Effect of Pin Length and Rotation Speed on the Microstructure and Mechanical Properties of Friction Stir Welded Lap Joints of AZ31B-H24 Mg Alloy and AA6061-T6 Al Alloy

Abstract

In the present work, friction stir welding of 3-mm-thick AA6061-T6 Al alloy and AZ31B-H24 Mg alloy sheets was carried out in lap configuration with Al alloy on top. Experiments were carried out by varying the pin length (3.25, 3.75, and 4.25 mm) and tool rotation speed (600, 800, and 1000 RPM). Microstructures of the joint cross-section revealed the presence of a compound intermetallic layer along the interface consisting of Al-rich (Al 3 Mg 2) intermetallic compound near the Al alloy side, and Mg-rich intermetallic compound (Mg 17 Al 12) near the Mg alloy side. The formation of a eutectic mixture of Mg solid solution and Mg 17 Al 12 was observed at the hook on either side. Two modes of failure were observed; Mode 1 through the interface and Mode 2 due to failure of the sheet. The Mode 1 failure strengths obtained were the highest (148 ± 6 N/mm) compared to the literature and were obtained for the 3.25 and 3.75 mm pin at 800 RPM. A lap shear strength of 212 ± 6 N/mm was obtained with 4.25 mm pin length at 600 RPM, which is the highest reported so far and was attributed to Mode 2 failure. The effect of interface pull-up, angle of the interface at the advancing side, and distribution of intermetallic compounds on the lap-shear strength are discussed.

Influences of Welding Parameters on Friction Stir Welding of Aluminum and Magnesium: A Review

“…A.Bandi et al concluded that the intermetallic joining layer and the intermetallic mixture of AA6061-T6 and AZ31B-H24 magnesium plates were presented. It was also observed that Al3Mg2 was obtained in the Aluminium side and a major Eutectic mixture of Al12Mg17 was derived in the Magnesium side [5].…”

Effect of a novel pin tool with shoulder at the top of the needle on the formability, microstructure and properties of Al/Mg dissimilar friction stir lap joint

“…As shown in Fig. 7(a),in the Al/Mg lapping mode, the interstitial layer grows perpendicular to the interface in the fusion zone, indicating insu cient mixing [5] , while the Mg/Al lapping joint shown in Fig. 7(b) shows obvious onion ring structure, indicating good mixig at the interface.…”

“…When aluminum-magnesium alloys are welded by traditional welding procedure, it is easy to form defects such as pores and thermal cracks in the joint due to the large difference between the physical and chemical properties of the two. Moreover, it can be found from the Al-Mg phase diagram that the eutectic reaction temperature of aluminum-magnesium is low, and intermetallic compounds are easy to form in the joint [4,5] .Not only in the traditional arc welding, but also in the laser welding with high energy density and high cooling rate, the formation of intermetallic phase due to the high temperature deteriorates the joint property of the weld [6] . Therefore, to realize the connection of Al-Mg dissimilar alloys, it is necessary to control the heat input during the welding process, so as to reduce the formation of intermetallic compounds.…”

“…Therefore, the stirring tool is the key factor of friction stir welding [8] .Bandi et al [5] studied the microstructure and properties of FSLW joints by changing the length of the stirring needle. They found that the depth of the stirring needle into the lower plate was directly proportional to the degree of mixing of the material at the core of the molten zone.…”

Joining of Dissimilar Al and Mg Metal Alloys by Friction Stir Welding

“…Mg AZ31B-H24 and AA6061-T6 Al were joined with FSW, with a pin range varying from 3.25 mm and 3.75 mm and welding speed (WS) 600–1000 rpm. Tunnel defects were produced at WS 4.25 mm pin at 800 and 1000 rpm [ 15 ]. It is troublesome when intermetallic phases occur during dissimilar welding using conventional welding, because—depending on the thickness—they can seriously compromise the joint integrity [ 16 ].…”