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Original Article
DOI: 10.1016/j.jmrt.2019.06.049
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Available online 14 August 2019
Investigating microstructural evolution at the interface of friction stir weld and diffusion bond of Al and Mg alloys
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M.A. Mofid
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moh.ammar_mofid@iauctb.ac.ir

Corresponding author.
, E. Loryaei
Department of Petroleum, Mining and Material Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
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Received 22 February 2019. Accepted 27 June 2019
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Abstract

Dissimilar Friction stir (FS) and Diffusion bond (DB) welds of Al alloy 5083 and Mg alloy AZ31 were produced at similar peak and bonding temperature of 435 °C. The weld had an irregular shaped region in the weld center of DB weld and layered interface in FS weld, having a different microstructure and hardness from the two base materials. The irregular shaped region in DB weld and interface of Mg and Al, in FS weld contained a large volume of intermetallic compound Al12Mg17 and showed significantly higher hardness in the weld center. The present study suggests that constitutional liquation resulted in the intermetallic compound (IMC) Al12Mg17 in the weld center.

Keywords:
Diffusion bond
Friction stir weld
Al alloy
Mg alloy
Interface
Microstructure
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1Introduction

Combining Al and Mg in one hybrid structure would make possible the use of these alloys for even more applications which will result in desirable weight saving. Given the increasing use of aluminum and magnesium alloys in aviation, aerospace, automotive, electrical and chemical industries, bonding these two alloys together is inevitable. In this regard, fusion joining and solid state methods are used to join the two alloys. Given the many problems of fusion welding of these alloys, such as thermal cracks, oxidative impurities and formation of brittle intermetallic compounds, attention is drawn to bonding the two alloys through solid state methods [1–4]. The solid-state welding processes used to bonding these two alloys include: friction welding methods, explosion welding, welding with transient liquid phase and diffusion welding [5–7]. Friction-stir welding (FSW) [8–12] of Mg–Al can achieve relatively high joining strength comparing other methods, but for the direct contact of base Mg and Al, there are also Mg–Al intermetallic compounds (IMCs) in the joints.

As mentioned, FSW was used to eliminate the intermetallic reaction layer, but the IMCs could only be reduced. There are two approaches to reduce the IMCs. One approach, used in our previous research is to conduct submerged FSW (SFSW) underwater or under liquid nitrogen [13]. The other approach is Diffusion bonding. DB is a solid state welding process which is applicable to the similar and dissimilar materials.

Formation mechanisms of liquid and intermetallics were investigated by some researchers, and the thermal behavior during the FSW was measured. In our previous research, the measured temperature rose to a maximum 435 °C in the vicinity of advancing side of the tool. A rotation speed of 400 rpm and travel speed of 50 mm/min were used for this specimen. Thermal cycle of this specimen showed a distinct plateau at about 430 °C, lasting for about 8 s. It was shown that, at the travel speed of 50 mm/min, the distance corresponding to 8 s is about 7 mm, which was the pin diameter. The presence of the temperature plateau indicated that a eutectic reaction occurred, and kept the temperature constant as the pin passed by Ref. [13]. Firouzdor and Kou confirmed that that the peak temperature was slightly below the eutectic reaction because the thermocouples were pushed downward during welding [1]. They also confirmed that the solidified droplets melted at 436 and 449 °C by differential scanning calorimetry, nearly identical to the eutectic temperatures.

Many experimental studies have been done for diffusion bonding of Al alloys to Mg alloys [4,5,14,15]. Predominant process parameters in diffusion bonding process are: bonding temperature, bonding pressure and holding time. It has been found that bonding temperature has a greater influence on shear strength and bonding strength of the joints followed by bonding pressure, holding time, and surface roughness [14]. Different microstructures formed under different parameters. However, the results showed that the presence of intermetallic compounds was confirmed even when the parameters were optimized [4,15].

Literature review indicated the feasibility of dissimilar FSW and DB of Al and Mg alloys, but no literature was found on comparison of details of microstructural evolution, during dissimilar FSW and Diffusion bonding of Al and Mg. The present study examines microstructural features in dissimilar FS and Diffusion weld of Al alloy 5083 and Mg alloy AZ31, at similar peak temperature, and discusses microstructural evolution during these two processes.

2Experimental procedure

The base materials used in this study were 3 mm thick sheets of 5083 Al alloy with the composition of Al–4.6 Mg–0.2Si–0.3Fe–0.1Cr–0.6 Mn (weight percent) and AZ31C-O Mg alloy with the composition of Mg–5.3Al–3.1Zn–0.2 Mn (weight percent).

For FSW, the rotating pin traveled along the butt line between the two base materials. The two plates were FS welded at a rotation speed of 400 rpm (low rotational speed to avoid liquation cracking) and travel speed of 50 mm/min. The welding tool rotated counterclockwise when viewed from above, and tilted 3° forward. The tool shoulder was 20 mm in diameter and concave. The pin was threaded, 7 mm in diameter and 2.8 mm in length. The pin was made of H13 steel. The specimens for OM were cut perpendicular to the welding direction.

