Analyzing defects and their effects on the strength of a three-layer FSW joint by using X-ray microtomography, localized spectrum analysis, and acoustic emission

The present work aims at proposing an experimental strategy involving three measurement techniques, namely X-ray micro-computed tomography, Localized Spectrum Analysis, and Acoustic Emission (AE) in order to detect the initiation of damage mechanisms within an FSW joint and to track its evolution during a mechanical test. The specimen was manufactured by superimposing and welding together three aluminum alloy sheets named AA6061, AA7075, and AA2024. First, the defects within the internal structure of the joint were identified by using X-ray micro-computed tomography. The joint was then subjected to a tensile test. The evolution of the defects as a function of the tensile stress was monitored by using acoustic emission coupled with non-contact strain fields measurements on two perpendicular faces of the specimen. The findings highlight a good correlation between the strain-concentration zones and those characterized by a high density of weld defects, as identified from the analysis of the microtomography results. The comparison between AE results with those obtained from the computed microtomography analysis enabled us to state that the first crack sources were caused by the weakness zones detected within the initial state of the welded joint. The findings from an unsupervised classification of the AE activity are that three main mechanisms governed the damage evolution of the studied FSW joint. The acoustic signature of each cracking mechanism is defined by a pair of values (peak frequency, amplitude), each within a specific range. A deep analysis of the experimental results highlights a good correlation between the AE results with those from the strain analysis.