Sequential Effects of Deep Rolling and Post-Weld Heat Treatment on Surface Integrity of AA7075-T651 Aluminum Alloy Friction Stir Welding
Abstract
:1. Introduction
2. Materials and Methods
- = an residual stress under aging temperature and aging time
- = an initial residual stress
- = an aging time
- = a numerical term dependent on the dominant relaxation mechanism( value between 0.1 to 0.3 for non-ferrous alloy)
- = a function dependent on the material and temperature according to Equation (2):
- = a material constant
- = a Boltzmann constant
- = the activation enthalpy for the relaxation process
- = an aging temperature
- For the first workpiece, only FSW was applied.
- For the second workpiece, FSW was applied, followed by DR, and then PWHT processes (FSW-DR-PWHT).
- For the third workpiece, FSW was applied, followed by PWHT, and then DR processes (FSW-PWHT-DR).
- σ = Bending stress at outer side (MPa)
- F = Load at defined point on the load deflection curve (N)
- L = Length of support span (mm)
- b = Width of test workpiece (mm)
- d = Thickness of test workpiece (mm)
3. Results and Discussion
3.1. Residual Stress
3.2. Microhardness
3.3. Surface Roughness
3.4. Microstructure
3.5. Fatigue Life
4. Conclusions
- Measured values were established by the last step of the treatment. It was found that the greatest benefit concerning both treatment process sequences was yielded when the workpiece was first subject to the PWHT process followed by the DR process (FSW-PWHT-DR).
- The FSW-PWHT-DR workpiece can enhance the fatigue life of the material by up to 239% when compared with FSW workpieces that have not undergone any improvement processes. Comparatively, it was found that the FSW-DR-PWHT workpiece resulted in the fatigue life of the material being reduced by up to 26% when compared with the FSW as-welded workpiece. The results of the DR process, the last step of treatment, are that the DR process can enhance the fatigue life.
- Near-surface compressive residual stress has confirmed that a great influence on fatigue life of the welded joint due to the near-surface compressive residual stress can prevent or reduce fatigue surface crack initiation as well as surface fatigue crack growth.
- The DR process improves the surface roughness (decreases the value of surface roughness) to a level equivalent to that of the grinding process with the Ra values in the longitudinal and transverse directions of the FSW-PWHT-DR workpiece at 0.15 µm and 0.07 µm, respectively. Comparatively, the PWHT process relaxes near-surface residual stress during aging treatments but will increase the hardness value as well as improve the hardness uniformity.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Abbreviations
AA | Aluminum Alloy |
AISI | American Iron and Steel Institute |
AS | Advancing Side |
ASTM | American Society for Testing and Materials |
BM | Base Material |
CDRX | Continuous Dynamic Recrystallization |
CNC | Computer Numerical Control |
DIN | Deutsches Institut für Normung e.V. (German Institute for Standardization) |
DR | Deep Rolling |
FSW | Friction Stir Welding |
FWHM | Full Width at Half Maximum |
HV | Vickers Hardness Number |
JIS | Japanese Industrial Standards |
PWHT | Post-Weld Heat Treatment |
RS | Retreating Side |
SZ | Stir Zone |
TMAZ | Thermo-Mechanically Affected Zone |
XRD | X-Ray Diffraction |
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Elements | Cu | Mn (Max) | Mg | Zn | Cr | Fe (Max) | Al |
---|---|---|---|---|---|---|---|
Standard | 1.2–2.0 | 0.30 | 2.1–2.9 | 5.1–6.1 | 0.18–0.28 | 0.50 | Balance |
Measured | 1.83 | 0.09 | 4.95 | 7.39 | 0.32 | 0.37 | Balance |
Mechanical Properties | Value |
---|---|
Hardness, Vickers | 175 |
Tensile strength, ultimate | 572 MPa |
Tensile strength, yield | 503 MPa |
Modulus of elasticity | 71.7 GPa |
Poisson’s ratio | 0.33 |
Shear modulus | 26.9 GPa |
Shear strength | 331 MPa |
Fatigue strength (5 × 108 cycles) | 159 MPa |
Parameters | Value |
---|---|
Rotational speed | 1600 rpm |
Welding speed | 30 mm/min |
Plunge depth | 0.1 mm |
Plunge feed rate | 6 mm/min |
Dwell time | 15 s |
Tool tilt angle | 0° |
Rolling Pressure (Bar) | Rolling Speed (mm/min) | Rolling Offset (mm) |
---|---|---|
150 | 1400 | 0.10 |
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Baisukhan, A.; Nakkiew, W. Sequential Effects of Deep Rolling and Post-Weld Heat Treatment on Surface Integrity of AA7075-T651 Aluminum Alloy Friction Stir Welding. Materials 2019, 12, 3510. https://doi.org/10.3390/ma12213510
Baisukhan A, Nakkiew W. Sequential Effects of Deep Rolling and Post-Weld Heat Treatment on Surface Integrity of AA7075-T651 Aluminum Alloy Friction Stir Welding. Materials. 2019; 12(21):3510. https://doi.org/10.3390/ma12213510
Chicago/Turabian StyleBaisukhan, Adirek, and Wasawat Nakkiew. 2019. "Sequential Effects of Deep Rolling and Post-Weld Heat Treatment on Surface Integrity of AA7075-T651 Aluminum Alloy Friction Stir Welding" Materials 12, no. 21: 3510. https://doi.org/10.3390/ma12213510