# Fatigue of Friction Stir Welded Aluminum Alloy Joints: A Review

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## Abstract

**:**

## 1. Introduction

## 2. Fatigue Failure Mechanism of FSW Weld

#### 2.1. Characteristics of Weld Zones

#### 2.2. Fractography

## 3. Factors Affecting Fatigue Performance

#### 3.1. Process Parameters

_{f}= 10 [6] with the optimal process parameters is shown in Table 1 [24,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56].

#### 3.2. Stress Ratio

_{a}/D

_{N}is the fatigue crack growth rate, ${K}_{IC}$ is the material fracture toughness, ΔK is stress intensity factor range, and C and m are constants.

^{−5}mm/cycle, with the stress ratio R ≤ 0.5, the fatigue crack growth threshold ΔK

_{th}decreased as the stress ratio R increased. When the stress ratio R ≥ 0.5, the fatigue crack growth threshold value tends to be stable; when da/dN ≥ 1 × 10

^{−5}mm/cycle, the fatigue crack growth rate was not affected by the stress ratio.

#### 3.3. Test Environment

#### 3.4. Residual Stress

#### 3.5. Weld Defects

^{5}cycles in contrast with the base metal. For the kissing bond (KB) weld with the weld root polishing, a reduction in fatigue strength by 17% was found comparing with defect-free welds [24]. It can be seen from Figure 9 that the fatigue lives of the 2024-T6 welded joints were greatly reduced [55,77]. Some researchers found that the kiss defect has no significant effect on the fatigue strength of 5083-H321 [57,78].

## 4. Crack Growth Rate

_{th}. When ΔK is larger than the threshold ΔK

_{th}, the cyclic crack gradually expands. The crack length gradually increases but the crack growth rate decreases to steady stage, before the fracture fails the crack expansion rate is accelerated. The Paris formula is used for calculating the crack growth rate:

^{1/2}), the fatigue crack growth rate of all welded samples is slower than that of the base alloy; however, at higher ΔK values, the fatigue crack growth rate of the welded sample is much faster than the fatigue crack growth rate of the base alloy. The compressive residual stress generated by laser peening decreased the average stress and closed the crack, which significantly reduced the crack growth rate, and the material fatigue life was improved. The influence of process parameters on the crack growth rate is mainly reflected in the unreasonable parameters which are prone to producing defects. The optimized process parameters can facilitate the material plastically movement, and break the hardened material precipitates and the surface oxide layer [7]. The existence of defects will reduce the crack initiation time and generate stress concentration to increase the crack growth rate. For material properties, fracture toughness can also affect the crack propagation, the higher material fracture toughness, the lower crack growth rate.

## 5. Fatigue Life of Friction Stir Welded Joints

#### 5.1. The Stress Cycle (SN) Analysis

_{f}is the number of cycles to failure, and a and C are constants. Basquin is not valid in the low cycle fatigue region.

#### 5.2. The Strain Cycle (EN) Analysis

_{f}is fatigue strength coefficient, E is young’s modulus, $\epsilon $

_{f}is fatigue ductility coefficient, and b and c are strength and ductility exponent, respectively.

## 6. Experimental Techniques

## 7. Conclusions

- The fatigue crack initiation generally started at the surface of the weld, due to the fact that FSW welds with optimized process parameters do not contain internal defects or flaws. In addition, the initiation site was mainly located between the TMAZ and the HAZ as a result of both high temperature and plastic deformation. The difference in hardness between the TMAZ and HAZ resulted in a weak zone, which is vulnerable to the formation of local slip bands.
- The fatigue performance of FSW joints is mainly affected by process parameters, stress ratio, environment, residual stress, defects, and so on. The process parameters can be optimized to increase the weld fatigue life. Residual stress has a large influence on the crack growth rate, and it is difficult to remove when the welds are complex. The effect of defects on the fatigue properties of materials is complicated and depends on the type of defects.
- Laser peening is recommended for the post weld treatment of friction stir welded joints. Multilayer laser peening can greatly decrease fatigue crack growth rate and improve material fatigue life. At ambient and elevated temperature, shot peening treatment has similar fatigue crack growth resistance as as-welded condition and crack growth rates were higher than the laser peened case.
- The fatigue life data in the literature are still limited. In the high cycle stress life analysis, more testing are required for different materials at various stress amplitude and mean stress combinations. For low cycle fatigue analysis where the plastic part dominates, considerable works such as dissimilar material joint assessment are needed in the future.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 2.**Metallographic cross-section of the FSW weld [23].

