Characterization of Steel-Ta Dissimilar Metal Builds Made Using Very High Power Ultrasonic Additive Manufacturing (VHP-U
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INTRODUCTION
ULTRASONIC Additive Manufacturing UAM is a solid-state processing technique that uses ultrasonic vibrations to bond metal tapes into near net-shaped components. A metal tape is initially bonded to a substrate material with additional tapes being subsequently bonded to the previous layer.[1] Ultrasonic vibrations of 20 kHz are locally applied to the metal foils through a sonotrode under a static normal force.[2] However, due to the limited power input (1.5 kW) of the conventional UAM system, adequate and consistent bonding is not achieved.[9] This shortcoming is addressed with the development of the very high power ultrasonic additive manufacturing (VHP-UAM) with greater ultrasonic power (9 kW). High power is realized through larger vibration amplitudes (52 lm as against 26 lm in the conventional UAM) and higher normal force levels (15 kN as against 2 kN in the conventional UAM). A machine delivering up to 9 kW of power at 20 kHz resonant frequencies has been developed by Fabrisonics.[9] The effect of the process parameters such as normal force, amplitude and travel speed on the bond quality has been investigated in detail using mechanical tests such as shear and transverse tensile testing[3] and NIYANTH SRIDHARAN, formerly Graduate Student, and SUDARSANAM SURESH BABU, formerly Professor with the Department of Welding Engineering, The Ohio State University, Columbus 43221, are now with the Department of Mechanical Aerospace Biomedical Engineering, University of Tennessee, Knoxville 37916. Contact e-mail: [email protected], niyanth.sridharan@ gmail.com MARK NORFOLK, President, is with Fabrisonic, Columbus 43221. Manuscript submitted July 31, 2015. Article published online February 24, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
using the concept of linear weld density (LWD). The LWD is the ratio of the bonded interface to the total length of the interface and is measured using an optical microscope.[4,5] It has been shown that the LWD and bond strength is related directly to the sonotrode horn texture.[1] Apart from the LWD, mechanical strength of these bonds also related to extent of metallurgical bonding between the foils.[3] Microstructural evolution is the key to understand the extent of metallurgical bonding. Work on VHP-UAM has been concentrated mostly on FCC metals. Recent publications indicate that the bond formation occurs due to complex interactions of many phenomena including mechanical interlocking due to plastic deformation,[6] frictional heating, deformation heating, recrystallization, diffusion, and contact between nascent surfaces.[2] Extensive microscopy conducted on the Aluminum builds using dual beam FIB in conjunction with TEM revealed that the microstructure at the interfaces are affected by severe plastic deformation (SPD) of bulk (5 lm) resulting in the grain refinement (0.5 lm to few nanometers).[1,7] The above observation confirms that the UAM process affects only the grains at the interface and does not have any major long-range effects into the bulk of materials. David Schic
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