Cold Metal Transfer joining of magnesium AZ31B-to-aluminum A6061-T6

Automotive manufacturers are faced with increasing pressure to reduce vehicle weight, improve fuel economy, reduce emissions, and enhance vehicle safety and performance. Therefore, an increasing number of vehicle structures are built using a combination of dissimilar materials such as steel, aluminum and magnesium. Though the advantages are potentially huge, this hybrid fabrication approach raises substantial technical challenges to the design of vehicle structures and the associated joining processes. Once two elements (e.g., magnesium-aluminum, aluminum-steel) are mixed in a high temperature welding pool, brittle intermetallic phases (IMP) can be easily formed. Experimental observations showed that a series of intermetallic phases will greatly reduce the mechanical performance of the welded dissimilar materials. In this study, welding of 1 mm thick magnesium AZ31B-to-1 mm thick aluminum A6061-T6 using a 1.6 mm diameter aluminum filler wire 4047 was investigated. Cold Metal Transfer (CMT) arc welding technique was adopted. The key feature of this process is that the motion of the wire has been integrated into the overall control of the process. The wire retraction motion assists droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the aid of the electromagnetic force. In this way the heat input and spatter can be controlled and the IMP formation minimized thereby improving the joint strength. Extensive experiments were performed and analyzed. It was found that although extensive efforts have been exercised to control the heat input, Mg-rich intermetallic gamma-Al12Mg17 and Al-rich intermetallic beta-Al3Mg2 were still produced in the weld. Fracture surfaces of CMT welded AZ31B-Al6061-T6 joints exhibited the Mg-rich intermetallic (gamma-Al12Mg17) which contributes to the weld strength degradation. To improve the joint, minimizing the content of the intermetallics especially Mg-rich intermetallics (gamma-Al12Mg17) is essential. (C) 2012 Elsevier B.V. All rights reserved.