Issue 48

R. Maciel et alii, Frattura ed Integrità Strutturale, 48 (2019) 269-285; DOI: 10.3221/IGF-ESIS.48.28 271 The combination of heat and resultant material softening and recrystallization along with material flow during the process result in a modified metallographic structure in the joint cross-section. The resultant material zones have different physical and mechanical properties. Generally, these areas can be subdivided in 4 zones, as shown in Fig. 3. These are, the base material (BM) where heating is not sufficient to cause metallographic changes and the material keeps the original morphology, heat affected zone (HAZ) where heating is sufficient to cause softening, the thermo-mechanically affected zone (TMAZ) is softened and material plastic flow is noticeable here by the elongated and reoriented grains and in the center of the weld is the recrystallization zone, usually called weld nugget (WN), where the combination of heating and mixing of the tool causes grain breakage resulting in small equiaxed grains[8]. Figure 3 : Resulting weld zones from FSW drawn on microscopic analysis image Even though friction stir welding presents several advantages over other welding techniques when joining aluminium alloys, it also presents its share of challenges. For example, in the case of overlap configuration joints, which are very common in structural design, the presence of a hook defect, Fig. 4, reduces the static and fatigue strength as this defect acts like a crack initiation point. In certain alloys, the question of chemical corrosion is also a factor requiring good sealant measures to avoid degradation [9]. Figure 4 : Hook defect in overlap configuration. The heat generation and plastic deformation resultant from the FSW process causes the softening of the weld center line [5, 8]. This FSW induced softening in precipitate hardened aluminium-magnesium-silicon alloys, as well as the typical W shape hardness profile. A considerable database of tensile tests is already available in literature. In Moreira et al. [10] a 68.5 % effectiveness( ratio of joints strength over bas material ultimate tensile strength) was obtained in AA6082-T6 welds. Braga in [5] compared the stress vs strain curves between the various material sections in FSW and base material AA6082-T6 aluminum alloy, using digital image correlation (DIC). In addiction, from [11] another tensile study for the same material was conducted, Fig. 5.