electrochemical properties of friction stir butt welded joint of dissimilar aluminum and steel alloys

Materials Characterization 154 (2019) 67–79

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Microstructure and mechanical/electrochemical properties of friction stir butt welded joint of dissimilar aluminum and steel alloys ⁎

Sam Yaw Anamana, Hoon-Hwe Choa, , Hrishikesh Dasb, Jong-Sook Leec, Sung-Tae Hongb,

T

⁎

a

Department of Materials Science and Engineering, Hanbat National University, 125 Dongseodae-ro, Yuseong-Gu, Daejeon, Republic of Korea School of Mechanical Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea c School of Materials Science and Engineering, Chonnam National University, Gwangju 61186, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: Friction stir welding Dissimilar alloys Intermetallic compound Microstructure characterization Corrosion resistance

The microstructure and mechanical/electrochemical properties of friction stir welded (FSWed) joints of two dissimilar alloys, 5052-H32 aluminum and dual-phase (DP) steel, are studied. The FSW joint is fabricated with an offset of the FSW tool towards the DP steel on the advancing side of the tool. In the stir zone (SZ), three distinct regions are observed: i) a top layer consisting of the aluminum matrix with scattered steel fragments, ii) a middle layer having a mixed lamellar structure of FeeAl solid solutions and intermetallic compounds (IMCs), and iii) a bottom layer consisting of the steel. IMCs are observed in the middle layer and at the interfaces between the aluminum and steel alloys. Very different hardness profiles are obtained in the three layers in the SZ. The electrochemical corrosion investigation reveals that the FSW joint exhibits a higher corrosion rate compared to the base materials due to the scattered steel fragments and the increase in martensite content and low-angle grain boundaries (LAGBs).

1. Introduction Conventional fusion welding methods may cause a large amount of brittle intermetallic compounds (IMCs) when joining dissimilar aluminum and steel alloys due to the large heat input [1,2] and the significantly different mechanical properties and melting temperatures of the materials [3–5]. Also, joining dissimilar alloys by conventional fusion welding often requires expensive equipment, such as laser welders and ultrasonic welders [6–9]. Recently, friction stir welding (FSW) has become a viable alternative to overcome these difficulties by employing its various technical advantages in the joining of dissimilar alloys [10,11]. FSW is a solid-state joining process that was invented by The Welding Institute in 1991 and uses severe plastic deformation without melting [12]. This process involves a rotating and traversing tool with a shoulder that creates frictional heat and a pin that goes into the material and plasticizes it during the process [13,14]. FSW has a variety of advantages compared to conventional fusion welding processes: a lower heat input (thereby preventing melting and solidification) [15], the ability to join dissimilar metals of any plate thickness with different joint configurations [16], relatively straightforward and cost-effective welding tools [17] and the production of decreased residual stresses and distortions [18].

⁎

Several researchers have studied the application of FSW to aluminum and steel alloys in terms of the IMCs. Liu et al. [19] investigated the effect of process parameters on the formation of IMCs during the FSW of 6061-T6 aluminum alloy and TRIP 780/800 steel alloys. They concluded that an increased rotational speed and a larger tool offset could increase the overall temperature of the weld, thus leading to the formation of IMCs, whereas the welding speed did not have a significant effect on the IMC formation. Pourali et al. [20] reported Fe-rich IMCs with a thickness of approximately 100 μm in the joint interface in friction stir welded 1100 aluminum alloy and St 37 low carbon steel. According to Pourali et al. [20], the Fe-rich IMCs were not detrimental to the joint strength. Instead, defects, such as voids, were more responsible for the decrease in the joint strength. Coelho et al. [21] studied two different high-strength steels (dual-phase (DP) 600 and HC 260LA) that were friction stir welded to a 6181-T4 aluminum alloy. They concluded that a certain offset value towards the aluminum side resulted in crack-free bonding, thus leading to the joint interface consisting of very fine ferrite grains and thin strips of Fe2Al5 IMC. Watanabe et al. [16] reported that an offset of 0.2 mm towards the steel side produced a sound joint after joining a 5083-O aluminum alloy to a SS 400 mild steel. They also demonstrated that a larger offset towards the steel side distributed steel fragments randomly in the aluminum matrix. Wan et al. [22] also reported that with a decreased welding speed and

Corresponding authors. E-mail addresses: [email protected] (H.-H. Cho), [email protected] (S.-T. Hong).

https://doi.org/10.1016/j.matchar.2019.05.041 Received 4 April 2019; Received in revised form 28 May 2019; Accepted 28 May 2019 Available online 30 May 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.

