Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 296 takes place well below the melting point of the joined materials, it can even be used to join such non-fusion-weldable aluminium alloys mentioned above. Characteristics of the weld depend strongly on welding parameters such as tool rotating speed and tool travel speed. High strength joints without defects such as voids or lack of penetration (LOP) can be produced when using appropriate welding parameters [2, 3]. However, other types of weld imperfections or weld flaws can arise even under the optimized welding conditions, that may or may not compromise the integrity of the welded join. Especially at low heat input, a faint zigzag line is sometimes formed in the stir zone [4, 5]. Sato et al. revealed by TEM observation that this zigzag line comprised a high density of amorphous Al 2 O 3 particles and suggested that they originated from the initial butt surfaces and its native oxide layer [6]. So, the zigzag line is also called “joint line remnant (JLR)” or “Lazy S”. In the following, the term of JLR is used for this zigzag line microstructural feature in the weld. The intensity, spatial distribution and location of JLR depend mainly on the aluminium alloy system, welding parameters (esp. welding speed) and tool configuration. “Root flaw” (sometimes “kissing bond”) is a weld defect with partially unwelded or only weakly bonded butt surfaces on the root side of the weld due to insufficient plunging of the tool, poor joint to tool alignment or inappropriate welding parameters [2, 7, 8, 9]. JLR can be formed accompanying root flaws depending on the welding conditions. These insufficient bonds at the bottom part of the JLR (i.e. root flaws) are not always detectable by non-destructive inspection but generally they open at root bend tests, while the JLR with sufficient root bonding does not deteriorate either root bending properties [4, 8] or tensile properties under as-welded conditions [10, 11]. Fatigue properties of the FS welds with JLR formation have been investigated for various Al alloys, e.g. 5083 and 6082 [8], 2024 and 5083 [12], 2024-T4 [13], 2198-T8 [14], and 1050-O [15]. Some researchers reported that kissing bonds or weak bonds on the root side caused fatigue crack initiation resulting in lower fatigue strengths [8, 12, 13]. In [14] and [15] it has been observed that fatigue fracture can take place independently from the existence of the JLR and cause hardly influence on fatigue strengths. Uematsu et al. investigated the fatigue initiation site in 1050-O with JLR and showed that localized plastic deformation at the boundary between TMAZ and HAZ at the advancing side of the FSW line caused fatigue crack initiation [15]. They also investigated fatigue behavior in FSWed samples of both heat-treatable and non-heat treatable alloys and concluded that fatigue fracture location was dependent on alloys due to their different microstructures and hardness distributions. While FSW is a promising candidate as future joining technology, Al-Mg-Sc alloys are promising candidates as next baseline material for metallic fuselage structures. They have lower density and better corrosion resistance than conventional fuselage material, e.g. 2024, and they basically show good weldability [16]. The characteristic Al3Sc dispersoids in Al-Sc alloys show high thermal stability and act as recrystallization inhibitor [17]. Due to these thermally stable precipitations in Al-Mg-Sc alloys, they are expected not to have pronounced degradation of mechanical properties by FSW processing, which are generally found in heat-treatable 2XXX alloys without post-welding heat treatment. It has been demonstrated that the formation of joint line remnant and the mechanical integrity of the FSW joint are controlled by the welding parameters [4]. Consequently, in this study, the friction stir welding technique was applied to an Al-Mg-Sc alloy at two different tool travel speeds. Formation of the joint line remnant was examined, tensile and fatigue tests were performed and the influence of the JLR on the fracture behavior under tensile as well as fatigue loadings of the FSWed Al-Mg-Sc alloy was investigated. E XPERIMENTAL PROCEDURES he material used in this study was Al-Mg-Sc alloy 5024 –H116 plate with a nominal thickness of 3.3 mm. Its nominal chemical composition is listed in Tab. 1. The FSW tool consisted of a conical threaded pin (diameter 4.5 mm) and a cylindrical shoulder (diameter 12.5 mm). Two plates were butted and position-controlled friction stir welded at constant tool rotation speed of 1200 rpm. The surface of the plates to be welded was ground directly before the FSW process to remove the native oxide layer, and thus, minimize the influence of initial surface conditions on the joint quality. However, as a common matter of fact, an amorphous aluminum oxide film layer can be rapidly formed in laboratory air [18]. The tool travel speed was controlled at two levels: 480 and 720 mm/min (hereafter welds produced with these parameters are referred to as weld-480 and weld-720, respectively). Temperature during welding was monitored with a thermocouple plunged at 7 mm away from the joint line in the retreating side plate at a depth of 1 mm from the top surface. The root bending tests revealed no cracking, i.e. no root flaws were produced with those welding parameter sets. Structural features of the welds including the JLR were examined with a light microscope on the polished cross sections etched with 5 wt% NaOH aqueous solution. Hardness profiles on the mid-section in the welds were measured by means of a micro-Vickers hardness tester under a load of 9.8 N. T