A Method for the Estimation of the Interface Temperature in Ultrasonic Joining
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ULTRASONIC joining is a rapid process in which parts/materials clamped under normal compression are metallurgically bonded by the application of local highfrequency vibration, usually in a fraction of a second. The process takes place at ambient conditions, without heating or a protective atmosphere.[1] Thus, it is widely adapted in electronics, auto, and aerospace industries.[2–4] New applications have also evolved in ultrasonic additive manufacturing[5,6] and more recently ultrasonic powder consolidation (UPC).[7] Despite the wide industrial acceptance and potential for future innovations, however, understanding of the fundamental mechanism(s) of ultrasonic joining is still lacking. It is known that this process involves local, high plastic strains applied at high cyclic rates.[1–7] Researchers have thus explored the consequences of such strains at an interface between two materials, including solidstate bonding,[6,8–15] local melting,[10,16] and mechanical interlocking.[8,12] One major debate is whether or not ultrasonic joining is truly solid state; if this is the case, the rapidity of bonding indicates greatly enhanced diffusivity, due to (i) increased vacancy concentration and/or (ii) increased temperature at the interface Tint. High strain-rate plastic deformation of a metal, as in ultrasonic joining, introduces large amounts of excess vacancies in the metal by the nonconservative motion of jogs on screw dislocations[17–19] and/or possibly by other mechanisms.[20–23] Vacancy mole fractions many orders of magnitude above the thermal equilibrium value have been estimated by electron microscopy,[20,22,24] calorimetry,[25] TIANYU HU and SOHEIL ZHALEHPOUR, Graduate Students, ANDREW GOULDSTONE, Associate Professor, and SINAN MUFTU and TEIICHI ANDO, Professors, are with the Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115. Contact e-mail: [email protected] Manuscript submitted July 15, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
electrical resistivity measurements,[25,26] X-ray diffraction (XRD),[25] and nuclear magnetic resonance (NMR).[27] Figure 1 shows a TEM micrograph of pure aluminum wire subjected to ultrasonic vibration for 1 second at 773 K (500 C) nominal temperature.[26] Numerous vacancy clusters and Frank loops about 3 to 10 and 20 to 30 nm in diameter, respectively, are found, attesting to the prior presence of excess vacancies in a very high concentration. The minimum value of prior mono-vacancy concentration XV can be assessed from the density of clusters and loops with XV = (4/3)pr3n/h (clusters) and XV = pr2bn/h (loops),[28] where r is the radius of clusters or loops, b is the Burgers vector, n is the number of clusters or loops per unit TEM view area, and h is the thickness of the TEM specimen. This gives XV ~ 104 for the specimen in Figure 1. Other TEM studies[18–20,22] also report XV ~ 104. However, these XV values do not include monovacancies that annihilate at sinks, especially dislocations that should be created in high density in a
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