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Article

Effect of Al-Si Coating on the Interfacial Microstructure and Corrosion Resistance of Dissimilar Laser Al Alloy/22MnB5 Steel Joints

by
Lingqing Wu
1,
Joao Pedro Oliveira
2,
Jin Yang
1,*,
Ming Xiao
1,
Min Zheng
3,
Wenhu Xu
1,
Yixuan Zhao
1,*,
Feifan Wang
4 and
Hua Zhang
1,3
1
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
2
CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal
3
School of Advanced Manufacturing, Nanchang University, Nanchang 330031, China
4
Beijing Institute of Astronautical Systems Engineering, Beijing 100076, China
*
Authors to whom correspondence should be addressed.
Metals 2024, 14(3), 328; https://doi.org/10.3390/met14030328
Submission received: 1 February 2024 / Revised: 7 March 2024 / Accepted: 9 March 2024 / Published: 13 March 2024
(This article belongs to the Special Issue Additive Manufacturing Process and Laser Welding of Metals)
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Abstract

:
This investigation employed different laser powers to conduct the laser welding–brazing process of 5052 aluminum alloy to both Al-Si coated and uncoated 22MnB5 steel. The flux-cored Zn-Al22 filler metal was employed during the procedure. The influence of Al-Si coatings on the microstructure and corrosion resistance of Al/Steel welded joints was investigated using microstructural characterization and electrochemical tests. It was noted that the interfacial microstructure of the laser Al/steel joints was significantly altered by the Al-Si coating. Moreover, the Al-Si coating suppressed the formation and growth of the interfacial reaction layer. Electrochemical corrosion tests showed that the impact of Al-Si coating on the corrosion resistance of laser joints depended on the laser powers and thickness of the interfacial intermetallic compound (IMC) layer. The research suggests that galvanic corrosion occurs due to the differences in corrosion potential between fusion zone (FZ), steel, and Fe-Al-Zn IMCs, which accelerate the corrosion of the joint. The IMC layer acts as a cathode to accelerate the corrosion of the FZ and as an anode to protect the steel from corrosion.

