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TRODUCTION

LIGHT weighting is one of the most important measures for cutting down greenhouse gas emissions, increasing fuel efficiency, and reducing automotive and aircraft component costs.[1–6] Advanced high-strength steels, aluminum alloys, and polymers are being used to reduce weight, but substantial reductions could be achieved by greater applications of magnesium (Mg) alloys. Mg alloys are the lightest structural metallic material with high strength-to-weight ratio and thus are attractive for use in the transportation and mobile electronics applications. Sheet forming processes of wrought Mg alloys have been used to manufacture some sheet-like components, such as door inner components and outer skins of car bodies.[7] In the application of Mg alloys in the automotive and aircraft industries, welding and joining would inevitably be involved. A variety of welding and joining techniques has been used to join Mg alloys including tungsten-inert S.M. CHOWDHURY, Graduate Student, D.L. CHEN, Professor and Ryerson Research Chair, and S.D. BHOLE, Professor, are with the Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada. Contact e-mail: [email protected] E. POWIDAJKO, Graduate Student, D.C. WECKMAN, Professor, and Y. ZHOU, Professor and Canada Research Chair, are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. Manuscript submitted August 2, 2010. Article published online December 15, 2010 1974—VOLUME 42A, JULY 2011

gas welding (TIG), metal-arc inert welding, CO2 laser welding, and solid-state Nd:YAG laser welding.[8–21] For example, Weisheit et al.[17] carried out CO2 laser welding of different Mg alloys and showed that most Mg alloys could be welded easily without serious defects, except AZ series and AM alloy series, which exhibited extremely high levels of porosity. Zhao and Debroy[18] investigated the formation of porosity in the AM60 Mg alloy during laser welding and concluded that hydrogen in the parent material was the main origin of porosity in the welds. They suggested remelting as a remedial measure. Sun et al.[19] evaluated TIG, CO2, and pulsed Nd:YAG laser-welded joints of AZ31 sheet and reported that TIG welding could be used to achieve welds without defects, but they noted that coarser grain sizes in TIG welds could reduce the mechanical properties. Sun et al.[21] employed Nd:YAG laser on AZ31B Mg wrought alloy and observed that weld penetration increased by adding oxide fluxes (TiO2, SiO2, Cr2O3) on the materials to be joined. Although the as-welded fusion zone (FZ) with oxide fluxes had a deeper weld penetration, the tensile strengths of the welded joints with activating fluxes were lower because of the presence of larger grain sizes in the FZ. This was attributed to the fact that the absorptivity of laser energy increased, i.e., more energy was absorbed by the flux coat that was transmitted to the specimen in the early period of laser interaction. These observations indicated that si