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Metall. Res. Technol.
Volume 117, Number 3, 2020
Article Number 311
Number of page(s) 6
DOI https://doi.org/10.1051/metal/2020012
Published online 03 June 2020

© EDP Sciences, 2020

1 Introduction

As increasingly important engineering structural materials, Mg alloys have several excellent properties, such as low density and high specific strength, and are widely applied in automotive, high-speed train, aerospace, and other industries [1]. To further enhance the flexibility and expand the applications of Mg alloys, Al alloys have been combined with them, as the joining of Al alloys and Mg alloys on proper portions of the structure can give play to their respective advantages and ultimately produce a lightweight structure. Recently, there has been a rapidly growing interest in joining Al alloys and Mg alloys via friction stir welding (FSW), as joining dissimilar metals via fusion joining is difficult.

FSW is a solid-state welding method invented by The Welding Institute (TWI) in the UK, and was initially designed for joining Al and its alloys, which are difficult to be connected with fusion welding [2]. During FSW, a cylindrical or other-shaped (such as a threaded cylinder) stirring pin is inserted into the interface of two butting workpieces. During the high-speed rotation of the tool, heat is mainly generated by the friction between the tool and the workpieces, and the temperature around the tool becomes sufficiently high to soften the materials; therefore, the material becomes plastically deformable. The rotating tool then traverses along the butting line of the workpieces, and the surrounding materials are simultaneously transported from the leading side to the trailing side to produce a joint [36]; this is why it is referred to as “solid state”. Due to the low temperature of FSW, the deterioration of joint performance during fusion welding, including that due to coarse grains, high residual stress, and severe welding distortion, can be decreased or even avoided. Today, FSW is regarded as a revolutionary resource-saving and environmentally friendly welding technology.

Much research has been conducted on the joining of dissimilar Al/Mg alloys with FSW. Zettler et al. [7] investigated the microstructures, chemical compositions, and tensile strength of joints. They observed that a rather complex vortex microstructure with intercalated bands of Al and Mg occurred in the stir zone. Moreover, brittle intermetallic phases of Al3Mg2 and Al12Mg17 were formed, and the maximum tensile strength of the joint achieved approximately 80% that of the base material. Firouzdor and Kou [8,9] demonstrated and explained the effects of the welding conditions, including the material position, travel speed, and rotation speed, on the heat input and joint strength with butt-welded sheets of 6061 Al and AZ31 Mg. Mohammad et al. [10] researched submerged friction stir welding (SFSW), in which FSW is first performed in air, and then in water and liquid nitrogen. They reported that the stir zone of underwater-welded and under-liquid-nitrogen-welded specimens presented a much smoother interface and underwent less intermixing, and due to a decrease in the peak temperature, the formation of intermetallic compounds was suppressed significantly and grain growth was limited. Ji et al. [11] introduced ultrasonic vibration into the dissimilar FSW of 6061 Al alloy and AZ31 Mg alloy with thicknesses of 3 mm based on a stationary shoulder. The ultrasonic vibration was found to induce a much higher mixing degree of Al/Mg alloys and longer interface joining, which increased mechanical interlocking. In addition, ultrasonic vibration resulted in the fracture path being located near the thermo-mechanically affected zone (TMAZ) of the AS and partially across the Mg alloy. Abdollahzadeh et al. [12] examined the effects of changing the chemical composition on dissimilar Mg/Al FSW butt joints using a zinc interlayer with 5-mm-thick sheets of AZ31 Mg and 6061-T6 Al alloys. It was found that both UTS and elongation were improved in the Mg/Al dissimilar FSW joint with the addition of the Zn interlayer. Yan et al. [13] explored the microstructure characteristics and performances of dissimilar welds between Mg alloy and Al alloy formed via friction stirring. They found that the joints showed complex a vortex flow characterized by intercalation lamellae. Yamamoto et al. [14] focused on the effect of an intermetallic compound layer on the tensile strength of the dissimilar FSW of a high-strength Mg alloy (RCPAZ31 Mg alloy) and Al alloy (5083 Al alloy). They illustrated that the formation of the IMC layer was controlled by the reaction diffusion, and the tensile strength decreased remarkably with the increase in the thickness of the IMC layer. Sato et al. [15] elucidated constitutional liquation during the dissimilar FSW of Al and Mg alloys.

