Elsevier

Materials & Design (1980-2015)

Volume 65, January 2015, Pages 675-681
Materials & Design (1980-2015)

Microstructural characterizations and mechanical properties in underwater friction stir welding of aluminum and magnesium dissimilar alloys

https://doi.org/10.1016/j.matdes.2014.09.046Get rights and content

Highlights

Abstract

Formation of intermetallic compounds in the stir zone of dissimilar welds affects the mechanical properties of the joints significantly. In order to reduce heat input and control the amount and morphological characteristics of brittle intermetallic compounds underwater friction stir welding of 6013 Al alloy and AZ31 Mg alloy was carried out. Microstructures, mechanical properties, elements distribution, and the fracture surface of the joints were analyzed by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, etc. The result shows that sound dissimilar joint with good mechanical properties can be obtained by underwater friction stir welding. Al and Mg alloys were stirred together and undergone the process of recrystallization, forming complex intercalated flow patterns in the stir zone. Tensile strength of the dissimilar joint was up to 152.3 MPa. Maximum hardness (142HV) appeared in the middle of the centerline of the specimen. Intermetallic compounds layer consisting of Al3Mg2 and Mg17Al12 formed in the Al/Mg interface and resulted in the fracture of the joint.

Introduction

As light alloys with low density, high special strength and good anti-corrosion properties aluminum and magnesium are widely used in transportation and electronic communications industries. In certain applications, the successful welding of dissimilar metals of aluminum and magnesium is advantage for producing lightweight structures, pushing forward the lightweighting technology and the project of energy-saving and emission-reduction [1]. Researchers had tried various fusion welding methods such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) [2], laser welding [3] and solid state welding technology such as vacuum diffusion bonding [4] to join Al–Mg. However, these welding methods are not widely used because of the poor weld joint strength caused by pores, cracks and Al–Mg intermetallic compounds with high hardness and low ductility. In addition, some specific process requirements which are difficult to carry out cause these methods limited.

Being different from fusion welding, friction stir welding (FSW) is a solid-state welding process. In the process of FSW, shouldered tool with a pin rotates and moves between sheets of the pieces to be welded and friction heat which is not sufficient to melt the materials generates. In consequence, materials to be welded reach the plastic state and are joined together under the action of stirring and pressure [5], [6]. Naturally, the obvious lower temperature helps FSW to avoid many defects appearing in fusion welding and the nature features FSW owns make it get higher joint strength. All these make FSW be a potential welding technique in dissimilar materials joining. More details about the process of FSW were described in many earlier publications [7], [8], [9], [10], [11].

Recent years, many efforts had been focused on the application of FSW in dissimilar materials such as dissimilar Al alloys [12], [13], [14], dissimilar Mg alloys [15], Al to steel [16], [17], and Al–Mg [18], [19], [20], [21]. Some researchers had tried to weld Al and Mg alloys under water using the method of FSW and got better mechanical properties than in air [22]. While joints of Al alloys and Mg alloys obtained by FSW under water were sound with good mechanical properties, the details of microstructural evolution of dissimilar joints of Al and Mg alloys have not been fully understood. In this paper, 6013 Al alloy and AZ31 Mg alloy were welded by FSW under water. The microstructure, distribution of elements, hardness, tensile strength and fracture feature of the joints were investigated to make a comprehensive analysis of the microstructures and mechanical properties of the FSW joints of 6013 Al alloy and AZ31 Mg alloy.

Section snippets

Material and experimental procedures

Commercial available base materials (BM) AA6013 aluminum alloy and AZ31 magnesium alloy sheets (100 × 90 × 2.5 mm) were welded in the study. Their chemical compositions and mechanical properties are listed in Table 1. The weld process was done under water at a FSW-3LM-002 machine produced by China Friction Stir Welding Center in this experiment. The shouldered tool which was made of H13 steel was 16 mm in diameter and concave, with a 2.5 mm long and 5 mm in diameter threaded cylindrical probe. During

Appearances and microstructures

Fig. 1 shows the surface appearance of the friction-stir joint of Al 6013 and Mg AZ31 performed under water. It can be seen that sound weld without obvious defects such as cracks or tunnel type defects was obtained when rotation speed was 1200 rpm and travel speed was 80 mm/min with the pin moving along the center. This is contrary to some other researcher’s result. Yan et al. [23] reported that if rotating pin traveled along the butt line between the two base materials cracks developed.

A

Conclusions

6013 Al alloy and AZ31 Mg alloy were FS-welded under water in this study and from the above investigations, the main conclusions can be summarized:

  • (1)

    Sound welding with good mechanical properties was produced between 6013 Al alloy and AZ31 Mg alloy through FSW under water with the welding speed of 80 mm/min and rotation speed of 1200 rpm.

  • (2)

    EPMA images show that elements of Al and Mg are stirred together and mixture structure form in the stir zone. The EDS analysis indicates that the mixture structures

References (29)

Cited by (147)

  • Material flow during dissimilar friction stir welding of Al/Mg alloys

    2024, International Journal of Mechanical Sciences
View all citing articles on Scopus
View full text