Advances in dissimilar metals joining through temperature control of friction stir welding

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Motivation Lightweighting is a topic of great importance to the automotive industry.1 In some instances, incremental improvements to existing designs are insufficient to achieve compliance with Corporate Average Fuel Economy standards.2 New systemlevel and component designs need to be developed in concert with associated manufacturing processes and materials to achieve lightweighting requirements and cost targets.1 Lightweighting is also an important consideration for military vehicles3 and portable structures, such as shelters and bridges. For combat vehicles, lightweighting results in improved range, mobility, and agility. For portable structures, lightweighting enables improved mobility and reduction in personnel and equipment requirements for assembly. In both the commercial automotive and military sectors, new lightweight system-level designs are mandating aggressive component design specifications. In many cases, a single material cannot meet the cost and performance targets demanded. Multimaterial design studies are therefore executed to identify suitable material combinations. Unfortunately, design engineers typically soon discover that appropriate joining methods do not exist for their identified materials combinations. Developing joining techniques to enable multimaterial

design is a thus a challenge of great importance for both commercial1 and military3 vehicles. The joining of aluminum to steel is an example of a material combination that is desirable for component design. Steel is used where increased strength is required and the remainder of the part is lightweight aluminum. However, it is challenging to join these using traditional joining processes, such as fusion welding, because their material properties are dramatically different. Relevant material properties affecting welding processes include density, melting temperature, coefficient of thermal expansion, and flow stress. Another challenge is that brittle intermetallic compounds (IMCs) can form at the Al-steel interface at an elevated temperature;4–6 these reduce the joint strength. These challenges are further amplified by the high heat input and melting associated with traditional fusion welding. Researchers are solving these issues for dissimilar material joining with two approaches—developing improved fusion welding processes7 and using solid-phase joining processes, which require low energy input and do not involve melting. This article provides an overview of friction stir welding (FSW), solid-phase processes, and FSW variants used for dissimilar metals joining (Figure 1). Friction stir dovetailing (FSD) is a new process, developed by the Pacific Northwest

Kenneth Ross, Pacific Northwest National Laboratory, USA; [email protected] Md. Reza-E-Rabby, Pacific Northwest National Laboratory, USA; [email protected] Martin McDonnell, US Army CCDC Ground Vehicle Systems Center, USA; [email protected] Scott A. Whalen, Pacific Northwest National Laboratory, USA; [email protected] doi:10.1557/mrs.2019.181

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