Al-to-Cu Friction Stir Lap Welding

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JOINING dissimilar materials has been suggested as a top priority in materials joining technology,[1,2] for instance, Al-to-steel or Al-to-Mg for weight reduction, and Al-to-Cu for electric connections. Recently, friction stir welding (FSW)[3] has been used to join dissimilar metals by plunging the pin at the bottom of a rotating tool into the workpiece and traversing it along the joint to cause bonding by stirring and mixing them together. Conventional lap[4–8] and butt[9–18] FSW of Al alloys to pure Cu FSW have been investigated frequently. As shown in Table I, in lap FSW, Al has always been put on top. For convenience of discussion, the Al-Cu phase diagram[19] is shown in Figure 1. According to the phase diagram, when Al and Cu are heated together, several intermetallic compounds can form in the solid state by diffusion, including Al2Cu. Upon additional heating, liquation (i.e., liquid formation) occurs at 821 K (548 C) by the Al + Al2Cu fi L reaction. Elrefaey et al.[5,6] lap welded 2-mm-thick 1100 Al to 1mm-thick commercially pure Cu. The tool was made of a tool steel. The shoulder was concave and 10 mm in diameter. The pin was 3 mm in diameter, 1.7-mm long, and threaded. The tilt angle was 3 deg. The travel speed ranged from 200 to 300 mm/min and the rotation speed ranged from 1000 to 2500 rpm. The resultant welds were peel tested. The welds failed prematurely before testing VAHID FIROUZDOR, formerly Research Assistant, with the Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706, is now a Research Assistant with the Department of Engineering Physics, University of Wisconsin. SINDO KOU, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin. Contact e-mail: [email protected] edu. Manuscript submitted August 2, 2010. Article published online July 20, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

if the pin tip was positioned at 0 mm above Cu, but did not fail if the pin tip penetrated the Cu by 0.1 mm. A dark-etching zone appeared at the interface between 1100 Al and Cu, in which layers of intermetallic compounds existed along the interface and in the Curich fragments nearby in the stir zone. Intermetallic compounds were identified by X-ray diffraction (XRD), including CuAl (g), CuAl2 (h), and Cu9Al4 (c). CuAl2 (h) appeared black, CuAl (g) appeared gray, and Cu9Al4 (c) appeared layered. In one weld made with a 50-lm Zn foil inserted between Al and Cu, the strength was significantly higher because, as suggested, Zn possibly reduced the formation of harmful AlmCun. Abdollah-Zadeh et al.[7,8] lap welded 4-mm-thick 1060 Al to 3-mm-thick commercially pure Cu. The tool was made of a tool steel. The shoulder diameter was 15 mm. The pin was 5 mm in diameter, 6.5-mm long, and threaded. The tilt angle was 3 deg. The resultant welds were tensile tested (shear tensile). The travel speed ranged from 30 to 375 mm/min and the rotation speed ranged from 750 to 1500 rpm. Judging from the transverse macrograph of a weld made at 95 mm/min and 1180 rpm