technological benefits from aluminium to other materials. Efforts are on to make the process more flexible. In the new millennium there is no doubt that the automotive sector will find an increasing number of uses for this process as its cost effectiveness and ability to weld dissimilar material combinations with minimal distortion is more widely appreciated. The process has been an excellent substitute for alloys that have inherent fusion welding problems.