1. Introduction

Lightweight design and manufacturing of automotive structures are fundamental for the development of the motor industry [1,2,3]. Due to the advantages of being lightweight and exceptionally resistant to corrosion, aluminum alloys have good prospects for application in the weight reduction of automotive structures by replacing conventionally used steels [4,5,6]. At the same time, to satisfy the demand for weight reduction and to improve the safety and crashworthiness qualities of vehicles, automakers have made remarkable advancements in the production of sophisticated high-strength steels (AHSSs), including martensitic steel, dual-phase steel, and boron alloy steel to obtain higher strength and thinner structural sizes [7]. The AHSSs have a higher strength than conventional mild steels but have poor formability. The 22MnB5 is a boron-containing alloy steel. It has a completely martensitic microstructure and offers unparalleled strength compared to the AHSS currently employed in the industry. In order to eliminate the disadvantage of low-formation properties, 22MnB5 steel is formed by a hot-forming process, so it was called a hot stamping steel. The 22MnB5 steel shows a high ultimate tensile strength after hot stamping [8,9].
Extensive research has been conducted to investigate different types of coatings, including Al-Si and Zn coatings, to mitigate surface oxidation that may occur during high-temperature procedures [10]. The Al-Si coatings leave no oxide stripping during heating on the steel surface, can be dispensed with shot blasting or sand blasting after stamping, and possess a high corrosion resistance without nitrogen protection. Therefore, Al-Si coatings have been more widely used in hot stamping and forming processes than Zn coating. It was inevitable that scholars would consider the influence of Al-Si coating during the welding process. A study conducted by Kim et al. [9] aimed to examine the effects of Al-Si coating in laser welding on 22MnB5 steel (Martin Steel Inc., Huntington, WV, USA). The researchers discovered that, when an Al-Si coating was present, the aluminum and silicon elements dissolved into the weld. These elements primarily formed a solid solution within the weld. Furthermore, the aluminum and silicon precipitated on the interface, leading to a weak joint strength. Sun et al. [11] focused on the effect of Al-Si coating on the joint strength of steel joints. According to this investigation, the ferrite generated by Al-Si coating exhibits a low hardness and strength. It is beneficial for enhancing the joint’s fatigue life. Lin et al. [12] observed an increase in the aluminum content in the fusion zone (FZ) of 22MnB5 steel joints as a result of the Al-Si coating dissolving. Furthermore, aluminum promoted the generation of δ-Fe during solidification, which weakened the joint’s mechanical properties. In the case of dissimilar welding, Ding et al. [13] proposed to join Al/Al-Si coated steel by refill friction stir spot welding and found that the Al5Fe2 (Si) existing in the Al-Si coating can inhibit the formation of IMC in the joint. Zhang et al. [14] welded magnesium and Al-Si coated 22MnB5 steels by resistance spot welding. They investigated the impact of the Al-Si coating on the strength and microstructure of the joints. It was demonstrated that the molten Al-Si eutectic promoted the evolution of the reaction, generating a new IMC layer in the center of the weld, which significantly improved the mechanical properties of the joint. There are many welding methods for dissimilar metals, such as friction stir welding [15], explosion welding [16], gas tungsten arc welding (GTAW) [17], and laser welding [18,19]. The advantages of laser welding are a high energy density, concentrated energy, easy control of the heat source, deep weld seam, small welding deformation, and high efficiency, which are promising for aluminum/steel dissimilar welding [20,21,22]. Yang et al. [19] studied the association between laser power, interfacial microstructure, and joint strength of Al/Al-Si coated 22MnB5 steel joints. The results of the study demonstrated significant variations in the type and structure of intermetallic compounds (IMCs) within the interfacial layer under different laser power conditions. These findings emphasize the effect of laser power on the formation and characteristics of IMCs.
Based on the above literature, it can be summarized that Al-Si coatings have a significant impact on the microstructure and mechanical properties of welds. However, there is still little literature on the effect of Al-Si coatings on the corrosion resistance of dissimilar Al/steel joints. Al-Si coating can improve the corrosion resistance of 22MnB5 steel surface, but the impact on the corrosion resistance of Al/steel dissimilar joints is unclear. Corrosion between Al/steel dissimilar joints is more prone to occur, which is caused by the difference in self-corrosion potential between the two materials. Hence, investigating the effect of Al-Si coating on the corrosion resistance of Al/22MnB5 steel joints is necessary. In the present study, the interfacial microstructure was comparatively analyzed in Al/22MnB5 steel joints with and without Al-Si coating. In order to study the effect of coatings on the corrosion resistance of the joints, self-corrosion current density, self-corrosion potential, and impedance of Al-steel joints were measured by electrochemical tests. The morphology of the joints after corrosion was analyzed, and illustrated the corrosion mechanism of the Al/steel joints. This will provide theoretical support for enhancing the security and stability of Al/steel welded construction products in the manufacturing industry.

