Magnesium and Aluminium alloys Dissimilar Joining

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ScienceDirect Procedia Engineering 183 (2017) 239 – 244

17th International Conference on Sheet Metal, SHEMET17

Magnesium and Aluminium alloys dissimilar joining by Friction Stir Welding V. Paradisoa,*, F. Rubinoa, P. Carlonea, G. S. Palazzoa a

University of Salerno, Department of Industrial Engineering - DIIN, 84084, Fisciano (SA), Italy

Abstract Multi-material lightweight structures are gaining a great deal of attention in several industries, in particular where a trade-off between reduced weight, improved performances, and cost compression is required. Magnesium alloys, such as the zinc-rare earth elements ZE41A alloy, fulfill the first two requirements; however, they are susceptible to corrosion and relatively expensive. Lightweight structures hybridization, for instance combining Magnesium alloys and Aluminium alloys, is currently under consideration as a potential solution to this problem. Nevertheless, dissimilar joining of Magnesium and Aluminium alloys is challenging due to the significant differences in physical properties, as well as to the precipitation of brittle intermetallic compounds, such as Al12Mg17 and Al3Mg2. In this study, the dissimilar joining of Magnesium and Aluminium alloys by friction stir welding process is discussed. In particular, 4 mm thick plates of ZE41A Mg alloy and AA2024-T3 Al alloy were welded in the butt joint configuration. The feasibility of the process was assessed by means of microstructure and mechanical analysis. The formation of brittle intermetallic compounds was investigated as well. ©2017 2017The TheAuthors. Authors.Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of SHEMET17. Peer-review under responsibility of the organizing committee of SHEMET17 Keywords: Friction stir welding; dissimilar joining; Magnesium alloys; Aluminium alloys; intermetallic compounds.

1. Introduction Magnesium and Aluminum are two of the lightest structural metals and their alloys are finding increasing applications in industries mainly for their high strength to weight ratio. More specifically Aluminium alloys are widely used in many industrial fields because of their low density, good corrosion resistance, good workability, high

* Corresponding author. Tel.: +39 089968163. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SHEMET17

doi:10.1016/j.proeng.2017.04.028

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thermal and electrical conductivity [1, 2]. As extremely light alloys, Magnesium alloys have good castability, hot formability and recyclability [3-5]. Combining Al and Mg alloys in one hybrid structure would make possible the use of these alloys for even more applications which will result in desirable weight saving. Due to the large amount of potential applications of Al/Mg hybrid structures, the joining of these dissimilar materials is an important issue. For this aim several conventional welding techniques such as gas tungsten arc welding [6], electron beam welding [7], and laser beam welding [8] have been adopted. All of these conventional fusion welding techniques used to join Al to Mg alloys however frequently have some defects such as liquation induced cracking and porosity, developed as a consequence of entrapped hydrogen inside of the weld bead during solidification [9]. Furthermore the resulting formation of various types of intermetallic compounds (IMC) in the weld zone undermines the weld integrity. For these reasons many alternative welding practices have been studied and developed in order to obtain defect free joining of light alloys. These include solid state welding techniques such as diffusion bonding [10, 11], ultrasonic welding [12], resistance spot welding [13], linear friction welding [14], and friction stir welding (FSW). FSW is a solid-state welding technique that may provide a feasible process to join dissimilar Al to Mg alloys [15, 16]. Since FSW takes place below the melting temperature of the alloy several defects due to the solidification of the metals are avoided [17, 18]. Despite these advantages the literature indicates the formation of brittle intermetallic compounds at the Al-Mg dissimilar weld interface, which results in weakening of the joint with negligible ductility [19]. Indeed according to the Al-Mg phase diagram, when Al and Mg are heated up together, Al 3Mg2 and Al12Mg17 intermetallic compounds may form. Upon further heating, the eutectic reaction Mg+Al 12Mg17 ൺ L occurs at the eutectic temperature 437 rC and the eutectic reaction Al+Al3Mg2 ൺ L at the eutectic temperature 450 rC. This liquid formation is called constitutional liquation [20]. The eutectic temperatures 437 and 450 rC are about 200 rC 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. Due to the aforementioned reasons the joining of dissimilar Al/Mg alloys clearly poses a unique set of challenges and the FSW process between these selected alloys is expected to produce a weld bead with a very complex metallurgy. In literature, to the authors’ best knowledge, there are very few papers dealing with the dissimilar FSW between Magnesium and Aluminium alloys and no article can be found in the bibliography on the dissimilar joining between AA2024-T3 Al-alloy and ZE41A Mg-alloy. AA2024-T3 is a very common high strength Alluminium alloy extensively used in the aircraft and defence areas because of its light strength to weight ratio and good corrosion properties, while ZE41A is a popular Mg-Zn-RE alloy that exhibits a moderate strength, creep resistance and many other advantages related to rare earth (RE) elements addition, such as purifying alloy melt, modifying castability, refining the microstructure, improving the mechanical properties and anti-oxidation properties. ZE41A Mg alloy is used for components such as aircraft gearbox and generator housings, particularly in military helicopters, which are exposed to corrosive environments during service [21]. This paper studies the dissimilar joining by FSW between ZE41A Mg alloy and the high-strength Aluminium alloy AA2024-T3. The aims of this paper are to assess the feasibility of the FSW process to provide sound joints and to study the microstructure and the metallurgy of the joint. 2. Materials and methods AA2024-T3 and ZE41A 4 mm thick sheets were used as base material. AA2024-T3 was supplied as rolled sheets. ZE41A was manufactured by cast gravity process and supplied in form of strips by a foundry of the FINMECCANICA industrial group. The plate edges were deburred to prevent any prior inhomogeneities. ZE41A sheet was fixed in the advancing side of the butt joint in order to lower the welding heat input and to reduce as minimum as possible the brittle IMCs formation [15, 22]. Friction stir welding was carried out offsetting the tool of 1 mm towards the magnesium side using a machining center (MCX 600 ECO). The adopted high speed steel tool, consists of a shoulder with 20 mm in diameter with a conical unthreaded pin (height 3.80 mm, major diameter 6.20 mm, and cone angle 30 deg). A scheme of the experimental welding configuration is provided in Fig. 1.a. The following FSW processing parameters were adopted: rotating speed ranging from 1000 to 1400 rpm; feed rate ranging from 20 to 80 mm/min; tilt angle 2 deg; shoulder plunge depth 0.48 mm. The processing parameters reported in Fig. 1.b were chosen on the basis of the available literature and taking into account a previous investigation by some of the authors [22, 23].