**Figure 3.**Fatigue fractography of 6005 aluminum alloys joint after stationary shoulder friction stir welding [32].

**Figure 4.**Magnification of crack initiation position (

**a**) The crack location with whole morphology; (

**b**) detail of the crack initiation site [23].

**Figure 5.**Effect of different stress ratios on crack growth rate [60].

**Figure 6.**(

**a**) Longitudinal and (

**b**) transverse residual stresses as a function of lateral distance from the weld line [67].

**Figure 8.**Incomplete fusion paths in weld: (

**a**) at the advancing side of the weldment; (

**b**) towards the middle of the weldment; (

**c**) upper embedded flaw; and (

**d**) connectivity flaw [73].

**Figure 10.**Optical micrograph of a kissing bond (KB)-bearing weld, the dotted line indicates the part of the KB that is removed by grinding of the surface treated specimens [76].

**Figure 11.**Hook defect of the dissimilar joint (AA5754–AA6082) [82].

**Figure 12.**Fatigue growth data for FSW 7075-T7351 [85].

**Figure 13.**da/dN versus applied stress intensity factor range, ΔK, for 2195 aluminum at −140 °C [83].

**Figure 15.**The plot of strain amplitude against the number of cycles to failure for the base metal and friction stir welded joint [91].

**Figure 16.**Fatigue life for the dissimilar AA6061-to-AA7050 and comparison to results from literature [21].

Material | Rotating Speed (rpm) | Welding Speed (mm/min) | Thickness (mm) | Stress Ratio, R | $\mathbf{\u2206}\mathit{\sigma}/2\text{}({{\mathit{N}}_{\mathit{f}}}_{\text{}}=\text{}{10}^{6})$ | Ref. |
---|---|---|---|---|---|---|