2. Experimental Design

The thickness of the base metals, AA5052 aluminum alloy (Guangdong Xingfa Aluminium Co., Ltd., Guangdong, China) and 22MnB5 steel (with or without Al-Si coating), used in this experiment were 2.0 mm and 1.8 mm, respectively. These base metals had dimensions of 100 mm× 50 mm. For this experiment, the Zn-Al22 wire was chosen as the filler metal, which had a diameter of 1.6 mm. Table 1 provides the elemental content of the the22MnB5 steel (Baosteel Co., Ltd., Shanghai, China), AA5052 aluminum alloy, and Zn-Al22 filler wire (Shandong Szeshang Welding Materials Co., Ltd., Shandong, China). Figure 1 shows the microstructure of the 22MnB5 steel with and without coating. In terms of uncoated steel, pearlite and ferrite were observed (Figure 1a), while only martensite was observed due to the hot stamping process inducing phase transformation [14]. In addition, the Al-Si coating with the thickness of 28 ± 2 μm was clearly observed on the steel substrate (Figure 1b). This coating consisted of an Al-Si eutectic phase and Fe2Al7Si intermetallic compound, where the IMCs were continuously distributed between the Al-Si eutectic phase and steel substrate with the thickness of 7 μm [23,24,25].
The IPG YLS-5000 fiber laser system (IPG Photonics, Oxford, MA, USA), capable of producing a maximum output power of 5 kW, was utilized to conduct the welding procedure. A visual representation of the Al/steel laser welding–brazing process can be observed in Figure 2. To ensure the cleanliness of the workpieces, the base metal was wiped with ethanol after being smoothed by sandpaper to remove scraps and oils before welding. The welding joint determined the lap joint, the steel was placed on the bottom of aluminum, and the width of the overlap was 10 mm. The filler metal was fed from the front of the laser beam at an angle of 45°. The angle between the laser beam and the workpiece was approximately 90°. The filler metal was fed from the front of the laser beam at an angle of 45°. A total of 99.99% pure Ar shielding gas was used to prevent oxidation during welding with a flow rate of 15 L/min. The angle between the workpiece and the protective gas feed nozzle was set at 30°. In order to obtain a wider laser irradiation area, the laser defocus parameter can be set to +35 mm. To obtain a satisfactory weld appearance, the experimental parameters need to be optimized before the beginning of the test. Table 2 shows the optimized parameters for the dissimilar laser welding–brazing.
After welding, the laser joints were cut along the direction perpendicular to the laser movement and subjected to microstructure analysis. The samples were sequentially ground with 400#, 800#, 1200#, and 1500# metallographic sandpapers for coarse and fine grinding, and polished with 1 μm diamond suspension until becoming scratch-free. Then, metallographic specimens were ultrasonically cleaned in acetone. The welding samples were observed using an optical microscope (OM, Olympus 4XCJZ, Olympus Group, Milwaukee, WI, USA) and scanning electron microscope (SEM, Hitachi S3400-N, ATA Scientific Pty Ltd., Caringbah, Australia) in order to analyze the microstructure and ascertain the chemical composition of the interfacial phases. The scanning electron microscope was also equipped with energy-dispersive X-ray spectroscopy (EDS, Nogales, AZ, USA) analysis for further investigation.
A CS350H electrochemical workstation (Wuhan Corrtest Instruments, Wuhan, China) and three-electrode system were used for the electrochemical corrosion testing, as shown in Figure 3. The saturated calomel electrode was used as the reference electrode. The electrochemical corrosion sample was used as the working electrode, and the platinum electrode was used as the counter electrode. A neutral solution of 0.35 mol/L NaCl was used in the electrochemical corrosion. In the electrochemical corrosion test, except for the polished cross-section, the rest of the material was covered with epoxy resin, and only the cross-section of the sample was retained as the surface to be tested. The samples were immersed in the NaCl neutral solution for 30 min to obtain uniform corrosion. After the open-circuit potential (OCP) became stabilized, the potential dynamic polarization curve test scanned from negative potential to positive potential, and the applied potential varied in the range of −1.5 to +1 V. The scan speed and sampling frequency during the test were 2 mV/s and 2 Hz, respectively. The electrochemical impedance spectroscopy (EIS) was conducted when the open-circuit potential is relatively stable, with the amplitude of the sinusoidal voltage set to 10 mV and the frequency set to 1000 to 0.1 Hz.

