Microstructure and corrosion behaviour of Al coating deposited by cold spraying onto FSW AA2219-T87 joint

Abstract

This work evaluates the microstructure and corrosion behaviour of Al coating deposited by cold spraying onto FSW AA2219 alloy. The results showed that a dense coating could be deposited on the FSW joint surface with low porosity (0.77%). XRD analysis indicated that no new phases were formed during the spraying. Refined grains were observed in the coating as expected by SEM results. Electrochemical tests revealed the coating had positive corrosion resistance performance. Immersion tests confirmed that the coating effectively withstood the corrosion attack in an aggressive environment and a typical intergranular corrosion can be observed in the surface of the coating particles.

Keywords

Aluminium alloy cold spraying friction stir welding LEIS intergranular corrosion

Introduction

Aluminium alloy 2219 (AA2219) is diffusely used within the spacecraft industry due to their good mechanical properties based upon the requirements of application [1,2]. However, the welding of aluminium alloy has been one of the key factors hindering its rapid development. Friction stir welding (FSW), as an innovative solid-state process, greatly improves the welding quality of aluminium alloy, which is invented by The Welding Institute (TWI) at 1991. It is seen as a revolutionary progress to welding [3]. In the past decades, the studies of FSW have focused on joint organisation and mechanical properties [4,5]. However, some accidents happened related to the corrosion of welding, so the corrosion performance of joints has to be valued in-service environment.

At present, many studies on corrosion behaviour of joints were reported. Owing to the thermo-mechanical processing conditions during FSW, the joints present a different grain structure, resulting changes in the corrosion susceptibility of joints [6]. Numerous studies [7–9] have shown that the corrosion resistance of joints is worse than that of the base material, which is usually the worst in the heat-affected area. There are some methods to improve the corrosion resistance of joints: post-weld heat treatment (PWHT) [10], micro-arc oxidation (MAO) [11] and laser surface melting [12]. Sivaraj [13] and Paglia [14] found that PWHT could increase the size of intragranular and precipitate-free zone. The discontinuous structure could be observed in the grain boundary, thus the corrosion properties, the mechanical properties and microhardness of FSW 7075 joint were improved. MAO coating has been successfully applied on FSW 6061 [15], 2219 [16] joints in order to resist corrosion. The effect of laser surface melting on corrosion protection of FSW 2219 [17], 7449-T7951 [18] and 2024-T351 [19] have been studied, respectively. Their results showed the improvement of corrosion resistance has been achieved because of the formation of a melting layer. Therefore, surface treatment is a feasible way to improve corrosion performance.

Cold spraying (CS) technology has been developing rapidly since 1990 as a new type of surface treatment. In CS, a solid powder is deposited on the substrate by a low temperature and high speed, resulting intense plastic deformation of the particle and the substrate [20]. CS is the same as FSW, which is a solid-state processing technique. CS coating has been formed on aluminium alloys [21,22], magnesium alloys [23], even stainless steel [24], which has been proven as an effective and practical surface protection method. At present, CS technology is mainly applied to the surface of various materials, but for the application of special structural parts such as welded joints is rare. To date, only Li [25] and Trahan [26] applied cold spraying technology to FSW joint protection. Their results revealed CS is a promising method to improve the corrosion property of FSW joints. However, the electrochemical properties and corrosion behaviour in severe solution of CS coating and FSW joint are not mentioned.

CS coating was deposited on the surface of FSW 2219-T87 joint. The aim of this work is to investigate the microstructure and corrosion behaviour of CS coating onto FSW joint, including electrochemical corrosion and exfoliation corrosion.

Material and methods

The base metal selected for this work was AA2219-T87, which is a high-strength aluminium. Two plates of 5 mm thick plate butt-welded together by FSW process. The welding speed and rotational speed were 150 mm min^–1 and 400 rev min(1, respectively. The spraying of the coating was performed at Xi'an Jiaotong University, China. The gas-atomized Al 2219 alloy powders with spherical geometry (Figure 1(a)) were used as the spraying feedstock. The composition of the particles is shown in Table 1. It can be seen from Figure 1(b) that the inside of powder particle is composed of equiaxial crystals. The size distribution of the powders mainly ranged between 5 and 50 μm (Figure 1(c)). After several trials on spray condition, the feeding rate of powders was about 30 g min^–1. Nitrogen was worked as the accelerating gas at a high pressure of 2.8 MPa and low temperature of 350°C. The standoff distance and the traverse speed of spray gun were 30 mm and 60 mm s^–1, respectively.

Table 1

The chemical composition of Al 2219 powders (wt-%).

