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

  • Xiang Xiao
  • Wei Li
  • Jian Zhao
  • Cheng Liu
Conference paper

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

The material is 70 mm thick 7050 alloy plate in hot rolled status. The plate is solution heat treated under different conditions, as shown in Table 1, and then age at 121 °C/6 h + 163 °C/12 h.
Table 1

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 tensile tests are conducted on Instron 5882 testing machine according to the GB/T 228-2002 standard in the 1/2 thickness location of the plate. The plane-strain fracture toughness tests are performed in the T-L orientation based on the ASTM E399 standard. Fatigue crack growth testing are followed by ASTM E647 standards. The CT specimens for testing is located at the 1/4 thickness in the T-L orientations as shown in Fig. 1. Fatigue crack growth testing is conducted at R = 0.1. Pre-cracking is conducted at a ΔK of 8 MPa \( \surd {\text{m}} \) at the range of 10–20 Hz.
Fig. 1

Dimensions of CT specimen

The Results and Discussion

Microstructure

The typic microstructure of the hot rolled plate is shown in Fig. 2. There are a large amount of second phase in the matrix (Fig. 2b). The EDX analysis shows that these particles are AlZnMgCu phase (particle d), Al2CuMg (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.
Fig. 2

The microstructure in the rolled plate, a 500×; and b 2000×

Table 2

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

The particles are almostly dissolved after solution heat treatment as shown in Fig. 3. Figure 4 presents the comparison of the area fraction of constituent particles (include Al2CuMg and Fe-rich phase) after different solution heat treatments. The results show that the content of particles decrease with the temperature increasing.
Fig. 3

The microstructure of the alloy after different solution heat treatments: a A-type; b B-type; c C-type; d D-type

Fig. 4

Quantitative results of the coarse particles after different solution heat treatments

The images of the aged alloy exert a partial recrystallization microstructures as shown in Fig. 5. The recrystallization fraction increases with increasing solution temperature as shown in Fig. 6. It is worth noting that the recrystallization fraction under D-type two-stage solution treatment is lower than those of the B-type and C-type one-stage solution treatments. According to [8] reported that in the low temperature stage, most of deformation energy stored in the rolling process can be released, and the recrystallization can be effectively suppressed in the second stage solution process.
Fig. 5

The images of the aged alloy after different solution heat treatments: a A-type; b B-type; c C-type; d D-type

Fig. 6

Recrystallization fraction of the alloy after different solution heat treatments

The Tensile Properties

Figure 7 shows that, under the one-stage solution heat treatment, the ultimate tensile strength and yield strength increases with the increasing in temperature. The strength reaches peak under the C-type heat treatment. With the temperature increases, the coarse particles redissolves more fully, increasing the solution supersaturation and precipitate strengthening effect, which has contributed to the improvement of the strength.
Fig. 7

The comparison of the tensile properties of the alloy after different solution heat treatments

Fracture Toughness

Figure 8 shows the value of KIC of the aged alloy with different solution heat treatments. The results show that under one-stage solution treatment, the value of KIC increases with the increasing of temperature.
Fig. 8

The comparison of fracture toughness of the alloy with different solution treatment

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

The relationship of da/dN versus ΔK and the crack length versus cycles is respectively shown in Figs. 9 and 10. The curve are follow the equation:
Fig. 9

The relationship of da/dN versus ΔK under different solution treatment

Fig. 10

The relationship of the crack length versus cycles under different solution treatment

$$ \frac{\rm{da}}{\rm{dN}} = {\rm{C}}(\Delta K)^{\rm{n}} $$
(1)

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.

The da/dN values at different 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.
Table 3

The da/dN values refers to various ΔK levels under different solution treatment

Solution treatment

da/dN (mm cycle−1)

ΔK = 11 MPa \( \surd {\text{m}} \)

ΔK = 15 MPa \( \surd {\text{m}} \)

ΔK = 18 MPa \( \surd {\text{m}} \)

A-type

9.24E−5

4.27E−4

8.75E−4

B-type

8.39E−5

3.51E−4

7.86E−4

C-type

3.25E−5

2.90E−4

9.07E−4

D-type

1.95E−5

2.05E−4

5.96E−4

Continuous fatigue striations can be observed in the fatigue crack propagation stable region in Fig. 11. The fatigue striation spacing in the early stage was measured respectively as: A-type/314 nm; B-type/225 nm; C-type/172 nm; D-type two-stage solution treatment/168 nm. Generally, the larger the striation width is, the faster the crack propagation rate is. The resistance of fatigue crack growth of the alloy is better with the increasing of temperature. Under the two-stage solution treatment, the alloy has the best fatigue crack growth resistance compared with the one-stage solution treatments.
Fig. 11

The fatigue striation morphology of the alloy under different solution heat treatments: a A-type; b B-type; c C-type; d D-type

Figure 12a, b shows that there are a large amount of coarse constituent particles (Al7Cu2Fe and Al2CuMg) 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.
Fig. 12

SEM morphology of the sample treated with A-type one-stage heat treatment (a) and corresponding backscattered SEM morphology and the particle EDX analysis (b)

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. (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. (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).

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Copyright information

© Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Chinalco Materials & Application Research InstituteBeijingChina
  2. 2.Suzhou Research Institute for Nonferrous MetalsSuzhouChina

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