Effect of Pin Length and Rotation Speed on the Microstructure and Mechanical Properties of Friction Stir Welded Lap Join
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INTRODUCTION
WEIGHT reduction has been a continuous focus of the automobile and aerospace industries. Both aluminum alloys and magnesium alloys are potential candidates for replacing steels due to their excellent strength to weight ratio.[1] Magnesium alloys have lower density (~ 1.7 g/cm3) than aluminum alloys (~ 2.7 g/ cm3) but suffer from poor crash resistance and poor corrosion resistance.[2,3] Hence, the joining of aluminum and magnesium alloys could have potential applications in the fabrication of hybrid components making use of their unique properties. Automobile companies have been using Mg/Al combination for making hybrid boot lid and hybrid door.[3] Intermetallic compound formation is inevitable during fusion welding as well as solid-state welding of these alloys, as indicated by the Al-Mg phase diagram.[4] It is well known that the low-melting mixture of intermetallic compounds leads ANIL BANDI and SRINIVASA R. BAKSHI are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Contact email: [email protected]. Manuscript submitted February 19, 2020; accepted September 6, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
to solidification cracking during fusion welding of Al to Mg alloys. Solid-state welding techniques have been explored for dissimilar metal joining in which the temperatures are below the melting point of the alloys. Friction stir welding (FSW) is becoming very popular due to the versatility of joint configurations and its ability to do very long and thick welds. In FSW, a rotating non-consumable tool is plunged into the interface of the two base materials and traversed along the desired path to accomplish the joint. The heat generated due to friction between the rotating tool and the workpiece and from the plastic deformation helps the base materials to reach the plastic regime. The tool exerts hydrostatic pressure on the plasticised material resulting in a sound weld.[5] Various dissimilar combinations such asaluminum to steel,[6] aluminum to copper,[7] aluminum to titanium,[8] and titanium to steel[9] have been joined successfully using FSW in butt configuration. Several studies on dissimilar joining in the butt configuration have shown the formation of alternating bands in the nugget zone.[10] Somasekaran et al. have joined 6061-T6 Al alloy and AZ31B-H24 Mg alloy in butt configuration, and noticed an intercalated microstructure with Al-rich and Mg-rich bands in the nugget zone.[11] Sato et al. have joined AA1050 Al alloy and AZ31 Mg alloy and observed increased mutual diffusion between the
alternating bands in the nugget zone due to the temperature and plastic deformation, leading to constitutional liquation and the formation of intermetallic compounds.[12] Yamamoto et al. have joined AA5083 Al alloy and AZ31 Mg alloy in butt joint configuration. They reported that intermetallic compounds Al3Mg2 and Mg17Al12 were formed in spite of the fact that the temperature was below the eutectic temperature by diffusion. It was sug
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