Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 297 For tensile tests, specimens with a parallel section length of 70 mm, a gauge length of 30 mm, width of 12 mm and nominal thickness of 3.3 mm were prepared (Fig.1). The weld center line passed through the center of the gauge length. The tensile tests were carried out at room temperature with the specimens as welded without removing any surface features of the weld such as weld flashes. The geometry of the fatigue specimens is shown in Fig. 2. The weld center line lies perpendicularly to the loading direction in the center of the specimen. Tool marks and flashes formed by FSW on the surface were removed to eliminate their geometrical influence (e.g. notch effect) on crack initiation behavior. Prior to the fatigue tests, the specimen surfaces were mechanically polished with emery papers and finished with buff-polishing using 1µm-diamond suspension. The fatigue tests were carried out under cyclic loading conditions with a stress ratio of R = -1 at a frequency of 10 Hz. Si Fe Cu Zn Mg Mn Ti Sc Al 0.25 0.4 0.2 0.2 3.9 0.25 0.2 0.4 Bal. Table 1 : Nominal chemical composition of Al-Mg-Sc alloy in wt%. Figure 1 : Tensile test specimen. Figure 2 : Fatigue test specimen. R ESULTS AND DISCUSSION Weld structures ig. 3 shows optical micrographs of the cross sections of weld-480 and weld-720. In order to verify formation of JLR in those welds, stir-in-plate friction stir welding (one plate welding) was performed in Al-Mg-Sc plate with the tool travel speed of 600 mm/min. The macroscopic structure of the stir-in-plate weld is shown in Fig. 3(c). Dashed lines in the figures indicate the pin width. All welds had a stir zone (SZ) in the center surrounded by a thermo- mechanically affected zone (TMAZ). Fine recrystallized and equiaxed grains were formed in SZ around the weld center in all welds. Based on EBSD-measurements the average grain sizes in SZ of weld-480 and 720 were determined as 1.12 µm and 1.63 µm, respectively. Obviously, no zigzag line was observed in the stir-in-plate weld, see Fig. 3(c). On the contrary, zigzag lines were observed in the butt weld for all weld conditions, as seen in Figs. 3(a) and (b), where the zigzag lines were highlighted with freehand lines for aid of visualization. Although the native oxide layer had been removed before the welding process, it seems that a new thin oxide film grew rapidly on the butting surfaces in laboratory air condition. This new oxide film was so thick that incomplete breakups remained resulting in formation of the zigzag line i.e. joint line remnant (JLR) in the welds. The degree of breakup of the oxide layer can sometimes be estimated based on the heat input during FSW [4, 5]. Generally, the heat input in FSW is considered to correlate with tool rotational speed and tool travel speed, where higher heat input is caused by higher tool rotational speed or lower tool travel speed, respectively [3, 19]. Consequently, in this study, the F