Although dissimilar Al/Mg FSW has been investigated from many aspects, including the relationships between the parameters, microstructure, and mechanical properties, there has been a lack of agreement on the deeper explanation of the joint formation mechanism. The reasons attributed to this are as follows: (1) the process cannot be viewed directly; (2) the process tends to be influenced by the geometry of the pin tool, the welding parameters, and the properties of the materials, etc.

Therefore, the dissimilar Al/Mg joint formation mechanism as related to the plastic metal flow has become a key technological problem that has recently attracted much research interest. Some research methods of visualizing the flow during FSW have been developed, and primarily include the marker technique, the welding of dissimilar materials, the stop-action technique, and numerical simulation.

In the present study, a butt FSW process was performed on 6061 Al and AZ31 Mg alloy plates. After the welding of these two dissimilar materials, differential etching can be used to produce a sufficiently high contrast to allow for flow visualization, and can be carried out by etching the two dissimilar materials separately. By applying this method, the flow pattern can be directly observed via the sufficient contrast [5,16]. To further clarify the microstructure evolution mechanism of the Mg/Al joint during FSW, the microstructure evolution mechanisms in different areas of the nugget zone are deeply explored. The obtained findings not only aid in the understanding of the effects of the most fundamental welding principle of FSW on the formation of the joints of dissimilar materials, which may contribute to academic advancements, but may also provide guidance for the microstructural control of the joining interface, which is essential for the fabrication of sound FSW joints.

2 Materials and methods

In this study, 6061 Al alloy plates and AZ31 Mg alloy plates of 200 × 150 × 4 mm in size were used for the butt FSW process at a tool rotation of 900 rpm and welding speed of 30 mm/min. The Mg alloy was placed on the advancing side (AS) to lower the input welding heat and restrain the formation of brittle IMCs. Their nominal chemical compositions are listed in Table 1. The oxide layers of the workpieces were cleaned before welding.

The FSW tool was made of H13 tool steel. The shoulder diameter of the tool was 12 mm. The pin of the tool had a tapered threaded shape with a maximum diameter of 4.3 mm, a minimum diameter of 3.4 mm, and a height of 3.8 mm. The stirring pin was tilted forward by 2.5° from the normal direction (ND), and the plunge depth during welding was 0.2 mm. There was an offset of 0.3 mm from the centerline to the Mg advancing side. Figure 1 presents a schematic illustration of the experimental system for FSW. After each welding pass, the tool was plunged into a fresh piece of Al alloy to remove the material stuck on the tool from previous welds.

The specimens were transversely sectioned, ground, and polished. The metallographic samples were then etched in two steps. The first step was to etch the samples with a solution consisting of 10 ml acetic acid, 10 ml distilled water, 100 ml ethanol, and 6 g picric acid for 6 s. The second step was to etch them with a solution consisting of 20 g NaOH in 100 ml distilled water for 60 s. The macrostructures were examined using optical microscopy (OM, Leica DM2700P), and microstructure evaluation was conducted via a scanning electron microscope (SEM, JSM-6700F) equipped with an energy-dispersive spectrometer (EDS). The phase composition was detected by X-ray diffraction (XRD) analysis.

Table 1

Nominal chemical compositions and mechanical of base metals.

thumbnail Fig. 1

The schematic of the Al/Mg dissimilar alloys FSW experimental system.