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Fig. 1. Experimental set-up: (a) Welding configuration; (b) FSW processing parameters.

The FSW process was carried out following plunging, dwelling, and welding stages. The feed rate of the tool during the plunging step was set as 5 mm/min, while the duration of dwell was set as 10 s. Metallographic specimens were cut away from the welded sheets in order to observe the sectional microstructure of the Al/Mg dissimilar joints and to perform the microhardness tests. Specimens were mounted in a proper thermoset resin and lapped using abrasive disks (P320, P600, P1200, P2000) and subsequently polished with polycrystalline diamond suspension (from 9 to 0.05 micron) on tissue disk until the surface exhibited a mirror like finish. Afterwards, the specimens were chemical etched by a modified Keller’s reagent (150 ml H2O, 2 ml HNO3, 6 ml HCl, 6 ml HF) to unveil the significant features of the Aluminium metallurgical microstructure. The same procedure was repeated to investigate the Magnesium side using a solution based on 4.2 g picric acid in mixture of 70 ml ethanol, 10 ml acetic acid and 10 ml distilled water. Microstructural observations were performed by means of optical microscope equipped with a digital camera to evaluate the weld bead morphology and to reveal the microstructure of the joint. Vickers microhardness measurements were performed along the cross-section of the weld joint in order to assess the influence of microstructural alteration on local mechanical properties of the joint. Three linear patterns, orthogonal to the weld line, were programmed on a LEICA VMHT-AUTO machine, respectively, at the mid-thickness of the joint cross-section and at a distance equal to 1 mm towards the top and bottom surfaces. The following parameters were adopted: distance between two consecutive indentations 1 mm, indentation load 100 gf (0.98 N), loading time 15 s, and indentation speed 60 μm/s. 3. Results and discussion Material flow occurs differently in both the alloys during friction stir welding process essentially due to the different microstructure of the base materials (Fig. 2). ZE41A Mg alloy exhibits higher brittleness with respect to Al alloy and therefore the attitude to the plastic deformation in ZE41A is lower compared with AA2024-T3. Therefore, the joint formation between AA2024-T3 and ZE41A alloys is complex in nature. The different thermal conductivity of both alloys also dictates the success of dissimilar joining [24]. In our case study the AA2024-T3 alloy presents nearly twice the thermal conductivity of ZE41A Mg alloy. Due to this significant difference, the dissipation of heat generated during the welding of AA2024-T3 and ZE41A is nonuniform hence high level of thermal stresses are developed. If these thermal stresses are not balanced, they may lead to hot cracking as observed in the joint processed at 1400 rpm and 20 mm/min of feed rate (Fig. 3.a).

