WELDING RESEARCH JULY 2016 / WELDING JOURNAL 267-s electric ceramic. Piezoelectric ceramics were installed in a stainless steel probe with a diameter of 4 mm, which was smaller than the inner diameter of the glass tube applied. The acoustic pressure test in the ultrasonic activated water rise process is illustrated in Fig. 2. Results Characteristics of the UltrasonicInduced Solder Rise Figure 3 illustrates the capillary rise of the filler metal without exposure to ultrasonic vibration. Limited solder rise was observed in the capillary; specifically, it was markedly lower than the solder bath level. Evident cracks appeared between the solder and the base material, demonstrating a nonwetting status between them. When the capillary was immersed in the molten solder, the solder rise was depressed because the surface oxides of the base material hindered the wetting and the capillary force did not work. However, solder rise still occurred under this condition. In fact, the rise resulted from a competition between dewetting effect and hydrostatic pressure. Figure 4 shows the effective capillary rise of the filler metal as a function of dwell time for the 6-mm-thick specimen exposed to 20 μm of ultrasonic vibration. The clearance value was 300 μm, and the heating temperature was 300°C. In all cases, the filler metal reached a level that exceeded that of the bath. The filling height increased with ultrasonic exposure time and reached a relatively constant level when the ultrasonic exposure time exceeded 16 s. Figure 5 reveals the overall appearance of the filler metal in the clearance under different ultrasonic exposure times. Except at the capillary bottom, the filler metal was not compact and remained unbonded to the base metals when the ultrasonic exposure time was 4 s — Fig. 5A. The filler metal appeared to be injected from the capillary bottom in this case. The filler metal in the clearance was compact and did not contain any void or porosity when the ultrasonic exposure time exceeded 4 s. Whether or not the solder wet the base metal is difficult to detect in Fig. 5. Detailed examinations of the wetting interface between the solder and the base metal are illustrated in Figs. 6–8. After 4 s of ultrasonic exposure, poor wetting was observed along the solder/ base metal interfaces; this phenomenon involved a number of small cavities and oxide inclusions — Fig. 6. The wetting of the solder to the base metal at the bottom of the capillary was markedly improved when the ultrasonic exposure time was prolonged to 8 s. The solder/base metal interfaces were free of oxide inclusions and contained nearly negligible cavities — Fig. 7A. In fact, only one suspected cavity of less than 50 μm was observed at the wetting interfaces (inset of Fig. 1). The backscattered image of the clearance bottom shows that aluminum dendrites nucleated at the wetting interfaces and grew into the solder — Fig. 7B. This finding indicates that wetting occurred between the solder and the base metal. However, the diffusion of solder into the aluminum substrate was limited. This result can be inferred to the Sn-Al and Zn-Al phase diagrams (Refs. 10, 18) showing that the solder constituents (i.e., elemental Sn and Zn) do not react with the aluminum substrate to form intermetallic compounds and that the solubility of Sn in pure Al is very low from ambient temperature to 300°C. In stark contrast to the bottom of the capillary, poor wetting was still observed at the solder/base metal interfaces of the solder head — Fig. 7C. Numerous oxide inclusions and pores were present at these zones, preventing metal-metal Fig. 7 — Microstructure of the wetting interface after 8 s of ultrasonic exposure: A — clearance bottom; B — backscattered image of the clearance bottom; C — solder head. A B C
Welding Journal | July 2016
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