3 Results and discussion

3.1 Transverse cross-sections

Figure 2a presents the schematic of the sample machining position, and Figure 2b presents the schematic view of the transverse cross-section of the Al/Mg joint after FSW. The nugget zone (NZ) is located in the center of the welded joint. Due to the severe stirring effect of the stirring pin, the zone experienced a thermal cycle of a higher temperature and dynamic recrystallization occurred [17], transforming the original fibrous structure to a fine equiaxed recrystallized microstructure. During FSW, the pin not only generated enough heat to convert the parent material to a plastic state, but plastic flow also occurred due to the relative motion along the welding direction. Because the flow speed and direction changed with time and location, the material around the pin did not experience static plastic deformation, but rather a dynamic process of random deformation. The NZ was composed of different regions due to the intense plastic deformation and material flow of dissimilar Mg/Al alloys, i.e., the shoulder affected zone (SAZ), banded structure (BS), and severely deformed zone (SZ). Figure 2c displays the optical macrograph of the transverse cross-section of the weld. The three divided zones were subsequently further studied, and respectively designated as locations 1, 2, and 3.

thumbnail Fig. 2

(a) The schematic of the sample machined position; (b) The schematics of transverse cross-section of the Al–Mg joint in FSW; (c) Optical macrograph of the transverse cross-section of the Al–Mg joint in FSW.

3.2 Shoulder affected zone

The shoulder affected zone (SAZ) is the region on the top that corresponds to what was compacted by the shoulder. The typical OM and SEM images of the SAZ (location 1 in Fig. 2) are respectively displayed in Figures 3a and 3b. Figure 3a reveals that the material on the advancing side (in this case, it was Mg) was transferred and extended a long distance towards the retreating side (in this case, it was Al). In particular, it can be observed that there were light, narrow, long strips inserted into the Mg matrix. Figure 3b shows the amplified figure of the light bands. It can be seen that the band was tortuous and made up of alternating thinner layers. To derive the chemical nature of the thinner strips, EDS analysis was carried out at the locations marked as points 1–4 in Figure 3, and the results are presented in Table 2. According to the EDS analysis, the light thinner layers were Al3Mg2, and the dark thinner layers were Mg17Al12 or α (Mg).

The SAZ, located on the upper part of the NZ, was in direct contact with, and seriously affected by, the shoulder. In this work, a 2.5° tilt angle was used, which increased the amount of top shoulder flow above the welded NZ. The shoulder moved in a circular motion, which softened the upper material to fill the gap between the two plates during FSW. It has been reported by Murr et al. [17] that the FSW of dissimilar Al/Mg alloys involves dynamic recrystallization as the mechanism for accommodating the superplastic deformation that facilitates the bond.

The schematic view of the SAZ microstructure evolution is presented in Figure 4. The Al plate was placed on the AS, and Mg was placed on the RS, as shown in Figure 4a. It can be seen in Figure 4b that almost all the material in the SAZ was driven by the shoulder, and only a small amount of the material was driven by the pin. Therefore, the Mg on the advancing side was transferred to the retreating side with the rotation of the shoulder. In the meantime, a strip of Al on the retreating side driven by the pin was intercalated into the thick Mg layer. According to diffusion theory, the self-diffusion coefficient of Mg is higher than that of Al at the same temperature, i.e., the diffusion rate of Mg is greater than that of Al. Based on the binary Al–Mg phase diagram [8,18], Al and Mg can react with each other in the solid state to form the intermetallic compounds Al3Mg2 and Mg17Al12 depending on the local composition. Furthermore, the eutectic reaction Mg + Mg17Al12 → L occurs at the eutectic temperature of 746 K, and the eutectic reaction Al + Al3Mg2 → L occurs at the eutectic temperature of 723 K. Because this liquid formation occurs far below the melting point of the parent material, it is called constitutional liquation [9,15]. As can be seen in Figure 4c, after cooling, Al3Mg2 and Mg17Al12 were formed from liquid.

thumbnail Fig. 3

Optical macrograph (a) and SEM micrograph (b) of SAZ.

Table 2

Compositions determined by EDS at locations demonstrated in Figure 3.

thumbnail Fig. 4

Schematic view of the SAZ microstructure evolution.

3.3 Banded structure

The typical OM and SEM images of the BS (location 2 in Fig. 2) are respectively shown in Figures 5a and 5b. It can be seen that the BS zone was adjacent to the AS, and significant material intermixing between Mg and Al occurred in this zone. Figure 5a shows alternating gray and dark bands parallel to the interface between the NZ and the TMAZ in the AS. Continuous intercalated bands originated from the top surface and extended to bottom in a direction that was tilted approximately 45° from the perpendicular direction.

