Non-Destructive Testing Techniques for Detecting Imperfections in Friction Stir Welds of Aluminium Alloys

Abstract

The invention of Friction Stir Welding (FSW) (Thomas, 2009) has contributed for a significant push forward in the weldability criteria for many engineering materials and the concept of how they can be mechanically processed in solid state. From the many materials processed by FSW the most remarkable results were obtained for the aluminium and all its alloys including wrought and cast original conditions. The main technological procedures and parameters are presented and most relevant tool features and architecture are established (Vilaca, 2003). Some industrial applications of FSWelds of aluminium alloys are also summarized. The quality of FSWelds of aluminium and its alloys is easy to reproduce and usually excellent, exceeding for some particular conditions the performance of base materials. Nevertheless, some imperfections can occur. The geometry, location, and microstructural nature of these imperfections will be established and classified for butt and overlap joints, which are the basic geometries enabling the production of all the remaining joint configurations, by combination of the previous. These imperfections bear no resemblance to the imperfections typically found in aluminium fusion welds. Consequently, it is difficult or even impossible, to identify all the FSWelds imperfections with commercially available conventional and advanced non-destructive testing (NDT) techniques. A paradigmatic example is the micro defects located at the root of conventional FSWeld joints with less than 100-50 μm. This state-of-the-art has been revealed as one important drawback preventing a wider transition of FSW to industrial applications, mainly focusing those where the quality standards are highly demanding and pursue the total quality assurance paradigm. The review of physical fundaments and some technological features of the different NDT concepts is included supporting the presentation of the following content of the present chapter (Santos, 2009). The assessment of the applicability of the conventional and advanced available NDT techniques to the most relevant FSW imperfections is established. The lack of assertiveness of the available NDT techniques in detecting the FSW joint imperfections are identified emphasizing the importance of the most recent new advances in the NDT technological