Effects of High-Speed Deformation on the Phase Stability and Interdiffusion in Ultrasonically Joined Aluminum and Zinc F
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Effects of High-Speed Deformation on the Phase Stability and Interdiffusion in Ultrasonically Joined Aluminum and Zinc Foils Ibrahim Emre Gunduz1, Emily D. Shattuck2, Teiichi Ando1, Peter Y. Wong2, Charalabos C. Doumanidis3 1 Mechanical and Industrial Engineering Department, Northeastern University, Boston MA 02115 2 Department of Mechanical Engineering, Tufts University, Medford, MA 02155 3 Department of Mechanical and Manufacturing Engineering, University of Cyprus, Cyprus
ABSTRACT The effects of high strain-rate deformation on the phase stability and interdiffusion were investigated for Al-Zn welds produced by ultrasonic welding at 513 K. The welds exhibited three distinct regions: a featureless region indicative of local melting on the zinc side, a solidified mushy layer and a layer of fcc grains enriched with zinc. Al-Zn phase diagrams calculated from vacancy-modified Gibbs free energy curves indicate that local melting at the weld interface may result even at 513 K if the vacancy concentrations in the fcc and hcp solutions approach 0.07 as a result of high strain-rate deformation. EDS analysis of the weld interface yielded an interdiffusivity of 1.9 µm2/s, which is five orders of magnitude larger than the normal diffusivity of zinc in aluminum at 513 K. Application of the mono-vacancy diffusion mechanism to the diffusion data also yields a vacancy concentration of 0.07, indicating that such a high vacancy concentration may indeed resulted during the ultrasonic welding at 513 K.
INTRODUCTION Ultrasonic metal welding is a process for joining similar and dissimilar metals by introducing a high-frequency vibratory energy into the weld zone of the metals to be joined [1]. Two metals, usually in the form of thin foils or thin wires, are clamped together and displaced with respect to each other at ultrasonic frequencies to apply shear deformation at the interface to disperse surface contaminants and oxides, providing pure metal contact and subsequently forming a bond. Further application of the ultrasonic vibration results in repeated deformation of the metals. The metals experience high-rate cyclic plastic deformation with strain rates up to ~103. The process is ideal for investigating the effects of high-speed plastic deformation on metals due to the ability to repeatedly deform the metals with minimal heat generation. Studies on high speed deformation of fcc metals [2,3] provide direct evidence for generation of excess vacancies in the metals, the concentration of which may reach as high as 10-1. Such a huge amount of vacancies created should have a substantial effect on the thermodynamics and microstructural evolution of the metals being deformed. In the present study, the effects of high strain-rate deformation on the phase stability and interdiffusion in the Al-Zn system is investigated. The effects of excess vacancies on the stability of the phases involved in the Al-Zn system were assessed by thermodynamic calculations using a solution model that accounts for the presence of the excess vacancies.
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