3. Results and Discussion

3.1. Macroscopic Morphology and Interfacial Microstructure

Figure 4 shows the macroscopic morphology of welding joints with and without Al-Si coating. It was evident that the welding joints were continuous and well-formed, with no visible surface defects such as spatter or porosity. The width of the filler metal on both uncoated and Al-Si coated steel increased gradually with increasing laser power, indicating that the heat input improved the wetting and spreading of the filler metal.
Figure 5 shows the SEM analysis at the FZ/steel interface of the laser joints with and without Al-Si coating obtained with various laser powers. It was obvious that the reaction layer was generated at the interface under both conditions.
Figure 5a–c depicts the SEM analysis of the laser Al/uncoated steel joints. In the case of the uncoated steel, the morphology and phase composition of the reaction layer changed drastically from 1500 to 1900 W (refer to Figure 5a–c). At the low laser power of 1500 W, the reaction layer was very thin and discontinuous, and the edge of the reaction layer close to the steel base metal was relatively straight, while it was uneven near the FZ. This indicates that the reaction layer has grown from the steel side towards the FZ [26]. According to the EDS analysis in Table 3, the chemical composition was 0.82 at. % Fe, 48.89 at. % Al, 50.11 at. % Zn, and 0.18 at. % Si. These measurements indicated that existing phases were mainly Fe2Al5−xZnx. The Fe2Al5−xZnx was a kind of variation of Fe2Al5 which has been reported previously [27]. A thicker reaction layer was observed consisting of a continuous dark lamellar layer at 1700 W. According to EDS analysis in Table 3, the chemical composition was 23.10 at. % Fe, 70.24 at. % Al, 60.42 at. % Zn, and 0.24 at. % Si, which was determined to be Fe2Al5−xZnx [28,29]. Upon increasing the laser power to 1900 W, a continuous reaction layer was observed, which comprised a thicker dark layer with dispersed light-colored microstructure. The elemental composition and content of the two phases was 11.46 at. % Fe, 57.41 at. % Al, 31.04 at. % Zn, 0.09 at. % Si and 19.58 at. % Fe, 70.6 at. % Al, 9.62 at. % Zn, 0.21 at. % Si (Table 3). The existing phases were determined to be Fe2Al5−xZnx and FeZn10. Similar phase determinations have also been reported in a related investigation [30,31,32].
Figure 5d–f shows the SEM analysis of laser Al/Al-Si coated steel joints. At the low laser power of 1500 W, a relatively flat reaction layer at the interface was found, consisting of a dense dark phase as shown in Figure 5d. Through the process of analysis, the main element of the dark phase was Al, with some Fe and Si and a small amount of Zn, which was determined to be Fe2Al7Si (Table 3). In fact, Fe2Al7Si was the IMC phase that pre-existed in the Al-Si coating (Figure 1b), which indicated that the laser heat input was insufficient to melt the IMCs where only dissolution was expected to occur. At the medium laser power of 1700 W, a continuous reaction layer was also present. However, the difference was that a small amount of light-colored microstructure was within the dark reaction layer as shown in Figure 5e. According to the results of EDS, the dark phase consists mainly of iron, aluminum, and zinc elements, which were determined to be Fe2Al5−xZnx. By analyzing the elemental composition and content of the light-color phase, it was indicated that the phase mainly consisted of FeZn10. This suggests that the compounds in the coating melted and generated new IMCs. Upon increasing the laser power to 1900 W, a dark phase and a light-color dispersion phase were observed at the interface. In the local area, the light-color phase was observed, which was irregularly distributed in the dark phase (Figure 5f). Through the process of analysis of the EDS results, the dark phase and the light-color phase were determined to be Fe2Al5−xZnx and FeZn10, respectively (refer to Table 3). Therefore, it was concluded that the Al-Si coating was completely melted or dissolved in the laser powers from 1700 to 1900 W, where new phases were then generated at the interface.
Except for the phase composition, the morphological characteristics of the interfacial intermetallic compound (IMC) layer changed significantly. In order to better understand this, a quantitative analysis was conducted on the change of the interfacial reaction layer. The thickness of the IMCs in both cases with the laser power range from 1500 to 1900 W is shown in Table 4. Due to an elevated temperature, an intensified diffusion reaction occurred at the interface, which led to an increase in intermetallic compound thickness with the rise in laser power [33]. The intermetallic compound thickness in the joint of Al/coated steel was larger as compared to the joint of Al/uncoated steel (7.1 μm vs. 1.5 μm) at 1500 W. This was because the interfacial metallurgical reactions were relatively weak at low laser power, only generating a 1.5 μm thick IMC layer which was much thinner than the pre-existed 7 μm thick Fe2Al7Si layer within the Al-Si coating. The thickness of the IMCs of the Al/Al-Si coated steel joints was thicker than that of the Al/uncoated steel joints with the laser power rising from 1700 to 1900 W (Table 4). This was primarily attributed to the Al-Si coating of 22MnB5 steel acted as a thermal and diffusion barrier between the molten pool and steel substrate that retarded the growth of the reaction layer [7,11]. It was suggested that the Al-Si coating was able to effectively suppress the formation and growth of interfacial IMCs.