Al	Cu	Si	Fe	Mn	Mg	Zr	Zn	Ti
Remainder	6.4	0.2	0.3	0.3	0.02	0.15	0.1	0.07

Microstructures of the powders and the coatings were observed using optical microscope (OM, ECLIPSE MA200, Nikon, Japan) and scanning electron microscope (SEM, SU8020, Hitachi, Japan). The phase composition was analysed by X-ray diffraction (XRD) using a BRUKER D8Focus. The coating thickness was measured from cross-sectional SEM images. The porosity of the coatings was calculated by analysing the images using Image Pro Plus software.

Electrochemical measurements were carried out in a typical three-electrode system. The system was driven by an electrochemical corrosion workstation (OGF500, Origalys, France). Saturated calomel electrode (SCE) was used as the reference electrode and the auxiliary electrode was a Pt meth. The working electrodes were the FSW joint, coating and base material. 3.5% NaCl solution was selected as the test solution, the specimen area exposed to the electrolyte solution was 1 cm², according to the ASTM G69-12 standard. Localised EIS (LEIS) measurements were supported by an electrochemical scanning system (AMETEK, VersaSCAN, USA). In order to compare the impedance of each area, a cross section of sprayed FSW joint was used as a test surface. The specimen was inlaided in epoxy resin and had a diameter of 3.15 mm (Figure 2(a)). The scanning area was set to 3.5 mm by 20 mm (marked by a red rectangle in Figure 2(b)). The step distance was set to 100 μm both in X-direction and Y-direction as required by the system. The scan order is from CS coating to FSW joint, one line by one line. Scan trajectory is shown in Figure 2(b).

Exfoliation corrosion (EXCO) test was adopted according to the ASTM G112-92. In order to compare the corrosion resistance of CS coating and FSW joint, the cross-sections of sprayed FSW joint were exposed in solution for 2, 6, 12, and 24 h, severally. The corrosion products on the samples’ surface were removed through the solution (50 mL H₃PO₄, 20 g CrO₃, reagent water to make 1000 mL) for 10 minutes at 90°C according to the ASTM: ASTM G1-03. The corrosion morphology was obtained by the digital microscope (DM, KH8700, HiRox, Japan) and SEM.

Results and discussion

Microstructure and morphological characterisations

On the lower surface of FSW joint, a CS coating was densely formed with the thick of 500 μm. Figure 3 shows the cross section of the specimen. It can be seen that the joint is divided into four characteristic zones, base material (BM), heat-affected zone (HAZ), thermos-mechanically affected zone (TMAZ) and weld nugget zone (WNZ). The BM zone is composed of elongated grains along the rotating track. The obvious coarsening phenomenon occurred in the grains of HAZ because of the thermal process during FSW. There is a slight thermo-mechanical processing in TMAZ, grains are clearly elongated and warped along the rotating track. By contrast, an intensive thermo-mechanical processing existed in the area of WNZ, so the fine equiaxial recrystallized grains were formed in this area during the FSW process.

The CS Al2219 coating exhibited a high quality with very low porosity levels and no cracks and pores at the interfaces with the FSW joint (Figure 4(a,b)). It is seen from Figure 4(b) that there is no delamination at the interfaces. Meanwhile, a higher number of second phases Al₂Cu were distributed in the FSW joint, which can help to distinguish the interface. Figure 4(c) shows that the inner of the coating was composed of splats formed by refined grains. The grains range from the submicron to a few microns in size. This result is also observed in the paper [27], evidencing of plastic deformation in the progress of cold spraying. This is a process of refining grains, which is expected to be seen after cold spraying. Elongated grains and equiaxial grains also can be observed in Figure 4(c). Elongated grain formation is caused by vertical force in the spray process. Equiaxial grains are the original state of feedstock powders, and this part is not affected by cold spraying. Only a small area belong to equiaxial grains, so plastic deformation occurred in most areas of the particles during the spraying.

Figure 4(d) shows the top of the coating, some pores can be clearly seen. This phenomenon also observed in the paper [25], it is a so-called tamping effect in the process of cold spraying, subsequent incoming particles have an impact on previously deposited particles. This effect can effectively reduce the porosity of the inner coating. However, tamping effect was not apparent in the top area of the coating. Hence, porosity values of 0.34% and1.56% were estimated on the inner coating and the top of coating, respectively. Eventually, the coating porosity was counted to be 0.77%.

The XRD images for the powders and CS coating are shown in Figure 5, and only the peaks corresponding to the Al FCC structure were identified. The powders and CS coating had the same characteristic peaks. Therefore, there was no change in the material composition during the process of cold spraying. Effect of low thermal energy and high kinetic energy, oxygen-free environment was created and the chemical reaction that led to the formation of new phase did not occur when the coating was formed.

Electrochemical corrosion studies

The open-circuit potential (OCP) curves of the CS coating, FSW joint and base material are shown in Figure 6, respectively. In order to avoid the instability of samples in the initial test, the samples were exposed to the solution for 30 min before the test. Hence, from all the OCP curves, almost no apparent variation was observed. The potential of the BM was stabilised at –0.80 V_SEC, which is typical values for aluminium alloys in 3.5% NaCl solution. From the curve of the BM, some weeny potential oscillations were observed, which may be connected with the attack of the oxide layer by chloride ions and repassivation of the surface. The potential behaviour of FSW joint was absolutely different to that of the BM, the potential of FSW joint present obvious differentiation in different zones, WNZ and HAZ/TMAZ were stabilised at –0.65 V_SEC and –0.75 V_SEC, respectively. The value of HAZ/TMAZ was close to BM because of the similar grains. However, the grains of WNZ were refined during the FSW process, the size of the grains decreased significantly. More importantly, the originally continuous thick grain boundaries were broken, which as a fast channel for corrosion. Therefore, WNZ has the best potential value. For CS coating, the potential value was stabilised at –0.68 V_SEC, which is between curves of WNZ and HAZ/TMAZ, and more closed to WNZ. It is because that the coating and WNZ has the same chemical composition and similar grains and grain boundaries, however, some pores can be seen in the top of CS coating (Figure 4(d)), resulting in a relatively low potential. Above all, CS coating has a more powerful potential than BM and HAZ/TMAZ.

Figure 7 shows the potentiodynamic polarisation curves of all samples. Corrosion potential (E _corr) and corrosion current density (I _corr), which are derived by Tafel method, are shown in Table 2. The E _corr values exhibit a variation trend similar to the OCP curves. Small data differences can be observed between E _corr and OCP values, which is a normal phenomenon because of the test sequence. The samples of polarisation were exposed more time than that of OCP. It is well known that the sample with more positive corrosion potential tends to be more stable. The corrosion potential of WNZ is the most stable. CS coating has a more positive corrosion potential than HAZ/TMAZ and BM. HAZ/TMAZ and BM are the areas of corrosion sensitivity, CS coating reveals a relatively stable protection. Interestingly, it can be seen from Table 2 that CS coating (6.03 μA cm^–2) exhibits smaller I _corr than that of WNZ (7.03 μA cm^–2), which is the opposite trend of corrosion potential. Compared with CS coating, HAZ/TMAZ (9.90 μA cm^–2) and BM (15.0 μA cm^–2) have a more negative corrosion current density. This indicates that the corrosion rate of CS coating is significantly lower than that of FSW joint and BM.

Table 2

Corrosion potential and Tafel extrapolation results.

Area	E _corr (V _SCE)	I _corr (μA cm⁻²)
BM	–0.81	15.00
WNZ	–0.65	7.03
HAZ/TMAZ	–0.75	9.90
CS	–0.70	6.03

The 3D map of LEIS is shown in Figure 8, |Z| represents the impedance value of the sample, which is an indicator of the corrosion resistance at each point. As shown in Figure 8, the value of |Z| in the CS coating (2.5 × 10⁵ Ω) is much higher than that in the FSW joint and BM, which is a contrary trend with the corrosion current density, indicating the better corrosion resistance behaviour of the CS coating. While, the |Z| values of FSW joint and BM are about 1.2 × 10⁵ Ω and 8 × 10⁴ Ω, respectively. The |Z| value difference between WNZ and BM is not as large as expected. This is because the test was started from CS coating to FSW joint, the FSW joint and BM had been tested when the sample had been exposed in solution for 3 h. An amount of corrosion products were accumulated on the sample surface under the combined action of chloride ions and electrochemical workstation currents, resulting in some deviation of impedance value. However, the error won't exceed 10%, the variation trend of LEIS test is undoubted. It can be clearly seen that the CS coating has better corrosion resistance behaviour compared with FSW joint and BM, which could effectively improve the corrosion sensitivity of the FSW joint as expected. Conclusively, a superduper corrosion protection of the CS coating on FSW joint is proved by electrochemical corrosion studies.

Corrosion behaviour

In order to compare the corrosion resistance, CS coating, FSW joint and BM were exposed to EXCO solution for 24 h, respectively. Figure 9 shows the 3D maps of corrosion morphology by scanning the surface with an optical digital microscope. The maximum corrosion depth can be clearly expressed by the limit coordinates, the values of BM and HAZ/TMAZ are 110.1 and176.4 μm, respectively. Large pits can be seen in the BM and HAZ/TMAZ. Differently, the value of WNZ is only 45.0 μm, a relatively flat surface can be obtained in Figure 9(c). CS coating also showed good corrosion performance, with a maximum depth of 67.5 μm.

