Friction stir welding is a method of materials processing that enables the joining of similar and dissimilar materials. The process, as originally designed by The Welding Institute (TWI), provides a unique approach to manufacturing—where materials can be joined in many designs and still retain mechanical properties that are similar to, or greater than, other forms of welding. This process is not free of defects that can alter, limit, and occasionally render the resulting weld unusable.
Friction stir welding (FSW) is a joining process that was first demonstrated by The Welding Institute (TWI) of Great Britain in 1991 
. Since that time, FSW use has soared, and by the end of 2007, TWI had issued 200 licenses for the process. Its applications continue to grow. In addition, approximately nineteen hundred patent applications have also been filed relating to aspects of FSW 
. The popularity of the process can be related to the multiple advantages that FSW offers, when compared with other jointing modalities, including the ability to join vastly different metals, when performed correctly, achieve minimal defect creation, maintain much higher material strength along the bonds than typical for other joints, and provide smooth surfaces after joining.
In performing the FSW process it typically involves two metals clamped on a rigid surface that serves as an anvil together and a rotating shoulder, a mechanical “stirring” device that resembles a drill bit. The anvil serves to react the downward pressure, i.e., the plunge force, during the FSW process. By forcing the rotating shoulder into the materials along the weld interface, a frictional force is generated due to the high speeds and maintained downward pressure of the rotating shoulder against the metal plates. The resulting frictional heating creates a softened zone which is mechanically plasticized at the location of joining (Figure 1). The welding tool is simultaneously rotated and moved along the desired weld line, blending the materials along the path. The resulting weld is typically stronger than that given by traditional fusion welding methods because it is formed at a lower temperature, which minimizes a heat-affected zone and this also reduces distortion and resulting residual stress. In addition, FSW is an environmentally friendly process, as it does not use shielding gas or filler material and it involves minimal energy input.
Since FSW is typically implemented in an automated process, when using correctly designed tools and parameters, defects should not occur. However, if the process is incorrectly controlled, the resulting quality of the weld can be degraded. For any material joining technique—No process is perfect, and defects can potentially occur. For FSW wormholes, kissing bonds, and defects caused by lack of penetration are the typical defects of current concern in industry 
. With the use of FSW soaring, there is a need for nondestructive evaluation (NDE) processes that are superior to those currently available in the market 
to provide adequate quality control 
, particularly for safety critical applications.
The nondestructive testing (NDT) demands by industry when FSW is used are that fast and cost-efficient methods are provided to assess the weld quality. Although welds are generally of high quality, some heterogeneity may arise due to the improper stirring of the parent material, lack of penetration of the tool pin, poor choice of tool pin design or improper choice of the process parameter window. When defects do occur, they are very different in form from those typically found in a conventional fusion welding process.
A further constraint, in terms of inspection needs, is that the FSW method is typically a high-end fabrication method. Destructive evaluation of such welds in most cases is not recommended for evaluating quality since they are costly, in terms of lost items, and time consuming to conduct 
. When used destructive examination techniques generally involve bending tests and metallography/macrographs. In these techniques, the welded samples are removed from the original welding surface, but such samples only provide data for the region where measurements, such as micrographs, are taken. In general, evaluation of weld quality is most commonly performed post-weld using conventional non-destructive testing methods, such as X-rays, ultrasonic testing, eddy current and dye penetrant, although the later are limited to detecting surface defects. The specific type, size, shape and orientation of a defect all affect the detectability and characterization of the specific anomaly, and this depends on the specific nondestructive method used