3.2. Corrosion Resistance

3.2.1. Open-Circuit Potential

In order to accurately measure the voltage sweep range of the specimen’s polarization curve, it is necessary to identify the open-circuit potential (OCP) and thus determine the voltage sweep range [17]. The open-circuit potential is the difference between the working electrode and the reference electrode without load, which is mainly related to the material and the corrosive solution. The change in the OCP reflects the gradual transformation of the electrode from an unstable to a stable state in the electrolyte solution.
Figure 6 shows the OCP curves of base materials and laser joints. The OCP of the Al base metal was −0.74 V, with slight fluctuations due to the generation and breakdown of the passivation film on the aluminum alloy surface during the corrosion process [34]. The OCP of the Al-Si coated steel was −0.65 V, which was significantly higher than the uncoated steel (−0.72 V). It was indicated that the Al-Si coating increased the potential and reduced corrosion tendency on the steel surface. Figure 6b shows the OCP curves of the laser joints at different laser powers. Certain fluctuations were observed in all the OCP curves, due to the corrosion dissolution of the interfacial IMC layer. With the increase in the laser power from 1500 to 1900 W, the steady-state OCPs of Al/uncoated steel joints were −0.849 V, −0.830 V, and −0.884 V. The steady-state OCPs of Al/Al-Si coated steel joints were −0.827 V, −0.832 V, and −0.860 V. As a whole, it was concluded that the Al/uncoated steel joint exhibits a slightly lower steady-state OCP in comparison to the Al/Al-Si coated steel joint at the same laser power.

3.2.2. Dynamic Potential Polarization

Figure 7 depicts a schematic diagram of the kinetic potential polarization curve of a typical laser Al/steel joint. The dynamic potential polarization curve consists of a cathodic semi-log polarization curve and an anodic semi-log polarization curve, where the intersection of the two polarization potential curves is the self-corrosion potential, Ecorr. Ecorr represents the stable corrosion state of the potential in the absence of an applied current; to some extent, it is also a thermodynamic parameter that reflects the corrosion tendency of the joint. The relatively small Ecorr indicates that the joint in the natural state is more prone to corrosion. The self-corrosion potential, Ecorr, only reflects the corrosion trend, and there is no certain relationship with the corrosion rate [35].
The Tafel extrapolation method was used to fit the strongly polarized region of the polarization curve (the position where the polarization potential deviates far enough from the self-corrosion potential) to obtain the intersection of two Tafel curves. The value of the horizontal coordinate corresponding to the intersection was lg(icorr), which is the self-corrosion current density. The self-corrosion current density icorr is typically used to indicate the corrosion rate in electrochemical corrosion. In the natural state, the corrosion rate of the joint increases as the icorr value becomes larger. The polarization potential as a function of the applied polarization current density was in accordance with the Tafel equation:
E − Ecorr = −balgicorr + balgia
Ecorr − E = −bclgicorr + bclgic
where, E is the polarization potential, Ecorr is the self-corrosion potential, icorr is the self-corrosion current density, ba, and bc are the anodic and cathodic Tafel slopes, and ia and ic are the anodic and cathodic corrosion current densities.
Figure 8 depicts the comparative analysis of the dynamic potential polarization curves of the laser Al/steel joints. The dynamic potential polarization curves of the joints with and without coating were both very smooth and followed a similar trend, i.e., initially, the current density decreased as the potential increased, but then rose again. To further investigate the influence of the coating on the corrosion resistance of the joints, the Tafel recursive method was used to calculate the self-corrosion current density and self-corrosion potential as shown in Table 5. It can be concluded that the self-corrosion potential of the Al/Steel laser joints varied little with the increase in laser power, but the self-corrosion current density gradually increased. At the same time, the addition of Al-Si coating decreases the self-corrosion current density of the Al joints. This indicated that the increase in laser power decreases the corrosion resistance of the joints and the Al-Si coating improves the corrosion resistance of the joints.
Figure 9 details the self-corrosion current density with the change in power for the Al/steel welded joints with or without coating. With the increased laser power, the self-corrosion current density of Al/uncoated steel were 2.02 × 10−5 A/cm2, 5.72 × 10−5 A/cm2, and 7.47 × 10−5 A/cm2, and the self-corrosion current density of Al/Al-Si coated steel were 3.35 × 10−5 A/cm2, 3.83 × 10−5 A/cm2, and 6.22 × 10−5 A/cm2. It was obvious that the self-corrosion current density of both joints increased with the increase in laser power, which further indicated that increasing the laser power was likely to reduce the corrosion resistance of the joints. By comparing the self-corrosion current density under the same power, opposite trends were observed for various laser powers. The self-corrosion current density of the laser Al/Al-Si coated steel joints were higher as compared to the joints of Al/uncoated steel (3.35 × 10−5 A/cm2 vs. 2.02 × 10−5 A/cm2) at 1500 W. On the contrary, at a laser power of 1700 and 1900 W, the self-corrosion current density of the Al/uncoated steel joints was greater as compared to the Al/Al-Si coated steel joints. Combining the effects of laser power and steel surface condition on the IMC thickness, there was evidence of a high correlation between the IMC thickness and self-corrosion current density in Figure 9. Therefore, according to the findings, interface intermetallic compound thickness and subsequently the corrosion properties of the laser joints were influenced by both the laser power and coating [36,37]. In a word, the thinner the IMC layer, the lower the self-corrosion current density, and the better joint corrosion performance.