For Diffusion bonding, Square shaped specimens (50 mm × 50 mm) were machined from plates of 3 mm thickness magnesium (AZ31C-O) and aluminium (AA5083) alloys. The specimen surfaces were prepared by conventional grinding techniques with final grinding on 1200# emery paper. The specimens were ultrasonically cleaned in an acetone bath to remove adhered contaminants and then dried in air. DB was carried out under a constant bonding pressure of 1 MPa and at the bonding temperature of 435 °C for a bonding time of 60 min. The bonding temperature of 435 °C is equivalent with maximum temperature, experienced during FSW with rotation speed of 400 rpm and travel speed of 50 mm/min [13]. Vacuum pressure was less than 6 × 10−3 Pa. In the bonding process, heating rates of the experiment were kept at 15 °C/min, the assemblies were cooled in the processing chamber under vacuum. The bonding specimen was sectioned by a cutting machine.

Following FSW and DB, cross-sections of two specimens were observed by optical microscopy (OM). The specimens for OM were etched in a 5 ml acetic acid + 5 g picric acid + 10 ml water + 100 ml ethanol solution for revealing Mg side of the weld. The chemical composition of the second phases in the weld zones was analyzed by Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) analysis system.

Additionally, second phases in the stir zone were identified by the X-ray diffraction (XRD) method. Samples for XRD were made by powdering blocks of stir zone of FSW weld and interface of Diffusion bond weld that mainly contained second phases. The blocks were carefully cut from a region containing only the second phases. However, they actually contained a small volume of the two base materials.

A Portable Hardness Testing by the ultrasonic contact impedance method (ASTM A1038) [16] was used to obtain Vickers microhardness measurements throughout the weld zones. Measurements were taken along 2 cm on the cross section (stretching from one base material, through the transition and weld regions, and into the other material) using a load of 10 N.

3Results

Fig. 1a shows weld map from friction stir weld of Al to Mg. The macrostructure is characterized by severe plastic deformation of the Al alloy and the Mg alloy resulting in their thorough mixing. Fig. 1b illustrates the microstructure of the Al/Mg interface for the friction-stir welded specimen. The static grain growth can be clearly seen in the recrystallized Mg grains in this zone. The average grain size of Mg in this zone is 20 µm. Fig. 1b shows that significant intermetallic material exists in the stir zone as light-etching and dark-etching phases. The lighter etching layer is on the Al side and the darker one is on the Mg side. The latter, because of its higher Mg content, is much more susceptible to corrosion and is preferentially etched out as dark pits. This region (Fig. 1b) was studied by means of SEM (Fig. 1c). SEM observations indicated that the interface zone of the Mg/Al joint included the transition region on the Mg substrate (Fig. 1c, region 1) and the transition region on the Al substrate (Fig. 1c, region 2). Al and Mg distributions in these regions analyzed by EDS are shown in Fig. 1c. Regions 1 and 2 have different chemical compositions; i.e., region 1 has lower Al and higher Mg contents than region 2. According to a phase diagram of the aluminum–magnesium system [17] and Fig. 1, layers of intermetallic phases are expected to be formed. The average thickness of the interaction layers in FS weld is 55 µm. The new phases are Al12Mg17 and Al3Mg2 intermetallics. These phases are all brittle compounds, which are the main reason for the formation of the weld crack. Both Al12Mg17 and Al3Mg2 phases have been reported in the literature as products of solid-state joining [1,18,19]. Fig. 1d shows many cracks in the light-etching phase, indicating a brittle weld owing to intermetallics.

Fig. 1.

(a) Weld map and interface of Al/Mg alloy macrostructure (b) Microstructure of Al/Mg interface (c) SEM micrograph and EDS results of Al/Mg interface microstructure (d) Cracks formation in interface of Al alloy and Mg alloy in stir zone from friction stir weld of Al 5083 to MgAZ31 performed in air.

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A transverse cross section of dissimilar diffusion bond performed under bonding pressure of 1 MPa and at the bonding temperature of 435 °C for a bonding time of 60 min is presented in Fig. 2a. The weld had no large defects, but contained a large irregular shaped region in the weld center. An optical micrograph of the irregular shaped region is shown in Fig. 2b. It seems that the irregular shaped region has a solidified microstructure. The microstructure consists of two regions “A” and “B” as shown in Fig. 2b. Region “A” consists of only a white phase, while region “B” has a eutectic microstructure consisting of bright and black phases. This microstructure was significantly different from the two base materials. Al and Mg distributions in the irregular shaped region analyzed by EDS are shown in Fig. 3. Quantitative analysis of the chemical composition by EDS showed that the bright phase in Fig. 2 consisted of 38 wt.% Al and 62 wt.% Mg, while the black phase contained 35 wt.% Al and 65 wt.% Mg. This result suggests that the white and black phases in Fig. 2 are the intermetallic compound Al12Mg17 and the Mg solid solution, respectively.

Fig. 2.

(a) Low magnification micrograph (b) optical micrograph of the irregular shaped region of dissimilar DB weld of Al 5083 to MgAZ31 at the bonding temperature of 440 °C for a bonding time of 60 min.