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Table 1 Chemical composition of DP 1200 steel and 5052 – H32 aluminum alloys (in wt%).

DP 1200 Al 5052 - H32

C

Si

Mn

Cr

Mo

Cu

Nb

Ti

V

Mg

Al

Fe

0.12 –

1.42 0.25

2.07 0.10

0.02 0.15

0.004 –

0.009 0.10

0.02 –

0.004 –

0.005 –

– 2.4

– Bal

Bal 0.40

Fig. 1. Schematic diagram of the FSW process. WD, TD, and ND indicate the welding, transverse and normal directions of the FSW process, respectively.

dissimilar joints. According to Sarvghad-Moghaddam et al. [24], the IMCs formed in the FSW joint of 3003 aluminum alloy and pure copper can be more susceptible to corrosion crack initiation. However, Akinlabi et al. [25] reported that the corrosion rate of the joint was lower compared to that of the Cu base metal due to the presence of the Al2Cu IMC phase when they joined a 5754 aluminum alloy and pure copper. Dissimilar FSW joints of 5083-O and 6082-T6 aluminum alloys also showed similar characteristics [26]. In Al/steel FSW joints, Thomä et al. [27] reported that no galvanic corrosion and low corrosion current densities were observed in EN AW-6061 aluminum and DC04 steel joints. In the present study, we aim to comprehensively investigate the FSW joint of aluminum (5052-H32) and steel (DP 1200) alloys in a butt joint configuration with a tool offset of 0.5 mm towards the steel side. We investigate the resulting microstructure by applying optical microscopy (OM) and electron back-scatter diffraction (EBSD) equipped with field emission scanning electron microscope (FE-SEM). We also report on how the IMCs developed in the SZ and how the weld joint interface affects the strength of the joint based on the micro-hardness tests across the weld cross-section Finally, the corrosion behavior is comprehensively investigated in the dissimilar Al/steel joint.

Table 2 FSW process parameters. Welding parameter

Value

Welding speed Rotational speed Plunging depth Tool offset Tilt angle

75 mm/min 2000 rpm 0.9 mm 0.5 mm 2°

Table 3 Dimension and geometry of the FSW tool. Tool geometry

a

Material Shoulder diameter (mm) Pin height (mm) Pin diametera (mm) Shoulder type

WC 14.3 0.6 2 Convex scrolled shoulder

Measured at the root of the pin.

an increased tool rotation speed, a chaotic mixed layered structure of aluminum- and steel-rich layers and also IMCs were formed in the stir zone (SZ) of a 6082-T6 aluminum and a Q 235A steel joint. A more significant issue associated with the joining of aluminum and steel alloys is the different corrosion rate between the FSW region and the base materials (BMs). Active or less noble metals tend to corrode by leaving the more noble metals intact at the joint [23]. Several researchers have recently studied the corrosion behavior of FSW

2. Experimental method 2.1. Materials As-rolled 5052-H32 aluminum and DP 1200 steel alloy sheets

Fig. 2. Schematic diagram showing geometry of samples and exposed areas of the top surface for the electrochemical corrosion measurements. 68

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Fig. 3. Microstructural features of the aluminum BM: (a) WD-IPF map, (b) {1 1 1} pole figure and (c) misorientation-angle distribution (the blue line represents Mackenzie random distribution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

applied using argon (Ar) to minimize surface oxidation. The principal directions of the FSW geometry are denoted as the welding direction (WD), transverse direction (TD), and normal direction (ND), as shown in Fig. 1. The offset used in this welding process was 0.5 mm towards the steel side, indicating that the tool pin was plunged into the steel side at a distance of 0.5 mm from the faying surface of the steel sheet.

(thickness of 1.2 mm) were cut into rectangular shaped samples with a length of 150 mm and a width of 50 mm. Their chemical compositions are listed in Table 1.