Figure 5b is the magnified view that shows the microstructure of the BS, and reveals that the aforementioned dark bands were actually composed of abundant of light-grey particles inset in a dark background. The chemical compositions of the typical microstructure in the BS were confirmed via EDS at the locations marked as points 1–4 in Figure 5, and the results are presented in Table 3. The EDS results indicate that, according to the percentages of the Al and Mg elements, the grey particle phase (points 3) was Mg17Al12, and the dark background (point 1) was α-Mg. Additionally, the relatively continuous thick gray lamellae (point 2) were actually Al3Mg2. Thus, it is evident that the alternating gray and dark bands observed in the OM after etching are attributable to the alternating arrangement of the continuous gray Al3Mg2 lamellae and gray Mg17Al12 particles inset on the dark α-Mg.

The banded structure featured by the alternating lamellae of the Al3Mg2 and Mg17Al12 is attributable to the stirring action and the tilted angle of the threaded tool. It can be seen in Figure 5b that Al/Mg intermixing occurred mainly via the transportation of the Al pieces from the RS to the AS. The mutual mixing in the dissimilar alloy weld was intimate, and constitutional liquation and diffusion then occurred to form intermetallic compounds. During welding, the material sheared by the pin was pulled toward it and into the grooves of the pin’s threads as it turned to the RS. A schematic of the pin thread that illustrates the material flow and the formation of the banded structure is presented in Figure 6.

As the pin rotates counterclockwise and tilts forward, the lower surface of the thread will shear the material and exert a force F on the material toward the RS. The pin will then move forward and hence detach from the layers, and only one layer dependents and detaches completely in each revolution, during which a cavity forms on the upper surface.

It is widely known that FSW is a constant-volume process, and that the shoulder, the pin, and the undeformed base material restrict the flow path. Thus, the material (in this case, it was mainly Al) that transferred from the RS filled this cavity. Finally, Al and Mg encountered and reacted with each other in a layer shape under enough heat was input, and the intercalated lamellae made by the IMCs thus became developed. The featured microstructure was vividly produced by differential etching.

On the other hand, the tool was inclined, and the threaded tool therefore tended to pull the material downward toward the bottom in the vertical direction. However, it is significant to realize that the vertical movement is the secondary motion overlaid over the primary material transport around the rotating tool on the horizontal plane of the weld. Therefore, the net result of the material flow pattern was a banded structure tilted downward at approximately 45°.

thumbnail Fig. 5

Optical macrograph (a) and SEM micrograph (b) of BS.

Table 3

Compositions determined by EDS at locations demonstrated in Figure 5.

thumbnail Fig. 6

The schematic of the pin thread.

3.4 Severe deformed zone

The presence of onion ring patterns in the weld (location 3 in Fig. 2) can be observed in Figure 7a. It can be clearly seen that the onion ring patterns arose from different etching contrast. Figures 7b and 7c are the magnified images of the local microstructure of the onion rings, from which it can be seen that there were some gray strips on the dark matrix. The strips were curly and continued with the banded structure; the onion rings can thus be regarded as a special curly banded structure. As is known, FSW can be roughly described as an in-situ extrusion process in which the tool shoulder, the pin, the backing plate, and the cold base material form an extrusion die. Considering the aforementioned elaborations, the formation mechanism of the onion ring structure can be explained as follows. When the material flowed from the top to the bottom, the lower part of the cold base metal blocked material flow and reflected it upward, acting as the wall of the die, as did the cold base metal of the RS. The formation of onion rings is illustrated by the schematic of material flow presented in Figure 9.

To precisely confirm the phases, XRD testing was conducted on the NZ, and the result is presented in Figure 8. Except for the major phases of Al and Mg, relatively large quantities of Al3Mg2 and Mg17Al12 were detected, and these results are in agreement with those reported by Chen et al. [19] and Yan et al. [13], as well as with the EDS analysis results presented previously.

