# Effect of Solution Treatment on the Microstructure and Fatigue Properties of 7050 Aluminum Alloy

## Abstract

The primary requirement of aircraft design is safety. The fatigue fracture is difficult to detect and prevent, which is a major safety hazard during aircraft service. In this paper, the influence of solution heat treatment on the microstructure and resistant of fatigue crack growth of 7050 alloy is investigated. For one-stage solution heat treatment, with the solution temperature increases, the area fraction of the coarse constituent phase decreases and the recrystallization fraction increase, and conresponsively the strength and fracture toughness increase and the fatigue crack growth rates decrease. Under the two-stage solution heat treatment, the alloy has the best fatigue crack growth resistance compare with the one-stage solution heat treatments. The effect of solution heat treatment on the properties can be understood on the basis of the combined influence of the constituent phase and the recrystallized fraction.

## Keywords

Aluminum alloy 7050 Solution treatment Microstructure Fatigue properties## Introduction

Based on the aircraft design principles, the high strength aluminum alloys with better fatigue resistance is developed [1, 2, 3]. 7xxx series aluminum alloys has excellent performance including high strength, and excellent fracture toughness, which are applied as structural engineering materials [1, 2, 3, 4, 5]. Many investigations focus on the microstructures and properties of 7xxx alloy [3, 4, 5, 6, 7]. Robson [3] investigate the microstructural evolution in AA7050 during post homogenization cooling, and its influence on behaviour during preheat, hot rolling, and solution treatment. Lin and Starke [4] developed the effect of the Cu content and recrystallization fraction on the property of fatigue crack growth in 7xxx alloys.

Generally, the heat treatments of 7xxx alloys is very important to improve the combination properties [8]. During solution treatment, the coarse phase dissolves and thus increase the supersaturated solid solubility of the matrix. Han et al. [8] find that the enhanced solution treatment can greatly decrease the content of the coarse particles, and thus improve the fracture toughness of the 7050 alloy. Despite detailed studies of the influence of solution heat treatment on the microstructure and properties, less attention is paid to the effect of solution treatment on the resistance of fatigue crack growth in the 7xxx alloy. In this paper, the influence of different solution heat treatment on the microstructure and resistance of fatigue crack growth of the 7050 alloy are studied.

## Materials and Methods

Solution treatment parameters of 7050 alloy

Solution treatment | The first stage | The second stage |
---|---|---|

A | 465–470 °C/2–4 h | |

B | 470–477 °C/2–4 h | |

C | 477–485 °C/2–4 h | |

D | 460–470 °C/1–2 h | 475–480 °C/2–4 h |

The samples location for metallographic observation is at the 1/4 of the thickness. The microstructure observation of the samples is examined on JSM-6480 scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). The recrystallization fraction and the area fraction of particles is analyzed by Image-Pro-Plus software. Nine images of each specimen are observed at a magnification 200 times.

## The Results and Discussion

### Microstructure

_{2}CuMg (particle a and b), and Fe-rich phase (particle c), as shown in Table 2. During solution treatment, most particles dissolve into the matrix, while the higher temperature heat treatment increases the recrystallization fractions.

The composition of the particles in the rolled plate (wt%)

Particle | The alloy elements contents (at.%) | ||||
---|---|---|---|---|---|

Mg | Cu | Zn | Fe | Al | |

a | 22.49 | 21.27 | 2.19 | – | 54.05 |

b | 13.00 | 11.00 | 2.25 | – | 73.75 |

c | 1.81 | 15.83 | 2.00 | 7.65 | 73.63 |

d | 5.27 | 1.64 | 6.99 | – | 86.10 |

_{2}CuMg and Fe-rich phase) after different solution heat treatments. The results show that the content of particles decrease with the temperature increasing.

### The Tensile Properties

### Fracture Toughness

_{IC}of the aged alloy with different solution heat treatments. The results show that under one-stage solution treatment, the value of K

_{IC}increases with the increasing of temperature.

Many researchers present that the constituent coarse particles fracture occurs previously [9, 10, 11]. The less the coarse particles on the fracture surface, the better the facture toughness. The incoherent particles precipitate along grain-boundary make the slip transfer difficulty, and benefits the strain localization. In 7050 alloy, the crack mainly propagates along the high-angle grain boundaries [11, 12].

With the two-stage solution heat treatment in this paper, the content of constituent particles and recrystallization fraction is lower than those of the one-stage solution heat treatment, as shown in Figs. 4 and 6. It is reasonable that the two-stage solution treatment resulting in the excellent fracture toughness.

### The Resistance of Fatigue Crack Growth

In the equation, C and n are respectively as the Paris constants. *ΔK* is the stress intensity factor range. It seems that the fatigue crack growth rate (da/dN) decreases and the cycles increase with the solution temperature increases under the one-stage solution treatment. The fatigue crack growth rate (da/dN) under two-stage solution treatment is lowest and the lifetime is longest among other one-stage solution treatments.

*ΔK*(11–18 MPa \( \surd {\text{m}} \)) is shown in Table 3. The comparison reveals that the crack growth rates decrease with the increase of the solution temperature. Especially, the crack growth rate of the two-stage solid solution heat treatment is lower than that of one-stage treated. It indicates that the resistance of fatigue crack growth under two-stage solid solution heat treatment is the best among than the one-stage treated ones.

