# Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy

^{*}

## Abstract

**:**

^{2}, 4.8 GW/cm

^{2}and 6.4 GW/cm

^{2}) of LSP were chosen and tested using manufactured fretting fatigue apparatus. The experimental results show that the LSP surface treatment significantly improves the fretting fatigue lives of the fretting specimens, and the fretting fatigue life increases most when the laser power density is 4.8 GW/cm

^{2}. It is also found that with the increase of the laser power density, the fatigue crack initiation location tends to move from the surface to the interior of the specimen.

## 1. Introduction

_{0}can be described as Equation (1) [21], where E is the pulse energy, R means the radius of the laser spot and τ is the laser pulse duration. It is recognized that a better residual compressive stress distribution can be obtained by selecting the appropriate laser power density [21,22], and an excessive laser power density will lead to indentation on the surface of the material, and even ablation of the absorbing layer [23,24].

## 2. Experiments

#### 2.1. Specimen Materials

#### 2.2. Laser Shock Peening (LSP) Specimen and Procedure

^{2}, 4.8 GW/cm

^{2}and 6.4 GW/cm

^{2}. The details of the parameters of laser shock peening are listed in Table 3.

#### 2.3. Surface Topography and Residual Stress

^{2}ψ method, and the range of the ψ were −39 to +39 degrees. The spring constants used here for the stress calculations were: S

_{1}= −2.97 × 10

^{−6}MPa

^{−1}, S

_{2}= 23.78 × 10

^{−6}MPa

^{−1}. The typical example of the d (interplanar spacing) − sin

^{2}ψ diagram was shown in Figure 7.

_{3}, 14% HF and 62% H

_{2}O (volume ratio). The diameter of the etched area was 10 mm, and each period of electro polishing was 15 s, and the total polishing measurement was taken five times. The thickness of the material layer removed by etching was measured after each time of electro polishing. Then, the residual stress inside of the material could be measured by proto LXRD X-ray diffractometer.

#### 2.4. Fretting Fatigue Test

_{min}and Q

_{max}was applied to the specimen. Figure 11 shows a simple load configuration of the test. Table 4 reports the experimental parameters of the tests. Eight fretting fatigue tests were carried out and the lives of the crack initiation were recorded. Strain gauges bonded on both sides of the main specimen (Figure 12) were used to monitor the initiation of the crack. Once the crack occurred, the strain curves would rise immediately until the strain gauges were broken. At the same time, the hole in the middle of the pad fixture could help us to observe the crack initiation.

## 3. Results and Discussion

^{2}, 4.8 GW/cm

^{2}and 6.4 GW/cm

^{2}, and the average surface compressive residual stresses of the treated part are −554 MPa, −650 MPa and −645 MPa respectively. It seems that with the increase of the laser power densities, the average surface compressive residual stresses will reach a peak value and change a little. Continuing to increase the laser power density will cause the peak pressure of shock wave to be too high, which will affect the surface quality of the material. In addition, the residual compressive stress on the surface of the material will be reduced to a certain extent due to the generation of a surface unloading wave.

^{2}, and the residual compressive stress affects the depth of the layer by at least 0.8 mm.

^{2}shows the best effect to prolong the fretting fatigue life. As mentioned before, the compressive residual stress of the laser power density of 4.8 GW/cm

^{2}and 6.4 GW/cm

^{2}is similar, but the surface condition of the laser power density of 6.4 GW/cm

^{2}is worse, which causes a shorter fatigue life. All the fatigue crack initiation location of the specimen was on the edge of the contact area near the lower grip, and the crack propagation direction was almost perpendicular to the moving direction, which is shown in Figure 16.

^{2}laser power density are at the surface of the contact area. However, the fatigue crack source of the LPS specimens with 4.8 GW/cm

^{2}and 6.4 GW/cm

^{2}laser power density are at the subsurface or in the interior of the specimens. This phenomenon shows that the surface compressive residual stresses improve the fatigue resistance of the surface, and lead to the transfer of the crack initiation position to the interior of the specimen.

