WELDING RESEARCH A B C D capillary effects in a liquid metal system, we conducted a simulation experiment involving water capillary rise in a glass tube under ultrasonication. The driving force for nonwetting filling and the relationship between wetting (i.e., surface oxide removal) and ultrasonic capillarity were determined. Experimental Procedures An Al-Cu-Mg alloy (2024 Al-T3) was used as the base material in this study. The chemical composition of the base material is given in Table 1. Rectangle coupons with a dimension of 40 × 10 mm and a thickness of 6 mm were machined, and their surfaces were ground 266-s WELDING JOURNAL / JULY 2016, VOL. 95 Fig. 6 — Microstructure of the wetting interface after 4 s of ultrasonic exposure. with a 500-grade emery paper prior to ultrasonic cleaning. A capillary parallel to the long surface of the coupon was prepared in each coupon through electric-sparking wire-cutting. The capillaries possessed sizes ranging from 300 to 800 μm and a constant length of 30 mm. A Sn-9Zn alloy was used as the filler metal. This alloy is a highly potential lead-free solder that presents a melting point (198°C) close to that of a Sn-37Pb solder (183°C) but exhibits superior shear strength and fatigue resistance. The base material and filler metal were ultrasonically cleaned in acetone for 30 min before the capillary rise test. The experimental conditions are summarized in Table 2. Figure 1 shows the schematic of ultrasonic assisted capillary filling. The specimen was vertically fixed and dipped into a solder bath (depth, 2 mm). The solder was held in a titanium alloy container and heated by a resistance heating system. Ultrasonic energy was transmitted to the solder bath by coupling the sonotrode to the solder container at 0.2 MPa. The sonotrode was operated at 20 kHz, with amplitudes of 10–22 μm on the top surface. The dwell time of the ultrasonic vibration ranged from 4 to 24 s, and the heating temperature ranged from 250° to 350°C. The soldering temperature of Sn-9Zn is generally about 250°C when using flux. The viscosity of the liquid solder would increase without flux since the solder is easily oxidized and covered by an oxide film in air. Heating temperatures higher than 250°C were used in this work. Moreover, examination of the effect of a wide range of heating temperature on soldering is beneficial for fundamental research. Each capillary was cut after the filling test, and the filling height was measured. The microstructure of the filler/base metal interface was observed under an optical microscope and a scanning electron microscope equipped with an energy-dispersive x-ray spectrometer. Specimens for microstructural analysis were prepared using conventional metallographic techniques. A glass tube with a length of 325 mm and dual inner diameters/capillaries was used for simulation. The top capillary was 1.5 mm in diameter and 8 mm in length, followed by a capillary with a diameter of 5 mm to the end. The glass tube was immersed gradually and vertically into the deionized water activated in a normal ultrasonic cleaning tank, and the water rise in the capillary was observed. The maximum power of the ultrasonic cleaning machine was 99 W. The deionized water bath was 50 mm deep. In particular, an acoustic pressure gauge with a customized probe was used to test the acoustic intensity inside and outside of the capillary. The acoustic-intensity measuring device was based on the direct piezoelectric effect of the piezo- Fig. 5 — Overall appearance of the filler metal in the clearance under different ultrasonic exposure times: A — 4 s; B — 8 s; C — 16 s; D — 24 s. Table 2 — Experimental Test Matrix Description Value Coupon size 40 mm × 10 mm × 6 mm Capillary size (300–800 μm) × 30 mm Sn9Zn melting point 198°C Solder bath depth 2 mm Sonotrode pressure 0.2 MPa Ultrasonic frequency 20 kHz Ultrasonic amplitude 10–22 μm Ultrasonic vibration time 4–24 s Heating temperature 250°–350°C
Welding Journal | July 2016
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