Figure 9 shows the full material flow in the transverse section of the FSW butt joint. The arrows indicate the local flow path of the plasticized material. The flow pattern reveals that material flowed from the AS towards the RS, and was stronger in the upper layers due to the strong friction of the shoulder. In addition, a slight vertical material movement towards the bottom of the joint was observed as the tool was inclined; an upward change of direction was observed for this material, and a descendant laminar flow in the AS then occurred. Due to the reflection of the cold base metal, the laminar flow changed the direction upward to form an onion ring structure. Finally, onion rings were observed in the bottom part of the joint, thus denoting effective material flow driven by the pin and the reflection of the imaginary die wall.

thumbnail Fig. 7

The micrograph (b) of SDZ. (a) Formation of a complete onion ring pattern and (b,c) local microstructure of the onion ring.

thumbnail Fig. 8

XRD spectrum of the NZ for the welding joint.

thumbnail Fig. 9

The schematic of overall flow pattern.

4 Conclusions

In this study, a butt FSW process was performed using 6061 Al and AZ31 Mg plates. The microstructure evolutions of the different regions in the NZ were systematically investigated via different etchings during the fabrication of the FSW joint to clarify the formation mechanism of the joint. This provided vivid images that illustrated the flow visualization and complex flow patterns. The obtained findings are as follows:

  1. During FSW, the material in the SAZ was mainly driven by the shoulder, and little material was driven by the pin. A strip of Al in the RS transferred by the pin was inserted into the thicker Mg layer. Thus, Al and Mg contacted with each other, and IMCs (Mg17Al12, Al3Mg2) were formed;

  2. The banded structure was tilted approximately 45° from the perpendicular direction and characterized by the alternating lamellae of the Al3Mg2 and Mg17Al12 that was produced, which is attributed to the stirring action and the tilted angle of the threaded tool;

  3. The onion ring structure in the SDZ was produced during welding, and can be referred to as a curly banded structure. The mechanism of the formation of the onion ring structure can be explained by the reflection effect of the imaginary die wall;

  4. Finally, the overall flow pattern of the nugget zone during welding was obtained by integrating the microstructure analyses of the three different regions.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant 51805359 and 51775366), China Postdoctoral Science Foundation (Grant 2018M631772), Natural science foundation of Shanxi Province (Grant 201901D211015) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province (STIP)(2019L0333).

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Cite this article as: Wei Chen, Wenxian Wang, Zepeng Liu, Decheng An, Ning Shi, Tingting Zhang, Min Ding, Microstructure evolution mechanism of Al/Mg dissimilar joint during friction stir welding, Metall. Res. Technol. 117, 311 (2020)

All Tables

Table 1

Nominal chemical compositions and mechanical of base metals.

Table 2

Compositions determined by EDS at locations demonstrated in Figure 3.

Table 3

Compositions determined by EDS at locations demonstrated in Figure 5.

All Figures

thumbnail Fig. 1

The schematic of the Al/Mg dissimilar alloys FSW experimental system.

In the text
thumbnail Fig. 2

(a) The schematic of the sample machined position; (b) The schematics of transverse cross-section of the Al–Mg joint in FSW; (c) Optical macrograph of the transverse cross-section of the Al–Mg joint in FSW.

In the text
thumbnail Fig. 3

Optical macrograph (a) and SEM micrograph (b) of SAZ.

In the text
thumbnail Fig. 4

Schematic view of the SAZ microstructure evolution.

In the text
thumbnail Fig. 5

Optical macrograph (a) and SEM micrograph (b) of BS.

In the text
thumbnail Fig. 6

The schematic of the pin thread.

In the text
thumbnail Fig. 7

The micrograph (b) of SDZ. (a) Formation of a complete onion ring pattern and (b,c) local microstructure of the onion ring.

In the text
thumbnail Fig. 8

XRD spectrum of the NZ for the welding joint.

In the text
thumbnail Fig. 9

The schematic of overall flow pattern.

In the text

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