# Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{a}= 150 MPa of FSWed specimen was about 1/20 of the bulk sheet, cavitation peening was able to extend the fatigue life of the non-peened FSW specimen by 3.6 times by introducing compressive residual stress into the FSWed part. This is the first paper to demonstrate the improvement of fatigue properties of FSWed metallic sheet by cavitation peening.

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Friction Stir Welded Aluminum Alloy AA5754

#### 2.2. Cavitation Peening Using a Cavitating Jet in Air and Water

_{p}, was defined by the following equation:

_{p}= 0.25 s/mm was chosen, as the maximum speed of the cavitation peening system using CJA, which was developed for the treatment of tool steel alloy [51], was v = 4 mm/s.

_{1}was 30 MPa. Note that cavitation number σ, which is a key factor of the cavitating flow, was defined by Equation (2):

_{2}and p

_{v}are downstream pressure of the nozzle and the vapor pressure of the water, respectively. As p

_{1}» p

_{2}» p

_{v}, σ is simplified to Equation (2). According to a previous report [33], when s/d satisfies Equation (3), the cavitation peening condition is met:

^{−0.6}= 55.1, as σ = 0.0033, then the used condition was cavitation peening condition. In the present experiment of CJW, n = 2, v = 1 mm/s, t

_{p}= 2 s/mm was chosen, as the density of the plastic deformation pits induced by CJW was lower than that of CJA.

#### 2.3. Evaluation of Fatigue Properties and Surface Characteristics

_{f 150}at σ

_{a}= 150 MPa was obtained by the following procedure. Note that S-N line of base metal and FSWed specimen were straight at σ

_{a}= 150 MPa as mentioned later. Then, σ

_{a}= 150 MPa was chosen to discuss the fatigue life at the present experiment. It was assumed that the S–N curve for low-cycle-fatigue base metal (BM) specimens is described by Equation (4) and that that for FSWed specimens is described by Equation (5), where c

_{1}, c

_{2}, and c

_{3}are constants. Namely, these curves are parallel to each other, as shown in Figure 4.

_{1}and c

_{2}were obtained from 8 experimental datasets of base metal specimens using a least square method. The number of fatigue specimens for base metal and FSWed sheet was 8 and 17, respectively. The c

_{3}of FSWed specimen at each condition was obtained from c

_{1}and experimental dataset of σ

_{a}

_{FSW}and N

_{f}

_{FSW}of FSWed specimens. Therefore, N

_{f}

_{150}for FSWed specimens is given by Equation (6):

_{f}

_{150}. The σ

_{a}at N

_{f}= 10

^{5}is also obtained by Equation (8):

## 3. Results

#### 3.1. Aspect of Cavitation Peened Surface

#### 3.2. Improvement of Fatigue Properties of FSWed AA5754 by Cavitation Peening

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}for base metal and FSWed AA5754 obtained using Equations (7) and (8). c

_{1}and c

_{2}obtained by the present experiment were 62.1 ± 10.8 and = 514.7 ± 56.5, respectively. As shown in Figure 9, the fatigue life and strength of FSWed AA5754 were considerably smaller than those of the base metal. For example, whereas N

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}of base metal were 7.45 × 10

^{5}and 204.2 MPa, those of FSWed AA5754 without peening, excluding “corner of root side”, were 3.38 × 10

^{4}and 120.7 MPa for ω = 1500 rpm and 2.10 × 10

^{4}and 107.9 MPa for ω = 2500 rpm, respectively. N

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}of FSWed AA5754 without peening were 4.54% and 59% for ω = 1500 rpm and 2.82% and 53% for ω = 2500 rpm compared with base metal. This result suggests that FSW at ω = 1500 rpm was slightly better than that of ω = 2500 rpm at the present condition.

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}. As shown in Figure 9, datapoints treated by CJW were scattered. This might be caused by the relatively large surface roughness as shown in Figure 7. Note that both data points of CJW in Figure 9 were used to obtain N

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}of CJW in Table 1. In the case of CJA, N

_{f}

_{150}and σ

_{a}at N

_{f}= 10

^{5}were 1.23 × 10

^{5}and 155.7 MPa for ω = 1500 rpm and 5.56 × 10

^{4}and 134.2 MPa for ω = 2500 rpm, respectively. In other words, in the case of FSW at ω = 1500 rpm, cavitation peening using CJA improved the fatigue life by 3.6 times and the fatigue strength by 1.3 times, compared with the non-peened one.