The da/dN values refers to various Δ*K* levels under different solution treatment

Solution treatment | d | ||
---|---|---|---|

ΔK = 11 MPa \( \surd {\text{m}} \) | ΔK = 15 MPa \( \surd {\text{m}} \) | ΔK = 18 MPa \( \surd {\text{m}} \) | |

A-type | 9.24E | 4.27E | 8.75E |

B-type | 8.39E | 3.51E | 7.86E |

C-type | 3.25E | 2.90E | 9.07E |

D-type | 1.95E | 2.05E | 5.96E |

_{7}Cu

_{2}Fe and Al

_{2}CuMg) in the fracture surfaces. These coarse particles have no effect on strength, and easily produce crack at the interface between the particle and matrix. This crack paths can decrease the propagation energy of fatigue crack [13, 14]. Therefore, decreasing the content of constituent particles in the alloy increases the resistance of fatigue crack propagation.

It is worth noting that with the solid solution temperature increases, the area fraction of constituent phase and recrystallization fraction shows the opposite trend. Figure 9 shows that the da/dN values decreases with the solid solution temperature increasing. It depicts that the impact of constituent phase on the resistance of fatigue crack propagation is effective than that of recrystallization. Although the area fraction of coarse particles under two-stage solution treatment is close to those of the high temperature C-type one-stage solution treatment, seen in Fig. 4, the resistance of fatigue crack propagation is better than those of this sample, seen in Fig. 9. This is mainly due to the reduction of the recrystallization fraction in the two-stage solution treatment, as shown in Fig. 6. The fatigue cracks easily propagate along the recrystallized grain boundaries. The two-stage solution heat treatment improves the solution supersaturation without increasing the total content of alloying elements, while reducing the coarse phase content and decrease the recrystallization fraction, results in the improvement of the resistance of fatigue crack growth, which can improve the comprehensive performance of the 7xxx aluminum alloy.

## Summary

- (1)
With the solution temperature increase, the coarse constituent phase dissolves into the matrix, while the recrystallization fraction correspondingly increases. The strength, fracture toughness increases and the fatigue crack growth rates decrease with the higher of solution temperature. The effect of constituent phase on the property of fatigue crack growth resistent is more significant than that of recrystallization fraction under one-stage solution heat treatment.

- (2)
The two-stage solution heat treatment is effective in decreasing the area fraction of the constituent phases and the recrystallized grains compared with the one-stage solution treatment, and hence in comprehensively improving the strength, and resistance of fatigue crack growth of the 7050 alloy.

## Notes

### Acknowledgements

This work was supported by The National Key R&D Program of China (Project No. 2016YFB0300900).

## References

- 1.J.C. Williams, E.A. Starke, Progress in structural materials for aerospace systems, Acta Materiali, 51 (2003) 5775–5799.Google Scholar
- 2.J.S. Wang, Y.W. Kang and Ch.R. Li, Phase Diagram and Phase Equilibrium Studies on Ultra High Temperature Alloys of Nb-Si-Ti, Materials Science Forum, 849 (2016) 618–625.Google Scholar
- 3.J.D. Robson, Microstructural evolution in aluminium alloy 7050 during processing, Materials Science and Engineering A, 382 (2004) 112–121.Google Scholar
- 4.F.S. Lin and E.A. Starke. The effect of copper content and degree of recrystallization on the fatigue resistance of 7xxx-type aluminum alloys, Materials Science and Engineering, 43 (1980) 65–76.Google Scholar
- 5.J.J. Schubbe. Fatigue crack propagation in 7050-T7451 plate alloy, Engineering Fracture Mechanics, 76 (2009) 1037–1048.Google Scholar
- 6.T.S. Srivatsan, S. Anand, S. Sriram, V.K. Vasudevan. The high-cycle fatigue and fracture behavior of aluminum alloy 7055, Materials Science and Engineering A, 281 (2000) 292–304.Google Scholar
- 7.S.S. Singh, C. Schwartzstein, J. J. Williams, X.H. Xiao, F.D. Carlo, N. Chawla. 3D microstructural characterization and mechanical properties of constituent particles in Al 7075 alloys using X-ray synchrotron tomography and nanoindentation, Journal of Alloys and Compounds, 602 (2014) 163–174.Google Scholar
- 8.N.M. Han, X.M. Zhang, S.D. Liu, D.G. He, R. Zhang. Effect of solution treatment on the strength and fracture toughness of aluminum alloy 7050, Journal of Alloys and Compounds, 509 (2011) 4138–4145.Google Scholar
- 9.N.U. Deshpande, A.M. Gokhale, D.K. Ddnzer, and J. Liu. Relationship between Fracture Toughness, Fracture Path, and Microstructure of 7050 Aluminum Alloy: Part II. Multiple Micromechanisms-Based Fracture Toughness Model, Metallurgical and materials transactions A, 29A (1998) 1203–1210.Google Scholar
- 10.O.E. Alarcon, A.M.M. Nazar, W.A. Monteiro. The effect of microstructure on the mechanical behavior and fracture mechanism in a 7050-T76 aluminum alloy, Materials Science and Engineering A, 138 (1991) 275–285.Google Scholar
- 11.S. Suresh, R.O. Ritchie. Propagation of short fatigue cracks, International Metals Review, 29 (1984) 455–476.Google Scholar
- 12.J. J. Schubbe. Fatigue crack propagation in 7050-T7451 plate alloy, Engineering Fracture Mechanics, 76 (2009) 1037–1048.Google Scholar
- 13.R. Bao, X. Zhang. Fatigue crack growth behaviour and life prediction for 2324-T39 and 7050-T7451 aluminium alloys under truncated load spectra, International Journal of Fatigue, 32 (2010) 1180–1189.Google Scholar
- 14.J.J. Schubbe. Evaluation of fatigue life and crack growth rates in 7050-T7451 aluminum plate for T-L and L-S oriented failure under truncated spectra loading, Engineering Failure Analysis, 16 (2009) 340–349.Google Scholar