A356-T6 | 500/1000 | 150 | 5 | −1 | 155/130 | Tajiri et al. [39] |

AA5083-H321 | - | - | 6 | 0.1 | 82.5 | Tovo et al. [40] |

5083-O | - | - | 6 | 0.1 | 102 | Threadgill et al. [41] |

2024-T3 | 2400 | 240 | 1.6 | 0.1 | 159.2 | Biallas et al. [42] |

2014A-T6 | - | - | 6 | 0.1 | 47.77 | Threadgill et al. [41] |

6013-T6 | 2000 | 208 | 1.6 | 0.1 | 111.88 | Magnusson and Kallman. [43] |

A6N01-T5 | 12 | 0.1 | 91.18 | Kawasaki. [44] | ||

7475-T76 | 950 | 110 | 2 | 0.1 | 115.58 | Magnusson and Kallman. [43] |

AA6082-T6 | 2500 | 1400 | 4 | 0.5 | 51.51 | Ericsson and Sandstro [45] |

5083-H321 | 500 | 80 | 8 | −1 | 144.92 | James and Bradley [46] |

AA5083-H3214 | - | 450 | 5 | 0.1 | 94.34 | Pocaterra and Tovo [47] |

ALUSTAR-H321 | - | 350 | 5 | 0.1 | 70.08 | |

AA6082-T5 | - | - | 5 | 0.5 | 53.02 | Maddox [48] |

6005A | 2100 | 1000 | 4.5 | 1 | 105 | Zhang et al. [32] |

5024-H116 | 1200 | 720 | 3.3 | −1 | 180 | Besel et al. [50] |

AA2195-T8 | 800 | 54 | 5 | 0.1 | 185 | Boni et al. [51] |

AA2198-T851 | 1000 | 80 | 4 | 0.33 | 178 | Cavaliere et al. [52] |

AA6082-T6 | 1500 | 300 | 4 | −1 | 170 | Costa et al. [53] |

6061 | 1000 | 80 | 2 | 0.3 | 38 | Hrishikesh et al. [24] |

7050-T7451 | 800 | 150 | 12 | −1 | 202 | Deng et al. [54] |

2024-T4 | 800–1000 | 150–250 | 4 | 0.1 | 73.71 | Di et al. [55] |

Specimen | Rotation-Welding Speed (rpm–mm/min) | Fatigue Strength Coefficient, σ_{f} (MPa) | Fatigue Strength Exponent, b | Fatigue Ductility Coefficient, ε_{f} | Fatigue Ductility Exponent, c | Ref. |
---|---|---|---|---|---|---|

6061Al-T651 | Base Metal | 760 | −0.12 | 0.22 | −0.72 | Feng et al. [88] |

1400–600 | 509 | −0.09 | 0.29 | −0.71 | ||

1400–400 | 476 | −0.09 | 0.34 | −0.73 | ||

1400–200 | 436 | −0.08 | 0.56 | −0.79 | ||

1000–200 | 419 | −0.08 | 0.24 | −0.69 | ||

600–200 | 404 | −0.08 | 0.41 | −0.75 | ||

2219-T62 A: Air cooling W: Water cooling | Base Metal | 751 | −0.10 | 0.04 | −0.50 | Xu et al. [90] |

300-100-A | 517 | −0.09 | 0.64 | −0.79 | ||

1000-100-A | 555 | −0.1 | 0.75 | -0.84 | ||

1000-100-W | 575 | −0.11 | 0.59 | −0.80 | ||

750-60-A | 353 | −0.05 | 0.04 | −0.49 | ||

750-200-A | 448 | −0.08 | 0.05 | −0.60 | ||

Thixomolded AZ91D alloy | Base Metal | 494 | −0.12 | 0.034 | −0.39 | Ni et al. [91] |

800-50 | 549 | −0.16 | 0.081 | −0.58 | ||

Dissimilar FSW of AA6061-to-AA7050 | 270-114 | 196.7 | −0.03 | 0.16 | −0.75 | Rodriguez et al. [21] |

360-114 | 218.3 | −0.04 | 0.0.13 | −0.69 | ||

410-114 | 238.7 | −0.04 | 0.14 | −0.68 |

**Table 3.**Summary of fatigue life and energy dissipated during cyclic deformation [89].

Total Strain Amplitude (%) | Total Energy (MJ/m^{3}) | Average Energy per Cycle (MJ/m^{3}) | Total Number of Cycles to Failure (N_{f}) |
---|---|---|---|

0.6 | 299.0 | 0.20 | 1736 |

0.6 | 338.8 | 0.27 | 1406 |

0.8 | 735.5 | 2.45 | 478 |

0.8 | 600.9 | 2.18 | 469 |

1.0 | 504.4 | 5.80 | 173 |

1.0 | 420.8 | 5.19 | 107 |

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## Share and Cite

**MDPI and ACS Style**

Li, H.; Gao, J.; Li, Q.
Fatigue of Friction Stir Welded Aluminum Alloy Joints: A Review. *Appl. Sci.* **2018**, *8*, 2626.
https://doi.org/10.3390/app8122626

**AMA Style**

Li H, Gao J, Li Q.
Fatigue of Friction Stir Welded Aluminum Alloy Joints: A Review. *Applied Sciences*. 2018; 8(12):2626.
https://doi.org/10.3390/app8122626

**Chicago/Turabian Style**

Li, Hongjun, Jian Gao, and Qinchuan Li.
2018. "Fatigue of Friction Stir Welded Aluminum Alloy Joints: A Review" *Applied Sciences* 8, no. 12: 2626.
https://doi.org/10.3390/app8122626