J. Cent. South Univ. (2021) 28: 1058-1067

DOI: https://doi.org/10.1007/s11771-021-4679-9

Comparative study on microstructure and electrochemical corrosion resistance of Al7075 alloy prepared by laser additive manufacturing and forging technology

ZHANG Jin-liang(张金良)¹, YE Jie-liang(叶界梁)¹, SONG Bo(宋波)¹,LI Rui-di(李瑞迪)², SHI Yu-sheng(史玉升)¹

1. State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: Al7075 alloy is a typical aviation aluminum with good mechanical properties and anodic oxidation effect. Laser engineered net shaping technology has unique advantages in the integrated forming of high-performance large aircraft structural parts. The manufacturing of 7075 aluminum alloy structural parts by laser engineered net shaping technology has become an important development direction in the future aerospace field. Electrochemical corrosion resistance of aluminum alloys is of vital importance to improve reliability and life-span of lightweight components. A comparative study on microstructure and anti-corrosion performance of Al7075 alloy prepared by laser additive manufacturing and forging technology was conducted. There are hole defects in LENS-fabricated Al7075 alloy with uniformly distributed η phase. No defects are observed in Al7075 forgings. The large S phase particles and small ellipsoidal η phase particles are found in Al matrix. The corrosion mechanisms were revealed according to the analysis of polarization curves and corrosion morphology. It was found that compared with that prepared by forgings, the additive manufactured samples have lower corrosion tendency and higher corrosion rate. Corrosion occurred preferentially at the hole defects. The incomplete passivation film at the defects leads to the formation of a local cell composed of the internal Al, corrosion solution and the surrounding passive film, which further aggravates the corrosion.

Key words: Al7075 alloy; laser engineered net shaping; forging; electrochemical corrosion resistance

Cite this article as: ZHANG Jin-liang, YE Jie-liang, SONG Bo, LI Rui-di, SHI Yu-sheng. Comparative study on microstructure and electrochemical corrosion resistance of Al7075 alloy prepared by laser additive manufacturing and forging technology [J]. Journal of Central South University, 2021, 28(4): 1058-1067. DOI: https://doi.org/10.1007/ s11771-021-4679-9.

1 Introduction

It is well known that 7075 (Al-Zn-Mg-Cu) aluminum alloys have received great interest in lightweight applications due to their low density, high strength, high specific stiffness and excellent anti-corrosion resistance [1-4].

Integral structural parts are commonly inclined to be adopted in the high-end manufacturing industry. Compared with the traditional riveted structure, the integral structural parts can not only improve the overall strength and assembly accuracy, but also greatly reduce the weight and the use of connectors [5]. However, integral parts with complex structures have high processing difficulty and manufacturing requirements, which poses a great challenge to the manufacturing process [6]. At present, the manufacturing processes of aluminum components are composed of casting, forging, hot rolling and machining, which are complicated and high-cost [7]. In order to shorten the manufacturing cycle of components and meet the requirements of complex structure and high performance, additive manufacturing (AM) technology based on the laywise principle can be used. Laser engineered net shaping (LENS) based on synchronous powder feeding is one of the AM technologies for metal materials [8-10]. This technique uses high-energy laser beam to melt the synchronous feeding metal powder, and the molten material rapidly cools and solidifies layer by layer, eventually forming a three-dimensional component.

