Abstract During solidification subsequent to welding process, segregation of Ti towards interdendritic region of the fusion zone (FZ) occurs which, in turn, causes dendrite core to become impoverished in Ti and, consequently, also in the strengthening γ’, which is consistent with the fact that welded tensile test specimens break at the FZ rather than at the base metal (BM), where there has been no segregation phenomenon associated with the solidification process. The fact that the interdendritic region in the FZ is enriched in Ti makes it a zone where the η Ti–rich phase precipitates preferentially. Cellular η phase appears at very early stages of aging, which raises the possibility that it may form not only from γ’ phase but also directly from the matrix. The dilution in the FZ varies with the point where it is analysed; this phenomenon is more pronounced for those solute elements whose content differs more between the filler metal and the BM, namely Ni and Fe, and may affect the nucleation of the γ’ and η phases, which, in addition to being rich in Ti, are also rich in Ni. Vickers microhardness tests confirm: (i) greater dispersion in microhardness values in the FZ, associated with Ti segregation; (ii) that the minimum microhardness values are lower in the FZ than in the BM (the fracture occurs at the FZ); and (iii) the lower microhardness values for the higher aging temperature, due to the greater presence of η phase and to the coarsening and coalescence of γ’ precipitates.
Tiedra et al. (Sat,) studied this question.