Fig. 1. (a) Schematic illustration of the FSW-processed and TIG-welded plates, showing the milling location and dimensions of the specimens for the Charpy impact test and microstructural analysis. (b) Detailed dimension of Charpy 45 ° V-notch specimens. TD: transverse direction; RD: rolling direction; WD: welding direction; ND: normal direction.

Fig. 2. (a) Process window map after FSW as a function of rotational and traveling speeds under the plunging load between 2000 and 4500 kg. (b) OM and EBSD IQ-phase images of defect-free steels after FSW as a function of rotational and traveling speeds under the plunging load of 4500 kg. In the process window map, the circle marker denotes successful welding, and the X marker means failure. The EBSD IQ-phase maps were taken at the stir zone center from the rolling//welding direction. In the EBSD IQ-phase map, the yellow is retained austenite (γR), the gray is martensite (α′) or ferrite (α), the blue lines are high-angle boundaries (> 15°), and the green lines are low-angle boundaries (3°-15°). RD: rolling direction; WD: welding direction; ND: normal direction; D: grain diameter; VγR: volume fraction of retained austenite.

Fig. 3. (a) OM and (b-d) EBSD images of FSW-processed specimen, which were observed from the rolling//welding direction of steels. EBSD observations were conducted at the region marked in the OM image. In the IQ-phase maps, the color representation is the same as that used in Fig. 2.

Fig. 4. (a) OM and (b-d) EBSD images of TIG-welded specimen, which were observed from the rolling//welding direction of steels. EBSD observations were conducted at the region marked in the OM image. In the IQ-phase maps, the color representation is the same as that used in Fig. 2. The prior austenite grain boundaries (d) were highlighted as the black lines by misorientation angles of 20°-50°

Fig. 5. (a) Vickers hardness profiling of FSW-processed and TIG-welded samples along the dashed lines in the OM images (Figs. 3(a) and 4(a)). (b) XRD patterns of BM, FSW-processed, and TIG-welded samples.

Fig. 6. Microstructural evolutions of the thermo-mechanical affected zone of FSW-processed specimen. The observation area was near the stir zone as marked in Fig. 3(a). Low magnification EBSD (a) IQ-phase map and IPF maps of (b) ferrite (α) or martensite (α′) and (c) retained austenite (γR). (d-g) High magnification EBSD IQ-phase maps at the marked areas in Fig. 6(a). In the IQ-phase maps, the color representation is the same as that used in Fig. 2.

Fig. 7. Charpy impact absorbed energy vs test temperature curves for BM, FSW-processed, and TIG-welded samples.

Fig. 8. SEM fractographs of TIG-welded samples fractured at (a) 25 °C and (b) -196 °C. SEM fractographs of FSW-processed samples fractured at (c) 25 °C and (d) -196 °C.

Fig. 9. EBSD IPF maps of (a) TIG-welded sample, and IQ-phase plus IPF maps of (b) FSW-processed samples fractured at -196 °C, which were taken from the normal direction. The black lines are the prior austenite grain boundaries (Fig. 9(a1)). The color representation of IQ-phase maps (Fig. 9(b1, b2)) was the same as that used in Fig. 2.

Fig. 10. SEM fractographs of BM fractured at (a) 25 °C and (b) -196 °C. (c) EBSD IPF and IQ-phase maps of the BM sample fractured at -196 °C, which were taken from the normal direction. In the IPF map, the black lines are the prior austenite grain boundaries. The color representation of IQ-phase maps was the same as that used in Fig. 2.

Fig. 11. (a) Elemental analysis of prior austenite grain boundary (PAGB) of nugget in TIG-welded sample: (a1) SEM micrograph showing a location for APT tip lift-out; (a2) three-dimensional APT result. (b) Elemental analysis of ferrite/ferrite (α/α) interface of stir zone in FSW-processed specimen: (b1) SEM micrograph showing a location for APT tip lift-out and TKD IQ map of APT tip; (b2) TKD phase and IPF maps of APT tip; (b3) three-dimensional APT result. One-dimensional elements profiles of (c) TIG-welded and (d) FSW-processed samples corresponding to black arrows in three-dimensional APT results.