Microstructural Investigation of the Impact Weld Interface of Pseudo Single Grained Cu and Ag
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pact welding is a solid-state joining technology that enables metallurgical bonding by collision of two metals. A high-speed oblique collision between flyer and target metallic components ejects the original surfaces in the form of a jet, thus the nascent metals come in contact and bond.[1] It has significant advantages for dissimilar metals joining as the heat of fusion welding (and even friction stir welding) generally has issues from thermal changes in the microstructure, such as formation of intermetallic compounds and heat affected zones. The weld interface typically shows an asymmetric wave-like morphology. The heat generated from the collision often shows some evidence of melting isolated into small pockets under the crest of the
TAESEON LEE, TAYLOR DITTRICH, ANUPAM VIVEK, and GLENN S. DAEHN are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, 43210. Contact e-mail: [email protected] MENGLIN ZHU and JINWOO HWANG are with the Department of Materials Science and Engineering, The Ohio State University and also with the Center for Electron Microscopy and Analysis, The Ohio State University, Columbus, OH, 43212. Manuscript submitted June 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
interfacial waves.[2] This is due to the heterogeneous temperature rise along the weld interface, as the tip of the interfacial waves requires more deformation.[2,3] Previous work with rigorous microscopic characterization has contributed to the understanding of interfacial zones, as they are the primary locations of intermetallic phases in dissimilar welds.[4–7] The morphology and composition at the interface is highly heterogeneous due to sliding, severe plastic deformation, jetting, and a rapid cooling rate.[4] The high pressure caused from collision also assists formation of metastable phases.[4,8] While many aspects of the interface remain unclear, the best mechanical properties are likely from the regions where there seems to be no obvious melting. There is debate on the mechanism of interface structure formation in impact welding. Li et al.[7] claim that some amount of melting always occurs in an aluminum-steel weld, even in the sharpest interfaces, and the rapidly cooled molten metals form a very thin (~ 30 nm) transition zone. The cooling rate in those transition zones is considered to be fast enough (up to 108 K/s, depending on composition and thickness) to form a non-crystalline structure, thus the amorphous structure of this layer[4] is considered to be indicative of melting. However, it is possible that an amorphous interface can still be created without melting, as severe plastic deformation can induce amorphization of metals.[5] The solid-state amorphization of a thin interfacial layer is also believed to be the bonding mechanism of ultrasonic welding.[6] Furthermore, a polycrystalline[9] structure is also observed in the impact weld interface, implying that the amorphization of the interface is not universal and may be related to the composition of liquid produced,
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