The effect of changing chemical composition on dissimilar Mg/Al friction stir welded butt joints using zinc interlayer

Abstract

In this study, solid state joining technology of friction stir welding (FSW) was carried out for Al and Mg butt joints. To improve the microstructural characteristics of the banded structure zone, before to the FSW process, a Zn interlayer was also inserted in the butt joints. Optimal conditions for friction stir welding were obtained using a combination of two travel speeds (25 and 35 mm/min) and three rotation speeds (550, 600 and 650 rpm). Mg-Zn and Mg-Al-Zn IMCs, Al solid solution and residual Zn were the most common phases in the stirred zone, eliminating Al-Mg intermetallic formation. The joint fabricated at 35 mm/min and 600 rpm, in which tensile strength of the weld improved from 141 to 175 MPa and an elongation from 0.7 to 1.9 was achieved, gave the maximum mechanical properties compared with the Zn free sample FSWed under identical conditions. The fracture micrographs were compatible with the corresponding ductility results and the fracture surfaces of Zn added samples possessed a fine texture combining brittle and ductile fracture features. Meanwhile, the hardness level was lower than that of the Zn free sample because of the change in the IMCs composition of the Zn added samples.

Introduction

Aluminum alloys have been extensively applied in different industries such as electronics, aerospace, shipbuilding and automotive, given their outstanding characteristics; namely good formability, high strength, good corrosion resistance and low weight [1,2]. Mg alloys have been used in electronic and transportation industries since they are the lightest among structural metals and they have high specific strength and rigidity and good damping capacities [3,4]. A major benefit for the production of light weight structures, energy saving and reduction of emission is the successful aluminum and magnesium dissimilar welding [5,6]. The high temperature in the fusion welding between Mg and Al alloys forms a large number of brittle Mg-Al intermetallic compounds (IMCs). This is more disastrous on the Al-Mg interface [7]. In addition, most of these dissimilar welds have drawbacks such as large heat affected zone (HAZ), porosities, solidification cracking, losses because of evaporation and high residual stresses [8]. The development of a perfect joining process between these metals is a decisive requirement to achieve a good combination of the characteristics of Mg and Al alloys. Therefore, solid state methods, as an alternative technology for joining Al and Mg, have been of grate interest [9,10].

The Welding Institute of the UK developed friction stir welding as a solid state joining technology in 1991. In this process a rotating tool formed by a shoulder and a pin is placed at a fixed rate inside a joint between two fastened parts of the material. Sufficient heat is generated by the friction between the material and the shoulder to plastify the material and a efficient bond between the materials is promoted by the pin stirring. Unlike other welding processes, this technique is characterized by the absence of melting, low temperature and low heat input [11,12]. These properties give rise to the wide application of FSW in dissimilar welding [[13], [14], [15], [16], [17]]. Much work has been carried out on the FSW of Al to Mg in butt and lap joints [[18], [19], [20], [21]]. The presence of mass brittle IMCs such as Al₁₂Mg₁₇ and Al₃Mn₂ may not be ignored in spite of successful FSW of Al-Mg and preventing the thick interlayer of brittle IMCs [[22], [23], [24], [25]]. Brittle fracture is caused by these IMCs.

The impact of interlayer or addition of alloying element on the reduction or elimination of Al-Mg-IMCs formation and the microstructure and mechanical properties of Al/Mg have recently been investigated [[26], [27], [28], [29], [30]]. At low temperatures, Zn preferably reacts with Mg to yield Mg-Zn IMCs, based on the phase diagrams of Mg-Zn and Al-Zn. Furthermore, a large solid solubility may be formed between Zn and Al, which causes Zn to function as an alloying element and improves the mechanical characteristics of the FSW Mg-Al joint [31,32]. According to the previous literature reports, the significant part of Zn interlayer in joining Al and Mg alloys is obvious since Zn can prevent the excessive reaction between Al and Mg atoms. This phenomenon inhibits Mg-Al and Mg-Zn binary IMC formation. Thus, the mechanical properties of joints are improved [[32], [33], [34], [35]].

In most of the published works, fusion welding methods have been used in order to join the dissimilar lap joints. In the case of lap joint, this is usually associated with the formation of a transition layer of the interlayer material as a barrier between the two base metals. However, to the best of the authors’ knowledge, the application of Zn interlayer in butt joints of 6061 A l alloy and AZ31 Mg alloy made by FSW as a solid state joining technology has not yet been reported.

Thus, the objective of this work is the investigation of the potential of using Zn interlayer in dissimilar FSW butt joint of 6061 aluminum alloy to AZ31 magnesium alloy. The effects of rotational and traveling speeds on macrostructure, microstructure and mechanical behavior (tensile tests and hardness) of the Zn added joints have been investigated to gain a perfect joint. An optimized joint free of Zn interlayer has also been prepared and tested for comparison purposes.