2.2. FSW procedure The 5052-H32 aluminum and DP steel samples were butt-joined along the length by FSW using a custom made FSW machine (RM-1, TTI, USA) as schematically shown in Fig. 1. The DP steel sheet was placed on the advancing side (AS), and the aluminum alloy sheet was placed on the retreating side (RS) of the tool travel direction. The welding was performed using the process parameters listed in Table 2, which were selected based on a separately conducted preliminary study. A spark plasma sintered tungsten carbide (WC) tool with a convex scrolled shoulder was used. Detailed information about the tool is presented in Table 3. During FSW, an inert gas atmosphere was

2.3. Microstructure assessment Samples for microstructural analysis were prepared according to the ASTM-E3 standard [28]. The samples were prepared perpendicular to the WD with dimensions of 10 mm (height) and 30 mm (width). They were carefully ground using 240, 600, 800, and 1200 grit papers and then polished using a 1 μm diamond paste suspension to produce the final polished surface. The samples for OM (GX41, Olympus, Japan) were etched using a 3 ml nitric acid +97 ml ethanol solution and a 1 ml 69

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Fig. 4. Microstructural features of the steel BM: (a) WD-IPF map, (b) {1 1 0} pole figure and (c) misorientation-angle distribution (the blue line represents Mackenzie random distribution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Optical macrostructure of the weld cross-section after etching. See text for details.

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Fig. 6. (a) SEM (BSE) image and (b) EDS chemical map of the SZ marked by the blue dashed rectangle in Fig. 5, (c) top, (d) middle and (e) bottom layers at higher magnifications. Steel fragment was identified in the top layer by EDS point scan with results presented in Table 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.4. Hardness test

Table 4 Composition of the steel fragment in the aluminum matrix in Fig. 6(c). Element

wt%

at.%

Al Si Mn Fe Total

1.90 1.55 2.11 94.44 100.00

3.80 2.97 2.07 91.16 100.00

A Vickers hardness test was conducted on a sample cross-section that was perpendicular to the WD using a hardness tester (JP/MMT-7, Matsuzawa, Japan). The hardness profiles were measured at the top, middle, and bottom regions of the weld cross-section in a 3 × 22 array at intervals of 1 mm horizontally and 0.20 mm vertically. The applied load, dwell time, and maximum depth were 0.98 N, 15 s, and 70 μm, respectively.

hydrofluoric acid +199 ml water solution to observe the steel and aluminum microstructures, respectively. Due to the intricate nature of the SZ, ion milling was applied to the samples for SEM and EBSD analysis using FE-SEM (SU500, Hitachi, Japan). Energy dispersive X-ray spectroscopy (EDS) analysis was also performed to measure the composition of the IMCs and identify steel fragments in the Al matrix. The operating conditions for the FE-SEM were as follows: accelerating voltage of 20 eV, probe current of 14 nA, tilt angle of 70°, and working distance of 20 mm. A mapping grid with a step size of 10 nm and a 4 × 4 bins were used. The range for the low-angle grain boundaries (LAGBs) was set between 2° and 15°, while the limit for the high-angle grain boundaries (HAGBs) was set at angles greater than 15°.

2.5. Electrochemical corrosion test Electrochemical experiments were conducted based on ASTM-3 and ASTM-5 standards to investigate the corrosion behavior resulting from the coupling of the two dissimilar alloys [29,30]. During the test, we compared the corrosion behavior of the FSWed Al/steel joint to that of the BMs. Three samples with dimensions of 20 mm × 20 mm × 1.2 mm were prepared, and corrosion measurements were performed on the exposed areas (approximately 1 cm2) on the top surfaces of the samples, as described in Fig. 2. The experiments were carried out in a three-electrode flat cell containing 1000 ml of 3.5% NaCl solution at room temperature (25 ± 1 ° C) with a Luggin capillary. The three samples were each used as a working electrode (in three separate tests), a platinum mesh was used as a counter electrode, and a saturated calomel electrode (SCE) 71

