Fig. 1. Schematic illustrations of temperature measurement by thermocouples and specimen preparations for macro- and micro-structural observations, tensile test and hardness test.

Fig. 2. (a) macrographs of longitudinal cross sections of the joints fabricated at 300 rpm, 4 mm, and different friction pressures of 180 MPa, 300 MPa, and 500 MPa; (b) corresponding SEM micrographs of weld interfaces center and EDS line analysis along the red solid lines perpendicular to the weld interfaces.

Fig. 3. Tensile strength of the joints fabricated at 300 rpm, 4 mm, different friction pressures of 180 MPa, 300 MPa, and 500 MPa in air, in comparison with that of the SUS316L BM.

Fig. 4. SEM microstructure of the weld interfaces of the joints fabricated at 300 rpm, 4 mm, and different friction pressures of (a-b) 180 MPa and (c-d) 500 MPa.

Fig. 5. (a-b) SEM-EDS results of the “mixed” layer formed at weld interface periphery fabricated at 300 rpm, 4 mm, and 500 MPa; (c) Vickers hardness results of the layer formed at weld interface periphery fabricated at 300 rpm, 4 mm, and 180 MPa.

Fig. 6. Schematic illustrations of processing modifications including rotation speed reduction and on-line CO2 cooling in friction welding.

Fig. 7. (a) macrographs of longitudinal cross sections of the joints fabricated at 500 MPa, 4 mm, 100 rpm in air and with on-line CO2 cooling; that of the joint fabricated at 500 MPa, 4 mm, 300 rpm in air is also shown for comparison; (b) corresponding thermal cycles measured at the weld interface center and periphery, and temporal evolution of burn-off length for the studied joints.

Fig. 8. SEM micrographs of the weld interfaces of the joints fabricated at 500 rpm, 100 rpm, 4 mm in air (a-b) and with on-line CO2 cooling (c-d).

Fig. 9. Tensile strength of the joints fabricated at 500 MPa, 4 mm, 100 rpm in air and with on-line CO2 cooling; those of the joint fabricated at 500 MPa, 4 mm, 300 rpm in air and the SUS316L BM are also shown for comparison.

Fig. 10. Macrographs of longitudinal cross sections of the joints fabricated at 500 MPa, 100 rpm, interrupted at burn-off lengths of 3.0 mm, 3.5 mm and 4.0 mm in air (a) and with on-line CO2 cooling (b), as well as the corresponding thermal cycles and burn-off length versus time during friction welding.

Fig. 11. SEM micrographs of the weld interface center and periphery of the joints fabricated at 500 MPa, 100 rpm, interrupted at burn-off lengths of 3.0 mm, 3.5 mm and 4.0 mm in air.

Fig. 12. SEM micrographs of the weld interface center and periphery of the joints fabricated at 500 MPa, 100 rpm, interrupted at burn-off lengths of 3.0 mm, 3.5 mm and 4.0 mm with on-line CO2 cooling.

Fig. 13. (a) schematic illustration for TEM foil specimen preparation and observation; (b-d) corresponding TEM results obtained at weld interface periphery of the joint fabricated at 500 MPa, 100 rpm, 4.0 mm with on-line CO2 cooling.

Fig. 14. STEM micrographs and corresponding STEM-EDS line analysis along the red lines at the weld interface center and periphery of the joint fabricated at 500 MPa, 100 rpm, 4.0 mm with on-line CO2 cooling.

Fig. 15. Temperature dependence of the yield strength of Ti64 and SUS316L [18,29].

Fig. 16. Schematic illustration for formation mechanism of mechanically mixed layers at the weld interface during friction welding.

Fig. 17. Tensile strength of the optimized Ti64/SUS316L joint fabricated in the present study as a function of test-to-weld area ratio; corresponding results of the other Ti64/stainless-steel friction welded joints published in literature [11,[17], [18], [19],23] are also plotted for comparison.