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Fig. 3.

SEM micrograph of the irregular shaped region of dissimilar DB weld of Al 5083 to Mg AZ31.

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The XRD spectrums of the powders of two specimens obtained in the irregular shaped region of DB weld and stir zone of FS weld, are indicated in Fig. 4. Large peaks of intermetallic compound Al12Mg17 are detected, though this figure contains some peaks obtained from matrices of Al and Mg alloys which were mixed into the powder of the second phases. The XRD spectrum confirms that the irregular shaped and stir zone regions of the dissimilar welds contain a large volume of the intermetallic compound Al12Mg17.

Fig. 4.

XRD spectrums of the powders of two specimens obtained in the (a) stir zone of FS weld (b) irregular shaped region of DB weld.

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The Vickers microhardness (HMV) profiles in the mid thickness cross sections of specimens, across the irregular shaped region of DB weld and stir zone of FS weld are shown in Fig. 5. Some fairly high hardness values are observed in the interfaces of the FS and DB welded specimens. Base materials of Al and Mg alloys have average hardness values of 128 and 72 Hv, respectively, while the stir zone and irregular shaped region in the weld center have hardness values between 120 and 224 Hv. This higher hardness is due to the intermetallic compound Al12Mg17.

Fig. 5.

Vickers hardness profiles in the mid thickness cross sections of specimens, across the irregular shaped region of DB weld and stir zone of FS weld.

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4Discussion

According to the Al-Mg phase diagram, when Al and Mg are heated up together, Al3Mg2 and Al12Mg17 intermetallic compounds may form; the former on the Al side and the latter on the Mg side. Upon further heating, the eutectic reaction Mg + Al12Mg17 → L occurs at the eutectic temperature 437 °C and the eutectic reaction Al + Al3Mg2 → L at the eutectic temperature 450 °C. This liquid formation is called constitutional liquation [11]. The eutectic temperatures 437 and 450 °C are about 200 °C below the melting points of Al and Mg, and they can be reached easily during FSW to form liquid films along the interface, and hence, lead to cracking [12].

Dissimilar FS and DB weld of Al alloy 5083 and Mg alloy AZ31 produced the intermetallic compound Al12Mg17 in the weld center, which resulted in significantly higher hardness in the weld compared to the base material. Results of OM and EDS suggest that the stir zone and irregular shaped region have a solidified microstructure experiencing the eutectic reaction, liquid → Al12Mg17 + Mg, after the primary solidification of Al12Mg17.

FS and DB weld are solid-state joining processes, which do not generally melt the welded material. In the case of dissimilar FSW and DB however, it may be possible that constitutional liquation occurs during the stirring in FSW and diffusion in DB. Since the welded material is simultaneously heated during stirring in FSW, Al and Mg atoms mutually diffuse at interfaces between the alternating bands. Based on our previous research, the present dissimilar FS weld should be exposed to peak temperatures higher than 435 °C during the stirring [13]. The peak temperature is sufficient for mutual diffusion between Al and Mg atoms. Additionally, the diffusion rates should be higher than that in the static condition, because Yashan et al. [20] suggested in a previous study on friction welding of 1100 Al and type 316 stainless steel that the diffusion is enhanced during plastic deformation with a high strain rate. The binary Al-Mg phase diagram [17] shows eutectic temperatures of 437 °C and 450 °C. Intensive mutual diffusion can form a liquid-phase constitutionally when the material is constantly held at temperatures higher than 435 °C.

The lack of formation of Kirkendall cavities in transition zone is in contrast with results of Lou and Acoff [21]. They reported that, in accordance with the Kirkendall effect, a few voids were formed in the Al-rich region of Al to Ti–6Al–4 V Transient Liquid Phase (TLP) bonded joint. This can be attributed to the proximity of the diffusion coefficients of aluminum and magnesium as much as 2.29 × 10−12 m2/sec and 1.89 × 10−12 m2/sec [22], and the proximity of their atomic mass as much as 27 g/mol and 24 g/mol [22]. According to the images, no micro-cavity was formed in the diffusion area.

5Conclusions

Dissimilar FS and DB weld of Al alloy 5083 and Mg alloy AZ31 was conducted at similar peak and bonding temperature of 435 °C. The results can be summarized as follows:

  • 1)

    Dissimilar FS and DB weld of Al alloy 5083 and Mg alloy AZ31 produced an irregular shaped region in the weld center of DB weld and layered interface in FS weld, having a different microstructure and hardness from the two base materials.

  • 2)

    The irregular shaped region in DB weld and interface of Mg and Al, in FS weld contained a large volume of intermetallic compound Al12Mg17. The intermetallic compound Al12Mg17 was probably formed by constitutional liquation during dissimilar FSW and it caused much higher hardness in the weld center.

  • 3)

    The IMC reaction layer could be significantly reduced due to the low welding temperature, but formation of brittle Al-Mg IMCs cannot be completely avoided.

Conflicts of interest

The authors declare no conflicts of interest.

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Copyright © 2019. The Authors
Journal of Materials Research and Technology

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