In addition, an interesting observation is that a stepped morphology appeared in the coating map (Figure 9(d)), the low of 3D map belongs to the top of coating, suffering more serious corrosion. The top half of 3D map belongs to inner coating, showing a low degree of corrosion. This is because the porosity of different coating areas is not consistent, more pores could be clearly seen at the top of the coating (Figure 4(d)), resulting a more serious corrosion occurred at the top of coating than inner coating.

The information of pitting is counted from 3D maps by the digital microscope (Table 3), including average depth of pits and per cent of pits as test area. It is easily seen that the zone of HAZ/TMAZ has an evidently large value in depth and per cent of pits, indicating a negative corrosion resistance. While, the values of CS coating are 32.18 μm and 30.89% independently, which are much more positive than HAZ/TMAZ (61.28 μm and 80.84%) and BM (33.64 μm and 46.52%). The best corrosion performance occurs in the WNZ (19.84 μm and 14.74%). It can be concluded that CS coating has corrosion resistance second only to the WNZ in EXCO solution (Acidic environment), which is more positive than HAZ/TMAZ and BM. So, CS coating has a high performance in protecting FSW joint, reducing the corrosion sensibility of FSW joint effectively.

Table 3

Corrosion pits information of each area.

Selected area	Average depth of pitting (μm)	Per cent of pitting area (%)
BM	33.64	46.52
WNZ	19.84	14.74
HAZ/TMAZ	61.28	80.84
CS	32.18	30.89

Corrosion mechanism

The structural morphologies of CS coating in EXCO solution at different time are present in Figure 10. At the beginning of the test (2 h), the coating particles showed an intact shape and were not corroded at all (Figure 10(a)). When the corrosion reached 4 h, the initiation of particles corrosion occurred. It can be clearly seen from Figure 10(b) that corrosion occurred at the elongated grains. Elongated grains are characterised by continuous wide grain boundaries (Figure 4(c)), there are more dislocation and vacancy in the elongated grain boundaries than equiaxial grain boundaries, which is caused by plastic deformation in the process of spraying. It is well known that segregation and element enrichment are easy to occur at elongated grain boundaries because of the structure defect, resulting in the significant potential difference between grain boundaries and grains [28]. When the particles were exposed in the corrosion solution, a corroded primary cell was constructed by grain boundaries and grains. Corrosion was preferred at the grain boundaries as anode. The areas of equiaxial grains were corroded when corrosion occurred 12 h, a typical intercrystalline corrosion morphology was shown in Figure 10(c). Most of the grain boundaries on the coating particles have been corroded. However, the spherical geometry structure of the particles remains intact. When the corrosion occurred 24 h, the degree of particle corrosion was further increased, corrosion ions invaded the inner of particles. Traces of shedding phenomenon can be excavated on the lower left of the particle in Figure 10(d), meaning the completely ineffective of the CS coating.

Conclusions

This work investigates the microstructure and corrosion behaviour of CS coating on FSW AA2219-T87 joint. The major conclusions result from the present work are as follow:

A dense CS coating can be obtained on the surface of FSW joint with a porosity of 0.77%. Numbers of refined grains appear in the CS coating as expected, evidencing of plastic deformation in the progress of cold spraying.

The electrochemical properties of CS coating are higher than FSW joint and BM, especially the value of impedance. Stable and superduper corrosion resistance performance is exhibited in the sample of CS coating throughout the tests. The CS coating effectively decreases the corrosion sensitivity of the FSW joint.

Though the immersion tests in the exfoliation solution confirm that CS coating has a high performance in protecting FSW joint, and the corrosion performance of inner coating is better than the top of coating because of the difference of porosity.

For the corrosion of CS coating particles, elongated grains have a negative corrosion performance in the exfoliation solution, and a typical intercrystalline corrosion morphology can be seen in the surface of the particles. At last, the particle is completely corroded along the grain boundaries.

Footnotes

Acknowledgements

The author would like to thank Xi'an Jiaotong University for cold spraying technical support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Changyu Zhao is a master student in Mechanical Engineering at Beijing Institute of Petrochemical Technology of China. His field of expertise is in friction stir welding, corrosion and material protection. He has participated in nine national and international congress for sharing the results of his investigations.

Hua Zhang is a professor in Mechanical Engineering at Beijing Institute of Petrochemical Technology of China. His field of expertise is in friction stir welding, corrosion and material protection. She has participated in severial national and international congress for sharing the results of his investigations. She has published more than 40 papers in total.

Tongge Shao is a master student in Mechanical Engineering at Beijing Institute of Petrochemical Technology of China. His field of expertise is in laser welding and friction stir welding.

Chengliang Dou is a master student in Mechanical Engineering at Beijing Institute of Petrochemical Technology of China. His field of expertise is in friction stir welding.

ORCID

Changyu Zhao

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