Author Contributions
Conceptualization, K.H.K., H.P. and J.-H.H.; methodology, K.H.K., H.P. and J.-H.H.; software, K.H.K., H.P. and J.-H.H.; validation, H.P. and J.-H.H.; formal analysis, K.H.K., H.P. and J.-H.H.; investigation, K.H.K., H.P., N.K. and J.-H.H.; data curation, K.H.K., H.P. and J.-H.H.; writing—original draft preparation, K.H.K. and J.-H.H.; writing—review and editing, K.H.K., D.J.L., H.P. and J.-H.H.; visualization, K.H.K., H.P. and J.-H.H.; supervision, H.P., N.K. and J.-H.H.; project administration, Y.N.K.; funding acquisition, Y.N.K. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Schematic of the diffusion bonding process (a) contact of two independent surfaces, (b) application of pressure from either side, (c) disappearance of void, and (d) grain growth with grain boundary movement.
Figure 1.
Schematic of the diffusion bonding process (a) contact of two independent surfaces, (b) application of pressure from either side, (c) disappearance of void, and (d) grain growth with grain boundary movement.
Figure 2.
Microstructural image of the as-received Ti-6Al-4V sheets.
Figure 2.
Microstructural image of the as-received Ti-6Al-4V sheets.
Figure 3.
(a) Geometry of the specimens used for the uniaxial tensile test. (b) Engineering stress and strain curves with respect to temperature. R5 and RT abbreviate 5 mm radius and room temperature, respectively.
Figure 3.
(a) Geometry of the specimens used for the uniaxial tensile test. (b) Engineering stress and strain curves with respect to temperature. R5 and RT abbreviate 5 mm radius and room temperature, respectively.
Figure 4.
Characteristic curve showing the general creep behavior of metals [
38].
Figure 4.
Characteristic curve showing the general creep behavior of metals [
38].
Figure 5.
Schematic of three creep tests in the (a) effective stress and effective stain domain, (b) effective strain and time domain, and (c) effective strain rate and time domain.
Figure 5.
Schematic of three creep tests in the (a) effective stress and effective stain domain, (b) effective strain and time domain, and (c) effective strain rate and time domain.
Figure 6.
(a) Geometry of the specimens used for the creep test and (b) apparatus used for creep test at high temperatures.
Figure 6.
(a) Geometry of the specimens used for the creep test and (b) apparatus used for creep test at high temperatures.
Figure 7.
(a) Experimental results obtained using each condition in the creep tests, and (b) calibrated viscoplastic model for each condition (the solid lines follow the creep model in Equation (2)).
Figure 7.
(a) Experimental results obtained using each condition in the creep tests, and (b) calibrated viscoplastic model for each condition (the solid lines follow the creep model in Equation (2)).
Figure 8.
Schematic of the HB/DBP process.
Figure 8.
Schematic of the HB/DBP process.
Figure 9.
Experimental pressure conditions depending on the temperature history during the HB/DBP.
Figure 9.
Experimental pressure conditions depending on the temperature history during the HB/DBP.
Figure 10.
(a) Photograph and (b) schematic of the HB/DBP apparatus.
Figure 10.
(a) Photograph and (b) schematic of the HB/DBP apparatus.
Figure 11.
Comparison between the ABAQUS 2023 built-in creep model and Equations (2) and (3) at 900 °C.
Figure 11.
Comparison between the ABAQUS 2023 built-in creep model and Equations (2) and (3) at 900 °C.
Figure 12.
Schematic of the finite element model and constraints applied.
Figure 12.
Schematic of the finite element model and constraints applied.
Figure 13.
(a) Schematic of the air-bending, initial contact, and bottoming processes and (b) the resulting force versus displacement curve.
Figure 13.
(a) Schematic of the air-bending, initial contact, and bottoming processes and (b) the resulting force versus displacement curve.
Figure 14.
Equivalent creep strain distribution depending on temperatures (4.0 MPa is used as an example): (a) 25 °C, (b) 700 °C, (c) 800 °C, (d) 900 °C, and (e) 900 °C as the last step.
Figure 14.
Equivalent creep strain distribution depending on temperatures (4.0 MPa is used as an example): (a) 25 °C, (b) 700 °C, (c) 800 °C, (d) 900 °C, and (e) 900 °C as the last step.
Figure 15.
Shear strength of the joints with respect to the bonding pressure with small error.
Figure 15.
Shear strength of the joints with respect to the bonding pressure with small error.
Figure 16.
Fractured specimens under conditions (a) B, (b) C, (c) D, E, and F.
Figure 16.
