Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 295 Focussed on Crack Paths Influence of joint line remnant on crack paths under static and fatigue loadings in friction stir welded Al-Mg-Sc alloy Y. Besel, M. Besel, U. Alfaro Mercado Institute of Materials Research, German Aerospace Center (DLR), Linder Hoehe, 51147 Cologne, Germany , , T. Kakiuchi, Y. Uematsu Department of Mechanical Engineering, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan , A BSTRACT . The influence of the joint line remnant (JLR) on tensile and fatigue fracture behaviour has been investigated in a friction stir welded Al-Mg-Sc alloy. JLR is one of the microstructural features formed in friction stir welds depending on welding conditions and alloy systems. It is attributed to initial oxide layer on butting surfaces to be welded. In this study, two different tool travel speeds were used. JLR was formed in both welds but its spatial distribution was different depending on the tool travel speeds. Under the tensile test, the weld with the higher heat input fractured partially along JLR, since strong microstructural inhomogeneity existed in the vicinity of JLR in this weld and JLR had weak bonding. Resultantly, the mechanical properties of this weld were deteriorated compared with the other weld. Fatigue crack initiation was not affected by the existence of JLR in all welds. But the crack propagated preferentially along JLR in the weld of the higher heat input, when it initiated on the retreating side. Consequently, such crack propagation behaviour along JLR could bring about shorter fatigue lives in larger components in which crack growth phase is dominant. K EYWORDS . Friction Stir Weld (FSW); Joint line remnant (JLR); Lazy S; Zigzag line; Aluminium alloy; Fatigue. I NTRODUCTION onventionally, riveting has been used to join high strength aluminium plates for fuselage structures. For increasing demand towards improvement of energy consumption of an aircraft, weight reduction of a fuselage is one of the most effective solutions. As some material overlap is necessary for rivet joining, it is feasible to achieve weight reduction by replacing those rivet joints by welded butt joints. However, some aviation aluminum alloys such as precipitation hardening 2xxx and 7xxx series have poor weldability in case of fusion welding. Friction stir welding (FSW) is solid state welding invented by TWI [1]. In the FSW processes, a non-consumable rotating tool consisting of cylindrical shoulder and pin is inserted into and moved along the butt joint line of two sheets. Frictional heating generated during this process causes softening of the material and local plastic deformation occurs. The plasticized material is stirred together by the rotation process resulting in a solid state join. Since the joining process by FSW basically C