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Introduction The automotive industry is addressing forthcoming legislated greenhouse gas emission targets with a two-pronged approach. Electrification of vehicles in the transportation sector has recently received major attention, however, the added mass associated with battery packs, for example, is significant. A second approach to achieving greenhouse gas emission targets is mass savings through the use of lightweight materials. Historically, mass saving initiatives have focused on specific material intensive solutions such as aluminum, magnesium, ultrahigh strength steels, and composites. However, over time, material selection has migrated to a strategy of identifying the right material, in the right form, for the right application. This then drives a mixed materials solution for automotive body structures where dissimilar materials need to be joined. The current industrial infrastructure is based upon the joining of similar materials, where resistance spot welding is the mainstream solution for steel to steel joints and self-piercing riveting or resistance spot welding are mainstream solutions for aluminum to aluminum joints. In order to take full advantage of the installed resistance spot-welding technology in

current production plants, General Motors (GM) has worked to create resistance spot-welding solutions for steel to steel, aluminum to aluminum, and aluminum to steel mixed material joints. By doing so, the introduction of a second joining technology as well as the accompanying investment in equipment and floor space in the body shop assembly plants is avoided.

History of aluminum resistance spot welding In the 1800s, Joule discovered that the passage of electricity through resistive elements produces heat, and this heat could be used to join these elements, such as metal wires, together.1 Thomson patented the combination of joule heating with the application of pressure in 1891,2 but it would not be until decades later that aluminum resistance spot welding would be used to produce aluminum aircraft structures,3 which helped bolster the American war machine during World War II.4 Resistance spot welding has seen little change since the technology’s inception; however, developments in power electronics have enabled precise control of heat generation in the weld region to overcome difficulties associated with welding aluminum. The electrical and thermal conductivities

Blair E. Carlson, Lightweight Systems Manufacturing Research Group, General Motors Global R&D, USA; [email protected] Amberlee S. Haselhuhn, Lightweight Systems Manufacturing Research Group, General Motors Global R&D, USA; [email protected] Jian Chen, Materials Joining Group, Oak Ridge National Laboratory, USA; [email protected] Zhili Feng, Materials Joining Group, Oak Ridge National Laboratory, USA; [email protected] doi:10.1557/mrs.2019.179

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