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Fig. 7. Microstructural features of region I marked at the top layer of the SZ in Fig. 5: (a) WD-IPF map, (b) {1 1 0} pole figure indicating shear texture components (marked by red triangles) and (c) misorientation-angle distribution (the blue line represents Mackenzie random distribution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion

was used as a reference electrode. Prior to the start of the experiment, the samples were exposed to the fresh electrolyte for 60 min to measure the open circuit potentials (OCP). After 60 min, the potentials of the test specimens reached a stable state in the solutions, and the anodic polarization tests began with a scanning potential rate of 0.167 mV/s. The potential was scanned from an initial voltage of – 0.2 V below the OCP to a final voltage of +1.00 V versus SCE. A potentiostat (SP-240, Bio-Logic, France) was used for the potentiodynamic polarization tests. During the anodic polarization tests, the applied potential was anodic (more positive direction), thus causing the samples (as working electrodes) to become the anode and lose electrons. The current response was plotted as a function of the applied potential on a semi-log graph to obtain potentiodynamic polarization curves [30]. From this graph, the corrosion behaviors of the samples were assessed.

3.1. Base materials The microstructural features, which include the inverse pole figure (IPF) maps, pole figures (PF) and misorientation-angle distribution charts of both BMs, are shown in Figs. 3 and 4. The aluminum BM has an average grain size of approximately 15.51 μm. It has a relatively high amount of LAGBs, indicating a high density of dislocations in the microstructure due to the initial cold rolling process [31]. The PF exhibits a strong {1 1 1} rolling texture in the aluminum BM [32]. The microstructure of the DP steel BM consists of a soft ferrite phase and islands of hard martensite particles [33] with an average grain size of approximately 4.54 μm. The DP steel BM is characterized by many HAGBs and exhibits a weak {1 1 0} rolling texture [34]. 72

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3.2. FSWed region 3.2.1. Stir zone Fig. 5 shows a cross-sectional macrograph of the FSWed zone after chemical etching. The FSW process with a tool offset of 0.5 mm to the steel side generated a SZ consisting of top, middle, and bottom layers due to severe stirring action of the tool and the heat (due to friction and plastic deformation of the material) [35]. The SZ shows a basin-like shape that widens considerably towards the upper surface. The SEM backscattered electron (BSE) image of the SZ that is marked by the bluedashed rectangle in Fig. 5 shows the distinct features of the three layers in Fig. 6(a). Also, the chemical composition map of the same region illustrates the distribution of aluminum and iron in the SZ. In the following sections, the three layers formed in the SZ are discussed in detail. 3.2.1.1. Top layer. During the FSW process, the upper region of the specimen gets rubbed by the tool shoulder, leading to a higher temperature compared to the other regions, thereby making the aluminum metal soften and settle at the top of the SZ as a matrix form, which is in accordance with literature [21,36]. Furthermore, the 0.5 mm offset towards the steel is large enough to remove the steel fragments, which get distributed randomly in the aluminum matrix by the stirring of the tool, as shown in the SEM (BSE) image (Fig. 6(c)). The composition of the steel fragments was measured by an EDS point scan and is listed in Table 4. The orientation map in Fig. 7(a), which was measured in region I in Fig. 5, shows that the average grain size of the aluminum matrix is approximately 14.6 μm, even though only a few grains exist. The steel fragments have an average size of 0.23 μm with many HAGBs, as shown in Fig. 7(c). Further analysis of the steel fragments in Fig. 7(b) indicates a {1 1 0} PF close to that of a shear texture component [37], which strongly suggests that the steel fragments underwent a severe shear deformation during the FSW process [38].

Fig. 8. Variations of Al and Fe elements in the mixed layer with an EDS line scan where the green line is marked in the inset as illustrated in the upper left corner. Composition of the IMCs are measured using EDS point scan in each point (displayed as numbers) of the inset and listed in Table 5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 5 Composition of IMCs in the mixed layer identified by EDS point scan in Fig. 8. Points

Fe (at.%)

Al (at.%)

Possible phase

Thickness (μm)

1 2 3 4

37.44 34.38 47.33 51.15

62.56 65.62 52.67 48.85

Fe2Al3 FeAl2 FeAl FeAl

4.1 4.9 3.2 9.4

3.2.1.2. Middle layer. The middle layer of the SZ has a maximum thickness of approximately 0.23 mm and is formed just below the aluminum matrix, as shown in the SEM (BSE) image in Fig. 6(a). As shown in Fig. 6(d), the middle layer appears to comprise a layered structure, which is a result of the stirring and mixing by the rotating tool during FSW. More specifically, it is speculated that the mixed layer is induced by the mixing of the base metals plasticized by frictional heat, which enhances the atomic diffusion and leads to the growth of the layer [18]. The IMCs tend to form in the mixed layer, which could be detrimental to the joint strength [21,22,39]. Variations in aluminum and iron content in the mixed layer were measured by an EDS line scan along the green line marked in the inset at the upper left corner, as shown in Fig. 8. This variation indicates the diffusion of the aluminum and iron in the mixed layer. The increase in the intensity of the aluminum peaks and the overlapping of the elements between the black dashed lines indicate the possibility of FeeAl IMC formation. To confirm the composition of the IMCs between the black dashed lines, each point (displayed as numbers) of the inset was analyzed by an EDS point scan, as listed in Table 5. From the results, it is confirmed that the mixed layer comprises an FeeAl solid solution and a chaotic mix of thick IMCs, such as Fe2Al3, FeAl2, and FeAl (Table 5). As reported by Rathod et al. [40], IMCs form in stages. First, a supersaturated solid solution of Fe and Al is formed by atomic diffusion. The Fe and Al atoms migrate to certain regions, and then they finally transform into IMCs as they reach a sufficient level. Furthermore, the stirring action of the tool can cause a hook-like structure to form in the RS of the SZ (region II in Fig. 5). Fig. 9 shows the variation of the aluminum and iron in the hooklike structure. It is confirmed that there is not a mixed layer in region II. Instead, the FeeAl solid solutions with high amounts of aluminum exist, and increase with distance. The results of the EDS point scans also confirm an aluminum-rich interface (60.3 wt% Al and 39.7 wt% Fe) and

Fig. 9. Variations of Al and Fe elements in the hook-like structure in region II of Fig. 5. EDS point scan confirms an aluminum-rich interface.

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Fig. 10. Microstructural features of region III marked at the bottom layer of the SZ in Fig. 5: (a) WD-IPF map, (b) {1 1 0} pole figure and (c) misorientation-angle distribution (the blue line represents Mackenzie random distribution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

phase [17,44]. The {1 1 0} PF shows a random texture due to the phase transformation during the FSW process (Fig. 10(b)). Furthermore, the martensite content and LAGBs noticeably increase. The martensite content in the steel BM is approximately 73%, whereas 88% is present in the deformed steel, indicating a ~15% increase. Each martensite volume fraction was calculated from image quality (IQ) maps by using indexed EBSD patterns [45]. Also, the length of the LAGBs in the steel BM is 5.02 mm, while that in the deformed steel is 7.30 mm, thus indicating a highly deformed structure in the bottom layer. As shown in the results, the FSW process introduced a significant amount of LAGBs and martensite into the microstructure of the bottom layer of the SZ. At the AS of the SZ, there is no visible mixed layer at the interface between the top and bottom layers. Variations of aluminum and iron in the area marked by the black chained rectangle in Fig. 5, were measured by an EDS line scan along the white dotted line, as

do not provide evidence of IMC formation. 3.2.1.3. Bottom layer. Although the estimated peak temperature is approximately 600 °C [41], it is possible that the temperature in the SZ during the FSW process could be higher than the intercritical temperature of the DP steel alloy. Based on the Trzaska and Park equations, the A1 and A3 temperatures of DP steel were estimated to be ~730 °C and ~897 °C, respectively [42]. Thus, a lath martensitic transformation [8] could occur with a small amount of ferrite grains possibly nucleating from the residual austenite phase during cooling [43]. Based on the orientation map in Fig. 10(a), which was measured in region III in Fig. 5, the measured average grain size is 3.07 μm, which is smaller than that of the steel BM. Smaller grains can be developed from discontinuous dynamic recrystallization in the austenitic phase due to the large plastic strain and lower stacking fault energy of the 74

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Fig. 11. Variations of Al and Fe elements along the white dotted line in the inset from the top layer to the bottom layer at the AS of the SZ marked by the black chained rectangle in Fig. 5. IMCs are measured at the interface between the black dashed lines by EDS point scans shown in the SEM (BSE) image at a higher magnification.

Fig. 12. Variations of Al and Fe elements at the joint interface measured from the SZ to the Al alloy marked by the red dashed rectangle in Fig. 5. IMCs are measured at the interface between the black dashed lines by EDS point scans shown in the SEM (BSE) image at a higher magnification. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 12 show that the majority of IMCs in this region can be FeAl3. Point A has an atomic composition of 76.83% Al and 23.17% Fe, while point B has an atomic composition of 77.48% Al and 22.52% Fe. It is speculated that the interface between the SZ and the aluminum BM experienced a lower temperature compared to the SZ due to the distance from the SZ and the high thermal conductivity of the aluminum BM. Aluminum-rich IMCs, such as Fe2Al3, FeAl2, Fe2Al5, and FeAl3, are reported to form at relatively low temperatures [20,46,47]. The brittle aluminum-rich IMC (FeAl3) can reduce the strength of the FSW joint [22,48].

shown in the inset of Fig. 11. The variations reveal that one of IMCs could be developed at the interface between the top and bottom layers (marked by black dashed lines). The SEM (BSE) image at a higher magnification in Fig. 11 shows two EDS point scans labeled X and Y, with atomic compositions of 73.7% Al and 26.3% Fe and 73.7% Al and 26.3% Fe, respectively. They indicate the presence of Fe2Al5 as one of the possible IMC phases at the interface.

3.3. Stir zone/aluminum base metal interface The variations of the aluminum and iron across the interface of the SZ and the aluminum BM, marked by a red dashed rectangle in Fig. 5, clearly show that the aluminum increases while the iron decreases with distance across the interface (between the black short dashed lines with an approximate maximum thickness of 4 μm), as shown in Fig. 12. This suggests that aluminum-rich IMCs could develop at the interface between the SZ and the aluminum BM. The results of two EDS point scans (labeled A and B) in the SEM (BSE) image at a higher magnification in

3.4. Microhardness assessment As shown in the hardness profiles across the welded sample obtained from the Vickers hardness test (Fig. 13), the DP steel BM and aluminum BM have average hardness values of 376 HV and 63 HV, respectively. The top layer of the SZ where the Al matrix formed has an average hardness value of 124 HV, which can be attributed to the steel 75

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Fig. 13. (a) Optical macrograph of the weld cross-section showing indentation marks across the top, middle and bottom regions with a uniform vertical spacing, d of 0.2 mm, (b) hardness profile in each line in Fig. 13 (a) across the welded sample.

were measured after 60 min at room temperature. The OCP of the DP steel BM decreases from an initial potential of – 567 mV to – 682 mV at 31 min. This potential is relatively stable for the rest of the measurement, and the final OCP is – 684 mV. The OCP of the Al BM sharply decreases from – 930 mV to – 1112 mV at 2.5 min. However, the potential quickly increases to a final OCP of – 807 mV. Considering the nature of the SZ of the FSWed sample, the top and bottom surfaces of the sample were tested independently to assess their corrosion behaviors. The top and bottom surfaces of the FSW sample in Fig. 14(a) are denoted as FSW_1 and FSW_2, respectively. The top surface of the FSWed sample has a similar pattern as that of the Al BM; its initial OCP of – 761 mV decreases substantially to – 900 mV at 3 min. It increases to – 834 mV at 8.2 min and increases slowly to the maximum potential of – 773 mV. In contrast, the bottom surface of the FSWed sample has an initial OCP of – 854 mV and quickly decreases to – 866 mV at 21 s. Between the 6 min and 10 min, it increases and decreases. Finally, it steadily increases to – 803 mV at 31 min and maintains this value until the end of the measurement. The OCP curves of the Al BM and the top surface of the FSWed sample (FSW_1) show almost similar nobilities with a potential difference of 34 mV. The OCPs of both samples decrease initially followed by an increase in the corrosion potential, and this cathodic behavior

fragments evenly distributed in the matrix. The steel fragments act as reinforcements in the Al matrix, thus achieving a relatively high hardness compared to the aluminum BM [16,49]. Furthermore, the middle layer has an average hardness of 549 HV due to the FeeAl solid solutions and IMCs in the mixed layer, thus making it the hardest part across the cross-section of the sample. Also, the bottom layer of the SZ, which consists of the deformed steel, has a hardness value of 383 HV. This increase can be attributed to the relative increase in the martensite content and deformation during the FSW process. A thermo-mechanically affected zone (TMAZ) developed at both the AS and RS of the SZ (7–8 mm from the weld center), as shown in Fig. 13(a). In particular, a TMAZ developed in the RS at the left region of the weld joint interface (Fig. 12) and is approximately 150 μm wide, as shown in Fig. 5. It also consists of steel; however, it is not substantially deformed. The TMAZ has an average hardness of 283 HV, which is lower than that in the SZ.

3.5. Electrochemical corrosion behavior 3.5.1. OCP versus time behavior The OCP measurements (Fig. 14(a)) were conducted to determine the electrochemical differences between the samples and to assess the free corrosion behaviors of the samples in a neutral solution. The OCPs 76

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steel; hence, it was expected to act anodically, like the DP steel BM. However, its corrosion behavior suggests cathodic behavior. This implies that corrosion products form whenever the OCP decreases, and the formation of corrosion products retards the migration of ions on the surface of the specimen, thus causing the potential to increase [53]. The risk of corrosion caused by the galvanic effect can be determined by the OCP differences between the materials [27]. The potential difference between the DP steel BM and the top surface of the FSWed sample is 89 mV, which is higher than that of the Al BM and the top surface of the FSWed sample (34 mV). Similarly, a potential difference of 123 mV between the DP steel BM and the bottom surface of the FSWed sample is higher compared to that between the Al BM and the bottom surface of the FSWed sample (4 mV). This indicates that the initiation of galvanic corrosion is possible in the coupled/welded region between the DP steel BM and the FSWed sample, especially at the bottom surface of the joint. 3.5.2. Potentiodynamic polarization measurements As mentioned previously, the corrosion behaviors of the samples in the corrosive media were assessed by monitoring the corrosion potentials (Ecorr) and current densities (icorr). The icorr values were determined from the measured Ecorr values by Tafel extrapolation from the obtained polarization curves (Fig. 14(b)) [54]. From the ASTM-102 standard [55], we were able to calculate the various corrosion rates in millimeters per year (mmpy) using the equation below. The results are listed in Table 6:

Corrosion rate (mmpy) = K

i corr EW ρ

(1)

where icorr is the corrosion current density in μA/cm , ρ is the density of the sample in g/cm3, EW is the equivalent weight and K is a constant with a value of 3.27 × 10–3 mm g/μA cm yr. The EW is dimensionless and is described as the mass of a metal that is oxidized during the corrosion process as the metal loses electrons. The EW can be calculated using the atomic weight, valence electrons and the mass fraction of the element in the alloy [55]. From Table 6, it is clear that the Al BM has the lowest corrosion rate, followed by the top surface of the FSWed sample, the DP steel sample and finally the bottom surface of the FSWed sample. The Al BM has the highest resistance to corrosion and it forms a continuous and uniform natural passive oxide layer during corrosion, which protects it against corrosion attack [56]. On the other hand, the DP steel BM has a relatively high corrosion rate, which is similar to the report by Sarkar et al. [52]. The phases of ferrite and martensite in the microstructure act as microgalvanic corrosion cells, and the corrosion rate increases depending on the amount of these cells. The top surface of the FSWed sample has a corrosion rate slightly higher than that of the Al BM. This higher rate can be attributed to the high amount of IMCs formed just below it and the steel particles randomly distributed in the Al matrix. Similar to the report by Thomä et al. [27], the steel fragments and FeeAl IMCs increase the corrosion rates in the weld zone. They form galvanic cells with the Al matrix due to the potential difference, thus causing a reduction in the corrosion resistance of the Al matrix [57]. The bottom surface of the FSWed sample experienced the highest corrosion rate of all the samples considered in this study. The high corrosion rate of 0.0346 mm/yr can be attributed to the increase in martensite content that occurred in the bottom layer of the SZ during the FSW process. As mentioned earlier, a ~15% increase in the martensite content was recorded in the bottom layer of the SZ as compared to the steel BM. Martensite content has a negative impact on the corrosion behavior of DP steels due to the residual stress from the distorted martensite structure and the microgalvanic corrosion cells that are formed between the martensite and ferrite phases in the microstructure [58]. An increase in the martensite content implies an increase in the cathode-anode ratio where martensite acts as the cathode and ferrite 2