#### 3.3. Introduction of Compressive Residual Stress by Cavitation Peening

## 4. Conclusions

- (1)
- Cavitation peening using a cavitating jet in air (CJA) and a cavitation jet in water (CJW) can improve fatigue properties of FSWed AA5754. The fatigue properties treated by CJA are better than those treated by CJW, as the surface roughness of CJA is smaller than that of CJW.
- (2)
- The fatigue life at σ
_{a}= 150 MPa can be more than doubled by CJA and CJW compared with non-peened FSWed specimen. CJA and CJW can also improve the fatigue strength at N = 10^{5}more than 1.2 times compared with non-peened FSWed specimen. - (3)
- Cavitation peening using CJA can introduce compressive residual stress into both surfaces of the welded side and the root side of the FSWed part.
- (4)
- In the present FSW condition, the surface residual stress of the FSWed specimen near the center part of the root side and boundary of FSW of the welded side reveals tension.
- (5)
- The fatigue life at σ
_{a}= 150 MPa and the fatigue strength at N = 10^{5}of tested FSW specimen were 3–5% and 53–59% of bulk sheet, respectively. In the present condition, the fatigue properties of FSWed specimen joining at a rotational speed of 1500 rpm were better than those of 2500 rpm.

## Author Contributions

## Funding

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

- Thomas, W.M.; Nicholas, E.D.; Needham, J.C.; Murch, M.G.; Temple-Smith, P.; Dawes, C.J. Improvements relating to friction stir welding. Eur. Pat. Specif.
**1991**, 615, B1. [Google Scholar] - Thomas, W.M.; Threadgill, P.L.; Nicholas, E.D. Feasibility of friction stir welding steel. Sci. Technol. Weld. Join.
**1999**, 4, 365–372. [Google Scholar] [CrossRef] - Ma, Z.Y. Friction stir processing technology: A review. Metall. Mater. Trans. A
**2008**, 39, 642–658. [Google Scholar] [CrossRef] - Nandan, R.; DebRoy, T.; Bhadeshia, H. Recent advances in friction-stir welding—Process, weldment structure and properties. Prog. Mater. Sci.
**2008**, 53, 980–1023. [Google Scholar] [CrossRef] [Green Version] - Gangwar, K.; Ramulu, M. Friction stir welding of titanium alloys: A review. Mater. Des.
**2018**, 141, 230–255. [Google Scholar] [CrossRef] - Cabibbo, M.; Paoletti, C.; Ghat, M.; Forcellese, A.; Simoncini, M. Post-FSW cold-rolling simulation of ECAP shear deformation and its microstructure role combined to annealing in a fswed aa5754 plate joint. Materials
**2019**, 12, 14. [Google Scholar] [CrossRef] [Green Version] - Cabibbo, M.; Forcellese, A.; Santecchia, E.; Paoletti, C.; Spigarelli, S.; Simoncini, M. New approaches to friction stir welding of aluminum light-alloys. Metals
**2020**, 10, 20. [Google Scholar] [CrossRef] [Green Version] - Thomas, W.M.; Nicholas, E.D. Friction stir welding for the transportation industries. Mater. Des.
**1997**, 18, 269–273. [Google Scholar] [CrossRef] - Dursun, T.; Soutis, C. Recent developments in advanced aircraft aluminium alloys. Mater. Des.
**2014**, 56, 862–871. [Google Scholar] [CrossRef] - Haghshenas, M.; Gerlich, A.P. Joining of automotive sheet materials by friction-based welding methods: A review. Eng. Sci. Technol.
**2018**, 21, 130–148. [Google Scholar] [CrossRef] - Mishra, R.S.; Ma, Z.Y. Friction stir welding and processing. Mater. Sci. Eng. R Rep.
