Thermo-Mechanical Effect on Poly Crystalline Boron Nitride Tool Life During Friction Stir Welding (Dwell Period)

Poly Crystalline Boron Nitride (PCBN) tool wear during the friction stir welding of high melting alloys is an obstacle to commercialize the process. This work simulates the friction stir welding process and tool wear during the plunge/dwell period of 14.8 mm EH46 thick plate steel. The Computational Fluid Dynamic (CFD) model was used for simulation and the wear of the tool is estimated from temperatures and shear stress profile on the tool surface. Two sets of tool rotational speeds were applied including 120 and 200 RPM. Seven plunge/dwell samples were prepared using PCBN FSW tool, six thermocouples were also embedded around each plunge/dwell case in order to record the temperatures during the welding process. Infinite focus microscopy technique was used to create macrographs for each case. The CFD result has been shown that a shear layer around the tool shoulder and probe-side denoted as thermo-mechanical affected zone (TMAZ) was formed and its size increase with tool rotational speed increase. Maximum peak temperature was also found to increase with tool rotational speed increase. PCBN tool wear under shoulder was found to increase with tool rotational speed increase as a result of tool’s binder softening after reaching to a peak temperature exceeds 1250 °C. Tool wear also found to increase at probe-side bottom as a result of high shear stress associated with the decrease in the tool rotational speed. The amount of BN particles revealed by SEM in the TMAZ were compared with the CFD model

Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba ₇S(VS ₃O) ₂(S ₂) ₃ and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study.

International audienceOxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q–Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba–V–Q–O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba7V2O2S13, which can be written as Ba7S(VS3O)2(S2)3, was substituted to yield three selenide derivatives Ba7V2O2S9.304Se3.696, Ba7V2O2S7.15Se5.85, and Ba7V2O2S6.85Se6.15. They represent original multiple-anion lattices and first members in the system Ba–V–Se–S–O. They exhibit in the first layer heteroleptic tetrahedra V5+S3O and isolated Q2– anions and in the second layer dichalcogenide pairs (Q2)2– with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q2– or (Q2)2– (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO3S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V5+, together with the presence/nature of the dichalcogenides (Q2)2– and isolated Q2–, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry