Fatigue Behaviour of 7N01-T4 Aluminium Alloy Welded by Ultrasonic-Assisted Friction Stir Welding
Abstract
:1. Introduction
2. Materials and Methods
3. Results
3.1. Microstructure
3.2. Residual Stresses Testing
3.3. Fatigue Strength
3.4. Fatigue Fracture Behaviour
4. Discussion
5. Conclusions
- (1)
- The application of axial ultrasonic vibration can promote the metal flow of the SZ, resulting in a greater deformation of the SZ and an increased strain rate of the micro-regions for refining grains. Using the UAFSW method can significantly improve the fatigue performance of the welded joint. The fatigue limit of the UAFSW specimen is 220 MPa, which is 20 MPa higher than that of the FSW.
- (2)
- The difference in the transverse residual stresses between the FSW and the UAFSW is very small. The maximum longitudinal residual stresses of the two joints is located near the edge of the tool shoulder on the AS, which can be reduced by 17.8% by ultrasonic vibration. This may be mainly due to the reduction in the boundary distortion of SZ/TMAZ and the local heat generation by ultrasonic vibration.
- (3)
- The joints show different fracture modes under different stress conditions. Both FSW and UAFSW specimens fracture at the TMAZ-AS under high-stress levels. Under alternating loads, second-phase particles on the TMAZ grain boundary are likely to cause stress concentration. Moreover, micro-zone high residual stresses and SZ/TMAZ microstructure difference also aggravate the nonuniform deformation, leading to crack initiation.
- (4)
- Under low-stress levels, fatigue crack initiation is prone to occur near the TZ and eventually fracture in the SZ. Axial ultrasonic vibration promotes the metal convergence from the SDZ and the PDZ, resulting in a more uniform and denser microstructure of TZ, thereby reducing the probability of forming loose defects in the TZ and improving the fatigue life.
Author Contributions
Funding
Conflicts of Interest
References
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Zhang, Z.; He, C.; Li, Y.; Wei, J.; Zhai, M.; Zhao, S.; Zhao, X. Fatigue Behaviour of 7N01-T4 Aluminium Alloy Welded by Ultrasonic-Assisted Friction Stir Welding. Materials 2020, 13, 4582. https://doi.org/10.3390/ma13204582
Zhang Z, He C, Li Y, Wei J, Zhai M, Zhao S, Zhao X. Fatigue Behaviour of 7N01-T4 Aluminium Alloy Welded by Ultrasonic-Assisted Friction Stir Welding. Materials. 2020; 13(20):4582. https://doi.org/10.3390/ma13204582
Chicago/Turabian StyleZhang, Zhiqiang, Changshu He, Ying Li, Jingxun Wei, Menggang Zhai, Su Zhao, and Xiang Zhao. 2020. "Fatigue Behaviour of 7N01-T4 Aluminium Alloy Welded by Ultrasonic-Assisted Friction Stir Welding" Materials 13, no. 20: 4582. https://doi.org/10.3390/ma13204582