**2005**, 50, 1–78. [Google Scholar] [CrossRef] - Simoncini, M.; Ciccarelli, D.; Forcellese, A.; Pieralisi, M. Micro-and macro-mechanical properties of pinless friction stir welded joints in AA5754 aluminium thin sheets. Proc. CIRP
**2014**, 18, 9–14. [Google Scholar] [CrossRef] [Green Version] - Feng, A.H.; Chen, D.L.; Ma, Z.Y. Microstructure and low-cycle fatigue of a friction-stir-welded 6061 aluminum alloy. Metall. Mater. Trans. A
**2010**, 41, 2626–2641. [Google Scholar] [CrossRef] - Xu, W.F.; Liu, J.H.; Chen, D.L.; Luan, G.H. Low-cycle fatigue of a friction stir welded 2219-T62 aluminum alloy at different welding parameters and cooling conditions. Int. J. Adv. Manuf. Technol.
**2014**, 74, 209–218. [Google Scholar] [CrossRef] - Wu, M.X.; Wu, C.S.; Gao, S. Effect of ultrasonic vibration on fatigue performance of AA2024-T3 friction stir weld joints. J. Manuf. Process.
**2017**, 29, 85–95. [Google Scholar] [CrossRef] - Li, H.J.; Gao, J.; Li, Q.C. Fatigue of friction stir welded aluminum alloy joints: A review. Appl. Sci.
**2018**, 8, 19. [Google Scholar] [CrossRef] [Green Version] - Uthayakumar, M.; Balasubramanian, V.; Rani, A.M.A.; Hadzima, B. Effects of welding on the fatigue behaviour of commercial aluminum AA-1100 joints. In Proceedings of the International Conference on Recent Advances in Materials & Manufacturing Technologies, Dubai, UAE, 28–29 November 2017. [Google Scholar]
- Chehreh, A.B.; Gratzel, M.; Bergmann, J.P.; Walther, F. Effect of corrosion and surface finishing on fatigue behavior of friction stir welded EN AW-5754 aluminum alloy using various tool configurations. Materials
**2020**, 13, 20. [Google Scholar] - Torzewski, J.; Grzelak, K.; Wachowski, M.; Kosturek, R. Microstructure and low cycle fatigue properties of AA5083 H111 friction stir welded joint. Materials
**2020**, 13, 14. [Google Scholar] [CrossRef] - Withers, P.J. Residual stress and its role in failure. Rep. Prog. Phys.
**2007**, 70, 2211–2264. [Google Scholar] [CrossRef] [Green Version] - Webster, P.J.; Oosterkamp, L.D.; Browne, P.A.; Hughes, D.J.; Kang, W.P.; Withers, P.J.; Vaughan, G.B.M. Synchrotron X-ray residual strain scanning of a friction stir weld. J. Strain Anal. Eng. Des.
**2001**, 36, 61–70. [Google Scholar] [CrossRef] - Peel, M.; Steuwer, A.; Preuss, M.; Withers, P.J. Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds. Acta Mater.
**2003**, 51, 4791–4801. [Google Scholar] [CrossRef] - Prime, M.B.; Gnaupel-Herold, T.; Baumann, J.A.; Lederich, R.J.; Bowden, D.M.; Sebring, R.J. Residual stress measurements in a thick, dissimilar aluminum alloy friction stir weld. Acta Mater.
**2006**, 54, 4013–4021. [Google Scholar] [CrossRef] - Haghshenas, M.; Gharghouri, M.A.; Bhakhri, V.; Klassen, R.J.; Gerlich, A.P. Assessing residual stresses in friction stir welding: Neutron diffraction and nanoindentation methods. Int. J. Adv. Manuf. Technol.
**2017**, 93, 3733–3747. [Google Scholar] [CrossRef] - Zhang, L.; Zhong, H.L.; Li, S.C.; Zhao, H.J.; Chen, J.Q.; Qi, L. Microstructure, mechanical properties and fatigue crack growth behavior of friction stir welded joint of 6061-T6 aluminum alloy. Int. J. Fatigue
**2020**, 135, 11. [Google Scholar] [CrossRef] - John, R.; Jata, K.V.; Sadananda, K. Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys. Int. J. Fatigue
**2003**, 25, 939–948. [Google Scholar] [CrossRef] - Nie, L.; Wu, Y.X.; Gong, H. Prediction of temperature and residual stress distributions in friction stir welding of aluminum alloy. Int. J. Adv. Manuf. Technol.
**2020**, 106, 3301–3310. [Google Scholar] [CrossRef] - Hatamleh, O.; Rivero, I.V.; Lyons, J. Evaluation of surface residual stresses in friction stir welds due to laser and shot peening. J. Mater. Eng. Perform.
**2007**, 16, 549–553. [Google Scholar] [CrossRef] - Hatamleh, O. Effects of peening on mechanical properties in friction stir welded 2195 aluminum alloy joints. Mater. Sci. Eng. A
**2008**, 492, 168–176. [Google Scholar] [CrossRef] - Hatamleh, O. A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints. Int. J. Fatigue
**2009**, 31, 974–988. [Google Scholar] [CrossRef] - Hatamleh, O.; DeWald, A. An investigation of the peening effects on the residual stresses in friction stir welded 2195 and 7075 aluminum alloy joints. J. Mater. Process. Technol.
**2009**, 209, 4822–4829. [Google Scholar] [CrossRef] - Nie, L.; Wu, Y.X.; Gong, H.; Chen, D.; Guo, X.D. Effect of shot peening on redistribution of residual stress field in friction stir welding of 2219 aluminum alloy. Materials
**2020**, 13, 13. [Google Scholar] [CrossRef] - Soyama, H. Surface mechanics design of metallic materials on mechanical surface treatments. Mech. Eng. Rev.
**2015**, 2, 14–00192. [Google Scholar] [CrossRef] [Green Version] - Hatamleh, O.; Lyons, J.; Forman, R. Laser peening and shot peening effects on fatigue life and surface roughness of friction stir welded 7075-T7351 aluminum. Fatigue Fract. Eng. Mater. Struct.
**2007**, 30, 115–130. [Google Scholar] [CrossRef] - Abdulstaar, M.A.; Al-Fadhalah, K.J.; Wagner, L. Microstructural variation through weld thickness and mechanical properties of peened friction stir welded 6061 aluminum alloy joints. Mater. Charact.
**2017**, 126, 64–73. [Google Scholar] [CrossRef] - He, C.; Yang, K.; Liu, Y.; Wang, Q.; Cai, M. Improvement of very high cycle fatigue properties in an AA7075 friction stir welded joint by ultrasonic peening treatment. Fatigue Fract. Eng. Mater. Struct.
**2017**, 40, 460–468. [Google Scholar] [CrossRef] - Dong, P.; Liu, Z.P.; Zhai, X.; Yan, Z.F.; Wang, W.X.; Liaw, P.K. Incredible improvement in fatigue resistance of friction stir welded 7075-T651 aluminum alloy via surface mechanical rolling treatment. Int. J. Fatigue
**2019**, 124, 15–25. [Google Scholar] [CrossRef] - 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, 19. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Sano, Y.; Masaki, K.; Gushi, T.; Sano, T. Improvement in fatigue performance of friction stir welded A6061-T6 aluminum alloy by laser peening without coating. Mater. Des.
**2012**, 36, 809–814. [Google Scholar] [CrossRef] - Sano, T.; Eimura, T.; Kashiwabara, R.; Matsuda, T.; Isshiki, Y.; Hirose, A.; Tsutsumi, S.; Arakawa, K.; Hashimoto, T.; Masaki, K.; et al. Femtosecond laser peening of 2024 aluminum alloy without a sacrificial overlay under atmospheric conditions. J. Laser Appl.
**2017**, 29, 7. [Google Scholar] - Kawashima, T.; Sano, T.; Hirose, A.; Tsutsumi, S.; Masaki, K.; Arakawa, K.; Hori, H. Femtosecond laser peening of friction stir welded 7075-T73 aluminum alloys. J. Mater. Process. Technol.
**2018**, 262, 111–122. [Google Scholar] [CrossRef] - Sasoh, A.; Watanabe, K.; Sano, Y.; Mukai, N. Behavior of bubbles induced by the interaction of a laser pulse with a metal plate in water. Appl. Phys. A
**2005**, 80, 1497–1500. [Google Scholar] [CrossRef] - Soyama, H. Comparison between the improvements made to the fatigue strength of stainless steel by cavitation peening, water jet peening, shot peening and laser peening. J. Mater. Process. Technol.
**2019**, 269, 65–78. [Google Scholar] [CrossRef] - Soyama, H.; Lichtarowicz, A.; Momma, T.; Williams, E.J. A new calibration method for dynamically loaded transducers and its application to cavitation impact measurement. J. Fluids Eng.