Aluminum alloy structural components are required to have not only good mechanical properties but also high anti-corrosion performance. Chloride ions, nitrate ions and sulfate ions commonly exist in humid environment. These ions adhere to the surface of components to form acid liquid film, which makes aluminum alloys easy to electrochemically corroded [11]. In particular, chloride ion has a very strong penetration ability and can cause the passivation film on the surface of aluminum alloys to break and pitting corrosion, which is the beginning of other types of corrosion (local corrosion or uniform corrosion) [12, 13]. Therefore, investigating the electrochemical corrosion mechanisms of 7075 aluminum alloys is a necessary step to establish a scientific anti-corrosion system and prolong the service life-span of components. Al7075 alloy powders with different particle sizes were sintered by TIAN et al [14]. It was found that compared with coarse grains, the intermetallic compounds in fine grains were distributed more intensively, which leads to the smaller potential difference and lower pitting growth rate. CABRINI et al [15] prepared AlSi10Mg alloy by selective laser melting. According to the test results of potentiodynamic polarization, it could be seen that the potential of Si particles is higher than that of Al matrix, leading to the selective dissolution of α-Al phase. This potential difference is more obvious at the edge of molten pool, which results in the preferential generation of pitting corrosion in these areas. GHARBI et al [16] compared the electrochemical corrosion resistance of 2024 aluminum alloy fabricated by selective laser melting and forging. The dissolution curve of electrochemical analysis showed that the corrosion rate of forgings is about 5 times that of SLM-fabricated samples. However, there are few studies on the anti-corrosion properties of LENS-fabricated Al7075 alloys.

In this study, the microstructure and electrochemical corrosion resistance of 7075 aluminum alloy manufactured by LENS and forging were compared. The electrochemical corrosion mechanisms were revealed according to the analysis of polarization curves and corrosion morphology.

2 Materials and methods

2.1 Raw material and LENS experiments

The chemical composition of 7075 aluminum alloy powder used in this experiment is shown in Table 1. The Al7075 alloy forgings with the same composition were provided by Shanxi Baoji Zhiyi Titanium Manufacturing Co., Ltd.

Table 1 Chemical composition of 7075 aluminum alloy (wt.%)

The technological parameters used in the LENS process are as follows: laser power is 1400 W; scanning speed is 800 mm/min; powder feed rate is 8.00 g/min; carrier gas flow rate is 6.00 L/min; shielding gas flow rate is 10 L/min; and layer thickness is 0.5 mm.

2.2 Microstructural characterization

The cubic samples were chemically etched by Keller reagent (2.5% HNO₃+1.5% HCl+1% HF+95% H₂O) after being ground and polished. Optical metallographic microscope (OM, Axiovert 200MAT, Carl Zeiss, Germany) and scanning electron microscope (SEM, JSM-7600F, JEOL, Japan) were used for microstructural characterization. Electron probe micro analyzer (EPMA-8050g, Shimadzu, Japan) equipped with wave dispersive spectroscopy was used to reveal the elemental distribution. A thin foil was prepared by focused ion beam (FIB, Quanta 3D FEG) for the further microstructural observation on a field-emission transmission electron microscope (FTEM, Tecnai G2 F30, FEI, Holland, 200 kV).

2.3 Electrochemical corrosion resistance tests

The potentiodynamic polarization (PP) was conducted in the 3.5 wt.% NaCl solution at 25 °C. The specimens were reduced potentiostatically at -1.0 V for 3 min initially to remove the air-formed oxides on the surface before the polarization. Then, the specimens were stabilized in the electrolyte, where the open-circuit potentials (OCP) were measured. The CPP test was performed at 0.5 mV/s from -1.2 V to 0.2 V (vs Ag/AgCl), and then the scan direction was reversed. After that, the E_corr and i_corr were calculated by extrapolating the Tafel line [17]. Each type of electrochemical measurement was repeated at least three times to ensure the reproducibility and consistency of the data. The diagram of three-electrode circuit is shown in Figure 1.

3 Results and discussion

3.1 Microstructure

Figure 2 shows the microstructure of Al7075 alloy prepared by LENS and forging technology, respectively. Pores are observed in the LENS-fabricated samples, and the average width of scanning tracks is about 300 μm. The precipitates exist both within the grains and along the grain boundaries. In contrast, the forging sample is fully dense with a uniform columnar microstructure.