3.2.3. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is usually used to characterize the charge transfer and substance transport processes during corrosion. Figure 10a shows the Nyquist plots of Al/Al-Si coated and Al/uncoated steel joints with laser power from 1500 to 1900 W. The radius of the semicircle decreased with increasing laser power in both cases of joints, which indicated a reduction in the corrosion resistance of the joints. At a laser power of 1500 W, the semicircle radius of Al/uncoated steel joints was larger than that of Al/Al-Si coated steel joints; the opposite was true at high laser power (1700–1900 W). It was indicated that Al/Al-Si coated steel joints possess a higher corrosion resistance compared with Al/uncoated steel joints at a high laser power. This was possibly related to interfacial intermetallic compound thickness. Nyquist plots of the Al/uncoated steel laser joints showed one induction loop at laser powers of 1500 and 1700 W. The inductive resistance at a low frequency was mainly attributed to the absorption and desorption of the material or the formation of corrosion pits on the electrode surface [38]. The curve peak in the Bode plots was normally used to determine relaxation time (Figure 10b). During corrosion, the relaxation time reflected the charge transfer from instability to equilibrium [39]. All samples tested in this experiment showed a shift in the characteristic minimum frequency toward a high frequency with increasing laser power, indicating that more time was needed for re-passivation, which suggested a decrease in the corrosion resistance. According to the above results, it can be summarized that the corrosion resistance of laser joints decreased with the increased laser power, and Al-Si coating improved the corrosion resistance of the joints especially at high laser powers.

3.2.4. Corrosion Morphology and Mechanism of Welded Joint

In order to further investigate the Al/steel welding joint corrosion mechanism, typical corrosion morphology at the interface of laser Al/coated steel joints at a laser power of 1900 W was observed (Figure 11). Figure 12 shows the schematic diagram of the corrosion mechanism of Al/steel welded joints in 3.5% NaCl solution. The corrosion pits were found on the steel surface, which was due to the reaction of Cl- with the hydroxide of Fe and the formation of complexes on the steel surface accelerating the pitting corrosion [40]. It can be seen that there is a darker color in the FZ and a brighter color of the joint steel side in Figure 11b. This indicates that the FZ corrodes more severely than the steel. This was because the galvanic corrosion occurred between the FZ and the steel, where the FZ with a lower corrosion potential exhibited a protective effect on the steel. At the same time, the FZ near the IMC layer experienced intense corrosion; this was because of the galvanic corrosion between the FZ and IMC layer. The IMC layer was a cathode which accelerated the corrosion of Al and Zn in the FZ. There was slight corrosion of steel adjacent to the IMC layer, which was due to the IMC layer as the anode by an oxidation reaction protecting the steel from corrosion [41]. During the corrosion process of the Al/steel joints, Zn and Al in FZ and IMC were dissolved in large quantities to generate Zn2+ and Al3+ cations, and slight corrosion occured in the surface of steel that generated Fe3+ cations, which react with OH− ions in the solution that generate products such as Al(OH)3, Zn(OH)2, and Fe(OH)3 [42].