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Fig. 2. Base materials microstructures: (a) Al 2024-T3; (b) Mg ZE41A.

By a morphological point of view the crack propagation was favored by the presence of Al matrix with dispersed IMCs narrow region clearly distinguishable within the stirred zone as reported in the same Fig. 3.a. It proves that for the same tool rotational speed and feed rate, the amount of heat generation in ZE41A alloy is different with respect to Al 2024-T3 alloy and hence obtaining a set of parameters which avoid development of hot cracks is an important task to get sound metallurgical continuity. Poor joints were observed at 1000 rpm for each condition of feed rate (Fig. 3.b).

Fig. 3. Representative observations of the encountered defects: (a) Hot cracks occurred within the stirred zone; (b) Bottom view of a poor joint.

Defect free sound joints were obtained at 1200 rpm with 20 mm/min of feed rate. As shown in Fig. 4, a complex flow pattern is formed in the stirred zone and large dynamic recrystallization was observed in the weld region as well as in the transition region, with a clear decrease in the grain size from the base materials through the transition zone and into the weld zone. The stir zone was found to be mixed with both alloys but the fraction of Al 2024-T3 alloy was appeared to be more compared with ZE41A alloy which is obvious as Al is more ductile compared with ZE41A alloy. The presence of these microstructure in the stirred zone is a clear evidence of liquid formation during FSW [20]. Furthermore it indicates higher level of material mixing from both alloys.

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Friction stir welding has a lower input compared with fusion welding techniques, nevertheless in literautre was found that the formation of Mg2Al3 and Mg17Al12 IMCs was commonly observed in the FSWed Al/Mg joint [15, 20]. As also shown in Fig. 4 very fine dispersed IMCs in Aluminium highly rich matrix were observed in the FSW joint stir zone. The combination of fine grains and smaller IMCs particles within the stirred zone, as clearly visible in a subfigure of Fig. 4, was confirmed by means of microhardness measurements. Indeed in Fig. 4 are also reported the microhardness values obtained by measuring across the weld joint. A gradual increase in the hardness from 69 HV0.1 mean value of ZE41A base material to 117 HV0.1 mean value of Al 2024-T3 base material can be observed from the results. Within the stirred zone large variations up to a maximum value of 223 HV0.1 have been found, which are mainly due to the combined effect of fine grain structure and the presence of hard IMCs particles with solid solution strengthening. On the other hand a slight decrease below the hardness value of base materials was observed immediately beyond the stir zone, more precisely in correspondence with heat affected zone (HAZ) of both Al 2024-T3 and Mg ZE41A sides. This is directly related to the fact that hardness decreases with increasing grain size in HAZ according to the Hall Petch equation [25].

Fig. 4. Micostructural and microhardness measurements of joint welded at 1200 rpm and 20 mm/min.

4. Conclusions This preliminary investigation has demonstrated the feasibility of joining the Aluminium alloy 2024-T3 to ZE41A Magnesium alloy offsetting the tool towards the Magnesium side and fixing the Aluminium in the retreating side. Several difficulties were encountered during FSW process due to the different behavior of the alloys in terms of attitude to the plastic deformation and thermal conductivity. Hot cracks and poor joints were obtained in some processing conditions.

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Instead sound joints were obtained with 1200 rpm of tool rotational speed and 20 mm/min of welding speed. Microstructure and microhardness of the sound joints were examined. Microstructural observations demonstrated that a complex vortex flow occurred in the stirred zone. The stirred zone was found to be mixed with both alloys but the fraction of Al 2024-T3 alloy was appeared to be more compared with ZE41A alloy. Furthermore very fine dispersed IMCs in Aluminium highly rich matrix was observed in the FSW joint stirred zone. The hardness distribution in stirred zone was found to be highly affected by the combined effect of fine grain structure and the presence of hard intermetallic compounds. The friction stir weld investigation indicates that exists a potential for further enhancements to join dissimilar AA2024-T3 to Mg ZE41A alloys. 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