Figure 1 Diagram of three-electrode circuit:

Figure 2 Microstructures of Al7075 alloy:

Figure 3(a) shows the elemental distribution of LENS-fabricated samples obtained by EPMA spectrum analysis. It can be seen that Zn is evenly distributed in the Al matrix, and Cu is segregated at the grain boundaries. Part of Mg is dissolved in the Al matrix, and the rest of Mg is involved to form intermetallic compounds at the grain boundaries. The environment-sensitive embedding energy (ESE) of Cu and Mg is relatively low at the grain boundary, while that of Zn is lower within the grains [18]. The lower ESE usually indicates more stable dopant atoms [19]. Therefore, a large amount of Cu segregation occurs at the grain boundary, while Zn is less distributed at the grain boundary. In addition, the existence of Zn also improves the solubility of Cu in Al and combines with Mg to form MgZn₂, which inhibits the diffusion of Mg atoms [20]. However, there is no significant enrichment of Zn in the spectrum analysis, which could be attributed to the low vapor pressure of Zn element. During the laser forming process, a considerable amount of Zn vaporizes, resulting in the burning loss of Zn content. The analysis accuracy of EPMA spectrum is difficult to identify residual Zn enrichment. Figure 3(b) shows the element distribution diagram of forgings. It can be observed that the content of Al, Mg, O and Zn is small in the coarse precipitates, and uniformly distributed in the surrounding Al matrix. Only Cu element enrichment is observed. Combined with element distribution, it can be inferred that the coarse precipitated phase may be θ phase (Al₂Cu). According to the available literature [21-23], the fine second phase is preliminarily concluded as η phase (MgZn₂) in forgings.

Figure 4 shows the TEM micrographs of LENS-fabricated samples. It can be observed that precipitate-free zone (PFZ) is formed at the grain boundaries. The Cu-enriched phases are seperated by the PFZs and form a discontinuous chain-like structure, which is closely related to the intergranular corrosion. The wide FPZ may make it difficult to sustain oxidation reaction at the grain boundary, which leads to low intergranular corrosion sensitivity [24]. The rod-like intermetallics particles are uniformly distributed within the grains. Figure 4(b) shows the element distribution of boxed area in Figure 4(a). The intermetallics particle is mainly composed of Mg and Zn, which is consistent with the element ratio of η phase (MgZn₂). In addition, a small amount of Cu is also dissolved in the η phase, which is beneficial to reduce the potential difference between the η phase and α-Al matrix.

Figure 5 shows the TEM micrographs of forgings, and it is observed that high density of dislocations is accumulated and entangled at the intermetallics particles. The reason for the formation of dislocations is large plastic deformation occurring in the forging process, which may promote grain slipping and dislocation entanglement. When the temperature is low, the driving force of recrystallization is insufficient, and the motion ability of dislocations is low. As a result, most of dislocations are retained [25]. Figure 5(b) shows mainly two types of precipitates in the forging grains. Cu and Mg are enriched in large intermetallics particles, while Mg and Zn are enriched in small ellipsoidal intermetallics particles. According to the existing literature and element analysis results shown in Figure 5(c), it can be determined that the large particles are S phase (Al₂CuMg) and small ellipsoidal particles are η phase [26]. The reason for the formation of this structure is that the absolute value of formation enthalpy of S phase is the highest among the main mesophases in Al7075 alloy such as Al₂Cu, S phase and η phases [27]. Different from the LENS process, Al₂CuMg in the forgings is retained, instead of being decomposed by high temperature. Mg atoms generally tend to form clusters with Zn atoms, so Zn atoms gather around S phase and combine with excessive Mg to form small granular η phase.

Figure 3 Elemental distribution of Al7075 alloy:

Figure 4 TEM micrographs of LENS-fabricated Al7075 alloy:

Figure 5 TEM micrographs of Al7075 alloy forgings:

3.2 Polarization curves and corrosion mechanism

The corrosion resistance performance can be compared by analyzing the parameters obtained from the potentiodynamic polarization curves, such as self-corrosion potential and self-corrosion current. The self-corrosion potential is used to characterize the difficulty of being corroded. Generally, under the same conditions, the electrochemical corrosion is more likely to occur when the E_corr value is more negative. While the self-corrosion current is a kinetic parameter of corrosion, which is used to measure corrosion rate. The higher self-corrosion current value indicates higher corrosion rate. The potentiodynamic polarization curves of Al7075 alloys are shown in Figure 6. After reaching the self-corrosion potential, the corrosion current density increases rapidly, which indicates that the passivation film on the surface is broken or dissolved, and pitting corrosion occurs. During the potentiodynamic polarization process, the cathodic reaction is the reduction of O₂ dissolved in NaCl solution (Eq. (1)), and the anodic reaction is the dissolution of Al from the alloy (Eq.(2)).

