Friction stir welding (FSW) is a relatively new welding process compared to electron beam or arc welding. Unlike most other welding processes there is no liquid state for the weld pool. For this reason the potential defect types present within the weld are quite different from conventional welding defects such as voids or lack of fusion. These can still be present, but defects such as slag or hot tearing due weld pool shrinkage cannot. Other defects more akin to those associated with resistance welding (joint line remnants) are more likely and can be more serious. TWI has run and taken part in a number of projects including the European “Qualistir” project lead by R/D-tech. The object of these projects was to develop a reliable inspection method for determining the quality of FSW in a butt and lap welded configurations. This paper describes the novel method developed by TWI for the detection of the joint line remnant defects. The paper describes the use of back scattered noise analysis for determining whether the weld has been correctly forged and whether the metallurgical structure indicates a good weld. Further to the difficult FSW specific flaws the paper also describes how the inspection procedure detects the more conventional defects.

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