^{2}) is shown in Figure 19. The formulation of the corrosive agent is as follows: HF:HNO

_{3}:H

_{2}O = 1:3:15, and the corrosion time is 20 s.

## 4. Conclusions

- In this paper, the TC11 specimens were under laser shock peening with three different laser power densities of 3.2 GW/cm
^{2}, 4.8 GW/cm^{2}and 6.4 GW/cm^{2}. The surface topography of the untreated and LSP samples were carried out by using a non-contact 3D optical profilometer, and it showed that with the increase of the laser power density, the surface roughness also increased, which meant a worse surface condition. - A proto-LXRD X-ray diffractometer was used to measure the surface residual stresses, and the result showed that the laser power density of 4.8 GW/cm
^{2}had the best effect with the introduction of compressive residual stresses of −650 Mpa, which meant the plastic deformation of the TC11 surface had reached saturation. The excessive increase of the laser power density would cause the peak pressure of the shock wave to be too large, which reduced the surface quality and induced tensile stress on the surface. - A specialized fretting pad fixture and fretting fatigue test rig were used to measure the initiation lives of the fretting fatigue crack. With the comparison between untreated and LSP specimens, it was found that the fretting fatigue life of LSP was significantly improved between 2 and 4 times. In addition, the 4.8 GW/cm
^{2}power density had the best effect on the improvement of fatigue life, although the average surface residual stress was similar at the power density of 4.8 GW/cm^{2}and 6.4 GW/cm^{2}, the high power density caused the bigger surface damage in the material, which led to the reduction of fatigue life. - The OM micrographs of the fractures of the specimens showed that with the increase of laser power density, the source of crack initiation was gradually transferred from the surface to interior of the specimen, which meant that the introduction of LSP improved the surface strength and reduced the surface damage of the specimen, and the fretting effect did not occupy the dominant position in the process of crack initiation. The metallographic of the main specimen showed that the crack arose due to the location of phases β, and passed through the grains (both α and β) in the process of propagation.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 5.**Surface condition after the LSP impacts. (

**a**) First step. (

**b**) Second step. (

**c**) Third step. (

**d**) Last step.

**Figure 13.**Three-dimensional surface morphologies of the main specimens. (

**a**) 3.2 GW/cm

^{2}(

**b**) 4.8 GW/cm

^{2}(

**c**) 6.4 GW/cm

^{2}(

**d**) untreated.

**Figure 16.**Pictures of the crack initiation and Specimen fracture. (

**a**) Crack initiation. (

**b**) Specimen fracture.

**Figure 17.**Pictures of the typical wear surfaces of the specimens and pads. (

**a**) Wear trace and accumulated debris of untreated specimen. (

**b**) Wear trace under laser power density of 3.2 GW/cm

^{2}. (

**c**) Wear trace under laser power density of 4.8 GW/cm

^{2}. (

**d**) Wear trace under laser power density of 6.4 GW/cm

^{2}.

**Figure 18.**Typical optical microscope (OM) micrographs of the fractures of the specimens. (

**a**) Untreated specimen. (

**b**) Specimen under laser power density of 3.2 GW/cm

^{2}. (

**c**) Specimen under laser power density of 4.8 GW/cm

^{2}. (

**d**) Specimen under laser power density of 6.4 GW/cm

^{2}.