Fractured specimens under conditions (a) B, (b) C, (c) D, E, and F.
Figure 17.
Bonding ratio, thickness strain, and shear strength under each condition.
Figure 17.
Bonding ratio, thickness strain, and shear strength under each condition.
Figure 18.
Optical image of the of α-case layer along the periphery of the cross-sectioned samples of Ti-6Al-4V.
Figure 18.
Optical image of the of α-case layer along the periphery of the cross-sectioned samples of Ti-6Al-4V.
Figure 19.
Schematic of the observation region (center/edge).
Figure 19.
Schematic of the observation region (center/edge).
Figure 20.
Optimal images of the edge line under conditions (a) B (0.5 MPa), (b) C (1.0 MPa), (c) D (2.5 MPa), (d) E (4.0 MPa), and (e) F (8.0 MPa).
Figure 20.
Optimal images of the edge line under conditions (a) B (0.5 MPa), (b) C (1.0 MPa), (c) D (2.5 MPa), (d) E (4.0 MPa), and (e) F (8.0 MPa).
Figure 21.
EBSD analysis and inverse pole figures along the center region for conditions (a) C (1 MPa) and (b) F (8 MPa).
Figure 21.
EBSD analysis and inverse pole figures along the center region for conditions (a) C (1 MPa) and (b) F (8 MPa).
Figure 22.
Schematic of (a) the gap-opening process after the HB/DBP and (b) the final shape of the prototype.
Figure 22.
Schematic of (a) the gap-opening process after the HB/DBP and (b) the final shape of the prototype.
Figure 23.
Volume and BTF ratios after the HB/DBP in comparison with those of the conventional machining process.
Figure 23.
Volume and BTF ratios after the HB/DBP in comparison with those of the conventional machining process.
Table 1.
Chemical composition of Ti-6Al-4V.
Table 1.
Chemical composition of Ti-6Al-4V.
Elements | Ti | Al | V | C | Fe | N | O | H |
---|
Weight (%) | 87.6–91 | 6.27 | 4.5 | 0.08 | 0.4 | 0.05 | 0.2 | 0.015 |
Table 2.
Typical mechanical properties of the as-received Ti-6Al-4V at room temperature.
Table 2.
Typical mechanical properties of the as-received Ti-6Al-4V at room temperature.
(GPa)
| (MPa)
| (MPa)
| (%)
| (%)
|
---|
113.69 | 1058.60 ± 1.2 | 1091.60 ± 2.0 | 9.94 ± 1.2 | 16.67 ± 0.5 |
Table 3.
Material parameters of the Voce-type hardening law under different temperatures.
Table 3.
Material parameters of the Voce-type hardening law under different temperatures.
Temperature | A | B | C |
---|
25 °C | 1060.92 | 326.03 | 6.93 |
450 °C | 560.28 | 165.02 | 29.76 |
500 °C | 520.82 | 128.75 | 22.11 |
550 °C | 449.45 | 89.99 | 24.03 |
600 °C | 369.17 | 50.75 | 46.60 |
650 °C | 279.97 | 38.36 | 294.15 |
750 °C | 100.17 | 11.32 | 4136.96 |
850 °C | 35.91 | 7.12 | 7.13 |
900 °C | 11.52 | 2.99 | 981.82 |
Table 4.
Material parameters of the creep behavior expressed in Equation (2) with respect to temperature.
Table 4.
Material parameters of the creep behavior expressed in Equation (2) with respect to temperature.
Temperature | | | n | m |
---|
700 °C | 1.4137 × 10−6 | 8.0 | 2.030 | −0.0093 |
800 °C | 1.3238 × 10−5 | −0.0512 |
900 °C | 7.4727 × 10−4 | −0.2532 |
Table 5.
Shear strength of joints under each pressure condition.
Table 5.
Shear strength of joints under each pressure condition.
Pressure Conditions | Shear Strength (MPa) |
---|
B (0.5 MPa) | 895 |
C (1 MPa) | 991 |
D (2.5 MPa) | 977 |
E (4 MPa) | 949 |
F (8 MPa) | 771 |
Table 6.
Bonding ratio, thickness strain, and shear strength under each condition.
Table 6.
Bonding ratio, thickness strain, and shear strength under each condition.
Condition | Bonding Ratio (%) | Thickness Strain (%) | Shear Strength (MPa) |
---|
B | 34.44 | 0.12 | 895 |
C | 97.56 | 0.25 | 991 |
D | 98.26 | 3.19 | 977 |
E | 98.82 | 8.77 | 949 |
F | 99.45 | 15.21 | 771 |