4. Conclusions

In this study, interfacial microstructures were observed, and electrochemical corrosion tests were designed to analyze the correlation between Al-Si coatings and corrosion resistance of aluminum/steel joints. The major research conclusions were as follows:
(1) In the Al/uncoated steel joints, Fe2Al5−xZnx and FeZn10 were generated at a laser power of 1500 to 1900 W; while, in the Al/Al-Si coated steel joints, pre-existing Fe2Al7Si within the Al-Si coating was observed at 1500 W, but Fe2Al5−xZnx and FeZn10 were detected while the laser power was 1700–1900 W.
(2) The Al-Si coating that consisted of Al-Si eutectic structure and Fe2Al7Si significantly depressed the formation and growth of the interfacial IMC layer because of the dual effects of thermal and diffusion barriers.
(3) The electrochemical corrosion tests indicated that the resistance to corrosion in Al/steel joints was noticeably enhanced by the Al-Si coating, specifically within the laser power range of 1700 to 1900 W, but had opposite effects on the joint corrosion resistance at 1500 W, which was mainly dictated by the thickness of interfacial IMCs layer.
(4) The galvanic corrosion that occurred between the FZ and the steel aggravated the corrosion process, while the galvanic corrosion between the FZ and IMC layer, the IMC layer as a cathode, accelerated the corrosion of the FZ; on the other hand, the galvanic corrosion between the steel and IMC layer, the IMC layer as the anode, protected the steel from corrosion.

Author Contributions

Writing—original draft, L.W.; conceptualization, F.W. and Y.Z.; methodology, J.P.O.; software, W.X.; validation, M.X. and M.Z.; formal analysis, J.P.O., Y.Z. and L.W.; investigation, M.Z. and W.X.; resources, H.Z. and J.Y.; data curation, L.W. and H.Z.; writing—review and editing, J.Y., J.P.O., F.W. and L.W.; visualization, M.X. and L.W.; supervision, H.Z.; funding acquisition, J.P.O. and J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