O₂+2H₂O+4e→4(OH)^-                     (1)

Al→Al³⁺+3e                             (2)

In the process of polarization, the rupture of passive film is mainly affected by Cl^- ions, which have small radius and strong penetrability. The Cl^- ions are easy to adsorb to the alloy surface and change the composition and properties of the passive film at the adsorption site. The uneven adsorption causes the uneven destruction of the passive film. After the passivation film breaks, Al is directly exposed to Cl^- ions, thus accelerating the anodic dissolution.

Figure 6 Potentiodynamic polarization curves of Al7075 alloys fabricated by forging and LENS

In order to further characterize the corrosion resistance of Al7075 alloy, self-corrosion potential and self-corrosion current density were obtained by Tafel extrapolation method, as shown in Table 2. It can be seen that the self-corrosion potential of forgings is -0.7790 V, which is lower than-0.6817 V of LENS-fabricated samples. Therefore, the forgings are more prone to corrode in terms of the lower self-corrosion potential. However, the self-corrosion current density of the forging is 1.917×10^-6 A/cm², which is lower than that of the LENS-fabricated samples (2.221 μA/cm²), demonstrating that the forgings have lower corrosion rate.

Table 2 Self-corrosion current density (i_corr) and potential (E_corr) of Al7075 samples

Generally, the smaller precipitated phase leads to more uniform anodic dissolution. In the LENS-fabricated Al7075 alloys, the fine rod-shaped η phase is evenly distributed, thus showing better anti-corrosion performance. In addition, η phase usually has the priority to be dissolved. The polarization resistance of η phase is low, while the corrosion current density is high. Therefore, compared with the Al7075 alloy forgings, the corrosion rate of LENS-fabricated samples is higher.

Figure 7 shows the corrosion morphology of after potentiodynamic polarization tests. Severe pitting and intergranular corrosion occur on the surface of aluminum alloy. In addition, the corrosion is especially concentrated at the hole defects. The passivation film at the defects is not complete, and the internal Al directly contacts with the corrosion liquid and forms a local battery with the nearby intact passive films. The schematic diagram of electrochemical corrosion process is shown in Figure 8. The cathodic reaction (Eq. (1)) occurs to the passive films, and the anodic reaction (Eq. (2)) occurs to the Al matrix. The anodic reaction of Al matrix leads to the expansion of pores, and the increasing concentration of Al³⁺ at the pores promotes the diffusion of Cl^- into the pores, which leads to the hydrolysis of Al³⁺ (Eq. (3)). The hydrolysis of Al³⁺ combines with OH^- to increase the acidity of the solution at the pores, resulting in more serious corrosion. Moreover, the loose flocculent Al(OH)₃ produced by hydrolysis forms a closed cell at the pores. The dissolved oxygen in the external solution is difficult to diffuse in, and the excessive Al³⁺ produced by internal dissolution is also difficult to diffuse outward. In order to maintain the neutral state, more Cl^- is needed to move into the etched pores, which further accelerates the corrosion process.

Al³⁺+3H₂O→Al(OH)₃+3H⁺                          (3)

Figure 7 Corrosion morphology of Al7075 alloy after potentiodynamic polarization test:

Figure 9 shows the element distribution of Al7075 alloy after corrosion. It can be found that O and Cl elements are enriched in flocculent corrosion products, while Mg and Zn are reduced. The enrichment of O and Cl is attributed to the oxidation product Al(OH)₃ and continuous diffusion of Cl^- into the corrosion pit due to the formation of occluded cell. The decreased Mg and Zn are owing to the existence of η phase, whose corrosion potential is more negative than that of Al matrix [28]. The η phase preferentially acts as anode and is dissolved in the solution at the initial stage of corrosion, resulting in the decreased content of Mg and Zn in the corrosion products. Therefore, the corrosion process can be regarded as follows: the incomplete passivation film at the defect lead to the formation of a local cell composed of the internal Al, corrosion solution and the surrounding passive film. Al is dissolved and then hydrolyzed to produce Al(OH)₃, and a large amount of Cl^- migrates into the hole to maintain the electrical neutrality. While the deposition of Al(OH)₃ in the hole leads to the formation of a blocked cell, which further aggravates the corrosion.

