Introduction Ultrasonic soldering/brazing is a fluxless method of joining difficult-tojoin materials, such as aluminum, magnesium, and titanium (Refs. 1–3). This technique has attracted considerable attention since its development in the 1940s and was used in the largescale production of joining aluminum heat exchangers during the 1970s (Ref. 1). Interest in ultrasonic soldering/ brazing has resurged in the last few years because of the increasing need for powerful and reliable methods to join new or dissimilar materials, such as metal matrix composites (Refs. 4, 5), ceramics (Ref. 6), amorphous alloys (Ref. 7), sapphires (Ref. 8), Ti/ stainless steels (Ref. 9), and Cu/ceramics (Ref. 10), which are extremely difficult to wet and join. Ultrasonic soldering/brazing studies have primarily focused on engineering applications (Refs. 1, 2). These investigations have been generally aimed at the engineering or commercialization of a particular procedure or product. Despite being considerably limited, fundamental investigations of solder wetting and joint formation are significant to guarantee or improve joint reliability. Noltingk and Neppiras (Ref. 11) first discovered that cavitation is a fundamental mechanism in ultrasonic soldering. Cavitation is induced when power ultrasonic is introduced into the molten solder. This process disrupts and disperses the oxide layer of a metal surface and allows the molten solder to wet the clean metal surface. The wetting mechanism in ultrasonic soldering has been firmly established and gained widespread acceptance (Ref. 12). However, capillary filling, another important aspect of ultrasonic soldering, has not been well documented, and studies on this issue still provide inconsistent results (Refs. 13–17). Fuchs (Ref. 14) graphically illustrated the interaction of solder and capillary with flux or ultrasonic energy. He stated that flux chemically removes oxides and functions as a wetting agent to reduce surface tension and promote the flow of solder alloy. As a result, the solder climbs in the capillary sometimes to a height that exceeds that of the surrounding solder through wicking. Ultrasonic energy could effectively remove surface oxides but cannot duplicate the wetting agent effect of flux to allow the solder to fill and wet to a height above the level of the surrounding solder. Vianco (Ref. 15) obtained similar results in his study on the wetting and capillary action between two parallel sheets of copper that were vertically dipped in an ultrasonic-activated Sn-Pb solder bath. A small volume of molten solder penetrated into the base of the joint clearance because of the hydrostatic pressure during immersion. Cavitation in the penetrated solder removed WELDING RESEARCH UltrasonicInduced Rising and Wetting of a SnZn Filler in an Aluminum Joint The feasibility of ultrasonicinduced solder capillary rise above the bath level was demonstrated and the underlying mechanism was proposed BY Z. XU, L. MA, J. YANG, J. ZHANG, AND J. YAN ABSTRACT The filling and wetting of a SnZn solder in a vertical aluminum joint clearance under ultrasonic agitation was investigated and the mechanism underlying ultrasonicinduced capillary rise was explored. The liquid solder was pumped into the capillary and rose above the solder bath level to a constant height after 16 s of ultrasonication, even though the base material was not wetted. The surface oxides of the base material were gradually removed from the bottom of the clearance after prolonging the ultrasonic exposure time. This phenomenon resulted in the formation of a metallurgical bond between the solder and the base material. Solder rise increased roughly linearly with the increase in applied ultrasonic amplitude but decreased with the increase in joint clearance and heating temperature. A physical model comprising water and a glass capillary tube was used to examine the acoustic pressure inside and outside of the capillary. Results showed the ultrasonicinduced capillary rise may be attributed to the considerable drop in the acoustic pressure at the entrance of the capillary and the decline in the acoustic pressure gradient along the capillary. KEYWORDS • Ultrasonic Soldering • Capillary Rise • Wetting • Oxide Film • Acoustic Attenuation Z. XU (xuzw@hit.edu.cn), L. MA, J. ZHANG, and J. YAN are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, China. J. YANG is with the Institute of Process Equipment and Control Engineering, Zhejiang University of Technology, Hangzhou, China. 264-s WELDING JOURNAL / JULY 2016, VOL. 95
Welding Journal | July 2016
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