Composition | Al | Mo | Zr | Si | Fe | Ti |
---|---|---|---|---|---|---|

Percent (wt./%) | 6.40 | 3.57 | 1.63 | 0.25 | 0.13 | Bal |

$\mathbf{E}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | $\mathsf{\nu}$ | ${\mathsf{\sigma}}_{-1}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathsf{\sigma}}_{\mathbf{b}}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathsf{\sigma}}_{0.2}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ |
---|---|---|---|---|

120,000 | 0.33 | 540 | 1139 | 866 |

Wave Length | Pulse Width | Diameter of Laser Spot | Overlapping Rate | Absorbing Layer | Restraint Layer | Laser Power Densities (GW/cm^{2}) |
---|---|---|---|---|---|---|

1064 nm | 20 ns | 2 mm | 50% | Black tape | Water | 3.2, 4.8, 6.4 |

Series | P (MPa) | Q_{max} (MPa) | Stress Ratio | Laser Power Densities (GW/cm^{2}) |
---|---|---|---|---|

1 | 65.45 | 400 | 0.1 | No treatment |

2 | 65.45 | 400 | 0.1 | 3.2 |

3 | 65.45 | 400 | 0.1 | 4.8 |

4 | 65.45 | 400 | 0.1 | 6.4 |

5 | 40 | 400 | 0.1 | No treatment |

6 | 40 | 400 | 0.1 | 3.2 |

7 | 40 | 400 | 0.1 | 4.8 |

8 | 40 | 400 | 0.1 | 6.4 |

Laser Power Density/(GW/cm^{2}) | 0 | 3.2 | 4.8 | 6.4 |
---|---|---|---|---|

Roughness/$\mathsf{\mu}\mathrm{m}$ | 0.94 | 1.10 | 1.28 | 1.52 |

I_{0}/(GW/cm^{2}) | Residual Stress/(MPa) | Point 3 | Point 2 | Point 1 | Point 4 | Point 5 |
---|---|---|---|---|---|---|

3.2 | σ_{x} | −138 | −521 | −563 | −559 | −121 |

σ_{y} | −121 | −488 | −601 | −589 | −135 | |

4.8 | σ_{x} | −171 | −676 | −671 | −670 | −168 |

σ_{y} | −161 | −639 | −630 | −619 | −164 | |

6.4 | σ_{x} | −132 | −674 | −735 | −633 | −130 |

σ_{y} | −90 | −569 | −696 | −561 | −99 |

**Table 7.**Experimental fretting fatigue initiation life of TC11 with different laser power densities.

Load | Series 1 (No Treatment) | Series 2 (I_{0} = 3.2 GW/cm^{2}) | ||
---|---|---|---|---|

Life (Cycles) | Average Life (Cycles) | Life (Cycles) | Average Life (Cycles) | |

P = 65.45 MPa, Q_{max} = 400 MPa | 105,223 | 88,863 | 152,527 | 198,752 |

82,750 | 237,018 | |||

78,616 | 206,710 | |||

Load | Series 3 (I_{0} = 4.8 GW/cm^{2}) | Series 4 (I_{0} = 6.4 GW/cm^{2}) | ||

P = 65.45 MPa, Q_{max} = 400 MPa | 357,900 | 323,652 | 195,221 18,823 | 192,022 |

333,157 | ||||

279,899 | ||||

Load | Series 5 (No treatment) | Series 6 (I_{0} = 3.2 GW/cm^{2}) | ||

P = 40 MPa, Q_{max} = 400 MPa | 133,233 | 106,599 | >400,000 | - |

79,450 | ||||

104,500 109,214 | ||||

Load | Series 7 (I_{0} = 4.8 GW/cm^{2}) | Series 8 (I_{0} = 6.4 GW/cm^{2}) | ||

P = 40 MPa, Q_{max} = 400 MPa | >430,000 | - | 282,828 | 282,828 |

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**MDPI and ACS Style**

Yang, X.; Zhang, H.; Cui, H.; Wen, C.
Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy. *Materials* **2020**, *13*, 4711.
https://doi.org/10.3390/ma13214711

**AMA Style**

Yang X, Zhang H, Cui H, Wen C.
Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy. *Materials*. 2020; 13(21):4711.
https://doi.org/10.3390/ma13214711

**Chicago/Turabian Style**

Yang, Xufeng, Hongjian Zhang, Haitao Cui, and Changlong Wen.
2020. "Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy" *Materials* 13, no. 21: 4711.
https://doi.org/10.3390/ma13214711