Figure 8 Schematic diagram of electrochemical corrosion process

4 Conclusions

1) There are hole defects in LENS-fabricated Al7075 alloy. The distribution of η phase is uniform, and the Cu-enriched phases are seperated by the PFZs and form a discontinuous chain-like structure. No defects are observed in Al7075 forgings. The large S phase particles and small ellipsoidal η phase particles are found in the Al matrix.

2) The self-corrosion potential of LENS-fabricated Al7075 alloy is -0.6817 V, and the self-corrosion current is 2.221×10^-6 A/cm². While the self-corrosion potential and the self-corrosion current of the Al7075 forging are -0.7790 V and 1.917 μA/cm², respectively. Compared with the LENS-fabricated samples, the forgings are easier to corrode but have lower corrosion rate in terms of the more negative self-corrosion potential and lower self-corrosion current.

Figure 9 Elemental distribution of corrosion surface

3) The corrosion occurs preferentially at the hole defects. The corrosion process is regarded as follows: the incomplete passivation film at the defect lead to the formation of a local cell composed of the internal Al, corrosion solution and the surrounding passive film. Al is dissolved and then hydrolyzed to produce Al(OH)₃, and a large amount of Cl^- migrates into the hole to maintain the electrical neutrality. While the deposition of Al(OH)₃ in the hole leads to the formation of a blocked cell, which further aggravates the corrosion.

Contributors

The overarching research goals were developed by SONG Bo, LI Rui-di, and SHI Yu-sheng. ZHANG Jin-liang and YE Jie-liang conducted the experiments, and analyzed the experimental data. The initial draft of the manuscript was written by ZHANG Jin-liang. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

ZHANG Jin-liang, YE Jie-liang, SONG Bo, LI Rui-di, and SHI Yu-sheng declare that they have no conflict of interest.

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(Edited by HE Yun-bin)

中文导读

激光近净成形与锻造制备7075铝合金的显微组织与电化学性能对比研究

摘要：Al7075合金是一种典型的航空铝合金，具有良好的力学性能和阳极氧化效果。激光近净成形技术在高性能大型飞机结构件的整体成形中具有独特的优势。利用激光近净成形制造7075铝合金构件已成为未来航空航天领域的重要发展方向。铝合金的耐电化学腐蚀性能对提高轻量化构件的可靠性和寿命具有重要意义。本文对激光近净成形和锻造工艺制备的Al7075合金的组织和耐蚀性能进行了对比研究。发现激光近净成形制造的Al7075合金中存在孔隙缺陷，η相分布均匀。在Al7075锻件中未发现任何冶金缺陷，铝基体中存在较大的S相颗粒和较小的η相颗粒。随后，通过极化曲线和腐蚀形貌分析，揭示了不同工艺制造Al7075合金的腐耐腐蚀性能和腐蚀机理。与锻件相比，激光近净成形制备的试样具有较低的腐蚀倾向和较快的腐蚀速率。腐蚀优先发生在孔洞缺陷处。缺陷处的钝化膜不完整，形成了由铝、腐蚀溶液和周围钝化膜组成的局部电池，加剧了腐蚀。

关键词：7075铝合金；激光近净成形；锻造；电化学腐蚀性能

Foundation item: Project(2016YFB1100101) supported by the National Key Research and Development Program of China

Received date: 2020-09-17; Accepted date: 2021-01-27

Corresponding author: SONG Bo, PhD, Professor; Tel: +86-27-87558155; E-mail: bosong@hust.edu.cn; ORCID: https://orcid.org/0000-0002-6730-3917