**1998**, 120, 712–718. [Google Scholar] [CrossRef] - Soyama, H.; Sekine, Y.; Saito, K. Evaluation of the enhanced cavitation impact energy using a PVDF transducer with an acrylic resin backing. Measurement
**2011**, 44, 1279–1283. [Google Scholar] [CrossRef] - Soyama, H. Key factors and applications of cavitation peening. Inter. J. Peen. Sci. Technol.
**2017**, 1, 3–60. [Google Scholar] - Soyama, H. Cavitation peening: A review. Metals
**2020**, 10, 27. [Google Scholar] [CrossRef] [Green Version] - Soyama, H. Cavitating jet: A review. Appl. Sci.
**2020**, 10, 7280. [Google Scholar] [CrossRef] - Enomoto, K.; Hirano, K.; Mochizuki, M.; Kurosawa, K.; Saito, H.; Hayashi, E. Improvement of residual stress on material surface by water jet peening. Zairyo
**1996**, 45, 734–739. [Google Scholar] - Soyama, H.; Chighizola, C.R.; Hill, M.R. Effect of compressive residual stress introduced by cavitation peening and shot peening on the improvement of fatigue strength of stainless steel. J. Mater. Process. Technol.
**2021**, 288, 116877. [Google Scholar] [CrossRef] - Soyama, H. Introduction of compressive residual stress using a cavitating jet in air. J. Eng. Mater. Technol.
**2004**, 126, 123–128. [Google Scholar] [CrossRef] - Soyama, H. High-speed observation of a cavitating jet in air. J. Fluids Eng.
**2005**, 127, 1095–1108. [Google Scholar] [CrossRef] - Soyama, H. Improvement of fatigue strength by using cavitating jets in air and water. J. Mater. Sci.
**2007**, 42, 6638–6641. [Google Scholar] [CrossRef] - Bevilacqua, M.; Ciarapica, F.E.; D’Orazio, A.; Forcellese, A.; Simoncini, M. Sustainability analysis of friction stir welding of AA5754 sheets. Proc. CIRP
**2017**, 62, 529–534. [Google Scholar] [CrossRef] - Casalino, G.; El Mehtedi, M.; Forcellese, A.; Simoncini, M. Effect of cold rolling on the mechanical properties and formability of fswed sheets in AA5754-H114. Metals
**2018**, 8, 15. [Google Scholar] [CrossRef] [Green Version] - Soyama, H.; Kikuchi, T.; Nishikawa, M.; Takakuwa, O. Introduction of compressive residual stress into stainless steel by employing a cavitating jet in air. Surf. Coat. Technol.
**2011**, 205, 3167–3174. [Google Scholar] [CrossRef] - Soyama, H. Enhancing the aggressive intensity of a cavitating jet by introducing a cavitator and a guide pipe. J. Fluid Sci. Technol.
**2014**, 9, 13–00238. [Google Scholar] [CrossRef] [Green Version] - Soyama, H. Enhancing the aggressive intensity of a cavitating jet by means of the nozzle outlet geometry. J. Fluids Eng.
**2011**, 133, 101301. [Google Scholar] [CrossRef] - Soyama, H.; Okura, Y. The use of various peening methods to improve the fatigue strength of titanium alloy Ti6Al4V manufactured by electron beam melting. AIMS Mater. Sci.
**2018**, 5, 1000–1015. [Google Scholar] [CrossRef] - He, B.B. Two-Dimensional X-ray Diffraction; John Wiley & Sons: Hoboken, NJ, USA, 2009; pp. 249–328. [Google Scholar]
- ASTM International. G134-17 Standard Test Method for Erosion of Solid Materials by a Cavitating Liquid Jet; ASTM International: West Conshohocken, PA, USA, 2017; pp. 1–17. [Google Scholar]
- Soyama, H.; Asahara, M. Improvement of the corrosion resistance of a carbon steel surface by a cavitating jet. J. Mater. Sci. Lett.
**1999**, 18, 1953–1955. [Google Scholar] [CrossRef] - Soyama, H. Luminescence intensity of vortex cavitation in a venturi tube changing with cavitation number. Ultrason Sonochem.
**2021**, 71, 105389. [Google Scholar] [CrossRef]

