Formation of Liquid and Intermetallics in Al-to-Mg Friction Stir Welding
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has been a growing interest in joining dissimilar metals,[1,2] for instance, Al-to-steel or Al-toMg for weight reduction and Al-to-Cu for electric connections. Joining dissimilar metals by fusion welding is often difficult because of the formation of massive brittle intermetallic compounds in the fusion zone that crack upon cooling. Recently, friction stir welding (FSW)[3] has been used to join dissimilar metals. FSW, which is considered as a solid-state welding process, produces joining by plunging the pin at the bottom of the rotating tool into the workpiece and traversing it along the joint to cause bonding by stirring and mixing. Dissimilar-metal FSW between Al and Mg alloys has been investigated frequently.[4–19] Most of the Al alloys studied were the 6000 series (essentially Al-Mg-Si with small amounts of Mg and Si), 1000 series (commercially pure Al), and 5000 series (essentially Al-Mg), and the Mg alloys were AZ31 Mg (essentially Mg-3Al-1Zn). Thus, the stir zone was essentially a Mg-Al alloy if the small amounts of Zn and Si are neglected. Thus, the binary Al-Mg phase diagram was almost always used to explain the microstructure in the stir zone. Figure 1 shows the binary Al-Mg phase diagram.[20] A schematic sketch consistent with other versions of the VAHID FIROUZDOR, Graduate Student, and SINDO KOU, Professor, are with the Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706. Contact e-mail: [email protected] Manuscript submitted March 8, 2010. Article published online July 27, 2010 3238—VOLUME 41A, DECEMBER 2010
Al-Mg phase diagram[21] is inserted in Figure 1 to show the eutectic on the Al-rich side more clearly. Upon heating up together, such as during FSW, the Al-rich phase (Al) and the Mg-rich phase (Mg) can react with each other in the solid state to form intermetallic compounds Al3Mg2 or Al12Mg17, depending on the local composition. Upon further heating, the eutectic reaction (Mg) + Al12Mg17 fi L occurs at the eutectic temperature of 710 K (437 C) and the eutectic reaction (Al) + Al3Mg2 fi L at the eutectic temperature of 723 K (450 C). These temperatures are more than 200 K (200 C) below the melting point of either Al or Mg. Depending on the welding conditions, they can be reached during Al-to-Mg FSW to form liquid films along the interface between (Al) and (Mg). This liquid formation far below the melting point is called constitutional liquation.[10,22–25] Upon cooling, the two eutectic reactions are reversed, and Al3Mg2 or Al12Mg17 form from the liquid L depending on the local composition. The formation of intermetallic compounds is an important issue in dissimilar-metal FSW because of the following two reasons. First, it can affect the joint strength significantly. Venkateswaran et al.[17] examined the formation of intermetallic compounds in the weld joints. Micro- and nanoindentation characterization methods were used to evaluate the mechanical properties at the interface. The fracture toughness (KIC) showed very low values, which is the primary reason for the b
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