13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- from ~1750 J to ~1500 J in the Mg/HSLA steel dissimilar joint. 4. Conclusions and Remarks 1. The ultrasonic spot welding of AZ31B-H24 Mg alloy to Al5754-O Al alloy and AZ31B-H24 Mg alloy to HSLA steel sheet with a Sn interlayer was successfully performed, with a characteristic composite-like Sn and Mg2Sn eutectic structure present in the interlayer of both dissimilar joints. 2. The lap shear strength of Mg/Al dissimilar joints with a Sn interlayer was achieved to be significantly higher than that of Mg/Al dissimilar joints without interlayer. This improvement was mainly attributed to the formation of solid solutions of Sn with Mg and Al as well as the composite-like Sn and Mg2Sn eutectic structure in the interlayer, which effectively prevented the occurrence of brittle Al12Mg17 intermetallic compound present in the Mg/Al dissimilar joints without interlayer. The fact that Sn and Mg2Sn were located on both Al and Mg sides of matching fracture surfaces indicated that the tensile shear failure occurred through the interior of the interlayer in the mode of “cohesive failure”. 3. The lap shear strength of Mg/HSLA dissimilar joints with a Sn interlayer was observed to be much higher than that of Mg/HSLA dissimilar joints without interlayer. Sn interlayer actively worked as an intermediate medium to join Mg to Fe by the formation of solid solutions of Sn with Mg and Fe as well as the composite-like Sn and Mg2Sn eutectic structure in the interlayer. 4. In addition to the beneficial role of enhancing the lap shear strength in both Mg/Al and Mg/HSLA steel dissimilar joints, the addition of Sn interlayer further led to energy saving since the welding energy required to achieve the maximum lap shear strength decreased from 1250 J to 1000 J in the Mg/Al dissimilar joint and from 1750 J to 1500 J in the Mg/HSLA steel dissimilar joint. 5. Further studies are needed to (1) explore other potential interlayers, such as, Zn, Ni, Cu, etc., for further improving the strength of the USWed Mg/Al and Mg/HSLA steel dissimilar joints and (2) evaluate the fatigue and dynamic (or impact) resistance of the dissimilar joints for the safe and reliable applications of the welded lightweight components. Acknowledgements The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and AUTO21 Network of Centers of Excellence for providing financial support. This investigation involves part of Canada-China-USA Collaborative Research Project on the Magnesium Front End Research and Development (MFERD). The authors thank Dr. A.A. Luo, General Motors Research and Development Center for the supply of test materials. One of the authors (D.L. Chen) is grateful for the financial support by the Premier’s Research Excellence Award (PREA), NSERC-Discovery Accelerator Supplement (DAS) Award, Canada Foundation for Innovation (CFI), and Ryerson Research Chair (RRC) program. The assistance of Q. Li, A. Machin, J. Amankrah, and R. Churaman in performing the experiments is gratefully acknowledged. References [1] J. Murray, D. King, Oil's tipping point has passed. Nature, 481 (2012) 433−435. [2] T.M. Pollock, Weight loss with magnesium alloys. Science, 328 (2010) 986−987. [3] R. Qiu, C. Iwamoto, S. Satonaka, Interfacial microstructure and strength of steel/aluminum alloy joints welded by resistance spot welding with cover plate. J Mater Proc Technol, 209 (2009) 4186−4193.
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