Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 303 Figure 13 : SEM observation of fatigue fracture surface of weld-480: (a) Magnified view of Area II in Fig. 11, (b) magnified view of Area III. Based on the evidence of the crack growth direction in the surrounding matrix as seen in Fig. 13(b), it can be concluded that when the crack approached there the fatigue crack propagated preferentially at JLR because of its weak bonding, and grew further from the JLR-fracture site into the matrix. As a result, the macroscopic crack growth direction in this area became not totally perpendicular to the through-thickness direction as seen in Fig. 11. The fine dimples in Area VII of JLR-fracture surface in the residual fracture regime resembled the monotonic tensile fracture as seen in Fig. 7(b). Taking into account the elevated net stress in this section due to the already long through-thickness crack, it can be concluded that the weakly bonded JLR was ruptured quasi-statically. Thus, when the fatigue crack approached JLR, it propagated firstly at JLR and then into the matrix. The fracture area at JLR was about 10% of the total fracture surface. The locally weak bonding clearly accelerated local crack growth rate around JLR. However, significant decrease in fatigue life of the two specimens where JLR-fracture occurred on RS was not observed as seen in Fig. 9. It is generally known that most part of fatigue lives of a metallic component is spent on initiation and early propagation phases. The fracture at JLR occurred in the phase of comparably fast striation growth as seen in Fig. 13. Therefore, the crack propagation at JLR could shorten the fatigue life to some extent but its contribution to the total life was insignificant in the laboratory size specimens used in this study. In contrast, when the component is large, i.e. crack propagation phase may occupy a significant amount of fatigue life, the crack growth along JLR could lead to significantly shorter life time of such components. S UMMARY AND CONCLUSIONS nfluence of joint line remnant (JLR) in friction stir welded (FSWed) Al-Mg-Sc alloy on tensile and fatigue fracture behavior was investigated. Butt welding was performed under two different heat input conditions by changing tool travel speeds. Fatigue tests were conducted with polished specimens, i.e. without welding flash and root flaws. Although the native oxide layer was removed before FS welding, a new oxide layer was immediately formed under lab air condition. This newly generated oxide layer was thick enough to form pronounced JLR in the welds. The distribution of JLR in the welds clearly were differed by the heat input levels: In the lower heat input weld, JLR distributed mainly around the weld center. Higher heat input facilitated the material flow during the welding process, and consistently JLR in the weld with the high heat input was more widely distributed and reached the boundary of stir zone (SZ) and thermo- mechanically affected zone (TMAZ) on retreating side (RS) of the weld, where microstructural mismatch exists. When JLR located within SZ, tensile fracture occurred independently of the existence of JLR. However, because of the local microstructural mismatch between SZ and TMAZ existing at JLR in addition to weak bonding of JLR, tensile fracture occurred partially along JLR in the higher heat input weld. As a result, this weld showed lower yield and tensile strengths and less ductility than the other weld. Fatigue crack initiation was not influenced by JLR in all welds. But in the higher heat input weld, when a crack initiated on RS and approached JLR, the fatigue fracture occurred preferentially along JLR. Due to the weak bonding, the fatigue crack propagated intergranularly at JLR. Contribution of this preferential fatigue crack growth at JLR towards the total fatigue life was insignificant because it happened in the striation growth stage in a comparably small specimen. In the case that the fatigue crack growth phase contributes significantly to fatigue lives, i.e. in large components under moderate I