The research undertaken in this study received funding from the National Natural Science Foundation of China (No. 52275155 and 52205395) and Class III Peak Discipline of Shanghai—Materials Science and Engineering. JPO acknowledges funding by national funds from FCT—Fundação para a Ciência e a Tecnologia, I.P.; in the scope of the project LA/P/0037/2020, UIDP/50025/2020, and UIDB/50025/2020 of the Associate Laboratory Institute of Nanostructures, Nanomodelling, and Nanofabrication—i3N.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Metals 14 00328 g001
Figure 1. Microstructure of 22MnB5 steel: (a) without coating; (b) with coating.
Figure 1. Microstructure of 22MnB5 steel: (a) without coating; (b) with coating.
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Figure 2. Schematic diagrams of laser welding–brazing of Al/steel.
Figure 2. Schematic diagrams of laser welding–brazing of Al/steel.
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Figure 3. Schematic diagram of electrochemical corrosion test.
Figure 3. Schematic diagram of electrochemical corrosion test.
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Figure 4. Macroscopic morphology of welding joints with the laser power from 1500 W to 1900 W: (a–c) Al/uncoated steel joints; (d–f) Al/Al-Si coated steel joints.
Figure 4. Macroscopic morphology of welding joints with the laser power from 1500 W to 1900 W: (a–c) Al/uncoated steel joints; (d–f) Al/Al-Si coated steel joints.
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Figure 5. SEM analysis at the FZ/steel interface in the Al/uncoated steel and Al/Al-Si coated steel joints with the laser power varying from 1500 to 1900 W: (a~c) Al/uncoated steel joints; (d~f) Al/Al-Si coated steel joints.
Figure 5. SEM analysis at the FZ/steel interface in the Al/uncoated steel and Al/Al-Si coated steel joints with the laser power varying from 1500 to 1900 W: (a~c) Al/uncoated steel joints; (d~f) Al/Al-Si coated steel joints.
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Figure 6. Open-circuit potential curves: (a) base metals; (b) Al/Al-Si coated and uncoated steel joints.
Figure 6. Open-circuit potential curves: (a) base metals; (b) Al/Al-Si coated and uncoated steel joints.
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Figure 7. Dynamic potential polarization curve of a typical laser Al/Al-Si coated steel joint.
Figure 7. Dynamic potential polarization curve of a typical laser Al/Al-Si coated steel joint.
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Figure 8. Potential dynamic polarization curves: (a) Al/uncoated steel joints; (b) Al/Al-Si coated steel joints.
Figure 8. Potential dynamic polarization curves: (a) Al/uncoated steel joints; (b) Al/Al-Si coated steel joints.
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Figure 9. Average IMC thickness and self-corrosion current density of joints with different laser powers.
Figure 9. Average IMC thickness and self-corrosion current density of joints with different laser powers.
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Figure 10. (a) Nyquist plots of laser Al/steel joints; (b) Bode plots of laser Al/steel joints.
Figure 10. (a) Nyquist plots of laser Al/steel joints; (b) Bode plots of laser Al/steel joints.
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Figure 11. Interface morphology of Al/coated steel joints in 1900 W: (a) before corrosion, (b) after corrosion.
Figure 11. Interface morphology of Al/coated steel joints in 1900 W: (a) before corrosion, (b) after corrosion.
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Figure 12. Schematic diagram of corrosion mechanism of Al/steel joints in 3.5% NaCl solution.
Figure 12. Schematic diagram of corrosion mechanism of Al/steel joints in 3.5% NaCl solution.
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Table 1. Chemical composition of base metals and filler metal (wt. %).
Table 1. Chemical composition of base metals and filler metal (wt. %).
MaterialsMgFeAlCrCuZnMnSiCB
22MnB5 steel-Bal.0.050.20--1.180.250.20.005
AA5052 Al2.2–2.80.4Bal.0.20.10.10.10.25--
Zn-Al22--22-0.8Bal.----
Table 2. Optimized parameters of laser welding–brazing.
Table 2. Optimized parameters of laser welding–brazing.
Laser Power
/W		Welding Speed
/m∙min−1		Wired Feed Speed
/m∙min−1		Defocused Amount
/mm		Shielding Gas Flow
/L∙min−1
15000.31.9+3515
1700
1900
Table 3. EDS results in selected regions in Figure 5.
Table 3. EDS results in selected regions in Figure 5.
PointsElements (at. %)Possible Phases
AlFeZnSi
148.890.8250.110.18Fe2Al5−xZnx
270.2423.106.420.24Fe2Al5−xZnx
357.4111.4631.040.09Fe2Al5−xZnx + FeZn10
470.619.589.620.21Fe2Al5−xZnx
568.7517.001.9512.30Fe2Al7Si
654.8116.2626.012.92Fe2Al5−xZnx + FeZn10
763.6719.9410.485.92Fe2Al5−xZnx
867.9220.687.234.16Fe2Al5−xZnx
916.0111.7471.890.35Fe2Al5−xZnx + FeZn10
Table 4. The thickness of IMC of the laser joints with a different laser power in both cases (μm).
Table 4. The thickness of IMC of the laser joints with a different laser power in both cases (μm).
Cases1500 W1700 W1900 W
Al/Al-Si coated steel joints7.18.210.3
Al/uncoated steel joints1.511.116.6
Table 5. Self-corrosion potential and self-corrosion current density of Al/Steel joints.
Table 5. Self-corrosion potential and self-corrosion current density of Al/Steel joints.
Laser Power
(W)		Corrosion Current Density
(10−5 A/cm2)		Corrosion Potential
(V)
Uncoated SteelCoated SteelUncoated SteelCoated Steel
1500 W2.02353.3538−0.945−0.9683
1700 W5.72343.8333−0.9512−0.952
1900 W7.47736.2223−0.9637−0.9854
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Wu, L.; Oliveira, J.P.; Yang, J.; Xiao, M.; Zheng, M.; Xu, W.; Zhao, Y.; Wang, F.; Zhang, H. Effect of Al-Si Coating on the Interfacial Microstructure and Corrosion Resistance of Dissimilar Laser Al Alloy/22MnB5 Steel Joints. Metals 2024, 14, 328. https://doi.org/10.3390/met14030328

AMA Style

Wu L, Oliveira JP, Yang J, Xiao M, Zheng M, Xu W, Zhao Y, Wang F, Zhang H. Effect of Al-Si Coating on the Interfacial Microstructure and Corrosion Resistance of Dissimilar Laser Al Alloy/22MnB5 Steel Joints. Metals. 2024; 14(3):328. https://doi.org/10.3390/met14030328

Chicago/Turabian Style

Wu, Lingqing, Joao Pedro Oliveira, Jin Yang, Ming Xiao, Min Zheng, Wenhu Xu, Yixuan Zhao, Feifan Wang, and Hua Zhang. 2024. "Effect of Al-Si Coating on the Interfacial Microstructure and Corrosion Resistance of Dissimilar Laser Al Alloy/22MnB5 Steel Joints" Metals 14, no. 3: 328. https://doi.org/10.3390/met14030328

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