Al-to-Mg Friction Stir Welding: Effect of Material Position, Travel Speed, and Rotation Speed
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E welding of dissimilar metals has been identified[1,2] as a top priority in materials joining technologies, for instance, Al to steel or Al to Mg for weight reduction and Al to Cu for electric connections. Friction stir welding (FSW)[3] is a solid-state welding process in which the pin at the bottom of the rotating tool is plunged into the workpiece and traversed along the joint to cause bonding by stirring and mixing. FSW can be used to join dissimilar as well as similar metals. Recently, interest has been growing rapidly in joining dissimilar metals by FSW. FSW of Al alloys to AZ31 Mg has been investigated frequently recently.[4–17] For butt welding, configurations involving 6061 Al either on the advancing or retreating side of the rotating tool with tool offset to 6061 Al, AZ31 Mg, or neither have been employed. The welding parameters employed in these studies are summarized in Table I. McLean et al.[4] welded 12-mm-thick plates of 5083 Al and AZ31 Mg at 60 mm/min to 100 mm/min (medium travel speed compared with most other studies) and 300 rpm to 400 rpm (low rotation speed compared with most other studies). The tool geometry was not 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 1, 2010. Article published online July 7, 2010 2914—VOLUME 41A, NOVEMBER 2010
specified except that the pin was conical in shape. Based on visual inspection (no tensile testing conducted), it was concluded that Mg on the retreating side results in better welds because the softer alloy (AZ31 Mg) was stirred into the cavity behind the tool when it was on the retreating side. It was suggested that tool offset to Mg was better probably because of the superior flow characteristics of Mg. Sato et al.[9] welded 6-mm-thick plates of 1050 Al and AZ31 Mg at 90 mm/min (medium travel speed) and 2450 rpm (high rotation speed) with a tool of unspecified geometry. No tool offset was used. Temperature measurements with thermocouples showed a peak temperature above 733 K (460 C), which is greater than both eutectic temperatures in the Al-Mg phase diagram (shown subsequently in Figure 3). The stir zone was irregular in shape, containing a large volume of a eutectic solidification microstructure of Mg + Al12Mg17. The eutectic was much harder (about 200 Hv) than either 5083 Al (about 25 Hv) or AZ31 Mg (about 50 Hv). Cracks were observed in the stir zone. These results confirmed liquation (i.e., liquid formation) and liquation cracking during FSW. It was observed that with Al on the advancing side, the welds failed after welding but not with Mg on the advancing side (no tensile testing conducted). No specific explanations for the observed differences were given. Zettler et al.[11,12] welded 2.6-mm-thick sheets of 6040 Al and AZ31 Mg at 200 mm/min and 225 mm/min (high travel speed) and 1400 rpm (medium rotation speed) using a tool of unspecified geometry. No tool offset was used. Tensile testi
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