**Figure 1.**Friction stir welded (FSWed) aluminum alloy AA5754 sheets for fatigue test specimens. The black line indicates the shape of the test specimen. (

**a**,

**b**): Rotational speed ω was 2500 rpm, and transverse speed v was 60 mm/min. (

**c**,

**d**): Rotational speed ω was 1500 rpm, and transverse speed v was 60 mm/min.

**Figure 2.**Geometry of the fatigue specimen for displacement-controlled plane-bending fatigue test and coordinates for residual stress measurement. The thickness was 2 mm.

**Figure 3.**Schematic diagram of cavitation peening systems. (

**a**) Cavitating jet in air. (

**b**) Cavitating jet in water.

**Figure 4.**Schematics to obtain N

_{f 150}of FSWed specimen using the S-N curve of the base metal (BM) specimen.

**Figure 5.**Aspect of the root side surface of cavitation-peened specimens to show plastic deformation pits taken by digital camera. (

**a**) Cavitating jet in air. (

**b**) Cavitating jet in water.

**Figure 6.**Aspect of the welded side surface of cavitation-peened specimens observed by digital microscope. (

**a**) Cavitating jet in air. (

**b**) Cavitating jet in water.

**Figure 7.**Surface roughness of cavitation-peened specimens by cavitating jet in air (CJA) and cavitating jet in water (CJW) comparing with non-peened specimen NP. (

**a**) Arithmetical mean roughness. (

**b**) Maximum height of roughness profile.

**Figure 8.**Surface hardness of base metal (BM) part and FSWed part of cavitation peened specimens by cavitating jet in air (CJA) and cavitating jet in water (CJW) compared with non-peened specimen NP.

**Figure 9.**S-N curve obtained by plane-bending fatigue test for the base metal and FSWed specimens treated by cavitating jet in air (CJA) and cavitating jet in water (CJW) and without peening (NP). The crack initiation point of the FSWed specimen was flat part of root side, except specimens indicated by “corner of root side” and “FSW boundary of welded side”.

**Figure 10.**Aspect of fractured surface observed by scanning electron scope SEM. The upper surface in the figure is the root side surface. (

**a**) ω = 1500 rpm, NP, σ

_{a}= 131 MPa, N

_{f}= 1.63 × 10

^{4}. (

**b**) ω = 1500 rpm, NP, σ

_{a}= 115 MPa, N

_{f}= 1.18 × 10

^{5}. (

**c**) ω = 2500 rpm, NP, σ

_{a}= 164 MPa, N

_{f}= 1.30 × 10

^{4}. (

**d**) ω = 1500 rpm, CJA, σ

_{a}= 130 MPa, N

_{f}= 2.14 × 10

^{5}. (

**e**) ω = 2500 rpm, CJA, σ

_{a}= 126 MPa, N

_{f}= 1.56 × 10

^{5}. (

**f**) ω = 2500 rpm, CJA, σ

_{a}= 117 MPa, N

_{f}= 2.57 × 10

^{5}.

**Figure 11.**Surface residual stress of FSWed AA5754 before and after cavitation peening (CP) measured by the 2D method of X-ray diffraction. (

**a**) The welded side of FSW by ω = 1500 rpm. (

**b**) Root side of FSW by ω = 1500 rpm. (

**c**) Welded side of FSW by ω = 2500 rpm. (

**d**) Root side of FSW by ω = 2500 rpm.

**Table 1.**Fatigue properties of FSWed specimen treated by cavitation peening. N

_{f}at σ

_{a}= 150 MPa, i.e., N

_{f}

_{150}, and σ

_{a}at N

_{f}= 10

^{5}are obtained by Equations (7) and (8) using experimental data. CJA and CJW are cavitation peening by using cavitating jet in air and water, NP means non-peened one.

Specimen | Peening | c_{2} or c_{3} | N_{f}_{150} (Cycle) (%) | σ_{a} at N_{f} = 10^{5} (MPa) | |
---|---|---|---|---|---|

Base metal | NP | 514.7 | 7.45 × 10^{5} | (100%) | 204.2 |

FSW (ω = 1500 rpm) | NP | 431.2 | 3.38 × 10^{4} | (4.54%) | 120.7 |

CJA | 466.2 | 1.23 × 10^{5} | (16.51%) | 155.7 | |

FSW (ω = 2500 rpm) | NP | 418.6 | 2.10 × 10^{4} | (2.82%) | 107.9 |

CJA | 444.7 | 5.56 × 10^{4} | (7.46%) | 134.2 | |

CJW | 440.9 | 4.83 × 10^{4} | (6.48%) | 130.4 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Soyama, H.; Simoncini, M.; Cabibbo, M.
Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754. *Metals* **2021**, *11*, 59.
https://doi.org/10.3390/met11010059

**AMA Style**

Soyama H, Simoncini M, Cabibbo M.
Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754. *Metals*. 2021; 11(1):59.
https://doi.org/10.3390/met11010059

**Chicago/Turabian Style**

Soyama, Hitoshi, Michela Simoncini, and Marcello Cabibbo.
2021. "Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754" *Metals* 11, no. 1: 59.
https://doi.org/10.3390/met11010059