Friction stir weld inspection using the motion induced eddy current testing technique

Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing work pieces without melting the work piece material. A non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and traversed along the line of joint. The tool serves two primary functions, heating of the workpiece, and stirring of material to produce the joint. The heating is accomplished by friction between the tool and the workpiece, leading to plastic deformations in the weld seam. The localized heating softens the material around the pin and combination of tool rotation and translation leads to movement of material from the front of the pin to the back of the pin. As a result, a joint is produced in solid state. During FSW process, the material undergoes intense plastic deformation at elevated temperature, resulting in generation of fine and regular recrystallized grains. The fine microstructure in friction stir welds produces good mechanical properties. Despite considerable interests in the FSW technology in past decade, the basic physical understanding of the process is adverse. Some important aspects, including material flow, tool geometry design, wear of welding tool, microstructural stability, welding of dissimilar alloys and metals, still require deeper understanding. However, it can be observed that new technologies are often commercialized before a fundamental science emphasizing the underlying physics has been developed. Thus, there is a need for nondestructive testing methods which allow to study these changes in the welding zone as well as to detect imperfections or defects in this region. Due to the complex structure of these materials, traditional NDT methods do not always satisfy the measurement requirements. In this paper, the capabilities of the application of the motion-induced eddy current testing technique to evaluation of friction stir welding are investigated. The specimen moves with constant velocity through a magnetic field, which is created by a fixed permanent magnet. The interaction of induced eddy currents and the primary magnetic field results in the Lorentz force acting on the specimen, but the reaction force acts in opposite direction on the magnet system as well. Here the magnetic force is measured in all three spatial dimensions. Every force component has a characteristic profile for a certain defect-free specimen. Anomalies in the specimen affect the eddy currents due to variations of local conductivity. These deviations influence the measured force profiles from which the location, size and type of the defect in the specimen may be determined.