PSI - Issue 61

Shahriar Afkhami et al. / Procedia Structural Integrity 61 (2024) 53–61 Shahriar Afkhami et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Table 3. Degrees of cold-forming, according to the FE approach, resulting from bending procedures. Bending radius: 10 mm 20 mm 30 mm Degree of cold forming: 14.1% 12.2% 9.9%

Fig. 1. Temperature distributions of the welded joints according to the FE models at 50% progress of the welding procedure under the same heat inputs (from Table 1) but different joint designs: (left) the design used in [4], and (right) the modified design used in this study.

Hardness fluctuations on the cold-formed side of the welded samples are shown in Fig. 2. According to this figure, the material did not experience any degrees of softening regardless of its DOC, unlike the case study in [4]. The absence of the HAZ softening phenomena in this study can be attributed to the modified joint design changing the thermal gradient of the joint area and hindering the appearance of relatively softer microstructural constituents by altering the cooling rate. Further, according to Fig. 2, increasing the DOC to 14% caused a slight increase in the strength ( σ y and σ u as yield and tensile strengths, respectively) of the welded sample but changed the failure location from the straight metal to the bent material (Fig. 3). It should be noted that the differences between the strength and ductility ( ε f ) values between the samples were insignificant, but there was a noticeable change in the hardening behavior of the welded sample with the 14% DOC on its bent side, especially after the point of stability (necking point). Consequently, increasing the DOC and imposing plastic strains higher than or equal to 14% can alter the dislocation network and micro-strains throughout the material to such a degree that manifests itself in the hardening pattern and change of failure location of the welded joint. However, this observation is in contrast with [4], where all the specimens (even the ones with DOCs as high as 20%) failed from the straight base metal, although some degrees of HAZ softening were also observed in the welded joints in [4]. The contradiction indicates that the DOC is not the sole determining factor in the fracture mechanism and location of the welded joints, and further research is required on the microstructural scale to identify other primary parameters related to the fracture mechanism of the cold-formed ultra high strength steel. Finally, as shown in Fig. 3, one specimen with a DOC of 14% failed from its fusion line; however, it is considered an exception since it only occurred in one instance and will be investigated further in the fractography evaluations.