OBRABOTKAMETALLOV Vol. 24 No. 4 2022 technology Fig. 4. The boundary between structural zones of FSW oints of Ti-4Al-3Mo-1V alloy, obtained by modes 1–3 Microstructural examination of welded joints also indicates the presence of a narrow zone of thermomechanical affection (TMAZ), which is characterized by the presence of not recrystallized grains, strongly deformed in the direction of the plastic flow of metal (Fig. 4). It should be noted that an increase in the axial force on the tool leads to an increase in the deformation degree of structural grains in the TMAZ. The line that indicates the boundary between the structural zones and is apparent in Figure 2 runs between the TMAZ and HAZ of the welded joint. An abrupt transition from one structural state of the material to another is related to the low thermal conductivity of the titanium alloy, which provides its local heating directly in the contact zone with the welding tool and prevents its plasticization, sufficient for the formation of an extended zone of thermomechanical effect. The difference between the structural states in the HAZ and TMAZ of the welded joint determines an abrupt change in the strength characteristics of its material. The interface of the characteristic zones acts as a factor determining the strength of a welded joint. The appearance of samples of welded joints tested for uniaxial static tension indicates that the failure of the material occurs along the line corresponding to above-mentioned boundary of characteristic zones (Fig. 5). Fractographic investigations showed that when samples are fractured, the surface of samples is formed, that is similar in texture to a fibrous fracture, with the formation of sharp passes (Fig. 6). A microrelief corresponding to a ductile fracture with formation of dimples is formed on the fracture surface. Sufficiently large and branched bridges of plasticity are observed between the dimples. The formation of small zones of material stretching is observed. Thus, the fracture occurred in a place containing a large number of lowdimensional and deformed structural elements, which corresponds to the partially fragmented and highly deformed grains of the TMAZ of the welded joint. According to the results of testing samples of welded joints for uniaxial static tension it was found that the ultimate tensile strength value of the obtained welded joints decreases to 84–93 % of the value for the initial alloy Ti-4Al-3Mo-1V (Fig. 7). According to the tension test results, an increase in the axial force on the tool during friction stir welding leads to a decrease in the ultimate tensile strength of the resulting welded joint of Ti-4Al-3Mo-1V alloy. This effect is associated with an increase in the degree of deformation of the material, which contributes to an increase in a strength characteristics gradient. When the mixing zone is strengthened, the zone of thermomechanical affection is softened due to deformation-induced processes of redistribution Fig. 5. Samples of FSW joints of Ti-4Al-3Mo-1V alloy, obtained by modes 1–3, after uniaxial tension tests
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