Fig. 14. (a) OCP and (b) electrochemical potentiodynamic polarization curves of the samples in 3.5% NaCl solution at room temperature, with the measured Ecorr and icorr presented in Table 6. (The top and bottom surfaces of the FSWed sample are represented by FSW_1 and FSW_2, respectively). Table 6 Corrosion current potentials and current densities measured from potentiodynamic anodic polarization curves in Fig. 14(b), and the corresponding corrosion rates. Sample

Ecorr (mV/SCE)

icorr (μA/cm2)

Corrosion rate (mm/year)

Al alloy FSW_1 DP Steel FSW_2

−693.8 −743.8 −647.5 −777.8

0.146 0.301 9.924 3.176

0.0001 0.0033 0.0103 0.0346

indicates the growth of passive layers on their surfaces [50,51]. The DP steel BM exhibits anodic behavior with a decrease in the corrosion potential in the solution after 1 h. A similar characteristic of DP steels in 3.5% NaCl solution was reported by Sarkar et al. [52]. The OCP of the bottom surface of the FSWed sample (FSW_2) shows an increase after every decrease in potential. The FSW_2 sample is made of deformed DP 77

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acts as the anode [52]. This is similar to Salamci et al. [59], who reported an increase in corrosion rate as the martensite volume fraction increased when they investigated the effect of microstructure on corrosion of DP steels. Also, a grain size reduction of 4.54 μm in the DP steel BM to 3.07 μm in the deformed steel affects the corrosion rate. A finer microstructure implies more interfaces, which increases the interconnectivities of the ferrite and martensite phases [60–62]. This leads to an increased number of microgalvanic cells that increase the corrosion rate.

[4]

[5] [6]

[7]

4. Conclusion [8]

In the present study, 5052-H32 aluminum and DP1200 steel alloy sheets were joined successfully by FSW with an offset of 0.5 mm towards the steel side. Three distinctively different layers were observed in the SZ. The top layer was an Al matrix reinforced with steel fragments randomly scattered within the Al matrix, which caused a relative increase in hardness of the top layer compared to the aluminum BM. However, the steel fragments decreased the corrosion resistance of the layer. The middle layer was a complex mixed layer of FeeAl solid solutions and couple of IMCs, such as Fe2Al3, FeAl2, and FeAl. It had a higher hardness than any other FSWed region. The bottom layer comprised of deformed steel. It had a higher hardness compared to the steel BM due to the grain reduction and increase in the martensite content. However, galvanic corrosion can be initiated by these microstructural characteristics. There must be an acceptable balance between the mechanical properties and corrosion resistance when joining aluminum and steel by FSW. The major IMCs at the weld joint interface and the interface between the top and bottom layers might be identified as FeAl3 and Fe2Al5, respectively. These IMCs were brittle Al-rich IMCs that could have adverse effects on the joint strength. Further corrosion investigations revealed that the region between the steel BM and the SZ might be susceptible to galvanic corrosion initiation. Also, electrochemical corrosion tests showed that the aluminum BM exhibits the lowest corrosion rate, followed by the top surface of the FSWed sample, the steel BM and then the bottom surface of the FSWed sample. A galvanic corrosion test is recommended in future studies to further assess the galvanic effects between the FSWed sample, steel BM, and aluminum BM.

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Data availability

[19]

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Acknowledgement [21]

This study was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (MSIT) (No. NRF-2018R1A5A1025224 and NRF-2015R1A5A1037627). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A3B03028386). Furthermore, this research was supported by the research fund of Hanbat National University in 2018. The authors thank the Korea Testing and Research Institute for electrochemical test.

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